My MK3 does not rev beyond 4500rpm

I finally received and installed the 260 main jets. After warming up the engine I adjusted idle mixture closing a bit the screws to improve warm fire up, both idle screws and idle mixtures screws respond precisely. The spark plugs where both quite clean and dry with some gentle pale brown color.
I tested the bike again along 50 miles and to my surprise keeps exactly with the same behaviour: not revving beyond 5000rpm not even on second gear. It just produces again an increasing vibration on handlebar, seat and pegs. It is a bit scary. The engine fires up cold easily and runs smooth up to 4000rpm.
Now the more bad news, after trying to keep 5000rpm along no more than 3 miles the engine stops, it gets hard to fire up, the pedal gets hard to push, it looks like the engine got very hot. I removed the spark plugs and fortunately it is possible to turn the engine over.
I am frustrated and perplex, it is clear that both ignition and carburetion are spot on, I already checked the camshaft marks and I adjusted it properly.
There must be something mechanicaly refraining the engine to rev.

1. Confirm chokes are opening fully and slides also.
You'll need to pull the air cleaner, open throttles fully, confirm chokes are fully in slides, nothing hanging down.
2. Then, confirm strobe timing @ 5k +.
3. After that?????? Weak coil, bad plug, bad wire.........

I just reread the entire post.
I read you put new pistons, rings head work etc.
Did you go into the bottom?
If not, it's entirely possible you have a bad cam that's not allowing full lift.
Mk 3's have a reputation of bad cams.

I would still confirm the carbs are fully opening and the timing.
After that, there are ways of checking cam lift.

My bikes easily rev to 7000 with about 1/2 throttle in lower gears.. I don't need to "grab the second handful"

I once had a Boyer rev limiter in a bike (It's a separate Boyer box that is adjustable) I wonder if the Boyer ignition box or stator is faulty and acting like a rev limiter. If you are still having weak spark on one cylinder and It doesn't trigger the timing light, that sound like an odd problem. That needs some research and investigation.

One common mistake with Boyers is hooking up the Black Yellow and Black white wires improperly. Look at that carefully!
And not having good battery voltage. 12+ volts

Do the carb slides fully open when checked with your finger? And you have good fuel flow to the carb bowls? Is the carb needle clip in the groove and being held properly. And moving with the slide up and down?

Are the float levels correct/ If you drain the bowls, do you get the same amount of gas out of each carb? Is there dirt/rust in the bowls? If they are new carbs,,, Are all the parts in them and tightened properly. Are the float levels correct?

Is the timing mark correct on the alternator rotor.
Piston BTDC .305 inch = 30 degrees BTDC Piston BTDC .270 inch= 28 degrees BTDC

If the engine gets real hot, Timing needs to be looked at.

IT is always easier when the bike is in my shop and I do the work and the test ride..

MichaelB said:

I just reread the entire post.
I read you put new pistons, rings head work etc.
Did you go into the bottom?
If not, it's entirely possible you have a bad cam that's not allowing full lift.
Mk 3's have a reputation of bad cams.

I would still confirm the carbs are fully opening and the timing.
After that, there are ways of checking cam lift.

Click to expand...

I've had worn cam lobes a few times and it's hard to tell performance wise, My racer friend Gary was surprised how good my MK3 could pull with 2 bad lobes. My MK2a did good 1/4 mile times with bad cam. Surprisingly hard to tell.

The carbs are brand new and I checked them deeply, both slides open go up and down in sincrony, they open fully and so do the needles. The choke slides also work perfectly. Anyway I will check the bowls level.
As per the Boyer ignition I have just installed brand new Lucas coils with their new HT wires and spark plugs, I checked the timing with strobe light and now both cylinders produce a good and sincronized spark so that problem is solved. Anyway if I get to run the engine again I will check the timing at 5K again.

Yesterday once I reached home I drained the engine oil because after 1000km I have just finished the break in. The oil was quite dark, from deep brown to almost black. And so was the transmision oil I also drained.

Bypass the ignition switch with a hot wire directly to the coil. Have had two occasions where the ignition switch went "open" around 4000 RPM due to vibration. Happened on my 750 Combat and a 350 Desmo Ducati. Neither bike would rev past 4 grand, vibrated like crazy Idled fine started fine.

hpshooter said:

Bypass the ignition switch with a hot wire directly to the coil.

Click to expand...

It has a Boyer so it should be hotwired to the box white not the coils which are supplied from the box.

javifoto70 said:

I finally received and installed the 260 main jets. After warming up the engine I adjusted idle mixture closing a bit the screws to improve warm fire up, both idle screws and idle mixtures screws respond precisely. The spark plugs where both quite clean and dry with some gentle pale brown color.
I tested the bike again along 50 miles and to my surprise keeps exactly with the same behaviour: not revving beyond 5000rpm not even on second gear. It just produces again an increasing vibration on handlebar, seat and pegs. It is a bit scary. The engine fires up cold easily and runs smooth up to 4000rpm.
Now the more bad news, after trying to keep 5000rpm along no more than 3 miles the engine stops, it gets hard to fire up, the pedal gets hard to push, it looks like the engine got very hot. I removed the spark plugs and fortunately it is possible to turn the engine over.
I am frustrated and perplex, it is clear that both ignition and carburetion are spot on, I already checked the camshaft marks and I adjusted it properly.
There must be something mechanicaly refraining the engine to rev.

Click to expand...

This seems very wrong to me, the main thing about the Commano that made it so much better than all other parallel twins was the ability to rev quickly and effortlessly to the red line through the gears WITHOUT increasing vibration.
You mentioned that the pedal gets hard to push, do you mean the kick starter won't easily turn the engine over ? That seems bad. If you mean gear lever that is also bad. Also the engine got very hot after a mere 3 miles. Also bad. Did you also check if the gearbox also got very hot? Also bad. Misfiring under load at 5000rpm would be obvious.

Time for a complete strip down of engine and transmission I think, before something really bad happens. Sorry.

javifoto70 said:

The carbs are brand new and I checked them deeply, both slides open go up and down in sincrony, they open fully and so do the needles. The choke slides also work perfectly. Anyway I will check the bowls level.
As per the Boyer ignition I have just installed brand new Lucas coils with their new HT wires and spark plugs, I checked the timing with strobe light and now both cylinders produce a good and sincronized spark so that problem is solved. Anyway if I get to run the engine again I will check the timing at 5K again.

Yesterday once I reached home I drained the engine oil because after 1000km I have just finished the break in. The oil was quite dark, from deep brown to almost black. And so was the transmision oil I also drained.

Click to expand...

Dark oil isn't a good sign. But I do my first oil change at 100 or 200 miles. I always take out the oil tank to clean the sludge out of bikes I rebuild to be sure the oil system is clean. If your Trans oil or gear lube is very dark?? that is very strange and scary. There is no normal reason for it to get dark. Do you actually have oil pressure? over 45 PSI cold. Another thing I always check.

If you have an oil filter cut it open to see if there is metal in it and carefully check the gear lube and clutch oil as well for metal. It should rev very easily when you time it and go well over 5000 RPMs and set the timing to 31 degrees 5000 rpms.

You have gotten a lot of good ideas and answers from people, and you have many very bad things going on all at once that are not normal. I wouldn't ride a bike for over a few miles if it wasn't running right before I drove back to the shop.

Dark engine can be good, it's the oil doing its job soaking up the carbon but after 1000km on a rebored engine its not telling you much due to the blowby you get before bedding in. Dark oil in the gearbox after only 1000km is a different matter and needs looking at.

All due respect but with all the work you say has been done i would be very skeptical as to how competent the engine builder was. New pistons and rings you said, was it rebored to the next available oversize? Whoever did the machine work did they hone the bore to the correct clearance? Who did the cylinder head work? A broken valve spring can limit the revolutions. Whose make of pistons were fitted? There can be quite a variation in the weights between the different makers.
If it has been bored and honed correctly you do not get blowby (at least not enough to black the oil in 1000km)
You mention nothing about the camshaft. The Mk3 soft camshaft is a known problem. Take the rocker covers off, measure and compare the valve lift between each cylinder. That will tell you if you have a cam problem. Inspect the magnetic sump plug, does it have loads of metal 'whiskers' stuck to it?
Lots of things to check

Carl H said:

I've had worn cam lobes a few times and it's hard to tell performance wise, My racer friend Gary was surprised how good my MK3 could pull with 2 bad lobes. My MK2a did good 1/4 mile times with bad cam. Surprisingly hard to tell.

Click to expand...

Interesting and thanks for posting that Carl.
I've had several Norton's, not sure if I had one with a bad cam or not.
Don't really know what the symptom's would be, other than spotty tuning.
What I've had is a Mark 3 that I chased these same sympton's several decades ago.
The ultimate issue was the choke. It would retract, but not stay retracted,. It would fall, needed to tighten the lever very tight to hold open the chokes.
Overheating seems like timing.

The 'pedal' stiffing up is a whole nother issue.........

Sounds like it is ready to seize up, if it hasn't already. Piston clearance needs to be at least .004". And rings gapped properly. Timing? Put a Trispark on it. Graham.

It sounds like it's ready to seize
Are the oil lines ok? I'm guessing if they were the wrong way round it would have gone bang long ago
Is the oil pump ok? Returning to the tank ok?
If you don't have an oil pressure guage you can slightly undo the oil banjo bolt in the head and start the motor it should piss out
I'd be very concerned by the overheating in such a short space of time,look at all the things again that can cause overheating like timing and mixture and lubrication

MichaelB said:

Interesting and thanks for posting that Carl.
I've had several Norton's, not sure if I had one with a bad cam or not.
Don't really know what the symptom's would be, other than spotty tuning.
What I've had is a Mark 3 that I chased these same sympton's several decades ago.
The ultimate issue was the choke. It would retract, but not stay retracted,. It would fall, needed to tighten the lever very tight to hold open the chokes.
Overheating seems like timing.

The 'pedal' stiffing up is a whole nother issue.........

Click to expand...

I had a combat with half the cam lobe worn away
You'd never know it the way it went but I was quite young at the time

dobba99 said:

All due respect but with all the work you say has been done i would be very skeptical as to how competent the engine builder was. New pistons and rings you said, was it rebored to the next available oversize? Whoever did the machine work did they hone the bore to the correct clearance? Who did the cylinder head work? A broken valve spring can limit the revolutions. Whose make of pistons were fitted? There can be quite a variation in the weights between the different makers.
If it has been bored and honed correctly you do not get blowby (at least not enough to black the oil in 1000km)
You mention nothing about the camshaft. The Mk3 soft camshaft is a known problem. Take the rocker covers off, measure and compare the valve lift between each cylinder. That will tell you if you have a cam problem. Inspect the magnetic sump plug, does it have loads of metal 'whiskers' stuck to it?
Lots of things to check

Click to expand...

New pistons and rings you said, was it rebored to the next available oversize.

Ring gaps could be butting, making it tight after that 3 mile run, low valve spring pressure as mentioned, is a good thing to measure, that piston is chasing the exhaust valve up the bore, motor power, will nose over too soon. Cheers

Try a new pair of 107 needle jets. Do you have the cut away spray tube?
Try raising the needles to the full rich position and see if conditions improve.

If you have pea shooters look down them and see if they are open all the way through. There was a batch of pea shooters that came with a restrictor plate which reduced the outlet to the size of a nickel. Atlases and earlier bikes had this restrictor, the best thing you can do is run a hole saw down the muffler.

Thank you for so many good ideas and thing to look at. I am also thinking there is something wrong inside the engine and/or transmission. I will start checking the transmission again. The cylinders and head are the only two parts of the engine I did not do myself because the previous owner told me it was done. Maybe this ends up dismantling the complete engine to see what has been done. I will see for at this point after more than 300 hours and 6000 pounds expended (not including the 4500 pounds I paid for the bike) I am starting to be really fed up of this very bike.
It is a petty because the handling of the bike is excelent on back roads and its low weight make it a peace of cake to ride it.