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Mk3 wont rev

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Mk3 wont rev

Posted by kevin_mcdermott at August 15. 2017

I have a problem with my Mk3 Commando, it wont rev above 4500 in 4th gear.  It will rev to 6000 in 1st and second and over 5000 in 3rd, but pop it into 4th and while it revs normally up to 4000, it then struggles for approx another 500 revs then stops.   The exhaust noise seems to increase though.

It has a Hemmings big valve head conversion, high comp pistons, 4s cam and belt drive.

Has anybody any ideas what the cause of this could be?  Any suggestions would be greatly appreciated.

Re: Mk3 wont rev

Posted by robert_tuck at August 15. 2017

Worn cam?.

Re: Mk3 wont rev

Posted by Katherine Scott at August 15. 2017

Previously kevin_mcdermott wrote:

I have a problem with my Mk3 Commando, it wont rev above 4500 in 4th gear.

Hi Kevin

Have you checked that your ignition system is advancing correctly?

 

Best best regards

 

Katherine scott

Re: Mk3 wont rev

Posted by Neil Wyatt at August 15. 2017

Has the gearing been changed? A Mk3 should pull to a shade over 5,000 rpm in top but that is on the standard set up.

Re: Mk3 wont rev

Posted by peter_shand at August 15. 2017

Get rid of the 4S cam. A Mk3 with a standard cam will rev over 5000 in top easily. even with a single carb.

The 4s will just wear your valve gear and itself out faster whilst making a lot of noise doing so!

 

 

Re: Mk3 wont rev

Posted by allan_walker at August 16. 2017

Kevin,

Have these symptoms just developed? Did it previously run well and reach max revs in 3rd and over 5000 in top?

When was the tuning work done? Is it run-in?

Re: Mk3 wont rev

Posted by richard_payne at August 16. 2017

It is clearly down on power as it should be able to 'red-line' in all the lower gears and certainly pull past 5000 in top. I'm not a heavy rider which helps, but I've had my Mk3 up to an indicated 7000 rpm in top with a 21t gearbox sprocket. (Yes, I know that it's not good for it).

Have you checked the timing ? Cam lobe wear as mentioned earlier is another favourite for gradually robbing perfomance although it's surprising how well they can pull even when worn.

Most belt drives though do raise the gearing. If you have a larger front pulley and a large (22t or above) gearbox sprocket then you may well be over-geared.

Re: Mk3 wont rev

Posted by kevin_mcdermott at August 17. 2017

Hi all thanks for all your comments.  Tuning work was done by Mick Hemmings a number of years ago, but not many miles done on the motor due to one thing and another, Aprilia's and Ktm's.

Its never been a prolific rev'er in the 37 years I've had the bike, but has been able to keep up with traffic, now because modern traffic the rev problem has become a issue.  It has a Boyer ignition fitted but this should'nt make any difference should it?

Will count the teeth on the sprockets on the belt drive belt and go from there.

Thanks again.

Re: Mk3 wont rev

Posted by david_evans at August 17. 2017

Over gearing I think 4500 in top should be 75 with a 22 tooth gearbox sprocket. 23 tooth sprocket and a windscreen. tank full of gas and a heavy rider with a big jacket, that'll do it.

Re: Mk3 wont rev

Posted by kevin_mcdermott at August 17. 2017

Ahh, at 4000 its doing 80-85 indicated, better check all sprockets and pulleys i think.

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