Misfire when warmed up

[Julesbug76]

I've got a 1970 with a fairly new engine, (4000 miles or so on it, previously running sweet) 1661 with twin carbs. On a trip back from Cornwall last week we started losing power about 3-4 hours into the journey, not long after a refuel stop. Started misfiring, running on 3, then cleared after a little while, continued to do this until losing all running power and the rest of the journey was completed on the back of a tow truck!
Have changed the coil today, started fine, ran beautifully for about 15 mins, idling fine, as soon as warmed up starts spluttering and misfiring again. Changed plugs, 2 were a bit sooty, same story again. We're running electronic ignition, do we go there next??

 

[ozziedog]

Try the condenser and points for a bit just to see if that sorts it. Elecky ignitions are black magic and you need a mad wizard to fix if broke, or get another one. Check valves and the feeler should be about 6 thou cold cold cold, check your book. If they are a little tight at cold, then they get tighter as they gets hotter then hey presto you got zilch compression in the noisey cupboard . Try a very very rough compression test first but only do this if you have the ignition keys in your pocket and the middle lead out of the dizzy. Turn your motor over by putting a ratchet on the alternator and just a finger on fan belt to tighten it slightly. Turn it over for a minute or so and see how easy it is to turn over, see how many revolutions you can get in a timed twenty seconds. Then get it hot and misfiring, then repeat the compression test. This is only rough but it might give you an idea if you is loosing compression. If it is, then get your rockers exposed and have a look for jammed valves etc, then do your clearances, etc etc

Ozziedog,,,,,,,,,,you've checked your fuel pressure ?

 

[ozziedog]

:evil: Oh, where did this motor come from ? Is it a Brazilian motor by any chance ?

Ozziedog,,,,,,,,,,, I got one of them :evil:

 

[thejinx]

I had this issue last year. changed Ht leads and coil (they were cheap ones) thought it might have been the coil or a lead breaking down. Timed the engine properly and all seems ok now. I still run a 009 with points and condenser I did have electronic but due to me being stupid I blew it by crossing the wires.

I initially thought I had a knackered cylinder but touch wood been ok this year.

 

[Julesbug76]

Ozziedog,,,,,,,,,,you've checked your fuel pressure ?

Thanks Ozziedog, this gives us plenty to try. Not checked the fuel pressure actually, we've got a regulator that's marked to a specific point, have tried fiddling with that, but it runs more like a bag of spanners when it's moved! :?

It's a Cali import, not been near Brazil as far as I'm aware