Misfire and backfire when hot

Al 29Tudor

Al 29Tudor

Well-known member
Hello all,
When my Tudor gets hot it starts to backfire and misfire so it will barely move.
This has been getting worse for a couple years. I have changed carburetors and distributors, coil, condenser, points and checked for 6 volts every place it should be.
With these changes the only thing that effects and helps slightly is opening the GAV. Could I have a cracked ignition part or manifold ?
I'm out of ideas.
Could use your suggestions - thank you.
Al
 
Back firing could be the valves are sticking when warm. How far are you opening the GVA and where are you running the spark lever? Have you done any of the tests to check for vacuum leaks; propane, WD-40 etc. How about a compression check when the issue happens while still hot?
 
I recently had all the valves out to remove a lot of carbon from the under side of the valves, new head gasket, new intake/exhaust gasket.
GAV open little more than 1/4 turn - then when it starts to miss and backfire open it 1 to two turns and that helps a little. Spark through the range retard to advance.
50 - 52 pounds compression range for all four cylinders.
Just ran it to warm it up half way on the moto meter in a pitch black garage (door open) looking for spark jumping and never saw one spark jump looking at both sides of the engine electrical system.
Still stumped.
Thanks for your suggestions.
Al
 
I suspect bad ignition condenser. They are often heat sensitive when bad.

I know you replaced it once.

Original pop-out style ignition switch? If so, try bypassing it temporarily, just to see how it runs. They can sometimes make a poor connection internally and get hot inside, which then gives even less volts to the coil (weak spark).
 
Does it run better if you slightly open the choke or does the backfire get worse? Sounds to me like the motor doesn't get enough fuel when warming up.
 
Jim/GA said:
I suspect bad ignition condenser. They are often heat sensitive when bad.

I know you replaced it once.

Original pop-out style ignition switch? If so, try bypassing it temporarily, just to see how it runs. They can sometimes make a poor connection internally and get hot inside, which then gives even less volts to the coil (weak spark).
Click to expand...




Additionally - if the wiring on the coil was removed, possible it was re-wired backwards? Causes weaker spark.
 
Exhaust manifold
Remove the manifold and lay a straight edge at the top or bottom of all the ports, they should line up.
If you find #4 is sagging...bingo, that's your problem.
Over the years the heat and weight from the muffler and tail pipe will pull that port down, especially if you are not using ring glands.
Replace manifold, use ring glands.
Just a thought
 
Try opening the gas cap when it does this. It’s possible the vent is plugged. I have always thought this was an oddball problem, but it was talked about in the latest restorer, and a close friend of mine who is it excellent mechanic Just had this happen with his downdraft weber carbureted fuel pump driven car. It would not even pump gas, until he open the gas tank cap, and away it went.
 
Hi all,
Manifold has always had gland rings and was machined when new and I checked the coil polarity tonight and it is correct.
Will check the gas cap though - it looks clean but could be clogged up inside.
I ran it in pitch dark garage last night looking for shorting or sparking and could not see any sparking.
Will give the ignition switch another look also but it is not a pop-out just on off.
Also buy more condensers
Thank you all.
 
Distributors can arc inside and you would never see it even in a dark garage. It’s probably not that anyway, but to be sure try a known good distributor shell.
 
with the engine idling spray around all intake manifold connections as redmodelt suggests.Vacuum leak is the first test. Another simple test is to attempt to change the air/fuel mixture screw adjustment at idle,if turning the screw in or out doesn't change the idle speed you have a vacuum leak.
 
I would take a compression check when the engine is cold and then when it is hot and acting up,could be a valve problem.
 
just sprayed propane around all vacuum connections and no change.
Idle adjust screw is working.
Still searching.
Thanks
 
Here is something that is easy to try. Connect a second condenser from the passenger side of the coil to ground. Easy with jumpers.
 
That action rules out a vacuum leak.Doubtful its a valve issue,those issues usually don't change with engine temperature. jackson is on to something with the condenser,and heres a couple more quick tricks..one,if your running a popout with an armored cable be sure its screwed into the distributor tightly.Second,if the condenser passes jacksons test run engine till the issues occur and gingerly feel the coil body,your coil could be getting hot and breaking down
 
If you lift the spark plug straps off of the dist about 3/16" you can watch the spark. Should be blue and steady. That'll help you discern if its fuel or spark.
 
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