Engine runs fine then stops when hot, then repeats, can be very difficult to diagnose. Sometimes it might be attributed to fuel or electrical problems, but every once and awhile, it turns out to be mechanical.
Sometimes, carbon will build up in the exhaust valve guide reducing the diameter of the guide bore. When the engine gets hot, the valve guide bore expands; further reducing the diameter of the bore till it grips the valve stem and prevents it from moving back to the seated position. This will cause the engine to backfire (thru the exhaust) many times till it stops running. At this point, if you try to re-start the engine, it will spin over very easy and faster than usual but will not start. This is due to the fact that the valve is being held open preventing the engine from building any compression. Although it may give you some really loud backfires thru the exhaust
After the engine cools down, the valve guide bore shrinks, reducing its grip on the valve stem. The valve returns to the seated position, compression is restored and the engine will once again run, until the engine gets hot enough to repeat the cycle over again.
The only way to fix this is to remove the valve and ream out the guide removing the built up carbon. It can be very difficult to remove the valve due to the excessive carbon build up but there are ways to do it.
Some ways to diagnose if this might be your problem
(WHILE OCCURING AND STILL HOT):
- Do a compression test while the engine is HOT. If you have NO to very LOW compression when hot and compression returns when cold, then you have found your problem.
- If you’re lucky enough to have an engine where the exhaust valve is located below the spark plug, simply remove the spark plug and rotate the engine while watching for full movement of the valve thru the spark plug hole. If you have less than full movement when hot and full movement returns when cold, then you have found your problem.
- Remove the valve cover and watch the other end of the valve, the valve stem for movement.
Some ways to diagnose if this might be your problem
(WHILE ENGINE IS COLD):
- Remove valve spring keepers, springs, washers and check for free movement of valve stem in bore for the full length of the valve. You will need to remove the valve cover and cylinder head for this check. The valve might move ok for a small distance but will be very tight when completely removing from its’ bore. If this sounds like your situation then you have found your problem.
Most times it
will be either electrical or fuel related, but before you go completely bald, after chasing all the electrical and fuel possibility’s, you might want to check your exhaust valves! The scenario I painted above is more for single cylinder engines however it still applies to twin’s .With twins you could have this issue with one cylinder and not the other and that will affect the symptoms slightly.
