My 289 has suddenly started racing when Idle. Drives decent enough, not really noticeable, but when ticking over it races up and down. Last week it was fine, but this has come into it. I think it's drawing in air through a carburettor gasket. It's suddenly swamping petrol too. WAY more than it did. Firing most of it straight out the exhaust!, fumes are unreal. I tried adjusting the timing with the distributor a bit, and played around with the adjusters on the carb. Couldn't get it right. Has anyone encountered this before? The gasket doesn't look the best. I'm gonna rip the whole thing off the engine over the weekend and clean the carb. The choke isn't sticking. Would appreciate any advice.

 

First thing I'd check is, as you mentioned, a vacuum leak. Spray some brakleen (or ether, or other hazardous aerosol) lightly around the carb baseplate. If the idle speed picks up, you've got your vacuum leak.

That said, the fume problem sounds like an overly rich mixture. It's possible you've got a sticky needle/float in the carb, or maybe a blown power valve. If you come up negative on the vacuum leak, I'd start looking at a carb rebuild. If you've still got the original Autolite carb, it's VERY simple to rebuild yourself.

 

Is the carb flooding like to the point where it may spill over when you stop? Just asking because I recently had my carb rebuilt and when they put the filter on they wrapped the thread in Teflon tape. The tape flaked and a piece literally less than half a millimeter got stuck right at the needle tip which ended up being the culprit of massive on and off flooding which led to an unsteady idle and horrendous fumes

 

From what you describe - skip the vacuum leak check and go straight to carb (stuck needle valve probably...)

 

+1 go directly to checking for an overflowing float or debris in the float.