Engine starts missing when warmed up

JasonNC

It runs great for about five minutes, then when it gets warm it starts missing. Give it gas and the miss goes away temporarily, but comes back at idle. I have spark tester on #3 piston and you can actually see the spark die down when it is missing. Any ideas as to what may be causing this?

cchancel

If the spark tester shows the same issue on the other cylinders it may be because your coil is going bad. As the dying coil heats up, the performance drops and not every cylinder gets a spark.

If the issue is confined to cylinder #3 try swapping the spark plug wire and/or spark plug to see if the issue goes away.

lostmind

Check the valve clearence

Jon B

I'd check Chancel's theory first. If all pistons show intermittent spark problems, go "upstream" and check the coil, coil wire, points and the ever-faulty condenser (maybe start there first!).

If only #3 has the spark problem, switching wires and plugs with another cylinder will definitely tell you if those components are at fault, as Chancel suggests.

Lostmind's theory (no clearance in #3 valves) will be a possibility if the miss (at #3) is constant once it gets hot. If intermittent, valve clearance would be less likely. If constant, a quick way to confirm too-tight valve adjustment, might be to take a compression reading of that cylinder when the engine's cold. Then, when hot, take another reading. If compression drops when hot, your #3 valve clearance is probably the culprit. (A compression check would save you from the hassle of checking the valve clearances.)

bob ward

A weak condenser can perform acceptably when cold and have insufficient capacity when hot.

Geoff

Also check coil polarity. Condenser would be main suspect.