Where' the power?

Hi folks, I'm hoping someone can help me. Just completed a full 750 Commando engine build & it's speced out for hot street settup with the following goodies:
Fullauto head
Complete JS1 cam & valvetrain
JS rods & 9.5:1 pistons
32mm Premier carbs
Full stock exhaust from Andover
Stock airbox
Trispark ignition.
Molnar crank & nikasil barrels
21 tooth gearbox sprocket
etc.

After putting in a few break in miles and starting to apply more liberal throttle applications, there no additional power after 1/2 throttle, just more noise in top gear. The bike struggles to achieve 100mph.
I set the valve timing with a degree wheel, the ignition timing and marks have been double checked, I bupmed the main jet size from 220 to 240 with no noticable difference.
I think this setup should work much better than this but I can't seem to figure out what the issue is. Please help!

MLW said:

Hi folks, I'm hoping someone can help me. Just completed a full 750 Commando engine build & it's speced out for hot street settup with the following goodies:
Fullauto head
Complete JS1 cam & valvetrain
JS rods & 9.5:1 pistons
32mm Premier carbs
Full stock exhaust from Andover
Stock airbox
Trispark ignition.
Molnar crank & nikasil barrels
21 tooth gearbox sprocket
etc.

After putting in a few break in miles and starting to apply more liberal throttle applications, there no additional power after 1/2 throttle, just more noise in top gear. The bike struggles to achieve 100mph.
I set the valve timing with a degree wheel, the ignition timing and marks have been double checked, I bupmed the main jet size from 220 to 240 with no noticable difference.
I think this setup should work much better than this but I can't seem to figure out what the issue is. Please help!

Click to expand...

My suspicion would be cam timing…

Before you blow the engine up tune the carbs and first get the main jet size right, 240 is likely too small.

MLW said:

Hi folks, I'm hoping someone can help me. Just completed a full 750 Commando engine build & it's speced out for hot street settup with the following goodies:
Fullauto head
Complete JS1 cam & valvetrain
JS rods & 9.5:1 pistons
32mm Premier carbs
Full stock exhaust from Andover
Stock airbox
Trispark ignition.
Molnar crank & nikasil barrels
21 tooth gearbox sprocket
etc.

After putting in a few break in miles and starting to apply more liberal throttle applications, there no additional power after 1/2 throttle, just more noise in top gear. The bike struggles to achieve 100mph.
I set the valve timing with a degree wheel, the ignition timing and marks have been double checked, I bupmed the main jet size from 220 to 240 with no noticable difference.
I think this setup should work much better than this but I can't seem to figure out what the issue is. Please help!

Click to expand...

What RPM are you reaching? What needle position? The main jet kicks in at 3/4 throttle. 110 is about the limit with a 21T front sprocket so something is not adding up.

Really

Well, it is a Norton 750. It's not going to blow anyone's skirt up if they are used to late model larger displacement motorcycles. The JS2 cam is where you should have started if you wanted to feel something. It'll get you over the 100mph hump with what you have. I would also dump the Amal carburetion if I wanted some power. The Keihin 35mm FCRs should work well with that head. Although I don't know if you could find any new jetted like SUDCO jetted them for the 750/850. I'll get my fair share of abuse for saying dump the Amals, but I don't care much for limitations and most of them have never used FCRs.

Is the noise a brighter exhaust note or the valve train?

The way amals work is there are components within the carbs that characterize the fuel flow (air/ fuel ratio) at different throttle positions..... Read this twice,..

So, if your bike won't idle, you look at the idle circuit
If you bike breaks up in the midrange you look at the needle position and needle jet
If you hit a wall at higher rpm's you look at the main jet size...

Of course you could always have an issue that shows up in a certain range that isn't specifically carburation causing the problem, but if you are troubleshooting the carbs, sometimes the range your problem happens in gives you a clue where to look.

I have an early 750 with smaller carbs and my bike used to run out of steam at 83mph. No matter what I did I couldn't get past 83. Someone mentioned that if you back off the wide open throttle slightly and it causes the bike to momentarily surge then the main jet might be wrong. I had 240's in the bike and switched them to 220's which is the recommended main jet for my carbs. I had the 260's in my pocket, ready to try them too, but the bike zoomed right past 83mph up to near 100mph with the 220's and I knew I had found the problem.

Usually, the recommended settings are correct for a stock bike. With your special build, there's a good chance you'll need to try some different settings and probably a different size main jet.

I own two Commandos, 750 (well used) and 850 (rebuilt), mostly stock, and they both easily top 100 MPH. Regarding Amal tuning: the circuits and mixture features in the carb are a function of throttle position, not RPM. But that said, they're related; obviously you're not going to hit peak RPM at low throttle position, or stay long at idle with open throttle, so they're often confused.

However, until you hit about 3/4 throttle, you could leave the main jets out with little difference, unless it's substantially too small. I would go bigger main jets to begin with and see if it's better or worse. I've had bikes surprise me with quite different from stock settings being the correct main jets depending on other factors.

Just in case, I have to mention, the chokes (if fitted) are not on are they? Cables must be pulled tight to take them off.

The 240s may be too rich. Combats were 220-230. Yours should be similar. Mine has a 19T and goes to 110 mph at the redline very easily (no waiting).

If it's gasping past 3/4 throttle main is likely too small, if it's a lumpy no-go the main may be too large. What do the plugs look like at the problematic range? Following FE's comment, did you dial the cam in per Jim's instructions? For comparison my hotted up 750 with a Web312 is set at 101/105 LSA and pulls past 100mph at around 5500rpm

Some things (not many) are simple. If bigger main jets make it worse at full throttle, try smaller main jets.

Are the mufflers straight through?

I would imagine the stock jetting on the Amal carburetors is not real close for that head and whatever exhaust you are using. Standard carburetor troubleshooting required.

Are you using the 0006 TriSpark? If you skipped the setup instructions for the 0006 and set it up like the 0005, ignition timing will be retarded. Did you verify timing with a strobe?

My 750 parts setup is similar to yours, but I have more cam, iron lined Molnar barrels, FCRs, and the head is older and ported by somebody else pre-dating Maney Full Auto heads. My 750 is better than stock performance wise. I hit a wall at 93 mph once, but it was because my overflow/atmosphere pipe hose to the float bowls was nearly pinched closed. You don't have that pipe with the Amal carburetors, so irrelevant.

Is this a another knowledge test disguised as a request for help?

Are you sure the carb slides are opening fully? I agree that it sounds like a fueling issue IF the ignition/cam timing is correct. A totally stock 750/850 Commando in proper running order has no trouble at all going easily past 100MPH with no hesitation.

It's quite a list of mods; are they based on reliable information that the specific setup works well on a 750 Commando? I have a LOT of experience with engine mods on cars and have seen everything from high performance cams to high compression pistons to free flow exhausts to larger carbs, etc, REDUCE performance compared to stock engines because the mod(s) did not work well together. TBH, I'm a little surprised to see a "performance cam" and higher compression pistons with stock exhaust/induction but IF that setup is well researched and shown to work well, that's all that matters.

Cam timing could certainly cause the problem as could retarded ignition timing but since they have been checked, one could ASSUME they are OK. Clogged intake/exhaust could also cause it but, again, being new that's extremely unlikely.

Go with richer main jets. Start with at least 260 or 280. Increase in size until it starts to miss at full throttle (8cycles) - this may take jets over 300. Then as soon as you find a size that 8 cycles go down 20 (from 300 to 280 etc). Going too rich is safe and cools the motor. Too lean is risky and can overheat and on a new motor that means possible siezure and piston damage. For reference when I was racing I was running 320s in amal 32s on a 750 with a combat cam (JS2) and ported head. Some people didn't believe I was using 320s but the bike was VERY fast and screamed bloody murder on the top end.

jseng1 said:

Go with richer main jets. Start with at least 260 or 280. Increase in size until it starts to miss at full throttle (8cycles) - this may take jets over 300. Then as soon as you find a size that 8 cycles go down 20 (from 300 to 280 etc). Going too rich is safe and cools the motor. Too lean is risky and can overheat and on a new motor that means possible siezure and piston damage. For reference when I was racing I was running 320s in amal 32s on a 750 with a combat cam (JS2) and ported head. Some people didn't believe I was using 320s but the bike was VERY fast.

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What he said. Just changed my jets on my 932 carbs from 220 to 260… different bike.

But as @Schwany said, why Amals vs FCR given you went his far?

I would go further until you've gone too far then back off. You could still be missing some HP.
FCRs are great carbs but 33s would work better on your 750 than 35s

jseng1 said:

I would go further until you've gone too far then back off. You could still be missing some HP.
FCRs are great carbs but 33s would work better on your 750 than 35s

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Depends a lot on the tuner, and what that tuner likes in a motorcycle. But 33s would be better in town and tolerate short shifting better. Your preconfigured 33mm CRS round slides would do the trick.

hhh said:

I own two Commandos, 750 (well used) and 850 (rebuilt), mostly stock, and they both easily top 100 MPH. Regarding Amal tuning: the circuits and mixture features in the carb are a function of throttle position, not RPM. But that said, they're related; obviously you're not going to hit peak RPM at low throttle position, or stay long at idle with open throttle, so they're often confused.

However, until you hit about 3/4 throttle, you could leave the main jets out with little difference, unless it's substantially too small. I would go bigger main jets to begin with and see if it's better or worse. I've had bikes surprise me with quite different from stock settings being the correct main jets depending on other factors.

Just in case, I have to mention, the chokes (if fitted) are not on are they? Cables must be pulled tight to take them off.

Click to expand...

The OP needs to answer this question about the chokes on Amal concentrics.

It would not be the first time or even the 10 th time people got that the wrong way around.

jseng1 said:

Go with richer main jets. Start with at least 260 or 280. Increase in size until it starts to miss at full throttle (8cycles) - this may take jets over 300. Then as soon as you find a size that 8 cycles go down 20 (from 300 to 280 etc). Going too rich is safe and cools the motor. Too lean is risky and can overheat and on a new motor that means possible siezure and piston damage. For reference when I was racing I was running 320s in amal 32s on a 750 with a combat cam (JS2) and ported head. Some people didn't believe I was using 320s but the bike was VERY fast and screamed bloody murder on the top end.

Click to expand...

Unless I miared, he says he's not getting past half throttle, so main jets have nothing to do with it.

Fast Eddie said:

My suspicion would be cam timing…

Click to expand...

That was my first thought, I timed it with a degree wheel as per instructions and checked again a second time, cam timing is spot on.