Love my Mark III - but how about a little more power?

If you want to stay in the realms of acheivable without the hassle.
2S cam, high compression pistons & twin 32mm Amals. You will notice hell of a difference.
Don't forget the layshaft bearing either, unless it has been already done.
As for the 999, I own a K1200s, these things are missiles.

for a pretty cheap expense and a big increase in performance, in order of priority:

1) Jim Schmidt's 32mm flat slides $450 for everything, Don't even think, just buy them and sell your Mikuni.

2) Got the flat slides yet, installed and loving them? ok, consider a Megacycle 5600 cam with matching lifters -
a little higher lift and longer duration cam makes a very nice difference

I agree with another post ditch the single carb. u will find a significant difference in 'get up and go' with a dual carb set-up instead of that laid back lazy feeling of a single.
Dox

1up3down said:

for a pretty cheap expense and a big increase in performance, in order of priority:

1) Jim Schmidt's 32mm flat slides $450 for everything, Don't even think, just buy them and sell your Mikuni.

2) Got the flat slides yet, installed and loving them? ok, consider a Megacycle 5600 cam with matching lifters -
a little higher lift and longer duration cam makes a very nice difference

Click to expand...

My list would go (from least expensive to most):

Check your timing and fuel mixture. Free.
Bigger aircleaner, that poor carb is trying to suck through a straw. I think a K&N or the like could be had for well under $100
Make sure you have free flow mufflers. Commando Specialities has a pair for $130
Dual carbs $450
Cam
920 kit

Requires a disc grinder for clearnce for dual throat Weber Etc .

An ounce of handlings worth a pound of horsepower , but HP costs .

Setting up the thing so its working smoothly is senseable , Decent valve springs clearanced correctly , and so on .

The drawings are from the factory tuneing notes , so as a guide arnt far out .Trying to find the whole Artical .

Going over 9:1 comp. ratio isnt to sane on pump gas .

Id toss out the elec start rubbish to loose weight . :?

Mk II Amals were said to be worth a few mm on Mk 1s .

Sock type or aftermarket air cleaner assy is worth a look , as are the offset velocity stacks for Mk 1s .
Notes show 36 mm Mk 1s , problem is you can start getting it a bit crankey , and are likely to ram something
getting under way, as it wont idle along at the same low rpm , as in the ' snatch speed ' is likely increased .
so ' under way ' clutch out is a bit brisker .

Mutes , shine a torch up the tailpipe . Most people threw em out , as it induced a bit of a ' flat spot ' in the carburation .

As I recall, a DCOE 45 is about an inch wider than the frame tubes, at least, so will require a hefty disk grinder.

If the peashooters have mutes fitted to them, the driver in the car behind can see they are fitted, so it won't require a torch anywhere. The factory manual notes the jetting required with or without them...

P.S. I haven't seen any mutes fitted to peashooter mufflers in 30+ years, are they still available ?

Whatever you do to increase performance, it's going to have to include going back to a twin carb setup. Why not make that Stage One? It may be sufficient for you. Stage Two would probably then be to add a cam and up the CR. An 850 as it came from the factory with a single Mikuni and a small aircleaner is the definition of 'hobbled'.

Here's what Jim Comstock did to hot rod a '75 850 with photo's from 2009.

viewtopic.php?p=28816

A '73 article on stage 1 and 2 for 12.1 et's 850. Also other breeds et's.

http://www.britbike.com/forums/ubbthrea ... ber=358385

Appreciate all the suggestions, and I am closer to coming up with a plan.

First though, in response to a couple of points :

- yeah, the header pipes in the photos seem overly "blue" The photos are actually from a couple of years back, and the pipes were only a few days old then. They blued up right away when new, but don't look anything like that now - quite normal as a matter of fact. The carb is not running lean.

- the air filter is a K & N, and no it wasn't dirty.

Here's my thinking on upping the power, over and above stock:

Phase 1: OK guys, I get it, I'll try the JS twin flat side Keihn carbs. I put on the single Mikuni 34 mm a few years back, after decades of fiddling with Amals on various Brit bikes over the years, and finally saying "F... it!" A single Mikuni seemed a "plug and play" dream in comparison, and I presumed I would be sacrificing something in terms of power. Made sense at the time, but if the twin flat sides are also low maintenance and get me some more oomph, I'll go for it. I'll put the Mikuni kit on E-bay.

COST = $445, offset by whatever I get for selling the Mikuni kit.

Phase 2: - Going for the gusto. Won't be in the coming weeks or months, but maybe by the time I tick over 30K miles on the stock motor (another year and a half at the current rate)

1. Megacycle cam (either the 560-00 or 560-NSS) COST = $349
2. JS pistons COST = $435
3. JS rods COST = $690
4. JS beehive valve spring and valve kit COST = $425

TOTAL $1,899
and maybe some rebuild work on the bottom end at that point too - 30,000 miles ?

Total cost of phase 2 is pretty reasonable, especially when I compare it to prices of goodies for my Ducati.

thanks again

keith

We think you should just try it with a decent big air cleaner to begin with.
Commandos have a fair bit of go as stock.

A lot of these missile cars around have 'hi-flow' air cleaner elements available, and gain quite a fair number of ponies just from that. Not that this has much to do with Commandos, but that air cleaner you have seems to be the reverse idea, bottling it up as it were...

Rather than messing around fitting different parts, the first thing to do with any bike that seems down on power is a compression check. If all is well there, the next thing to do is have the bike tuned on a rolling road dyno, where the carburetion and ignition can be accurately set up.

Didn't want to post after the last two but here goes.
I run a similar set up 73 850,and I know what you mean, 6000rpm is about the limit, about 100mph, but I enjoy riding in twisty hilly back roads so I dont want to be comming into corners at 7000rpm.
I ACTUALLY set my carb up with a good quality O2 sensor and meter and got good results, I checked a couple of different things and one was whether that pancake filter was restrictive, no difference, nearly the same reading fitted or not,when a jet change either way made a noticable difference.It seemed to me the bike ran hotter and was a concern, could smell hot oil,after 20000km I pulled the head and barrel to fix a couple of oil leaks, the under side of the piston is spotless, so all must be good, my header pipes are even worse than yours.
If I were you I"d be going back to duel carbs first, or sell the Ducati and buy another COMMANDO with them already fitted.
The only time I really noticed a lack of power was when riding with other bikes, good tyres , suspension , brakes and stupidity made up for the lack of power, but its hard to beat the torque in the twisties and hills, go for a ride with someone on an old gsx 400 impulse hes down 2 gears in every corner , commando same gear, roll throttle off and roll back on ,much more fun. It was pointed out on here that a single carb only makes about 38 horsepower.

If your air cleaner is too restrictive and hobbling your horsepower, it can still be perfectly jetted and show correct reading with an O2 sensor, and respond to jetting changes as described. ?

It would be ridiculous to dynotune it with that aircleaner fitted, if it is too restrictive.

Most bikes respond well to having some sort of still air box they can draw from, and that tiny little air filter is about the exact opposite of that. It could be that changing it will make no difference, but I'd hate someone to spend big on engine mods when all it needs is an adequate aircleaner...

P.S. I notice that Jim Comstocks setup had quite a substantial aircleaner, maybe for a reason.

Aren't the Mark III's about 60 lbs. heavier than the earlier bikes? You might have to underwrite serious engine modifications to even approximate the performance of the lighter bikes.

It might be worth a test ride or two on a 750 Combat or RH10 850 with dual carbs to find out whether they already have the performance you're looking for--if you don't need the electric start. Your Mark III as pictured would probably fetch close to $10,000 as is--which would be plenty to trick out an earlier, quicker bike if you so choose


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850

Rohan said:

If your air cleaner is too restrictive and hobbling your horsepower, it can still be perfectly jetted and show correct reading with an O2 sensor, and respond to jetting changes as described. ?

It would be ridiculous to dynotune it with that aircleaner fitted, if it is too restrictive.

Most bikes respond well to having some sort of still air box they can draw from, and that tiny little air filter is about the exact opposite of that. It could be that changing it will make no difference, but I'd hate someone to spend big on engine mods when all it needs is an adequate aircleaner...

P.S. I notice that Jim Comstocks setup had quite a substantial aircleaner, maybe for a reason.

Click to expand...

You are a fuckwit, if removing the air filter doesn't change the mixture how the fuck is it restrictive !!!!

no reasons to use explitives.... offensive and immature.

JD

splatt said:

Rohan said:

If your air cleaner is too restrictive and hobbling your horsepower, it can still be perfectly jetted and show correct reading with an O2 sensor, and respond to jetting changes as described. ?

It would be ridiculous to dynotune it with that aircleaner fitted, if it is too restrictive.

Most bikes respond well to having some sort of still air box they can draw from, and that tiny little air filter is about the exact opposite of that. It could be that changing it will make no difference, but I'd hate someone to spend big on engine mods when all it needs is an adequate aircleaner...

P.S. I notice that Jim Comstocks setup had quite a substantial aircleaner, maybe for a reason.

Click to expand...

You are a fuckwit, if removing the air filter doesn't change the mixture how the fuck is it restrictive !!!!

Click to expand...

Hi Jeff, Couldn't agree more about expletives- not what we come here for.

Can I ask where you got those rear sets from? They look great. Not easy to get for l/h gearchange. Did you ride the bike before these were fitted? If so what are your impressions?

I have a MK3 850, though not as nice as yours. I too have enjoyed the 'plug and play ' improvement the Mikuni VM34 offered, mainly as I rode it alot in heavy London traffic and a worn set of Amal makes any bike a misery.

I was thinking of fitting a 920 kit, but am now wondering about these JS Keihn flatsides. Can anyone report in abit more detail on their virtues? I don't want to lose the clean running and steady tick over of the Mikuni, but will they give back or exceed the Amal type performance on their own?

Mine would pull a good ton on the Amals before it ran out of steam. With the Mikuni it was great till I tried to out run a HD, which it managed but didn't want to do much more than 80mph.... 38 bhp is not much to tow around 460lbs :roll:

If you like your bike just the way it is, I would just tweak it as it is. The 750s might be more rapid but with or without the electric start, the mk3 has many other improvements, especially in terms of reliability, than you can shake a stick at. Let us know how you get on!



Many thanks

If removing the airfilter "made no difference to the mixture" but "the bike ran hotter" what does that tell you....

I have an 850 MKIII with a single MIkuni 34mm. I has a stock engine 8.5 compression, RH10 head (30mm), std cams, Lucas Rita, and a 22 tooth sprocket. It can go 95 MPH in third gear up a long steep hill. On the flat it can get up too 100 MPH really quickly, I don't find I want to go any faster even for passing. The engine is in good shape, everthing works right. It is faster than my friends 750 that has a PW3 cam in it and dual Amal 930s.

I am not really interested in making it go faster. If I want to go faster I will ride my VTR 1000 or my Triumph Sprint. But I prefer riding the Norton most of the time.