My MK3 does not rev beyond 4500rpm

Leave the pilot jets alone, you have a pilot air adjustment screw for tuning the pilot mixture and 19's are the recommended size for Commando engines.

You need to tune the pilot mixture when the engine is fully warm, as long as the engine idles best with the air screw between 1 1/4 to 1 3/4 out then you are good to go. I suspect you currently have the idle mixture too rich, hence the good starting from cold and difficult warm starts.

As FE says a 22T gearbox sprocket will limit acceleration and top speed in 4th unless you first rev to 7000 rpm in third which you cannot achieve at the moment.
 
javifoto70 said:
Oh yes, I mean crankshaft! And agree I am just thinking about possibilities, the engine runs smooth up to 4500rpm so I doubt the crankshaft could be so much unbalanced.
I will double check both carb slieds are fully opening.
I tried to rev on second and third gear with the same result: it does not rev beyond 5000rpm. Although I did not open the throttle fully because the engine has only 500 miles and I yet do not want to force it so much. But i think it should rev at least to 6000rpm easily which is not doing.

The only part of the bike I did not open myself was the crankcase so I do not know how the crankshaft and camshaft are.

I will revise slides opening, carbs and replace the main jets to 260 ones.
Click to expand...
If you want to know what it revs like, you have to open the throttle slides my friend…

If it’s done 500 miles it will happily take some short bursts of WOT (in fact it is good for the bedding in process).

You are really wasting your time looking for problems until you do this, and see what happens…
 
"I tried to rev on second and third gear with the same result: it does not rev beyond 5000rpm. Although I did not open the throttle fully because the engine has only 500 miles and I yet do not want to force it so much. But i think it should rev at least to 6000rpm easily which is not doing."



All I can say is:

WOW.
 
Fast Eddie said:
If you want to know what it revs like, you have to open the throttle slides my friend…

If it’s done 500 miles it will happily take some short bursts of WOT (in fact it is good for the bedding in process).

You are really wasting your time looking for problems until you do this, and see what happens…
Click to expand...
This.^
 
It's your main jet. You did mention Amal pack 113 in the very beginning, that means nothing to most of us.
That pack comes with 220 jets which is correct for a STOCK Mk3 and 2a with the restricted silencers.
Your bike is not stock with the Pea shooters, note the 260 jets listed in the 136 pack.
It's your main jet and has been all along..
andover-norton.co.uk

Andover Norton Parts Drawings - Andover Norton Factory Parts - Commando Parts

Andover Norton Parts are the only Norton spare parts in the market manufactured to original drawings, within the factory-approved tolerances, from the correct materials, and with the correct finish.
andover-norton.co.uk
 
The fact that your engine runs fine right up to a certain RPM range would tend to point to something that is particular ONLY to that range...... It only took me 30 years to figure out a similar symptom on my own bike, so I'll try not to be too much of an ass... Norton got the recommended spec's right. If you have 220 mains in an 850, and the recommended mains are 260, then that deviation fits exactly with the range that your problem is occurring in

Just another example of how people don't understand how amals work and why people give up on them...
 
o0norton0o said:
The fact that your engine runs fine right up to a certain RPM range would tend to point to something that is particular ONLY to that range...... It only took me 30 years to figure out a similar symptom on my own bike, so I'll try not to be too much of an ass... Norton got the recommended spec's right. If you have 220 mains in an 850, and the recommended mains are 260, then that deviation fits exactly with the range that your problem is occurring in

Just another example of how people don't understand how amals work and why people give up on them...
Click to expand...
True.

But even more fundamental is that revs are linked to throttle position.

You cannot expect an over-geared, under-tuned old engine to rev out unless the throttle is opened appropriately…

This is not meant to be facetious in any way, I’m just trying to be crystal clear.

Problem solving is a process of elimination. Until you have operated the equipment as intended (in this case, using WOT to achieve high rpm) there is nothing to be gained by doing further hypothesis testing.
 
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MichaelB said:
It's your main jet. You did mention Amal pack 113 in the very beginning, that means nothing to most of us.
That pack comes with 220 jets which is correct for a STOCK Mk3 and 2a with the restricted silencers.
Your bike is not stock with the Pea shooters, note the 260 jets listed in the 136 pack.
It's your main jet and has been all along..
andover-norton.co.uk

Andover Norton Parts Drawings - Andover Norton Factory Parts - Commando Parts

Andover Norton Parts are the only Norton spare parts in the market manufactured to original drawings, within the factory-approved tolerances, from the correct materials, and with the correct finish.
andover-norton.co.uk
Click to expand...

Fast Eddie said:
True.
But even more fundamental is that revs are linked to throttle position.
You cannot expect an over geared, under-tuned old engine to rev out unless the throttle is opened appropriately…
Click to expand...
At times like these, I reflect on the 300 pages of the guy (I can't remember the user name) who had us trying to "help" him time a magneto.

Only to find out it was all a ruse. Had all his content (and OURS!) removed by asking.
 
What Air box you running?
If still the Black box, probably need to remove the restrictors/snorkels with the 260 mains.
The Mk 3 guys can chime in on this better.
 
MichaelB said:
What Air box you running?
If still the Black box, probably need to remove the restrictors/snorkels with the 260 mains.
The Mk 3 guys can chime in on this better.
Click to expand...
“K&N air filter directly mounted on the carbs with no filter box”.
 
The Black Air box makes no difference except if fitted with the snorkels, which is unlikely as they were the first items to be ditched by owners on MK2a/3's. I run peashooters with a snorkelless black box and it revs out cleanly with 260's.

A Snorkel

andover-norton.co.uk

Andover Norton - AIR INTAKE VENTURI - 06.4839

AIR INTAKE VENTURI
andover-norton.co.uk

My MK3 does not rev beyond 4500rpm
 
javifoto70 said:
Oh yes, I mean crankshaft! And agree I am just thinking about possibilities, the engine runs smooth up to 4500rpm so I doubt the crankshaft could be so much unbalanced.
I will double check both carb slieds are fully opening.
I tried to rev on second and third gear with the same result: it does not rev beyond 5000rpm. Although I did not open the throttle fully because the engine has only 500 miles and I yet do not want to force it so much. But i think it should rev at least to 6000rpm easily which is not doing.

The only part of the bike I did not open myself was the crankcase so I do not know how the crankshaft and camshaft are.

I will revise slides opening, carbs and replace the main jets to 260 ones.
Click to expand...
Absolutely 260 main jets ASAP.

Have you done a proper valve adjustment yet? I'm guessing your valve lash specs are too large. Valves are not opening enough. That is all part of a basic tune-up.
 
kommando said:
As FE says a 22T gearbox sprocket will limit acceleration and top speed in 4th unless you first rev to 7000 rpm in third which you cannot achieve at the moment.
Click to expand...
I disagree, 22T was standard in Europa, usually I change gear around 4000 rpm even on steep hills.

Otherwise I agree, I have peashouters with balanced pipes and the Blackbox Airfilter and 250 Mainjets.
 
Marcelx said:
I disagree, 22T was standard in Europa, usually I change gear around 4000 rpm even on steep hills.

Otherwise I agree, I have peashouters with balanced pipes and the Blackbox Airfilter and 250 Mainjets.
Click to expand...

The comment should be taken in consideration with the whole thread for context…

Riding along in top gear at low rpm… with a too small main jet… not opening the throttle fully… whilst in top gear… with a 22t sprocket… if you’re doing all of this, you cannot expect it to romp up to 7k !
 
grandpaul said:
Absolutely 260 main jets ASAP.

Have you done a proper valve adjustment yet? I'm guessing your valve lash specs are too large. Valves are not opening enough. That is all part of a basic tune-up.
Click to expand...
I am vert meticulous with every default when I testore mu bikers. The valve adjustment Is spot on as per Norton manual and made every time I retorqued the head.

Definitely I will double check the carb sliders open fully, I will road test opening full throttle on low gears and eventually I will order 260 main keys.
 
javifoto70 said:
I am vert meticulous with every default when I testore mu bikers. The valve adjustment Is spot on as per Norton manual and made every time I retorqued the head.

Definitely I will double check the carb sliders open fully, I will road test opening full throttle on low gears and eventually I will order 260 main keys.
Click to expand...
‘Eventually’ ?

I would suggest fitting the 260s before you do some WOT runs.
 
javifoto70 said:
I am vert meticulous with every default when I testore mu bikers. The valve adjustment Is spot on as per Norton manual and made every time I retorqued the head.

Definitely I will double check the carb sliders open fully, I will road test opening full throttle on low gears and eventually I will order 260 main keys.
Click to expand...
You are already risking piston & cylinder damage with too-small main jets. You really should forgo any more running until they are replaced.
 
Burned piston might not be the best term. Detonation due to excess heat because of high air fuel ratio is what does the damage. Below 3/4 throttle the mixture is predominatly determined by the needle jet and needle taper. The main jet is the primary control from 3/4 to wide open throttle.
 
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