Misfire and backfire when hot

[Al 29Tudor]

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

[Tacoma Bob]

Fuel flow?

[Jacksonlll]

Look for Intake an air leak. Probably need a new manifold gasket. Easy job. Good luck.

[redmodelt]

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?

[Al 29Tudor]

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

[Jim/GA]

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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).

[bavArian]

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.

[Mulletwagon]

Might check valve lash.

[redmodelt]

Originally Posted by Mulletwagon

Might check valve lash.

That would be a good place to check. If too little as the valves expand the gap could be closing up and not fully seating.

[30 Closed Cab PU]

Originally Posted by Jim/GA

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).

Additionally - if the wiring on the coil was removed, possible it was re-wired backwards? Causes weaker spark.

[burner31]

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

[Chuck Sea/Tac]

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.

[Al 29Tudor]

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.

[Bill G]

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.

[Jack Shaft]

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.

[rocket1]

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.

[Al 29Tudor]

just sprayed propane around all vacuum connections and no change.
Idle adjust screw is working.
Still searching.
Thanks

[Jacksonlll]

Here is something that is easy to try. Connect a second condenser from the passenger side of the coil to ground. Easy with jumpers.

[Jack Shaft]

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

[eagle]

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.

[Al 29Tudor]

OK some more things I tried tonight.
Added one ounce of ATF fluid to about 1/3 tank of fuel, put the old dist cap and rotor back on and ran it with out the gas gap.
Pretty much the same results. opening the GAV more than one turn does help a bit as usual.
Now Jackson111 has a good idea. putting a second condenser on the coil in parallel to the one in the dist.
That gets it away from the heat. I like it. will try it tomorrow.
Thanks,
Al

[Big hammer]

For giggles verify that the gav knob is turning the shaft, some knobs in the past would just spin around the shaft.

[30 Closed Cab PU]

Does not happen often, try a different coil.

[jg61hawk]

So did it help..................

Now Jackson111 has a good idea. putting a second condenser on the coil in parallel to the one in the dist.
That gets it away from the heat. I like it. will try it tomorrow.
Thanks,
Al

[Al 29Tudor]

Hi Guys,
Update - Just now tried Jackson's trick with a second condenser hanging off the coil with a pigtail to ground and it did seam to help a bit. The engine had to get a little over the halfway mark on the motometer before it started missing and backfiring. Improvement !
Still It responded well to opening the GAV about a full turn.
The next thing I'll try is to use Ethanol Free fuel. It could be boiling on the carburetor bowl.
Thanks for all your suggestions - I'll let you know what happens.
Al

[old31]

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Originally Posted by Jim/GA

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).

Jim, What is the tests for bypassing the popout?

[johnbuckley]

here we are. .. hot wire to by-pass pop out ( and junction box if you so wish)... it's best to disconnect the exisiting pop out wires in case there's a short circuit in them

[Al 29Tudor]

Another step in the right direction - To explain, Just before this all happened the temp was about 1/8 inch showing on the motometer. Then for no reason it began to run about 1/3 the way up the motometer.
So today I took it out and the same, 1/3 the way up.
I filled up with E-Free fuel, (.71 more per gallon) it took 9.34 gallons.
To my amazement the engine temperature went down to about 3/16 up on the motometer.
I wonder if the fuel formulation here in south FL change in the Spring?
So no more backfiring. There is still some missing but it's very quiet.
I had changed to a new set of 3X plugs just before this happened. So next i'll put the old plugs back in and take a ride - lets see what happens.
Johnbuckley - I like that hot wire test circuit you sent. I printed it out - thanks.
Al

[Al 29Tudor]

OK - We got it now.
Changed the new 3X plugs for the old Autolite 3076 plugs with 18,000 miles on them and it runs great. No missing, no popping, just smooth! Well smooth for an "A".
Thank you all for your suggestions.
Al

[Sunnybrook Farm]

What did you last work on just before it started acting up? I know it has gotten worse but just wondering if some of the history might be a tip off for what direction to look. I had a new condenser from NAPA go bad in just a hundred miles so it could be something new that is screwing up.

[Jacksonlll]

Next time it acts up, toss about a bottle of water on the carb bowl. That would cool it off. Can’t hurt anything.

[Big hammer]

Originally Posted by Al 29Tudor

OK - We got it now.
Changed the new 3X plugs for the old Autolite 3076 plugs with 18,000 miles on them and it runs great. No missing, no popping, just smooth! Well smooth for an "A".
Thank you all for your suggestions.
Al

My A doesn't like to run with 3X plugs either and crap gas! Glad you figured it out, take a nice long ride!

[katy]

Johnbuckley - I like that hot wire test circuit you sent. I printed it out - thanks.

It wouldn't hurt to put an "on-off" switch in the wire between the coil and the starter switch.

[cr1248]

Originally Posted by Al 29Tudor

just sprayed propane around all vacuum connections and no change.
Idle adjust screw is working.
Still searching.
Thanks

I think that it may be vapor locking. If it backfires, it shows a lack of fuel. Get a 1/4” rubber fuel line and split it down one side and put it over the existing fuel line. The condenser is a good idea to change since the new ones go bad often but opening the Gac would have no change if it was the condenser.

[Al 29Tudor]

Big Hammer - how odd some cars don't like 3X and ethanol fuel. I still wonder if the fuel is more that 10% and it boiles at a lower temperature.

[Jack Shaft]

All this for plugs..made the assumption you threw a set in along time ago,porcelain insulators do fail. Press assembled plugs like the autolite were designed for a reason, the insulator is protected from expansion.Insulators secured by a nut like the 3X are vulnerable, the insulator nut can loosen over heat cycles,allowing the porcelain to expand beyond limit.

[30 Closed Cab PU]

Originally Posted by Al 29Tudor

OK - We got it now.
Changed the new 3X plugs for the old Autolite 3076 plugs with 18,000 miles on them and it runs great. No missing, no popping, just smooth! Well smooth for an "A".
Thank you all for your suggestions.
Al

Am happy you found the issue. As this string illustrates, for such simple systems as the A has, there are almost endless possibilities for this type of issue. Easy to miss something basic. I leaned quite a few things including Jack Shaft's plug design info.