Misfire mystery

[Dneill62]

My simple pinto with straightforward ignition and carb is not firing no 4. I have replaced everything in the ignition side. Coil, cap, rotor, accuspark stealth unit, lead and plug. There is a spark but it looks weak although its difficult to compare with the others as it won't run on 2 cylinders.
My question is, although compression is good(175) could i be looking at a burnt valve?
Driving the car the problem seems to go away above 2500 revs. Plug looks good colour. No oil.
Any thoughts?

[Matth93]

If you've got good compression on that cylinder then it says to me the valves are fine as you would lose compression if a valve wasn't sealing or damaged. I would check and compare the spark to the other cylinders it doesn't need to be running just get someone to turn the key while you look. Also have you checked the ignition timing is ok. What about air leaks on the inlet manifold is there a pipe that could be sucking air and once the revs get up it can overcome it?
Another thought has the engine been tuned? High lift cam maybe? Which might feel rough low down in the revs

[Dneill62]

Thanks for the ideas. Timing is good. Engine is very standard and ran fine on the way to the pub! Returning home is began. Will check out air leaks. Managed to compare sparks and they all look similar on turning over. You confirm my thoughts that good compression means good valve seals. Its a curious one!

[Nash]

What carb are you running?

Does it rev cleanly above 2500 all the way to the redline?

Is your air filter clean?

[Dneill62]

Hi. Its a weber 32/36 and the ait filter is fine. Yes revs happily up to the red line. Raining today so no chance to check things. Definitely just a no 4 cylinder problem.

[Dneill62]

Just did another compression check. No 4 is about 10% above all the others. But from memory i think it always has been. 185 as opposed to about 170. Can't think why this would make it not fire. Any more ideas. Checked spark again. Looks the same as all the other cylinders.

[Dneill62]

Solved thanks to matth93! Well done sir, air leak at the inlet manifold. I could blame my son who fitted it with a new carb last year but I should have done a check. Tightened the loose bolts but it is still leaking a bit. New gasket on the way. Phew! Thanks everyone.