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Jeep F-head

I have an older jeep-F-head Engine. Rebuilt the engine and it was running perfect until one day it started running rough . When I went to start it to check things out it wouldn’t start. have fuel-have spark at points-spark thru wires to plug but plug doesn’t fire. With plug out of block & pressed against a ground it fires with good spark but when installed it doesn’t fire. Checked with a timing light-light shows fire when plug is out but no fire when installed. I am befuddled–any suggestions ???

Have you tried a new plug in that cylinder? You might pull the insulator boot off the plug, disconnect the wire from the plug and hold the wire within 1/8 inch of the plug tip and check for spark there. And how old is the Jeep?

I second getting a new plug(s). It takes more energy to arc across a gap under compression than it does in air. You gap might be too large or the conductor inside the plug has deteriorated.

Have you checked the gap, it should be about 0.030". If it is, then try closing it to 0.025 or 0.020 and see if it sparks under compression. The engine may not run very good with this small of a gap, but at least you will know it is plug related, maybe.

If the points are pitted or out of adjustment, you could also get a weak spark, too weak to jump the normal 0.030 gap under compression. Same thing for a bad coil, but usually coils either work or they don’t work, a weak coil is kind of rare and won’t last very long.

@Bigfeet to test to coil, you need to use a spark plug tester, not a regular spark plug. Available at nearly any auto parts store.

Could it be when you rehooked up the coil you reversed the pos and beg wires?? This can cause a weak spark like you mentioned

If the spark plug fires when grounded out of the engine, then they must have a bad ground when installed. Go to a local parts store and pick up one of these and run in and out of the spark plug holes to see if that gets the spark plugs to fire.