SOHC.co.uk Forums > SOHC Singles & Twins

High revs won’t drop back down

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Toko_Jo:
Always worth checking carb rubber diaphragms on those CV carbs for degradation/splits.

Craizeehair:
What I’ve found out so far, the diaphragms weren’t great, one was split and one was wearing through, both have now been replaced for NOS from DS.

It still had the same issue after changing those, I did squirt some easy start around the new intake manifolds and the left hand one did cause the revs to drop slightly.

I went back to basics and adjusted the cam chain, checked the valve clearances, no issue there.

When I checked the compression it was rubbish, around 80! Squirted a new drops of oil down there from the oil can and it improved to around 120.

I’m going to do a top end rebuild at the least and may go deeper, so the hanging idle will be addressed if needed after that. I’m going to take my time and hopefully have everything in place to start it when the nights start drawing in and in the meantime, concentrate on riding the two bikes that are ready, willing and able through the summer months.

Thanks for all the responses, very much appreciated.

K2-K6:
From that description, it sounds like it could be a bit "windy" somewhere in the engine.

Before stripping it, you could try putting the crankcase breather into a glass of oil when you get conditions that cause it to rev. If it sucks the oil up into crankcase then that would suggest something amiss with rings in allowing it to draw air past piston on downstroke to supply even with throttle closed.

Could be worthwhile checking like this to give more indication before dismantling. 

It may have broken or badly worn rings/bore/piston in there.  Or, possibly very stuck rings from gumming/corrosion if sat for extended period.

Craizeehair:
That’s interesting, another thing I didn’t know. I knew I shouldn’t have started stripping it before updating here…

I’ve taken off the points, spark advancer, air filters and carbs already!

K2-K6:
Aah, ha.

It doesn't matter too much as any of those conditions I've mentioned above are going to be very apparent on inspection. Ultimately you'd have to do the same strip down if it ducked up oil, just that you'd have a little more evidence for "conviction" if that was present  :)

Hopefully you'll be able to pinpoint much in the way of untoward conditions easily.

It'll be interesting to see what you've got in there once apart.

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