Hey guys, I would like to put to bed a possible intake valve problem. Number 1 backfires through the carb, but when I pull the plug and hand crank the motor I can see the valve opening and closing. Also, with the engine running and the plug out I can see the valve opening and closing. The backfire is not random but occurring consistently. A compression check gives me 50 psi.
I have gleaned much useful information from the forum over the past 2 years, but would prefer someone to give me a call so we can discuss this matter.
First swap the plug and coil for #1 cyl just to eliminate them as the problem but I don't believe their the cause. It hints at a sticking intake valve. Was your compression test done hot or cold? Done cold the "shrunken" parts might have a bit of clearance at the valve/ cam follower area which disappears as they heat up causing the valve to stay open slightly. I talking thousandths here. 50 Lbs is quite good also hinting that the valve is closing fully at cranking speeds. I'd check the valves clearance. While the cover is off I'd also try to move the valve manually to check for possible sticking. I'm reasonably sure one of the items mentioned is your problem.
About 20 years ago I had a T which backfired. I don't remember what cylinder it was, but I found the problem. The wiring between the timer and coil box was grounding out. When a coil primary is grounded it will produce a spark regardless of the position of the piston. You might have a spark arrive while the valve is still open. However if you are the same person who posted last week, you stated that the compression was lower with the engine idling than when hand cranking. That would indicate to me that your valve is closing slowly on #1 cylinder. You might try replacing the spring on #1 intake valve and see if that corrects your problem.
Check for a "lazy" lifter, i have one on my 24, #3 exhaust doesn't close fast enough and causes a back fire through the exhaust, i found this the expensive way , checking coils, mag output, three carbs, three timers, took wires off and on, checked terminals, switch, finally took the head off and removed the valves, checked for any tightness in the valve function, and finally lifted the lifters one at a time with my fingers and pushed them back down, most just fell back when releasing but #3 exhaust had to be pushed firmly back down, it backfires at low idle and at a medium idle at speed it is not noticeable, i will just drive it more to wear in the lifter.Just my .01 cents worth.
Good luck tracing the problem.
Visually check for a broken valve spring.
Then try lifting each valve open with a large screwdriver & compare with the problem valve. Spring could have lost tension from heat, etc.
I vote for take the head off and pull the valve. Then you can see if it needs cleaning in the guide area of the stem (gum or rust or both) and or the spring or keeper parts are suspect. Heck, it may be a great time to replace all the valve springs/keepers with new ones (cheap to buy) and lap the valves, reset clearances. I bet the issue will be found along the way and the T will run much better.
I agree with Eric but only after you've checked the problem out. If it is a clearance problem and you have adjustable lifters, (it's a stretch,I know, but possible) an adjustment will cure you. Try to confirm what it is first before getting out the drain pan and the 5/8" socket. Keep it simple.
What is the history of this engine? Has it spent any time sitting? Original valves? Etc......
I'm with Norm on this. Over the years I have had way more ignition problems than other things.
My number one carb problem has been ignition!!!
I had an issue with a car that sounds quite similar. Car bought off ebay by a friend...runs great! (no it didn't).
Car had broken adjustable exhaust lifter on #3 and never opened that exhaust valve. Cylinder would fire and then recompress the spent charge (where it should have discharged it), and the moment the intake opened for suction, the compressed gases would expel through the intake quite violently. Engine, of course hammered pretty good too...
The center main bearing cap was obviously replaced before car was sold, and that new one was being beaten out too.
Fixed lifter and scraped and shimmed bearing cap and the car is now a wonderful running car.
Not saying this is your problem, but certainly mirrors the issue you seem to have, and would also show good compression where a leaky intake valve would show up. Usually spitting through the carb is a leaky intake valve, but that was not the case with this car.
I've sent you a PM regarding something you might want to try if you're unsuccessful in your search