Hello again. I know the choke on a cold engine causes higher rpm but mine revs so high when the choke is on I worry its ruining the engine. When the choke is then switched off the revs are fine. It certainly isn't healthy for a cold engine to rev that high. Carb is a dellorto. All the inlet rubbers are new so i don't think its an air leak. Could the choke itself be worn causing this high revving. Carb has been stripped-down and rebuilt. Any ideas ?

From what I've seen, thats normal for small displacement bikes.

From what I've seen, thats normal for small displacement bikes.

No previous bike has revved like this though. Its revs like the throttle is wide open. Not what my neighbours want to hear when im testing the bike and surely not good for a cold cylinder and piston rings.

Try starting without the choke. It may be rich enough already and this time of year ambient temp is much better.

It will always be a zingy bike though as that's a busy piston in there.

This seems to be a fault on many betas and on quite a number of bikes which have cold start enrichment systems rather than chokes. Check your carb for seal condition on the enrichment system. At this time you will be able to identify the air inlet passage for the cold start. Then try reducing the diameter of this air inlet with a piece of pvc tubing or similar.

Another trick is to get a friend to block / partially the exhaust until you turn off the cold start enrichment.

large capacity (500 cc) 2 stroke motocrossers could be a pig to start cold then they revved too much. A fix was to make a removable reducer for the exhaust tail pipe outlet. in addition to preventing over revving of a cold engine this increased back pressure in the exhaust, increased pressure in the cylinder and made the engine fire up easier.

cheers