4 replies to this topic

[Haynes]

Car is an 850 with single hs2 and points.

It was running ok but had tendency to cut out when pulling up at lights etc. I increased tick over to maybe 1000 rpm (no rev counter) and that solved it. I assumed the cutting out was down to the cold weather.

I tried reducing tick over and it seemed ok but then it started cutting out again. Increasing the tickover hasn't helped as it started running rough after you blip the throttle. I started it earlier and warmed it up until it ran ok with the choke in. Ticking over nicely up around 1000rpm. Blip the throttle and it dies. Start it again and it's running rough. Some times you can rev it and it's ok, other times you blip the throttle and it runs rough.

Plugs looks ok, nice brown colour. I've checked points gap. Compression test is 120psi across all 4 (a bit low but I have a suspicion my tester is reading low). I topped up dash pot and cleaned carb piston.

Electrical problems can be intermittent but cant see how theyd coincide with throttle. Mixture or timing neither? It's not just fluctuation in idle speed it's going from running ok to rough.

[T.Harper]

Check for a vacuum leak around the manifold where it mates with the carb and cylinder head. Also check the be carb itself and any pipework going into the carb or manifold for air leaks.

If none then also try to cover one side of the carb throttle spindle with your thumb to see if you still get as bad an issue when blipping - if not then could be indicative of worn throttle spindle.

[steeley]

Air leak

See if rubber ends of vacuum hose have perished / see if throttle bitterly is shutting fully

[andyapanel]

Use a bit of rubber tube to help detect a leak more easily. Hold one end in your ear, and investigate potential leak sites with the other end. 

Don't forget your H+S; when leaning over the engine, you could get caught up un the alternator drive belt and give yourself an HT shock off something downstream from the coil.

If you think its the exhaust manifold, dribble some oil down the back of the head to make a temporary seal. Sometimes squirting Plusgas/similar in the effected area will change the revs.

Have you got a fuel filter? If not, it could be rubbish in the carb.

Good luck and keep us posted on progress.

[Haynes]

Think I've found the problem, thanks to all the replies pointing me towards the carb and manifold.

No sign of a vacuum leak and spindle didn't have any significant play but with air filter off I could get to the dashpot piston. Turns out the dashpot piston was sticking and not falling properly. After giving it all a good clean it seems that tightening up the second dashpot screw is enough to make it bind. Spring seems very weak but it only an 850.