First impressions after rebuild: Back to back comparison

john robert bould · Aug 30, 2014

"more planted in the bends, hope the Lansdowne Dampers are assisiting this" :?:

Fast Eddie said:
I went out for a ride today with a mate who has an excellent, unrestored mk3 with about 20k under its belt. It's stock bar peashooters and a double K&N on the new twin Amals.

First thing, it is clear that my Commando vibrated a lot more than his prior to the rebuild. So the 'before and after' comparisons I had were a little 'tainted'. Mine is now much smoother than his below 3k. But above that, there's not much in it, and actually his is slightly smoother at some points than mine (we put this down to the compression).

We did some side by side 'roll on' comparisons, clearly I was very keen to see what all the mods I've done really equated to...

Well, I was wrong in my assessment that mine has lost its low end urge, even at low revs, mine had lots more pull. As the revs rose, the difference just got bigger!

I'm revving mine to 7k now, sometimes more (by accident) and it pulls SO strong. It was most noticeable for me when we swapped bikes and my mate just disappeared, with me riding his stock bike, I had no chance of keeping with him. The performance difference is huge!

The chassis work was noticeable too, with mine just feeling 'tighter' and more neutral and far more 'planted' in the bends.

So all in all, it was good confirmation. My intent was to build something of a hot rod for blasting round on and having fun, and with that in mind, it pretty much seems like 'mission accomplished'!

Final note: Building this bike simply would not have been possible for me without this web site, and all the info I've gleaned from it, and the advice and input I've had from many of you via emails etc. Thanks to all.

Fast Eddie · Aug 30, 2014

john robert bould said:
"more planted in the bends, hope the Lansdowne Dampers are assisiting this" :?:

Fast Eddie said:

I went out for a ride today with a mate who has an excellent, unrestored mk3 with about 20k under its belt. It's stock bar peashooters and a double K&N on the new twin Amals.

First thing, it is clear that my Commando vibrated a lot more than his prior to the rebuild. So the 'before and after' comparisons I had were a little 'tainted'. Mine is now much smoother than his below 3k. But above that, there's not much in it, and actually his is slightly smoother at some points than mine (we put this down to the compression).

We did some side by side 'roll on' comparisons, clearly I was very keen to see what all the mods I've done really equated to...

Well, I was wrong in my assessment that mine has lost its low end urge, even at low revs, mine had lots more pull. As the revs rose, the difference just got bigger!

I'm revving mine to 7k now, sometimes more (by accident) and it pulls SO strong. It was most noticeable for me when we swapped bikes and my mate just disappeared, with me riding his stock bike, I had no chance of keeping with him. The performance difference is huge!

The chassis work was noticeable too, with mine just feeling 'tighter' and more neutral and far more 'planted' in the bends.

So all in all, it was good confirmation. My intent was to build something of a hot rod for blasting round on and having fun, and with that in mind, it pretty much seems like 'mission accomplished'!

Final note: Building this bike simply would not have been possible for me without this web site, and all the info I've gleaned from it, and the advice and input I've had from many of you via emails etc. Thanks to all.

Click to expand...

I think they might be John! I have your dampers and Falcon shocks on the rear. Other factors are the new iso's and CNW head steady.

But yes, it is very clear that your dampers work extremely well. I haven't even altered them from your suggested 'starter' settings yet, so will see if they can be improved still further!

john robert bould · Aug 30, 2014

Simon,
You have lots to explore! oil grade,settings the lot...The lansdowne boys change the settings from track to track..dry or wet..check out Duncan on Mike Edwards youtube blog...Better than Moto GP :lol:

12gs · Sep 6, 2014

Fast Eddie

I am in the process of rebuilding my 1975 Commando and followed your build closely. I too am purchasing the JSMotorsports internals. I am curious as to the pistons you used. Medium or High compression? Looking forward to your next update.

Fast Eddie · Sep 6, 2014

12gs said:
Fast Eddie

I am in the process of rebuilding my 1975 Commando and followed your build closely. I too am purchasing the JSMotorsports internals. I am curious as to the pistons you used. Medium or High compression? Looking forward to your next update.

Well, there is no denying how good my motor feels with these internals, so I think you're onto a good thing.

I used the high comp pistons. My bikes CR measured lower than the book said prior to the rebuild, and this seemed to remain true afterwards, meaning that my hi CR JS pistons came out slightly lower than claimed. My CR is still a little higher than most seem to be happy with these days, but I was very keen to get a tight squish and so far it seems fine, no evidence of pinking and lots of go !

Hobot · Sep 6, 2014

Wait till ya work up to catching rubber on lower gear shift ups and it pisses ya off d/t the hesitation in pull. Should be darn near bullet proof too.

Fast Eddie · Sep 7, 2014

hobot said:
Wait till ya work up to catching rubber on lower gear shift ups and it pisses ya off d/t the hesitation in pull. Should be darn near bullet proof too.

Steve, there is no hestation. My first comments saying that I thought is had lost some initial bottom end pull just goes to show how in accurate 'seat of the pants' impressions can be. I think it felt less dynamic because it was so much snoother. It was very obvious, when run back to back with a stock mk3, that mine actually has more initial pull now.

Compared to a stock bike, mine just has more pull everywhere, but the gap between mine and stock gets bigger as the revs rise. So IT IS more fun in the above 4k regions, but NOT because of lack of low end pull.

Hobot · Sep 7, 2014

Gosh darn - its like reading a good sex story Fred so the only hesitation I meant was the tire spin on up shifts - to compensate for by a bit less throttle snap so power stays hooked up. Bad ass over lapped cams don't get into happy zone till above some rpm, nor does anyone try to sprint from off idle but by reving up to drop clutch. Its my dream to have race power let loose in public too. Hope to hear your complaints on rear tire life soon.

WZ507 · Sep 7, 2014

Fast Eddie said:
12gs said:

Fast Eddie

I am in the process of rebuilding my 1975 Commando and followed your build closely. I too am purchasing the JSMotorsports internals. I am curious as to the pistons you used. Medium or High compression? Looking forward to your next update.

Click to expand...

Well, there is no denying how good my motor feels with these internals, so I think you're onto a good thing.

I used the high comp pistons. My bikes CR measured lower than the book said prior to the rebuild, and this seemed to remain true afterwards, meaning that my hi CR JS pistons came out slightly lower than claimed. My CR is still a little higher than most seem to be happy with these days, but I was very keen to get a tight squish and so far it seems fine, no evidence of pinking and lots of go !

What was the actual static compression ratio, and what CR are your mates counseling you to stay below?

john robert bould · Sep 8, 2014

High compression is crap! Years ago i had a BSA Road Rocket and my mate had a Golden Flash..i was in collage and was 18 . I decided once and for all to show my mate a clean pair of heels...so off the head come...onto the miller and bingo 10-1 comp...This will do the trick! My mate's Flash is 7-1 no contest!!
Well that what i thought...first thing i noticed was the Knocking at low revs..The flash was sowing machine smooth :mrgreen:
On the road we did the head to head...the Flash out pulled me!! His flat top pistons had steam engine torque..my Rocket felt un-happy,
He laughed at my attempts to out gun him, i had a go on the Flash to compare...no contest it would pull like a train....rhino power compared to my knocky Rocket...that leaned me a good lesson ..no doubt the rocket would have gathered him up on the long bits...pity where i live we dont have any!

Any way not to be beaten easy..i fitted a Bob Joyner Cam...big square lobes ...this made no differance to the low down..infact it was worst! and knocked bad :twisted:
Today ..i would go for 8-1 ..in a commando .Norton tried 10.5 and look what happened! Its just added stress on a old design ..want power? get a BMW S1000R with a modest 170 BHP and 190 mph!

No doubt the Old die hards will be chiming in. my pre-respone is COMBAT!

olChris · Sep 8, 2014

john robert bould said:
High compression is crap! Years ago i had a BSA Road Rocket and my mate had a Golden Flash..i was in collage and was 18 . I decided once and for all to show my mate a clean pair of heels...so off the head come...onto the miller and bingo 10-1 comp...This will do the trick! My mate's Flash is 7-1 no contest!!
Well that what i thought...first thing i noticed was the Knocking at low revs..The flash was sowing machine smooth :mrgreen:
On the road we did the head to head...the Flash out pulled me!! His flat top pistons had steam engine torque..my Rocket felt un-happy,
He laughed at my attempts to out gun him, i had a go on the Flash to compare...no contest it would pull like a train....rhino power compared to my knocky Rocket...that leaned me a good lesson ..no doubt the rocket would have gathered him up on the long bits...pity where i live we dont have any!

Any way not to be beaten easy..i fitted a Bob Joyner Cam...big square lobes ...this made no differance to the low down..infact it was worst! and knocked bad :twisted:
Today ..i would go for 8-1 ..in a commando .Norton tried 10.5 and look what happened! Its just added stress on a old design ..want power? get a BMW S1000R with a modest 170 BHP and 190 mph!

No doubt the Old die hards will be chiming in. my pre-respone is COMBAT!

I bet we have all lost more races than we've one.. Its OK and we feel for you.. Just go to the Pub and talk out your unresolved issue.... (Joking OK)

First impressions after rebuild: Back to back comparison

Fast Eddie · Sep 8, 2014

john robert bould said:
High compression is crap! Years ago i had a BSA Road Rocket and my mate had a Golden Flash..i was in collage and was 18 . I decided once and for all to show my mate a clean pair of heels...so off the head come...onto the miller and bingo 10-1 comp...This will do the trick! My mate's Flash is 7-1 no contest!!
Well that what i thought...first thing i noticed was the Knocking at low revs..The flash was sowing machine smooth :mrgreen:
On the road we did the head to head...the Flash out pulled me!! His flat top pistons had steam engine torque..my Rocket felt un-happy,
He laughed at my attempts to out gun him, i had a go on the Flash to compare...no contest it would pull like a train....rhino power compared to my knocky Rocket...that leaned me a good lesson ..no doubt the rocket would have gathered him up on the long bits...pity where i live we dont have any!

Any way not to be beaten easy..i fitted a Bob Joyner Cam...big square lobes ...this made no differance to the low down..infact it was worst! and knocked bad :twisted:
Today ..i would go for 8-1 ..in a commando .Norton tried 10.5 and look what happened! Its just added stress on a old design ..want power? get a BMW S1000R with a modest 170 BHP and 190 mph!

No doubt the Old die hards will be chiming in. my pre-respone is COMBAT!

Sorry John but I can't agree with you on this. I think a lot often happens when people bung in HC pistons without a tad more work or thought, did you actually measure your CR before and after? Did you measure the CR of your mates bike? etc?
A lot of aftermarket HC pistons used to be utter crap, some of them wieghed a ton, balance factors would be seriously put out, causing vibration etc. Triumphs in particular would also suffer from over HC 'pointy' pistons with very poor flame path etc causing knocking and all sorts.
But all of these problems are due to issues other than the actual issue of CR.
My Commando is 10.25:1, so is definitely what one would call High Comp. But this is achieved with flat top pistons, and also means I run a tight squish, this all helps burn rate, part of the decision to go HC was also because of the extended duration of the cam I have (JS1) and the fact that the HC pistons are a third lighter than stock is also nice. I am in no doubt that the CR helps mine be as crisp as it is, but my point is that it is all a part of an overall 'package' that, frankly speaking, works really well!
We may all be forced to go 8:1 one day as fuel gets worse and worse, but I'm gonna enjoy my high comp fun for as long as I can until then!!
Your argument about the S1000R is a strange one matey... one could easily defend against the improvement of Norton forks thus: "want better handling? buy a S1000R" ...!

john robert bould · Sep 9, 2014

Nigel,
Comparing the "other" improvements to High Comp is not a good comparision, High comp puts more strain on the engine,otherwise there would be no extra power increase :?: Better brakes , starters, stronger gear box's etc improve the bike at no loss ...all gain. High comp is a trade off. with old design..as the combat engine clearly "demon"strated ,
As the Japan influx started ,Norton where under pressure to get some "cred" in a power hungry market, Having nothing else up their sleeves they tweeked the 750 up..High Comp was a cheap way to improve market status...to their expensive cost.
My fork mod was originaly designed to reduce the "Klonk" topping out, intended to do just that..the next step was to "try" and give some adjustment.
The Noise from just putting the bike on the stand was a big dislike to most owners ,as is that tiny oil drip!! I think keeping the oil in is more important than High comp :?: You asked for figures Comp etc . i had none ..well it was 40 years ago...i can only report the seat /pants experiance...you are quite right with the old style High doom pistions ,they produced pockets in the head and the flame path was not good.
Again ,the BMW S1000 comparision, was poor ..It was like comparing Boiling a pan on the fire against a Microwave. Speaking of microwaves its breakfast time ...Nige have a good in :!: and take care.

Fast Eddie · Sep 9, 2014

It's not the pistons that put extra strain on the engine...
It's me holding the throttle WIDE open that does that!
When I first fitted the FCRs I thought the throttle action was much too quick... Now I want it quicker still...
I'm 17 again.
Yeah baby...!

Roadrash · Sep 9, 2014

john robert bould said:
Nigel,
High comp is a trade off. with old design..as the combat engine clearly "demon"strated ,
As the Japan influx started ,Norton where under pressure to get some "cred" in a power hungry market, Having nothing else up their sleeves they tweeked the 750 up..High Comp was a cheap way to improve market status...to their expensive cost.
quote]

I am under the impression that the original main crank bearings were the weak spot on the hoped up combat engine. Once those are replaced with modern bearings the bike can be driven like it was intended.
I believe Fast Eddie is correct, it's not the higher compression that kills a well built engine, it's racing (and worth it)

auldblue · Sep 9, 2014

Roadrash said:
john robert bould said:

Nigel,
High comp is a trade off. with old design..as the combat engine clearly "demon"strated ,
As the Japan influx started ,Norton where under pressure to get some "cred" in a power hungry market, Having nothing else up their sleeves they tweeked the 750 up..High Comp was a cheap way to improve market status...to their expensive cost.
quote]

I am under the impression that the original main crank bearings were the weak spot on the hoped up combat engine. Once those are replaced with modern bearings the bike can be driven like it was intended.
I believe Fast Eddie is correct, it's not the higher compression that kills a well built engine, it's racing (and worth it)

Click to expand...

I thought the combat motor just chucked all it's oil out the breather and that's what screwed the bearings and wrecked the engines, but on the other hand I could be talking tosh and wasted time and money on breathers and case mods! Where's that Triumph catalogue mmmhh.

john robert bould · Sep 9, 2014

Fast Eddie said:
It's not the pistons that put extra strain on the engine...
It's me holding the throttle WIDE open that does that!
When I first fitted the FCRs I thought the throttle action was much too quick... Now I want it quicker still...
I'm 17 again.
Yeah baby...!

May the gods go with you

worntorn · Sep 9, 2014

One does wonder where the point of no return is with stock cases. Kenny Dreer clearly went by it with his 880s, and at least some of those had his own strengthened crankcase half. Steve Maney also located and went past it many times years ago.
Aside from the engine, I worry that the gearbox, designed originally for twenty horsepower +- machines, already is incredibly strained with my near stock 850.
But the itch to squeeze out just a bit more power from the old bike won't go away!

Glen

john robert bould · Sep 9, 2014

Glen, Is it like standing on a cliff edge on a windy day! :lol: Now i know you do plenty of lathe work...as the thought of sticking a finger in the spinning chuck ever drawn you..abit like rabbits and car headlamps..or am i just a lot nuts? Its like pushing the engine into danger zone 3.

worntorn said:
One does wonder where the point of no return is with stock cases. Kenny Dreer clearly went by it with his 880s, and at least some of those had his own strengthened crankcase half. Steve Maney also located and went past it many times years ago.
Aside from the engine, I worry that the gearbox, designed originally for twenty horsepower +- machines, already is incredibly strained with my near stock 850.
But the itch to squeeze out just a bit more power from the old bike won't go away!

Glen

First impressions after rebuild: Back to back comparison

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