Backfiring - getting frustrated

[cederholm]

As mentioned in another thread I’m having trouble starting my 1930 after a 2 year slumber. In the other thread I focused on the fuel system after having done some carb work (re-gasket to fix a leak). Having followed all the advice I’ve certain the fuel is being delivered as designed.

Since taking it out of storage I’ve been able to get it running correctly and up to temp only once. Now I can get her running for a bit with the choke, but then she dies. More concerning is the backfiring

She seem to backfire both out of the carb and out of the tailpipe, at different times. And she backfires often. I’ve search for a vacuum leak thinking she’s running lean, but have found none. And when enriching with the GAV she floods.

This car has always run well and I’m getting frustrated.

Thoughts welcome.

Thanks,
Carl

[Gary WA]

Site Index (Model-A.org) your answer may be here. Thanks to Model a dot org

[Mulletwagon]

Hibernating Model As often suffer from sticking/sluggish valves. Might shoot some Marvel Myster Oil directly into the carb at elevated RPM. Had a similar problem with my 31 Tudor that MMO fixed. Please share the fix when known. Good Luck !

[cederholm]

Originally Posted by Mulletwagon

Hibernating Model As often suffer from sticking/sluggish valves. Might shoot some Marvel Myster Oil directly into the carb at elevated RPM. Had a similar problem with my 31 Tudor that MMO fixed. Please share the fix when known. Good Luck !

If I can get it to run at a high RPM for a little while. Thanks for the tip!

[cederholm]

Originally Posted by Gary WA

Site Index (Model-A.org) your answer may be here. Thanks to Model a dot org

Very interesting. Thank you!

[jeepguy1948]

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I know that condensers don’t always like sitting around unused

[cederholm]

Originally Posted by jeepguy1948

I know that condensers don’t always like sitting around unused

I’ve considers that, does the condenser just pull out once the screw is removed?

[Gary WA]

Said you did some carb work? Make sure all the jets have only one gasket, NO gasket on idle jet! Ensure to check that there are no other gaskets still in the holes of jets. and make sure jets are very snug!

[Gary WA]

Originally Posted by Mulletwagon

Hibernating Model As often suffer from sticking/sluggish valves. Might shoot some Marvel Myster Oil directly into the carb at elevated RPM. Had a similar problem with my 31 Tudor that MMO fixed. Please share the fix when known. Good Luck !

And I always put MMO in the fuel 4oz/full tank helps with lubricating the valves when using ethanol gas.

[cederholm]

Originally Posted by Gary WA

Said you did some carb work? Make sure all the jets have only one gasket, NO gasket on idle jet! Ensure to check that there are no other gaskets still in the holes of jets. and make sure jets are very snug!

Yes, I plan to pull the carb tomorrow and double check all that. Thanks for the advice - and on the MMO.

[Big hammer]

Your having trouble so I would suggest double the MMO to 8oz in a full gas tank, she may smoke it little but the muffler bearings will like it ! Also are the points clean and gapped to .020 +or- .002

[Marshall V. Daut]

"I’ve considers that, does the condenser just pull out once the screw is removed?"

TWO screws need to be removed: one facing the manifolds and one inside the distributor body on the opposite side. While you're fooling around with the distributor, ensure that the wire connecting the lower plate to the points on the upper plate is not frayed and not touching the metal distributor body. That will cause backfiring as the upper plate moves through its arc. Also ensure that the little nut on the ignition points is tight. It would be best to pull off the top plate to inspect the condition of the connecting wire. That short, stiff wire has been a source of trouble for Model A owners since December of 1927!
Marshall

[Flathead Fever]

I'm not a Model A Mechanic but I was modern mechanic for 30-years. Engine basics are all the same. The most common thing that goes wrong after a car has been sitting for a long time is fuel problems. You say you are pretty sure that's not the problem. Ignitions don't usually go bad while sitting but it could happen. For some reason flathead style engines are more prone to their valves sticking open, while sitting in that open position for long periods of time than overheads are. I did this every day for 30-years and never saw an overhead with a sticking valve unless a spring broke. One clue can be a loud valve train noise from the excessive gap. I have a '32 V8 roadster that was my dad's in the garage that sat for 15-years and now it has a stuck valve. It ran fine when it was parked and now it runs horrible, like it has all its plug wires crossed. About all I've done was give it new fuel and start it. I'm pretty sure from the single loud lifter noise that an intake valve is stuck open. If it was an exhaust valve it would only effect one cylinder. It runs way too bad to be one cylinder. With a stuck intake valve what happens is as the piston comes up on the compression stroke it forces that mixture back into the intake through that stuck open intake valve and it disrupts the flow to the other cylinders. Then the plug for that cylinder fires and it can ignite the outgoing mixture and the flame travels through the intake and backfires out the carb. An extremely lean condition can also cause a carb backfire as can timing that is off. It's unusual to have both an intake and exhaust backfire. Usually, an exhaust backfire is from excessive fuel and oxygen being ignited by the exhaust. Like if a cylinder did not fire and then the mixture was forced out the exhaust manifold and lit by the exhaust flame from another cylinder. I think the first thing I would be doing is a compression check. Most people are not going to have the large Model A spark plug adapter for a compression gauge. You can place each cylinder on top dead center and take a blow gun and a rag to make a seal and force air into each cylinder. If you hear it coming out the carb, an intake valve is stuck open or burned, and it comes out the exhaust it's the exhaust valve. You can also pull all four plugs and crank it while sticking something over the plug hole with a little pressure and see if the compression blows it back. If doesn't there no compression in that cylinder. Do the tests that cost anything.

[Jim/GA]

While you are in the distributor, look also at the contact faces of the ignition points. When a car sits, you can get a fine white powdery film on the points that makes the spark weak. A quick pass with some super fine sandpaper between the points will knock off the corrosion and give you a good electrical contact again.

[1crosscut]

To rule out the problem being with the carburetor or the distributor borrow a known good working carburetor and distributor and swap them out one at a time.

[Rob Doe]

Random backfires, out of the muffler and the carb? I would be suspicious of the ignition switch. There is an inexpensive bypass cable for this.


For a compression test, there is an adapter sold by the vendors that screws into the spark plug hole first and then the modern compression tester screws into it.



If you had the carb apart and the jets out, the jets may be leaking at their bases. I've had that issue in the past after cleaning a carb.

[cederholm]

Very helpful info all.

Thank you!!

[Kurt in NJ]

when changing a condenser first break loose the screw in the hole, then take out the screw for the strap, with the new condenser close by (on the head in easy reach) then unscrew the screw in the hole, do not take screwdriver off the screw, put in the replacement condenser, start the screw, then put the screw in the strap.
This way the screw and insulation washer won’t fall inside or get out of alignment.
Before the A&L condenser i got real good at changing them, could do it while waiting at a ed light,
Does a little choke, or opening the gav more change the backfire?
If you look at the points contact area is it grey( good running normal), or have blue spots (usually condenser or too high voltage),

[jeepguy1948]

I mentioned condenser earlier and I see that you are considering that. I also have another memory; awhile back I was experiencing something that sounds like what you have going on. The coupe belonged to a new club member and had been sitting for awhile. I tried everything I could think of but no luck (I did find a number of other issues that I fixed along the way). I did get it running better but it still ran terrible. I kept asking around about the coil, everybody I talked to said that a coil works or it doesn’t so if the car runs at all it’s not the coil. Well, it turns out that I talked to the wrong people, a coil can be bad but still good enough for the car to run. I finally swapped the coil in desperation and that was it, ran like dream! That car had been sitting too.

[cederholm]

Progress!

So I think I have a couple issues, but things are getting better. I had a intermittently stuck float valve which would lean the mixture and cause me to backfire and stall. I also think I have some sticky valves but need to investigate further.

Got her running, warmed up to temp and drove around the yard a bit. The backfiring stopped, but she still seem to give off small pops and wont run at idle. I sent some Marvel Mystery oil into the carb and will do it again with a few more heat cycles.

I took a video of the running motor and would love for you guys to listed and provide some feedback.

Google photo link below.
https://photos.app.goo.gl/zRXSuQMbDqL27qhy7

Thanks!
Carl