Rough vibration on the overun

Check your carb:

Starting point: 106 needle jet 240 main jet, needle in middle position and 3 1/2 slide. I'll say it again...Starting point as no two bikes are the same. A well worn carb won't help either.

You would probably hear a rotor grinding away on a stator. I would suggest checking both chains. A worn gearbox sprocket and overtight or knackered chain gives the impression of a rough notchy ride.

It could be the carb....if so, first check the pilot jet is doing its business. Next go for a worn slide.

If the problem is engine bits....such as big ends, mains etc you would be hearing lots of tinkling or clanking noises.

You would probably hear a rotor grinding away on a stator. I would suggest checking both chains. A worn gearbox sprocket and overtight or knackered chain gives the impression of a rough notch y ride.

It could be the carb....if so, first check the pilot jet is doing its business. Next go for a worn slide.

If the problem is engine bits....such as big ends, mains etc you would be hearing lots of tinkling or clanking noises.

Long screw driver as a stethoscope so you can then hear any dull thudding noises after revving up and tell go of throttle then you may hear this dull deep thudding noise , then if you hear this then its time to get the bottom end a good looking at , if you hear just a rumbling noise thay ok its just the main bearings at work they all rumble a bit but any horrible banging or high frequency taping noises you need to look to the top end , or the clutch inside of the primary drive some where , hope some of this helps yours anna j

Probably too much of a coincidence, but Iâve recently had a Commando in with similar symptoms, fine under acceleration, but harsh & a bit of a rumble on the over-run or very light throttle. Turned out to be the gearbox mounting bolts were a bit loose. Under load, the final drive chain was able to pull the gearbox back against the adjuster & hold it where it should be, but with the load off the box was either pulled forward by the primary chain, or just moved forward of itâs own accord, tightening the final drive chain & letting the primary chain flap about. Found it when I checked the primary chain tension, it was about right until I pushed a bit harder with my finger through the inspection hole, when it went quite loose & I noticed the rear chain tighten up a bit.

Regards, Tim

Previously robert_tuck wrote:

Previously robert_tuck wrote:

Previously tim_gostling wrote:

Probably too much of a coincidence, but Iâve recently had a Commando in with similar symptoms, fine under acceleration, but harsh & a bit of a rumble on the over-run or very light throttle. Turned out to be the gearbox mounting bolts were a bit loose. Under load, the final drive chain was able to pull the gearbox back against the adjuster & hold it where it should be, but with the load off the box was either pulled forward by the primary chain, or just moved forward of itâs own accord, tightening the final drive chain & letting the primary chain flap about. Found it when I checked the primary chain tension, it was about right until I pushed a bit harder with my finger through the inspection hole, when it went quite loose & I noticed the rear chain tighten up a bit.

Regards, Tim

Hi Tim,Think it will be chain,I'm just pannicking a bit as i have just a week to fix problems with two bikes ,a car and the van. Not looked inside the primary for 20 years ,Perhaps its time. The primary is tight, why it should move now after years of use is a mystery,although i did give it some welly recently which has also resulted in an increase in compression to the point where i can't kick it over !,A change of oil to a 20/50 may be next.

In a tug o war between the primary chain and the rear chain I think the rear chain would win because of mechanical advantage (another word for "gearing down") so a loose gearbox would result in the primary tightening and rear chain loosening. That is my theory. I think I had it happen on a Velo years ago but the same principle applies. The high compression is an odd symptom. I have only known compression to reach a peak when new rings bed in and then go down from there; never up. Make sure it nreally is compression and not the engine stiffening up from partial seizure or something else. Kick it over with the plugs out and both chains adjusted right. It should spin freely.

The final drive chain will almost always, if not always win in a fight with the primary chain, which is why you should always overtighten the primary, then use the adjuster to push & hold the gearbox forward to get correct tension on the primary, then adjust the final drive chain. Doing it this way stops the drive chain from pulling the gearbox back due to any free play in the adjuster. Even if the g/box bolts are really tight, this can happen.

Regards, Tim