Bike is 1959 Dommi '99'
When I took the engine down for a rebuild I found that it had high compression (i.e. domed rather than flat tops) pistons fitted. In the course of the complete rebuild I have fitted appropriately sized standard pistons to fit the barrel which is now at + 0.060"
I have refitted the carburettor in exactly the same state it was in before the rebuild but I'm wondering if I should now revisit the jetting. I assume, although I haven't checked, that the previous owner would have add to re jet from standard to accommodate the higher compression.
In it's current state the engine seems to be eight stroking when trundling along with the throttle only slightly open. It seems to revert to normal behaviour when the throttle is opened up again. Does this indicate a weak mixture or a rich mixture ?
Thanks in anticipation,
8 stroking signifies that the mixture is too rich. Does the carb have a choke slide fitted?
Hi Bob - yes, carb has a choke slide. So, I guess I need to weaken the mixture. I believe this means just dropping the needle a notch before looking for other fueling issues ?
I'd suggest downloading the Amal set up guide. If the bike used to run fine, the compression change should not upset it nearly as much as you suggest, if at all. At small throttle the effective compression ratio is much less anyway. Sounds like you have accidentally altered something. Like pilot screw perhaps. The needle won't do a lot at the opening you describe. That will be slide cutaway. I suspect something adjustable has changed.