Where' the power?

[MichaelB]

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.

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

Does it have chokes?

I know I'm a broken record, sounds like the chokes are not fully with drawn.
Pull the air cleaner, open throttle fully, make sure chokes are not dangling.

 

[MLW]

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.

I'm getting about 6000 rpm in top gear. Needle clip position is on the lowest grove, richest postion up from the middle position from where I started.

 

[MLW]

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

I ordred in a selection of larger main jets as well as a 107 needle jet to try. I will limit my riding to test runs until this issue is sorted out.

 

[MLW]

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?

It's the exhaust that gets louder, the engine makes surprisingly little mechanical noise. Based on hours of reading the various threads on this forum, I felt that the JS1 cam would work best with the chosen settup.

 

[MLW]

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).

I started off with the 220s before trying the 240s. I'd be happy if mine could easily do 110. Right now mine takes about as long to get to 100mph as my lod KLR650 did!

 

[MLW]

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.

I completely agree. A performance engine must be built with components that work well together. My selection of components is based on the recomendations of the many experts found on this forum, including Jim Schmidt and Jim Comstock among others. Ignition timing marks were checked with a degree wheel for acuracy and set with a strobe. The cam timing was set with a degree wheel.

 

[MLW]

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.

I have a selection of richer main jets on order to try out, will report on the results.

 

[MLW]

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?

Call me weird, but I wanted to stay with the Amals and the original "ham can" airbox to maintain a stock appearance. A period look was my goal, I don't want any modern looking parts to be clearly visible.

 

[MLW]

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.

Not my first time with Amals, lol. The chokes are fitted and are in the full open position when they are not needed.

 

[MLW]

Are the mufflers straight through?

Straight through peashooters from Andover.

 

[johnm]

Not my first time with Amals, lol. The chokes are fitted and are in the full open position when they are not needed.

 

[MLW]

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?

Timing was checked with a strobe, the timing marks checked with a degree wheel for accuracy.

 

[MLW]

Does it have chokes?

I know I'm a broken record, sounds like the chokes are not fully with drawn.
Pull the air cleaner, open throttle fully, make sure chokes are not dangling.

When installing the carbs and setting them up, I checked that the choke slides were able to reach the top position before the operarting lever reached it's stop. I also checked that the throttle slides would reach full open with the twist grip.

 

[MichaelB]

Does it pull hard to 7k in the lower gears?

 

[2WheelsUp]

Call me weird, but I wanted to stay with the Amals and the original "ham can" airbox to maintain a stock appearance. A period look was my goal, I don't want any modern looking parts to be clearly visible.

Understand, when I rebuilt my bike, I had the same decision, hot-rod resto-mod or stock. I went stock plus some period correct mods, I'm happy but would be nice to have a hot rod. Good luck getting things resolved, hoping you'll post pictures of your bike at some point.

 

[MLW]

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.

 

[MLW]

This is the best photo I have of the completed bike, will post more in the future...

 

[MichaelB]

Looks Great!!

 

[marshg246]

I'm getting about 6000 rpm in top gear. Needle clip position is on the lowest grove, richest postion up from the middle position from where I started.

Ok, 6000 rpm with a 21T front sprocket is 102 MPH. Am I right that you are at half throttle and 6000 RPM? If so, move the needle and try again. The main jet has nothing to do with it.

I'm assuming that the carbs are perfectly synced, opening all the way, and the fuel height is correct in both carbs - any of those wrong and you'll never get there. Also, do a run with both petcocks open.

7000 RPM is 119 with a 21T front sprocket - I'll be shocked if you get there but then Jim is a magic man.

Someone asked if it will rev to 7500 in the lower gears - answer that.

 

[jseng1]

Don't thrash it until you get some richer main jets in there. Its too risky, Start rich to be safe. You generally need more fuel with a hotter cam etc.