Mk1 having carb/misfiring issues not holding constant revs

[PintsizePowerhouse]

Hello all-need some help figuring a problem with my 1980 Mk1 345. Basically if I try to hold the engine at any rpm (been trying 2000/3000 rpm) the rev counter needle (the one in the car and an independent one give exactly the same behaviour) fluctuates +/- 100 rpm approx. The engine seems to misfire, but with no regularity-I have checked/tightened all the carb screws and eliminated leaks as suggested in other posts (from the idle solenoid and flange from the air filter housing in my case),set the CO2 to 2%,dwell angle is just over 50 and timing to +6 degrees. Ignition system is original points mean prizes,tried a capacitor discharge system to eliminate the pesky condenser just incase but the behaviour is just the same. Idle speed is wonderful with no hiccups,although there is a slight engine wobble when it switches over to throttle,although have tried the spray with easy start trick and no sign of any more leaks or play in the spindles. Only non original component is the air filter-have put in a K&N (since getting the original type was becoming a pain), but the problem was there before. I was thinking the distributor,but the misfire ish thing is not regular. The plugs are firing properly at the not-so-constant revs,have checked with those funky light up HT extensions. Genuine 80k on the clock-this problem has been with me for a few years,and I really want to sort it in time for the MOT. Hope someone can help.

[macplaxton]

Random guess: Have a look past the points base-plate and see if the advance weights are slopping about. Some Ducelliers have crappy plastic bearings which disintegrate causing the advance curve to go wonky.

[pettaw]

I would suspect the carb throttle plate bushes have worn and are letting air in at certain throttle positions. Quite often this will be the 'light cruise' area where the car will have spent most of its time, but sometimes it can be the idle area too. If you have a blowtorch (cut the giggling at the back......) to hand, then in a ventilated area, get someone to hold the throttle at the position where the car misfires and wave the blowtorch, unlit, round the throttle spindle area and see if the engine changes tone at all. If that happens, then its time for a new carb. Depending on the exact type, new carbs are available for plenty less than it would cost a carb specialist to fit and ream new brass bushings, or you could try the 'suck it and see' and take a chance on a scrappy carby, but they tend to be very hit and miss.

HTH

[PintsizePowerhouse]

Thanks for the advice guys-believe or not i'm still working on this problem. I haven't tried the blowtorch idea yet,but I've noticed the dwell meter oscillates between 50 and 54 irregularly,suggesting the distributor is at fault. Has anyone experienced this?

[macplaxton]

Might be worth taking a peek under the base plate inside the distributor. Within lies the mechanical advance mechanism. I'm not sure on Ducelliers of that particular vintage, but I know some have nylon bushes on the advance weights, the bushes like to disintegrate and flop around.