500 domi exhaust tunning

I’m looking for some help / info on norton 500 domi exhaust tunning, I’ve got 1” 5/8 OD pipes at 38” long and 1”3/8 OD at 40” long with 14” long skinny megga that opens up to 60mm in to a silencer no reverse cone the silencer has a 50mm outlet and 8” long to keep to the UK 105DB rule
The engine revs to 7200rpm with 12.5” inlet total lengh from the valve seat. With 1”5/32 twin carbs
The engine has the full JS kit ie conrods 10.5-1 hi comp pistons with a 0.020” head gasket, js 2 cam timed to spec same as the PW3 profile but smoother, JS valves and springs, head has been ported both on the exhaust and inlet side one piece crank basically the best of everything including a TTI 5 speed gearbox and I can’t get it above 40BHP at the back wheel, I’ve tried different inlet lengths with this being the best in the midrange, but tried a couple of exhausts and made no difference, where am I going wrong? Any help would be appreciated.

Current exhaust set up and curved inlet manifold to get around the oil tank pics, all to club rules to the pre 1955 class of race bike

That looks lovely Brian !

What is the power delivery like? Are you suffering any flat or rough spots? Or is it smooth and clean all the way through?

I have no experience of 500 Norton twins, so am fully qualified to ask daft questions, like these:

What power are you expecting? For a relatively long stroke 500 twin I would have thought that a genuine 40rwhp was good. That’s as much as many road going 850 Commandos produce in real life ! Does your dyno man have a data base to compare against?

Are you running methanol ? If using petrol, is the A/F ratio good and what kind are you using ?

How is it on the track? Do you keep up with the opposition that supposedly has more power? I had this with a triple once, I was frustrated not to be getting the numbers that Pecket bikes all claimed, but then it dawned on me that my bike was no slower than them on the straights, even though I’m no racing jockey!

However, I would have been thought it should rev higher than that, so…

Carbs: do you have access to any bigger carbs to try? With such a big cam, I’d have thought it really warranted bigger carbs ? And if you haven’t already done so, I’d try those carbs without the long stacks, and on straight manifolds.

Exhaust pipes: when I tried 1.5” pipes on my 850 it de-tuned it. Considerably! The stock 1 3/8 definitely worked best. Using that evidence I’d ask if your downpipes are too big?

Megaphones: where did you get the mega dimensions from? Have you tried with simple straight pipes?

Cam: as I’ve already said, I know next to nothing about 500 Nortons, but a JS#2 is a big cam. Are you sure it’s good in a 500? IIRC the stock Commando cam is the same as fitted to 650SS and 500 racers?

CR: I know nothing about your piston to head or valve to piston clearance etc, but wonder if 10.5:1 is enough for a racing 500? I’d have thought 11 or 12 would be more like it? For ref I’m running 11:1 in a 920 and 10.8:1 in a 1007 with no ill effects.

Obviously, playing with a different cam is a big job, but doing some dyno work with some 30 or 32mm concentrics and some mock up old 1 3/8 pipes should be easy?

It is highly commendable that you are using a Domi 88. I have always believed it to be the best of the heavyweight twins. Even more so with a 5 or 6 speed cr box. If you are reving to 7200 a standard pre-alternator crank and rods are perfectly good enough. Mine survived 8 years of club racing.
I don’t like that exhaust. For mine I copied the Commando 750 Formula Racer 2 into 1 system and it was good. With no silencing it was much quieter than a twin pipe/open megaphone system. I suspect your silencer is having a disastrous effect on the power.
I used a 650SS head (Doug Hele said it didn’t give any more power than he got from the earlier head, he used it to clear the oil tank with the carbs). I tried Daytona, SS, and 4S cams. The 4S gave an enormous power increase over the SS. Your JS2 is probably similar to the 4S.
Good luck.

There is a section on Domi 500 s and iirc, some info about race tuning them in one of my Norton books. I will give that a read and report back.
40 rwhp seems a stout number for a 500.
Did the dyno operator use a correction factor such as SAE, Din etc? I am learning that most dyno results have a cf involved. Many are actually an estimation of crankshaft BHP but are almost always reported as RWHP since the reading was done at the rear wheel.
I recall that the stock 500ss, a hot little bike, was rated at about 40 bhp at crank, so you've done well if you now have 40 rwhp.

Glen

Here's a link from @johnm giving some advice. Hopefully he'll get that bike running again!

40 BHP at the rear wheel is plenty. Gearing and handling are more important than power. Raising the overall gearing in combination with close ratios can make the torque more usable. It might work better with a 2 into 1 exhaust, but with a featherbed frame that can be difficult.

acotrel said:

40 BHP at the rear wheel is plenty. Gearing and handling are more important than power. Raising the overall gearing in combination with close ratios can make the torque more usable. It might work better with a 2 into 1 exhaust, but with a featherbed frame that can be difficult.

Click to expand...

2 into 1 on a featherbed is easy - pipes go down, under engine, join up under gb near non drive side, megaphone comes out close to swinging arm and frame, bends up to give bags of ground clearance.

Fast Eddie said:

That looks lovely Brian !

What is the power delivery like? Are you suffering any flat or rough spots? Or is it smooth and clean all the way through?

I have no experience of 500 Norton twins, so am fully qualified to ask daft questions, like these:

What power are you expecting? For a relatively long stroke 500 twin I would have thought that a genuine 40rwhp was good. That’s as much as many road going 850 Commandos produce in real life ! Does your dyno man have a data base to compare against?

Are you running methanol ? If using petrol, is the A/F ratio good and what kind are you using ?

How is it on the track? Do you keep up with the opposition that supposedly has more power? I had this with a triple once, I was frustrated not to be getting the numbers that Pecket bikes all claimed, but then it dawned on me that my bike was no slower than them on the straights, even though I’m no racing jockey!

However, I would have been thought it should rev higher than that, so…

Carbs: do you have access to any bigger carbs to try? With such a big cam, I’d have thought it really warranted bigger carbs ? And if you haven’t already done so, I’d try those carbs without the long stacks, and on straight manifolds.

Exhaust pipes: when I tried 1.5” pipes on my 850 it de-tuned it. Considerably! The stock 1 3/8 definitely worked best. Using that evidence I’d ask if your downpipes are too big?

Megaphones: where did you get the mega dimensions from? Have you tried with simple straight pipes?

Cam: as I’ve already said, I know next to nothing about 500 Nortons, but a JS#2 is a big cam. Are you sure it’s good in a 500? IIRC the stock Commando cam is the same as fitted to 650SS and 500 racers?

CR: I know nothing about your piston to head or valve to piston clearance etc, but wonder if 10.5:1 is enough for a racing 500? I’d have thought 11 or 12 would be more like it? For ref I’m running 11:1 in a 920 and 10.8:1 in a 1007 with no ill effects.

Obviously, playing with a different cam is a big job, but doing some dyno work with some 30 or 32mm concentrics and some mock up old 1 3/8 pipes should be easy?

Click to expand...

Fast Eddie said:

That looks lovely Brian !

What is the power delivery like? Are you suffering any flat or rough spots? Or is it smooth and clean all the way through?

I have no experience of 500 Norton twins, so am fully qualified to ask daft questions, like these:

What power are you expecting? For a relatively long stroke 500 twin I would have thought that a genuine 40rwhp was good. That’s as much as many road going 850 Commandos produce in real life ! Does your dyno man have a data base to compare against?

Are you running methanol ? If using petrol, is the A/F ratio good and what kind are you using ?

How is it on the track? Do you keep up with the opposition that supposedly has more power? I had this with a triple once, I was frustrated not to be getting the numbers that Pecket bikes all claimed, but then it dawned on me that my bike was no slower than them on the straights, even though I’m no racing jockey!

However, I would have been thought it should rev higher than that, so…

Carbs: do you have access to any bigger carbs to try? With such a big cam, I’d have thought it really warranted bigger carbs ? And if you haven’t already done so, I’d try those carbs without the long stacks, and on straight manifolds.

Exhaust pipes: when I tried 1.5” pipes on my 850 it de-tuned it. Considerably! The stock 1 3/8 definitely worked best. Using that evidence I’d ask if your downpipes are too big?

Megaphones: where did you get the mega dimensions from? Have you tried with simple straight pipes?

Cam: as I’ve already said, I know next to nothing about 500 Nortons, but a JS#2 is a big cam. Are you sure it’s good in a 500? IIRC the stock Commando cam is the same as fitted to 650SS and 500 racers?

CR: I know nothing about your piston to head or valve to piston clearance etc, but wonder if 10.5:1 is enough for a racing 500? I’d have thought 11 or 12 would be more like it? For ref I’m running 11:1 in a 920 and 10.8:1 in a 1007 with no ill effects.

Obviously, playing with a different cam is a big job, but doing some dyno work with some 30 or 32mm concentrics and some mock up old 1 3/8 pipes should be easy?

Click to expand...

Thanks Eddie, from 3500Rpm it pulls very good and clean with no flat spots with those carbs on any lower than that there is a flat spot, when I had the GPs on it had a huge flat spot from 4200 to 5500Rpm

I’d be happy if I could get an extra 5-6bhp from it, I’ve heard these can go from 48 to 52bhp

Yes there’s an other 500 with 49bhp but I’ll not find out what he’s done with his as it’s all kept close to the chest. But my dyno guy uses this as a guide

I’m running 105 octane race fuel from Sunoco it seems good stuff. I’m pretty much bang on just slightly on the rich side to aid with cooling especially with sitting in the holding areas in the heat.

It handles good on the track but the 350’s and same class just pull away from me which is frustrating. I could Rev it to 8000RPm but there’s no point as the power band drops off at 7000RPM and with the 5 speed gearbox it goes through the gears pretty quickly

Carbs I’m at my max with them there’s no point in going any bigger for the 500, I’ve tried with no carb spacers the power drops off a cliff with the manifolds on I’m up 6-7bhp and 5 flbs torque in the midrange the longer you make the spacers the more power you will get in the mid range, that’s why the works domiracers in 61 had really long inlets.

Where they 1”3/8 OD? I’ve got a set of those at 40” might chop 10” or more off to see what happens on the dyno.

Unfortunately UK regulations it has to have a silencer on to 105DB and they are very strict on that and have static db meters at points around the track.
I will how ever try them on the dyno to see what it dose.

hybridracer said:

It is highly commendable that you are using a Domi 88. I have always believed it to be the best of the heavyweight twins. Even more so with a 5 or 6 speed cr box. If you are reving to 7200 a standard pre-alternator crank and rods are perfectly good enough. Mine survived 8 years of club racing.
I don’t like that exhaust. For mine I copied the Commando 750 Formula Racer 2 into 1 system and it was good. With no silencing it was much quieter than a twin pipe/open megaphone system. I suspect your silencer is having a disastrous effect on the power.
I used a 650SS head (Doug Hele said it didn’t give any more power than he got from the earlier head, he used it to clear the oil tank with the carbs). I tried Daytona, SS, and 4S cams. The 4S gave an enormous power increase over the SS. Your JS2 is probably similar to the 4S.
Good luck.

Click to expand...

Thanks, I thought I’d brake the mould as there’s not many 88’s out there racing anymore.
It definitely shifts but would like to get it up to 45bhp atleast to make it more competitive
I’ve got a mick Hemmings one piece crankshaft and Carrillo steel conrods with ultra lightweight pistons
I’ve got to use that exhaust as per club rules and UK race regulations as they are strict on 105db’s

worntorn said:

There is a section on Domi 500 s and iirc, some info about race tuning them in one of my Norton books. I will give that a read and report back.
40 rwhp seems a stout number for a 500.
Did the dyno operator use a correction factor such as SAE, Din etc? I am learning that most dyno results have a cf involved. Many are actually an estimation of crankshaft BHP but are almost always reported as RWHP since the reading was done at the rear wheel.
I recall that the stock 500ss, a hot little bike, was rated at about 40 bhp at crank, so you've done well if you now have 40 rwhp.

Glen

Click to expand...

That would be a great help if you could dig them out.

Brian Riley said:

Thanks Eddie, from 3500Rpm it pulls very good and clean with no flat spots with those carbs on any lower than that there is a flat spot, when I had the GPs on it had a huge flat spot from 4200 to 5500Rpm

I’d be happy if I could get an extra 5-6bhp from it, I’ve heard these can go from 48 to 52bhp

Yes there’s an other 500 with 49bhp but I’ll not find out what he’s done with his as it’s all kept close to the chest. But my dyno guy uses this as a guide

I’m running 105 octane race fuel from Sunoco it seems good stuff. I’m pretty much bang on just slightly on the rich side to aid with cooling especially with sitting in the holding areas in the heat.

It handles good on the track but the 350’s and same class just pull away from me which is frustrating. I could Rev it to 8000RPm but there’s no point as the power band drops off at 7000RPM and with the 5 speed gearbox it goes through the gears pretty quickly

Carbs I’m at my max with them there’s no point in going any bigger for the 500, I’ve tried with no carb spacers the power drops off a cliff with the manifolds on I’m up 6-7bhp and 5 flbs torque in the midrange the longer you make the spacers the more power you will get in the mid range, that’s why the works domiracers in 61 had really long inlets.

Where they 1”3/8 OD? I’ve got a set of those at 40” might chop 10” or more off to see what happens on the dyno.

Unfortunately UK regulations it has to have a silencer on to 105DB and they are very strict on that and have static db meters at points around the track.
I will how ever try them on the dyno to see what it dose.

Click to expand...

Yes I realise you need silencing, but trying straight pipes on the dyno will at least tell you if the megaphone shape is the issue. My understanding is that so long as the silencer is perforated, and a larger ID than the end of the pipe, it has little to no effect on power. A silencer designed to take Supertrapp discs allows a lot of tuning by adding / subtracting discs and different reverse cones.

Megaphones can have a huge impact, both positive and negative, especially so with a big cam, so def worth trying straight pipes IMO, I’d leave them long and cut of a bit at a time on the dyno.

I used to run very shallow megas on Triumphs, with silencers, and they worked great but when I tried them on my Commando they were no good at all.

I have a copy of Steve Maneys 2:1 design, it’s for a Commando but would probably fit on yours enough to test on the Dyno if you wanna borrow it sometime?

My dyno man claims that on numerous occasions bikes running 105 race fuel have gained a few (up to 4) BHP when switching to super unleaded! Seems totally wrong and counter intuitive, but def worth a try.

What about head porting? What’s been done and by whom? I built an 850 with JS parts and was happy with it. Then I got Comnoz to port the head and got an extra 9 BHP just from the porting work. It definitely ‘unlocked’ it !

What are you doing for sparks? Is that an actual mag, or a mag housing containing an EI? Are you 100% sure your sparks are good?

The higher octane fuels have a bit less energy than lower Octane.
I get a bit more speed on dyno hill with 91 ethanol free than with 94 ethanol free. It isn't a huge difference but it's there every time.
You really only want as much octane as is needed.
It might be that your bike does need the race gas to avoid pre-ignition.
I did some reading on 500s last night.
When the Lawton 650ss won the Thruxton the first time, the second place bike was a 88. That was its 3rd win of the 500 cc division. The amazing part was that it finished ahead of 650 Triumphs and Ajs etc.

I'm reading about the IOM Domiracer now. Of course that engine was a complete one off with special everything.

Afterwards Doug Hele wrote
" I remember we were doing a lot of work to improve the performance of the twins- initially with the 600cc engine then with the 500cc.
I remember that we prepared an 88 for Dennis Greenfield and Fred Swift to race in the Thruxton 500 in 1960.
They won the Thruxton 500 and that encouraged us to take things a stage further and for the 1961 TT we built a really serious engine.
This had short con rods, thicker crank pins, full race cam and the camshaft running on needle roller bearings, hollow pushrods and hollow tappets, eccentric rocker adjustment and a separate pressurized oil feed to the rockers."
According to the article this engine gave 58 bhp and revved to 8000 rpm. On the same engine dyno the then current works Manx engines made 51-52 bhp.

I would love to know what " full race cam" was used.

Glen

worntorn said:

I would love to know what " full race cam" was used.

Glen

Click to expand...

I've previously posted the '62 cam lift curve but I don't know how it compared with the '61 version

worntorn said:

I would love to know what " full race cam" was used.

Glen

Click to expand...

I've previously posted the '62 cam lift curve but I don't know how it compared with the '61 version

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Not intending to critisise your endevour , BUT . Ya gotta lokkatit like you were after a goal . If you want to bounce the ball off the posts , youve got it .

A good kick , it should be STRAIGHT DOWN the gurgler , its not a pool / snooker shot . IF your after R P M 's .

Dunno what yer carbs are ? 30 m.m. Id hope , or bigger . ALSO ya wanna get some length inem there maniFOLDs ( which they are . FOLDED . )

Youve got THE RIGHT stacks . They dont pressureise the floats down & burn out the internals . Tooned Lenff , some take it valve face to intake lip . BUT the Jet Posn. is a pulse senstive thingamybob .

Youd be yup 240's + Id hope . Fink Kevins 560 unit T100 R ran 260s . AND the short flare thingos adjacent the voitical TRIUMPH 59 Frame Tube . This thing walked . but was goodin the Gravle . 2 up .
Like any good Triumph .

So , Well Then . IF you wanna gettit Going , Id Recomend a redoo . BUT , Perry ran a T100 SidecaR SPEEDWAY , WOT GOTTA CHAMPIONSHIP INNA DER 50'S , with TWO CARBS .
Manifold was two tubes . One Cross Tube . Carbs Each end . ASin , a carb aimed in , tangental , each side . So He had EFFECTIVELY , Three Tuned Lengths per carb . Wot doneit .
+++++

My real lice Worlds Fastest 55 T 100 R ran 1 1/16th Monoblocks . On old Bulldozer ( 60s . as it Were ) Reinforced laminated straight raqdiator hose . Industrial stuff .And 8.000 r.p.m.
WITH he Intake on the Fwd. Tooth .

worntorn said:

I would love to know what " full race cam" was used.

Glen

Click to expand...

I've previously posted the '62 cam lift curve but I don't know how it compared with the '61 version

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Start conversation

Click to expand...

Grumpy old shites prefer the high lift short durason ( RELITIVELY ) period race cams . IF your running unmuffled or megaphones . It wool be wooley , ' off the cam ' . as in beloww 3.500 .
So needs milking
But ' On the Stop ' off the lines , all those things are ' its n it ' and going .

IF you run it right out ( and consider it needs hours to free off right 0 at a smidgeon backed off from full welly . from cruiseing along , you should ' find the lump '
When / where , it picks up ( comes on the cam ). And where it gets ' pulled right up ' . 90 in third . (Comes on the pipes . IF it does . Megas . Top end . PERCOLATEING . but. as said .
On the Stop , off the line . it should percolate . ( if you sort those intakes out ) IF its jetted up to feed it , when its hungry .Like That . Bedded in & freed off . MATE .

If its just for pussyfooting , dont bother . forget it . If you wanna sethe I.o.M. lap record . Get To It .

The BEST Manxes , they wrote . Were 60 horse . at the shaft . Hand Built custom , one piece driveside crank , in ailwoods megabuck 1000 mph lap job . So get to it .

Bit hard to see. But theye in there . Figured out , the other day , geared for 105 at 7.000 in top , it pulled 110 at 8.000 in third . & snatch TOP .
when ya gottit right .
Club Circuit . Pukekohe .



into the hairpin . Best Manx'd do 105 .

But this was way lighter than youll get . And the throttle cables were set so they wouldnt break . FORMLY on the Stop !!!

Youll see the sides ( on Monoblocks ) go up Just Past Full Open ! ! ! . So work ' from the top .
Throttle Grip ON THE STOP . With still a iota of FEE PLAY on the cables . Slides fully clear of carb bore .

1 5/8 pipes end around the tyre .

I figure youve glatre at all the woiks dommie stuff ? . https://www.accessnorton.com/NortonCommando/factory-race-shop-dustall-24-hr-record.6638/#post-628399




If you were Doug Hele , you would have done it like this .



Right , there you are , then .

If you buy an airfield , you can take that muffler off , and get it running properly . Use the runway .

If you used a 44 gallon drum , 2/3 full of water , as a silencer , it should be pretty quite !

Something like that . ANYWAY . just a exaple . BIGGER ( volume ) is better . SCIENTIFIC , a T150 / R III ray gun IS a ' MEGA .



At Daytona the ' Stock ' R III run 130 + BLUED it half way down the muffler .

What does THIS tell us . Afterburner . Valve Overlap , adequate fuel , to burn
to create vacum to letter rev rightout . Enhanced Exhaust Evacuation . Severely .

So , a Triple Pas Reverse Cone Mega , into pipe , clear of Reverse Flow plate , back ove ,
and out - over and out .



Ahem . Fronts the Mega . Rev Cone at join . Pipe i.d. of that to 1 inch off rear , into bigger pipe forward to oter Rev Cone , and OUT over that though out the back there . or sumat likethat .

So their intermixed at exit , overlayed to dampen , and fre 'n easy getting there !

VOLUME ( more time ta do it )

LENGTH ( Mulitple tooned lenffs , so its gotta lotta go , everywhere )

Outlet Area . ( It aint choked . as ya want NEGETIVE timed E G P pulse ! . Not BACKPRESSURE . No matter what the headmaster might think . )

Thats wotyer 2 stroke tooned pipe thingos get . Wassel perforated core , incandescent , enhance the effect well , on 270 mainjets , and come in at 7500 , give + 1000 rpm .

Old Chap .

Below .

t ingermanson said:

Here's a link from @johnm giving some advice. Hopefully he'll get that bike running again!

Click to expand...

Thanks to Mr ingermanson for adding this link about my Dommie 500 that raced in the NZCMRR clubmans series from around 2000 to 2010. I reread my posts and wouldn't change much that I said. At the end the bike produced about 50 rear wheel bhp from 6250 to 6800 rpm measured on a Dynojet inertia dyno. See the plot at end end of this post. Fuel was NZ pump 98 RON. Definitely not on methanol which I did use on occasion in the open class but not for this dyno run.

At the time I'd say my bike was the second fastest in the Clubmans class beaten by Peter Lodges Clubmans ES2. Peter's bikes are quick. His race class ES2 got third at the Manx Classic Senior last year lapping at over 111 mph. My clubmans class bike was never as good as his.

A couple of things to note about the plot. At 4250 rpm you see a strange peak. This is megaphonitis. Under around 5000 rpm the bike is unridable and therefore about 5000 to 7500 is the usable power band. This isnt very good and means only a good rider with preferably a 6 speed gearbox can use the engine to its full potential.

The megaphonitis could probably be fixed but I never got there because my company posted me overseas for 7 years and this bike has not been touched for 14 years now. Also my computer with most of my data and experiments has crashed so obtaining detailed info is a bit difficult.

So what follows is hopefully a partial answer to the OP question about why his bike currently tests at 40 bhp rear wheel. I really doubt there is a single solution to your question. Above 40 bhp you get little bits here and there.

First off as I said in the Britbike thread the motor must be blueprinted perfectly. You have got really good bones in that bike. Crank and camshaft must have perfect bearings and be perfectly linebored to rotate with minimal friction. Decked cases with barrels bored perfectly square and parallel. Conrods perfectly centred square and aligned. Timing gears also perfectly meshed and no friction. You need a really good shop or the skills and gear yourself to do this.

Next the head. I used a 650 SS head which was ported by a NZ guy with a great track record in bike and car racing. Your photo does not show an SS head. That's not great. As someone said above porting is not the same from everyone. There's a guy in NZ at the moment testing 45 bhp from a non desmo Ducati 350. His shop does porting for all sorts of race vehicles. But he spent 100s of hours on his head. He would charge thousands to do a full job on a Dommie head. I changed two things at once on my bike. A 4S cam to a PW3 cam plus a porting job that was done almost 25 years ago now. The bike went from 37 to 43 bhp on the same dyno afterward. I think most of that 6 bhp came from the porting work. I'm not sure how good the guy who ported your head was. Does he have a good track record of top performing machines?

Setting up the head. I think it's really important to set the valves up right. I set them a minimum of 60 thou off coilbind and about 220 lb over the nose of the cam. This should be good to at least 8500 rpm with standard sized valves. I did not use titanium valves and back then I did not use beehive springs. If you match the valve spring compression strength carefully it seems to make the bike smoother. Obviously check the valves for piston clearance. I went for minimum 40 thou. I used NGK BP7ES plugs with a Morris rotating magnet magneto. Check the projected tip clearance.

Valve timing. I used a PW3 which I understand is very similar to your camshaft. With the head off and a push rod onto the followers measure the lift profile of all the cam lobes using a degree disc on the crank. Plot them all on the same spreadsheet and inspect the overlap. Check the profiles both sides match and that the cam is perfectly machined with the correct timing as published. I fully expect the JS motorsport ones will be perfect but beleive me they are all not like that. This next bit is perhaps a bit strange and not necessary for you but the only PW3 cam I could get back then was a 750 one. A Dommie and a Commando have slightly different piston conrod geometry. Different rod stoke ratios. This means a 750 cam installed in a 500 will have valve opening at a slightly different percentage of piston stroke. Now does the JS 500 camshaft allow for this? I don't know. On my Dommie I actually RETARDED the cam about 7 crankshaft degrees to allow for this and picked up significant top end hp on the dyno. Unfortunately it destroyed bottom end below 5000 rpm which could only be regained using very long headers. Obviously you must check this cam retardation does not cause valves to hit the piston. I had the head on and off many times getting this correct. Do not use a vernier on the camshaft. They are not strong enough. Use the chain and sprocket remeshing method which give 5 degree steps. RGM make sprockets with slightly offset key slots or worse case use offset keys. This tuning of camshaft can give your 2 or 3 hp at least.

Carbs. I'm sorry but I don't like the way your carbs are splayed out. I cut big holes in my oil tank to keep my carbs straight. See the photos. Valve to bellmouth was about 13.7 inches. I would go longer of I had the room. Mostly I used 30 mm Amal Mk1s. 32 mm for methanol. Must be mounted on good quality isolation rubber joiners. If you put your hand on a solid mounted carb on a dyno run it feels like you are getting an electric shock. High frequency vibration is huge. I added fibreglass to the bellmouth openning to make a rounded entry just like those recomended by Vizard or Blair. Good carb settings are also good for a couple of hp. I used mains between 190 and 210 - 220 on my bike. These days I work with friends using lamda real time mixture measuring. This is great. Most dynos also have it.

Ignition timing. Test on the dyno. I settled on 28 degree btdc with 10.25 CR and 98 fuel. Use Gordon Jennings spark plug articles to perfect the ignition timing settings.

Lastly exhaust pipes. I did a lot of testing on the dyno to maximise max power and spread of power. Pretty much these two objective counter each other so it's a compromise to get the best for rider skill, no of gears and track type. I made up lots of header pipes lenghts and diameters and three different lenght megaphones. I tested without a silencer and was pleasantly surprised when it was added it didn't hurt power at all. I will attach photos of my pipes to my next post and will have to give the dimensions then because I will have to go to my shed to measure them up.

OK. I'm writing this on my phone so there may be lots of typos which I'll correct latter.

Your bike looks great but I suspect you may need to address several of the things I discuss above to find the missing 10 bhp. Plus always remember dynos differ. I used the same one to go from an initial 34 bhp to 50 bhp so I knew the trend. But I would not be surprised with a 3 to 5 bhp difference on a different dyno, different day, different weather , different operator.

Brian Riley said:

Thanks Eddie, from 3500Rpm it pulls very good and clean with no flat spots with those carbs on any lower than that there is a flat spot, when I had the GPs on it had a huge flat spot from 4200 to 5500Rpm

I’d be happy if I could get an extra 5-6bhp from it, I’ve heard these can go from 48 to 52bhp

Yes there’s an other 500 with 49bhp but I’ll not find out what he’s done with his as it’s all kept close to the chest. But my dyno guy uses this as a guide

I’m running 105 octane race fuel from Sunoco it seems good stuff. I’m pretty much bang on just slightly on the rich side to aid with cooling especially with sitting in the holding areas in the heat.

It handles good on the track but the 350’s and same class just pull away from me which is frustrating. I could Rev it to 8000RPm but there’s no point as the power band drops off at 7000RPM and with the 5 speed gearbox it goes through the gears pretty quickly

Carbs I’m at my max with them there’s no point in going any bigger for the 500, I’ve tried with no carb spacers the power drops off a cliff with the manifolds on I’m up 6-7bhp and 5 flbs torque in the midrange the longer you make the spacers the more power you will get in the mid range, that’s why the works domiracers in 61 had really long inlets.

Where they 1”3/8 OD? I’ve got a set of those at 40” might chop 10” or more off to see what happens on the dyno.

Unfortunately UK regulations it has to have a silencer on to 105DB and they are very strict on that and have static db meters at points around the track.
I will how ever try them on the dyno to see what it dose.

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Does a dyno give a good indication of mid-range torque ? With a heavy crank, an increase in torque sometimes does not reveal itself, until the overall gearing is raised and the bike goes quicker. Normally, if you lower the gearing on most bikes, they accelerate faster - my 850 does not do that. If I lower the overall gearing it does not accelerate faster - the crank always seems to winde-up at the same speed, regardless of the gearing. With 5 gears, you should be able to work with a narrower rev-range.

johnm said:

Thanks to Mr ingermanson for adding this link about my Dommie 500 that raced in the NZCMRR clubmans series from around 2000 to 2010. I reread my posts and wouldn't change much that I said. At the end the bike produced about 50 rear wheel bhp from 6250 to 6800 rpm measured on a Dynojet inertia dyno. See the plot at end end of this post. Fuel was NZ pump 98 RON. Definitely not on methanol which I did use on occasion in the open class but not for this dyno run.

At the time I'd say my bike was the second fastest in the Clubmans class beaten by Peter Lodges Clubmans ES2. Peter's bikes are quick. His race class ES2 got third at the Manx Classic Senior last year lapping at over 111 mph. My clubmans class bike was never as good as his.

A couple of things to note about the plot. At 4250 rpm you see a strange peak. This is megaphonitis. Under around 5000 rpm the bike is unridable and therefore about 5000 to 7500 is the usable power band. This isnt very good and means only a good rider with preferably a 6 speed gearbox can use the engine to its full potential.

The megaphonitis could probably be fixed but I never got there because my company posted me overseas for 7 years and this bike has not been touched for 14 years now. Also my computer with most of my data and experiments has crashed so obtaining detailed info is a bit difficult.

So what follows is hopefully a partial answer to the OP question about why his bike currently tests at 40 bhp rear wheel. I really doubt there is a single solution to your question. Above 40 bhp you get little bits here and there.

First off as I said in the Britbike thread the motor must be blueprinted perfectly. You have got really good bones in that bike. Crank and camshaft must have perfect bearings and be perfectly linebored to rotate with minimal friction. Decked cases with barrels bored perfectly square and parallel. Conrods perfectly centred square and aligned. Timing gears also perfectly meshed and no friction. You need a really good shop or the skills and gear yourself to do this.

Next the head. I used a 650 SS head which was ported by a NZ guy with a great track record in bike and car racing. As someone said above porting is not the same from everyone. There's a guy in NZ at the moment testing 45 bhp from a non desmo Ducati 350. His shop does porting for all sorts of race vehicles. But he spent 100s of hours on his head. He would charge thousands to do a full job on a Dommie head. I changed two things at once on my bike. A 4S cam to a PW3 cam plus a porting job that was done almost 25 years ago now. The bike went from 37 to 43 bhp on the same dyno afterward. I think most of that 6 bhp came from the porting work. I'm not sure how good the guy who ported your head was. Does he have a good track record of top performing machines?

Setting up the head. I think it's really important to set the valves up right. I set them a minimum of 60 thou off coilbind and about 220 lb over the nose of the cam. This should be good to at least 8500 rpm with standard sized valves. I did not use titanium valves and back then I did not use beehive springs. If you match the valve spring compression strength carefully it seems to make the bike smoother. Obviously check the valves for piston clearance. I went for minimum 40 thou. I used NGK BP7ES plugs with a Morris rotating magnet magneto. Check the projected tip clearance.

Valve timing. I used a PW3 which I understand is very similar to your camshaft. With the head off and a push rod onto the followers measure the lift profile of all the cam lobes using a degree disc on the crank. Plot them all on the same spreadsheet and inspect the overlap. Check the profiles both sides match and that the cam is perfectly machined with the correct timing as published. I fully expect the JS motorsport ones will be perfect but beleive me they are all not like that. This next bit is perhaps a bit strange and not necessary for you but the only PW3 cam I could get back then was a 750 one. A Dommie and a Commando have slightly different piston conrod geometry. Different rod stoke rates. This means a 750 cam installed in a 500 will have valve opening at a slightly different percentage of piston stroke. Now does the JS 500 camshaft allow for this? I don't know. On my Dommie I actually RETARDED the cam about 7 crankshaft degrees to allow for this and picked up significant top end hp on the dyno. Unfortunately it destroyed bottom end below 5000 rpm which could only be regained using very long headers. Obviously you must check this cam retardation does not cause valves to hit the piston. I had the head on and off many times getting this correct. Do not use a vernier on the camshaft. They are not strong enough. Use the chain and sprocket remeshing method which give 5 degree steps. RGM make sprockets with slightly offset key slots or worse case use offset keys. This tuning of camshaft can give your 2 or 3 hp at least.

Carbs. I'm sorry but I don't like the way your carbs are splayed out. I cut big holes in my oil tank to keep my carbs straight. See the photos. Valve to bellmouth was about 13.7 inches. I would go longer of I had the room. Mostly I used 30 mm Amal Mk1s. 32 mm for methanol. Must be mounted on good quality isolation rubber joiners. If you put your hand on a solid mounted carb on a dyno run it feels like you are getting an electric shock. High frequency vibration is huge. I added fibreglass to the bellmouth openning to make a rounded entry just like those recomended by Vizard or Blair.

Lastly exhaust pipes. I did a lot of testing on the dyno to maximise max power and spread of power. Pretty much these two objective counter each other so it's a compromise to get the best for rider skill, no of gears and track type. I made up lots of header pipes lenghts and diameters and three different lenght megaphones. I tested without a silencer and was pleasantly surprised when it was added it didn't hurt power at all. I will attach photos of my pipes to this answer but will have to give the dimensions in a second post because I will have to go to my shed to measure them up.

OK. I'm writing this on my phone so there may be lots of typos which I'll correct latter.

Your bike looks great but I suspect you may need to address several of the things I discuss above to find the missing 10 bhp. Plus always remember dynos differ. I used the same one to go from an initial 34 bhp to 50 bhp so I knew the trend. But I would be surprised with a 3 to 5 bhp difference on a different dyno, different day, different weather , different operator.

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With bigger ports, the result is usually more power up high in the rev range. Unless you have a short-stroke motor with a light crank, up high is not where you need the power increase - lower in the rev range with appropriate gearing, can be just as good pulling power, and easier to ride fast.
I wish I had started racing with a Domi 500 motor instead of my short stroke 500 Triumph motor - it was a never ending search for more midrange - I got some, but it was never enough. I still get nervous thinking about it. It was fast - but stuff it ! Nobody needs to ride like that.