Where' the power?

I may as well throw in my usual:

Final Tuning is to be done in a specific order, per the standard tuning guidelines in the shop manual, as each task is dependent on the previous task, for optimal effect.
 
For the folks talking about Jim's cams. Jim's cams are designed to work like they should when installed dot to dot just like a stock cam. The crank, intermediate, and cam sprockets would have to be really messed up to cause a performance problem if the cam was installed dot to dot. I don't dial in cams. Never had an issue making power not dialing a cam in. I spend a lot more time doing other things that improve flow. Can't teach an old dog new tricks. I don't have a shop manual, which could explain a lot. :)
 
grandpaul said:
I may as well throw in my usual:

Final Tuning is to be done in a specific order, per the standard tuning guidelines in the shop manual, as each task is dependent on the previous task, for optimal effect.
Click to expand...
Of course, true, but I look at it a little different.

Thinking for the moment of stock engines (750 non-Combat, Combat, 850) the standard, 2S and 4S cams are timed using the same method and therefore should be set per the manual. For all AMAL carbs, the fuel height is best when set to AMAL's specifications or at least set the same without flooding on both sides. The carbs must be 100% synced. The carbs cannot be worn out. The ignition timing must at least start off 28 degrees for points or 29 degrees for Tri-spark (or whatever the installed EI manufacturer says). New plugs installed starting with standard values (resistor or not as needed). Rocker gaps properly adjusted. And, of course, no air leaks, wiring problems, bad battery, etc.

In this case, a different cam is in use so it needs to be timed as specified by the manufacturer but hopefully since it's for a Norton Commando it was made to be timed by the same method as standard Norton cams.

Once all that is correct, then carb jetting is all that's left that can make a significant "seat of the pants" change. In theory, this engine has a higher compression ratio so some ignition timing change might be required for optimal performance once everything else is right, but this will make a minor change - I'm guessing 1-2 degrees more advance.

The other changes made to this engine should only affect vibration, ease of engine spinning up, and engine braking - not really performance.

Personally, I would not use a hotter cam without the increased compression ratio to go with it. Maybe the JS1 is good at 9.5/1 - I don't know. I also don't know what actual compression ratio the engine has since the head is non-standard and I don't know what cylinder base and head gaskets were used.

BTW, in Virginia at least, getting caught at 100 mph will get you a reckless driving ticket and possibly arrested. In Fairfax county, if you hire a lawyer, you'll get to turn yourself into jail after being convicted and you will pay $2500 plus court costs and your drivers license is gone for up to 6 months. Of course your insurance goes through the roof. The jail sentence is 30 days. The judge might suspend some time and/or money for a 1st offense. If no lawyer, you get shackled on the spot and you get to sit at the front of the courtroom until court is over and then proceed directly to jail. They treat reckless more harshly than drunk driving unless a wreck is involved.
 
SteveBorland said:
As has been suggested before, the best way to fix this is a session on a dyno. This will allow you to set the carbs up properly and check the power curve.
Click to expand...
I agree, but that is simply not an available option in my part of the world.
 
Schwany said:
For the folks talking about Jim's cams. Jim's cams are designed to work like they should when installed dot to dot just like a stock cam. The crank, intermediate, and cam sprockets would have to be really messed up to cause a performance problem if the cam was installed dot to dot. I don't dial in cams. Never had an issue making power not dialing a cam in. I spend a lot more time doing other things that improve flow. Can't teach an old dog new tricks. I don't have a shop manual, which could explain a lot. :)
Click to expand...
I installed the cam as per Jim's instructions, lobe center and valve lift @ tdc on overlap, which turns out it is just fine when installed as per shop manual simply lining up the dots etc. I might add that given the power suger @ 5000 rmp that I mentioned earlier,in first and second is not symptomatic of a cam that is too far advanced.
 
marshg246 said:
Of course, true, but I look at it a little different.

Thinking for the moment of stock engines (750 non-Combat, Combat, 850) the standard, 2S and 4S cams are timed using the same method and therefore should be set per the manual. For all AMAL carbs, the fuel height is best when set to AMAL's specifications or at least set the same without flooding on both sides. The carbs must be 100% synced. The carbs cannot be worn out. The ignition timing must at least start off 28 degrees for points or 29 degrees for Tri-spark (or whatever the installed EI manufacturer says). New plugs installed starting with standard values (resistor or not as needed). Rocker gaps properly adjusted. And, of course, no air leaks, wiring problems, bad battery, etc.

In this case, a different cam is in use so it needs to be timed as specified by the manufacturer but hopefully since it's for a Norton Commando it was made to be timed by the same method as standard Norton cams.

Once all that is correct, then carb jetting is all that's left that can make a significant "seat of the pants" change. In theory, this engine has a higher compression ratio so some ignition timing change might be required for optimal performance once everything else is right, but this will make a minor change - I'm guessing 1-2 degrees more advance.

The other changes made to this engine should only affect vibration, ease of engine spinning up, and engine braking - not really performance.

Personally, I would not use a hotter cam without the increased compression ratio to go with it. Maybe the JS1 is good at 9.5/1 - I don't know. I also don't know what actual compression ratio the engine has since the head is non-standard and I don't know what cylinder base and head gaskets were used.

BTW, in Virginia at least, getting caught at 100 mph will get you a reckless driving ticket and possibly arrested. In Fairfax county, if you hire a lawyer, you'll get to turn yourself into jail after being convicted and you will pay $2500 plus court costs and your drivers license is gone for up to 6 months. Of course your insurance goes through the roof. The jail sentence is 30 days. The judge might suspend some time and/or money for a 1st offense. If no lawyer, you get shackled on the spot and you get to sit at the front of the courtroom until court is over and then proceed directly to jail. They treat reckless more harshly than drunk driving unless a wreck is involved.
Click to expand...
The fuel level was set to Amal specs, the carbs synced with vacuum gauges to ensure even idle screw settings and then checked for even throttle openning. Timing is set @ 28 degrees as per JS recommendations.
 
Fast Eddie said:
MLW, there are still many variables that you haven’t clarified, and also your plot / storyline thickens !

You fitted 9.5:1 pistons… but you haven’t told us what your measured static CR is. Why is this important? Because the piston makers figure is only an estimate, the actual figure depends on many things like gaskets used etc. basically it is quite probable that you have less that that, perhaps only around the stock CR.

You have a FA head. Is it one of the original Australian ones, or the newer ones?

You have a JS #1 cam. You checked the timing, but haven’t divulged the numbers.

So… you could potentially have a stock CR and a sporty cam timed too advanced… which would definitely fit your symptoms.

However… assuming that’s not the case, and all was well, if your bike was put on the dyno I use it would be in the 50 to low 50s rwhp range. Now, even though that’s a healthy number for an old bike, it’s still not a lot in absolute terms, and will be an issue if you’re used to riding modern bikes.

With that cam, and the lighter crank, your engine should feel VERY responsive on the throttle, when you blip the throttle it should feel / sound very different to stock. That cam and crank complement each other in this regard. BUT they also compliment each other in being things that reduced torque to the wheel below around 4,500rpm. So, if you’re expecting to out drag a good stock bike when opening the throttle at 3,000rpm, you’re likely gonna be disappointed.

As others have already said, you’ve built a sporty engine, it needs revs. To get max rpm in top gear on a bike like yours means riding it to high rpm in the lower gears to get there.

If you’re a big guy, with a lot of weight and poor aerodynamics, and if you have high-ish western bars, and if you’re sat upright, you can’t just expect to open the throttle at 50mph in top and expect it to romp up to 120 quickly.

So, what I’m saying is, please address / report back on the ideas put forward thus far (carb settings, cam timing, CR numbers, etc) BUT also consider that some of your problem could be down to expectation management.
Click to expand...
I didn't measure actual CR, I assembled the engint to have exactly 50 thou squish band clearance. The head was from the last batch of the Aussi ones. I'm not a big guy, 185lbs @ 5'10".
 
marshg246 said:
Of course, true, but I look at it a little different.

Thinking for the moment of stock engines (750 non-Combat, Combat, 850) the standard, 2S and 4S cams are timed using the same method and therefore should be set per the manual. For all AMAL carbs, the fuel height is best when set to AMAL's specifications or at least set the same without flooding on both sides. The carbs must be 100% synced. The carbs cannot be worn out. The ignition timing must at least start off 28 degrees for points or 29 degrees for Tri-spark (or whatever the installed EI manufacturer says). New plugs installed starting with standard values (resistor or not as needed). Rocker gaps properly adjusted. And, of course, no air leaks, wiring problems, bad battery, etc.

In this case, a different cam is in use so it needs to be timed as specified by the manufacturer but hopefully since it's for a Norton Commando it was made to be timed by the same method as standard Norton cams.

Once all that is correct, then carb jetting is all that's left that can make a significant "seat of the pants" change. In theory, this engine has a higher compression ratio so some ignition timing change might be required for optimal performance once everything else is right, but this will make a minor change - I'm guessing 1-2 degrees more advance.

The other changes made to this engine should only affect vibration, ease of engine spinning up, and engine braking - not really performance.

Personally, I would not use a hotter cam without the increased compression ratio to go with it. Maybe the JS1 is good at 9.5/1 - I don't know. I also don't know what actual compression ratio the engine has since the head is non-standard and I don't know what cylinder base and head gaskets were used.

BTW, in Virginia at least, getting caught at 100 mph will get you a reckless driving ticket and possibly arrested. In Fairfax county, if you hire a lawyer, you'll get to turn yourself into jail after being convicted and you will pay $2500 plus court costs and your drivers license is gone for up to 6 months. Of course your insurance goes through the roof. The jail sentence is 30 days. The judge might suspend some time and/or money for a 1st offense. If no lawyer, you get shackled on the spot and you get to sit at the front of the courtroom until court is over and then proceed directly to jail. They treat reckless more harshly than drunk driving unless a wreck is involved.
Click to expand...
The alternative to reckless driving is TRACK DAYS. I assume there is a nearby racetrack with MC track days. I look forward to reports from OP on how he blazed past people with $30,000 Ducatis, causing much unhappiness and maybe even a minor riot.
 
Here's how to dial the complete carb in, because in reality an Amal is a sequence of parts working together through the throttle range. Put a piece of tape by the throttle and mark 1/4, 1/2, 3/4 and wide open throttle positions. At idle the mixture is controlled by the pilot jet. Coming off idle the slide cut away comes into play up until about 1/4 throttle. Above that you are on the straight part of the needle and the needle jet up until about half throttle, above that to roughly 3/4 you are on the needle jet and the tapered part of the needle. Above 3/4 the needle will be above the needle jet and mixture will be controlled by the main jet. The main jet should have no effect below 3/4 throttle unless it was extremely small, less than the clearance between the needle and the needle jet. The clearance between the straight part of the needle and the needle is about the width of a human hair. It should not matter what gear you are in, if the problems occur in the same throttle position it is a carb problem. Problems at the same rpm in different gears is more likely an electrical problem. Once you know what throttle position your problem is occuring you can tweak that part of the carb. Idle=pilot jet, off idle to 1/4=slide cut away, mid throttle=needle clip position,
wot=main jet.
 
nortonmargie said:
The alternative to reckless driving is TRACK DAYS. I assume there is a nearby racetrack with MC track days. I look forward to reports from OP on how he blazed past people with $30,000 Ducatis, causing much unhappiness and maybe even a minor riot.
Click to expand...
No Ducatis yet, but I did blaze past some $30,000 Camry's & Accords... Didn't stick around for the ensuing riots.
 
Have you consulted ChatGPT on where's the power on a Norton motorcycle?;) I'm thinking the answer would be useless. I don't use the app so maybe I'm wrong.

How's the conundrum going?
 
I still think its starving for fuel when doing a high speed run in top gear, fuel from the tank to the carbs why both fuel taps need to be open when doing high speed in 4th gear, its ok in the lower gears as the fuel there as you rev through the gears but when in top gear you are running with the throttle open on a longer run usually on a long straight road for a few miles, I found that out a long time ago, high speed runs in top gear the carbs starve for fuel if only one fuel tap is on, but remember to turn your reserve tap off when done with the high speed run, I got caught out once racing up Mt Mee with my mate he was on his old 1970 Ducati GT 750 had both taps on for the speeds we were doing and I ran out of fuel, went to turn on the reserve tap, oooops was already on lol (this was back in the 70s).
 
MexicoMike said:
I'm not sure I could count the number of times I saw "more carburation" reduce engine performance. As has been mentioned many times, all this mod stuff has to work together. Very few mods can actually add useful power on their own. Yeah, I know it's hard for us performance-oriented types to accept but most of the time the motorcycle/car manufacturer knows what they're doing. ;)
Click to expand...
I suggest that even the most mundane Commando can be made to perform by adjusting ignition timing and carburation. Often when there is a mod, it has a similar effect as leaning-off the jetting. It is probably wise to avoid jetting too lean. Some things are a compromise. For a road bike, I would probably jet slightly richer - carburation can be weather-dependent. With my bike, I use Mikuni 6D needles in 34mm Mk2 Amal carbs. It means I am forced to feed the throttle-on, but for better performance, most bikes require that. If you wobble around corners then whack the throttle open, you will always be slower. Riding my bike, is like riding a two-stroke, but better. For maximum torque, it is necessary to maintain vacuum in the inlet tract. Quicker taper needles compensate for loss of vacuum, be feeding more fuel. Loss of vacuum is caused by opening the throttle too quickly. I use a quick-action twist-grip so that I do not need to move my hand - not to open the throttle quicker, and my front brake is operated by my fore-finger.
 
MLW said:
My order of main jets arrived last night, will test out different mains on the weekend.
Click to expand...
Your problem is more likely to be needle jets rather than main jets, your carburetors might be flooding. Usually if the midrange jetting is lean enough, it does not matter if the main jets are too rich. If you are going to burn a piston or a valve, it will usually happen when you are using full throttle. If you feel the motor miss when you are riding, always stop and raise the needles. When I tune, I lower the needles until I get the miss then raise them one notch. Your needles will be quick taper, when you raise them one notch, that is a fairly big increase in fuel delivery. But they need to be lean to get acceleration. When the revs are high, the main jets do not make much difference, unless they are too small.
 
Many years ago, I had a fairly big fight with my brother over this stuff. He became the Australian Speedway Sidecar Long-track Champion in about 2001. When you think about the way most motorcycle carburetors work, it is quite logical but not obvious.
Some speedway bikes have Dell-Orto carbs which have no needles - my brother puts needles into them.
 

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