[Kalun D]

Problem Diagnosis is a process of elimination.

Basically you need 3 things for an engine to run.

compression

ignition

fuel

First try to determine which of these 3 has the problem then move on to determining the exact cause.

In this case you can eliminate compression because it's a starting, not a running problem, and it's intermittent. Chances are it's fuel or fire.

To test ignition.

Take a small screwdriver and remove a plug wire at a plug, insert the screwdriver where it touches the metal contact inside and hold the screwdriver by the handle with the blade about 1/4 inch away from any metal ground. Preferably nothing with fuel in or nearby.

Make sure your hand and any part of your body is at least a couple inches away from the metal blade of the screwdriver. It's also safer if you don't let any other part of your body touch any metal on the car, thus eliminating the electrical path to ground.

(don't recommend this method for heart patients!)

Have someone crank the engine over and look for blue spark, increase the distance of the blade to ground from 1/4" up to 1/2". It should spark nice and blue up to a 1/2" gap. You can get faked out here if you see spark at 1/4", it's not enough to fire under compression. There are certain problems that cause a weak spark, so you need to know if there is no spark or a weak spark.

It's an invalid test to use a spark plug because the gap is too small. When the plug fires in the cylinder the compression of the fuel/air mix causes resistance to the spark jumping the gap, thus requiring a hot enough spark that will jump a 1/2" gap in ambient air.

For heart patients you can also purchase a cheap spark tester with adjustable gap for about $5.00.

Once you have determined you have spark you can move on to fuel. There are other ignition problems that can cause no start when you do have fire such as bad plug wires and spark timing but those are doubtful with the "only when hot" symptom.

To test fuel.

Look down the carb barrels and pump the gas all the way 2 or 3 times to see if the accelerator pumps are squirting each time. You should see a small stream of fuel squirting into the barrel. This most likely tells you there's fuel in the carbs but not necessarily that it's reaching the cylinders in the correct amount.

A better test is to take some carb cleaner or starter fluid and squirt a smallish amount down the carb primary barrels while trying to start the engine. If no fuel is the problem the engine will run briefly on the fluid your supplying.

(This type carb (I believe) has primary and secondary barrels, the 2 primaries open first then the secondaries on full throttle.)

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The "only when hot" symptom most likely points to a coil or ignitor failure if it's ignition and a vapor lock or evaporation problem if it's fuel.

Coil and ignitors are best tested by replacement, they have electrical tests for them but these aren't always conclusive and sometimes misleading. Also others suggesting cooling via water mist or pressurized air is correct.

Vapor lock is a condition where the fuel boils and the pressurized vapor blocks passage of the liquid fuel. The infra-red temp gun will be handy in this case to find hot spots.