My MK3 does not rev beyond 4500rpm

MexicoMike said:
I think this entire thread should be deleted if the original poster doesn't give an update in the next couple of days.
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In fairness, the OP hasn’t been MIA for that long, he last posted on 3rd Sept, it just looks longer coz of the number of posts !

Hopefully he’s busy checking his cam timing and will provide a sitrep shortly …
 
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worntorn said:
All that fussing over the perfect cam profile and all you need is a piece of cold rolled round stock!
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Only need a keyway for the cam gear in the timing chest. Wouldn't matter how many rollers were between the dots on the sprockets either. An expert tuner's paradise. :rolleyes:
 
Alfred Hitchcock would never make a movie without revealing who raided the cookie jar. - Perhaps there were never any in it anyway ? He was just 'stuffing birds and animals' and peeping through a hole in the wall.
 
There are two very common reasons when a motor will not rev high, and both easy to find. If you disconnect the cable from the instrument on the handle bars - the end of the inner cable is supposed to be square, and fit into a square hole. The other reason is something which often happens to road racers - the need to move off the bitumen onto the grass to avoid dropping oil on to the track.
 
Lots of people get on the internet asking for help, get a few ideas, then fix what needs fixing and never say a word about making the fix. Hardly worth thinking much about it.

The OP might have the engine apart and be plugging away at it, or got rid of the bike. He did mention he was not happy with it, or was that some other post I was reading. :)
 
Fast Eddie said:
In fairness, the OP hasn’t been MIA for that long, he last posted on 3rd Sept, it just looks longer coz of the number of posts !

Hopefully he’s busy checking his cam timing and will provide a sitrep shortly …
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By post #57 it was clear that he had and replaced a bad coil and then the bike would rev without problem to 5000 rpm. In post #57 he stated cam timing correct, 220 main jets, non-crossover pipes, peashooters, and K&N air filter, 22T front sprocket. Therefore, IMHO, he is running out of main jet but as best I can tell never answered the throttle position question so hard to say for sure.

It has now been 10 days. I was on the cam timing bandwagon for a while, but re-reading this thread changed my mind. I'm no longer interested until he has 260 main jets and states the throttle opening and condition (Riding?, What gear?, etc.) where the problem starts if the jets do not fix it.

BTW, if I'm not mistaken, a 22T front sprocket at 5000 rpm is 89 MPH. If he's testing in 4th gear while riding, I hope he's testing on a track or Spain has the equivalent of the Autobahn.
 
kommando said:
Post 81 I finally received and installed the 260 main jets.
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Oops - don't know how I missed that. He states they changed nothing except making it harder to start which is not something a main jet can do. Still wish I knew the throttle opening and conditions.
 
Hello again gentlemen! I am sorry for the late reply. There is not mistery nor unpoliteness from my side. The fact is that after two years and 380 hours restoring my MK3 project facing that final problem really void me, I was so fed up of the Commando I needed to step back along some time.
Since nn the meantime I have been successfully restoring a Moto Guzzi 850 T3 California from 1979 I have spent a couple of months out of the workshop and just riding my bikes.



Last week I was happy again and on the mood to face the Commando issues again.
After voiding primary transmission oil and realizing it was pitch dark with aluminum particles I I dismantled the primary transmission case completely only to finally discover the origin of the engine low torq and laziness: the previous owner installed the gearbox wrongly making me mainshaft to be non aligned with the crankshaft, this created a huge tension on the primary chain.


After so many things I have tried, many parts bought new and replaced in the end the problem was quite simple. Funny enough more than two years ago L.A.B. realize something was not properly assembled on the gearbox looking at a side of a picture I posted about the carburetors. I did not consider the comment as something to important!! what an asshole am I for not to follow every comment of the Norton gurus.
 
javifoto70 said:
Hello again gentlemen! I am sorry for the late reply. There is not mistery nor unpoliteness from my side. The fact is that after two years and 380 hours restoring my MK3 project facing that final problem really void me, I was so fed up of the Commando I needed to step back along some time.
Since nn the meantime I have been successfully restoring a Moto Guzzi 850 T3 California from 1979 I have spent a couple of months out of the workshop and just riding my bikes.



Last week I was happy again and on the mood to face the Commando issues again.
After voiding primary transmission oil and realizing it was pitch dark with aluminum particles I I dismantled the primary transmission case completely only to finally discover the origin of the engine low torq and laziness: the previous owner installed the gearbox wrongly making me mainshaft to be non aligned with the crankshaft, this created a huge tension on the primary chain.


After so many things I have tried, many parts bought new and replaced in the end the problem was quite simple. Funny enough more than two years ago L.A.B. realize something was not properly assembled on the gearbox looking at a side of a picture I posted about the carburetors. I did not consider the comment as something to important!! what an asshole am I for not to follow every comment of the Norton gurus.
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Explain more detail about gearbox not properly assembled, and how that prohibited revving as expected.
 
Gearbox alignment should not limit revs. And how can it be out of alignment anyway ? Chain might be too tight, but the motor would still rev. Even cam timing will not stop a motor from revving high. If the main jet affects starting - check to see that it is not blocked.
 
L.A.B. said:
As I was mentioned it was because previously I said I thought the upper gearbox spacer 030023 looked as if it was fitted at the wrong (RH) end of the upper gearbox lug so I assume that's what it is.
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If that was the case, the binding that would be great enough to cause the engine to not rev out, would surely have destroyed Parts and they'd be smoking wreckage to view.
 
One horsepower is 746 watts. Two horsepower lost is like a baseboard heater on full. Picture the loss of horsepower and it converting to heat, concentrated at the parts that were misaligned. Things would get pretty hot to say the least.
 
It is great news that a fault has been found that can now be rectified before any serious damage ensues. But whether or not it was the cause of the inability to rev will only be ascertained when it’s finished and tested.
 
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hhh said:
That would depend on how much it's out, wouldn't it?
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Cam timing errors usually have more effect on where the power band is in the rev range, than the actual amount of power which is delivered. The E3134 race cam which is used in Triumph 650 Bonneville motors has a power band from 4000 RPM to 8000 RPM - an around 1970 Commando cam has similar timings, but cannot usually be revved that high without destruction. The exhaust system and gearing are important. When you first ride the bike after a different cam or exhaust system is fitted, you need to feel where the power band is and gear to suit. When you raise the gearing, it changes how much throttle you use, so you need to recognise the pattern, before you can optimise the set-up to suit the circuit on which you normally race. With race cams, I suggest close ratio gears are essential. With my 850, I stay between 5,500 RPM and 7000 RPM in races. Close ratios require less throttle, and I always use the leanest needles in the carburetors, and feed the throttle on. I use a quick-action twist grip, so I do not need to move my hand while racing - my front brake is operated with my forefinger.
 
acotrel said:
Cam timing errors usually have more effect on where the power band is in the rev range, than the actual amount of power which is delivered. The E3134 race cam which is used in Triumph 650 Bonneville motors has a power band from 4000 RPM to 8000 RPM - an around 1970 Commando cam has similar timings, but cannot usually be revved that high without destruction. The exhaust system and gearing are important. When you first ride the bike after a different cam or exhaust system is fitted, you need to feel where the power band is and gear to suit. When you raise the gearing, it changes how much throttle you use, so you need to recognise the pattern, before you can optimise the set-up to suit the circuit on which you normally race. With race cams, I suggest close ratio gears are essential. With my 850, I stay between 5,500 RPM and 7000 RPM in races. Close ratios require less throttle, and I always use the leanest needles in the carburetors, and feed the throttle on. I use a quick-action twist grip, so I do not need to move my hand while racing - my front brake is operated with my forefinger.
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Drum or disc when you're racing?
 

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