1942 plymouth backfiring

[chekone]

I am working on a 1942 Plymouth special deluxe. It has been sitting for about five years and everything was original. I had to rewire the entire car, rebuild the carb, replace the water pump and many other various projects. Now I have it almost completed exept for one problem, It is having a backfiring problem when accelerating. I rechecked all the carb linkages and the accelerator pump, and set the timing as best as I can without a timing light for 6 volt. anyone have any ideas? [color:\\"blue\\"]

[Guest DeSoto Frank]

Hi there, '42 Plymouth!

'41 De Soto here, who had to drive his antique to work today because his daily driver ('93 Escort) had frozen-up overnight after a day of wet snow then an overnight temperature drop to 10 deg. F - I was frozen-out! Didn't have time to mess with thawing-out locks, so I cleared-off the De Soto and away I went! .

A few things to check:

Does it do this all the time, or only before the engine comes up to operating temperature (160-180 deg F) ?

1) A '42 Plymouth should have a manual choke - it is usually necessary to run a manual choke engine with the choke partially closed until it warms-up...have you tried this - does it help ?

(Forgive me if you're an "older buck" and already know about choke technique ! )

2) verify that your accelerator pump circuit is really working: with the engine off, remove the air cleaner and look down the carb throat with a good light: open the throttle slowly - you should see a stream of gas begin to squirt into the carb throat almost immediately, and continue to do so as you open the throttle.

If it spits or just kind of weeps-out of the jet, then either the pump passages are restricted or the pump plunger is not seating properly in the bore.

Did your kit inlcude a leather pump plunger or a rubber one? If leather, did you soak it in clean motor oil (preferably overnight) before installing?

If the pump is working okay, next check the pump stroke adjustment - you have a choice of three holes in the pump actuating arm of the throttle shaft- the center one is the "standard" adjustment; the inner one (shortest stroke) is for very warm climates, the outer one (longest stroke) is for cold climates & winter. Try the outer hole and see how that is?

(Also, wear in the hole of the pump arm and/or the connecting link will delay the pump's response to the throttle opening, and cause a flat-spot of stumble.)

3) Manifold heat control valve: there is a butterfly in the exhaust manifold that is controlled by a thermostatic spring - when the engine is cold, the spring closes the butterfly, and the hot exhuast gas circulates around the center of the intake manifold, helping to vaporize the fuel air mixture, and preventing carburetor icing when the engine & outside are cold.

As the engine warms-up, the spring "relaxes" and the exhaust pressure forces the butterfly open.

Often, the butterfly shaft either is frozen, or the spring is gone; in either case, the valve is usually in the open position, causing longer warm-up time and contributing to spitting & stumble on acceleration.

Other things to check include the timing: basic timing for the flat-head MoPar six is TDC.

(If you desire, I can give you instructions for static timing the engine w/o a neon timing light.)

Also look for vacuum leaks in the distributor vacuum control line and the vacuum wiper line.

Another suggestion is to also visit the following website:

"www.P15-D24.com" - this site is centered around 1946-48 Plymouth & Dodge, but fanciers of other years and makes of flat-head MoPar (Chrysler Corp.) automobiles & trucks hang-out there...the discussion forum is very helpful.

Good Luck!

[chekone]

Thanks for the reply, I have tried all of those things exept for the manifold heat control. but I will try it. I also would like to know how to static time it. thanks

[Guest DeSoto Frank]

Static timing:

first, rig up a little test light with a 6-volt bulb (like an #1129 or #63) and a socket (from your box o' junk or the local auto store) with two leads about 2 ft. long, and alligator clips on the ends. This little gizmo will come in handy for other tests and you can stick a 12-volt bulb in it to work on Modern cars too.

Next, note where number one spark plug wire goes into ditributor cap, and chalk the distributor housing beneath that tower on the cap. Remove distributor cap.

Turn engine over by fan or bumping starter until the distributor rotor is coming up on the mark for number one cylinder.

Now you can find the timing mark on the crank pulley, cleaning it up with solvent or sanding the rust off, as appropriate. High light the markings with some white chalk or white china pencil and also hit the center groove of the pointer.

Make extra sure that the points are clean and properly gapped (.020").

Now, connect your test light as follows: one lead to the "distributor" terminal, the other to a good ground.

Take the high tension lead from the coil and remove it from the distributor cap and ground it (so that you don't get a nasty surprise), then turn the ignition switch "on".

Turn the engine over clockwise until the rotor is coming up to "1" cylinder, and bring your crank pulley mark up til "TDC" ( or "0" ) is lined-up with the pointer. At this point, your points should just open (as the pointer lines up with TDC), and the light will go on.

If the light does not come on, then loosen the distributor clamp bolt and turn the distributor against rotor rotation until the points just break (light comes on).

If the light comes on before the marks line-up, then turn the distributor in the same direction as rotor rotation until the light goes out, then turn the distributor back the other way until the points just break and the light goes on.

Now you can tighten the distributor clamp bolt, then turn the engine over one complete rotation and see if the timing is "on the mark".

There is another possibility as to your backfiring - the Autolite distributor used on Chrysler products has an insulated jumper wire running from the terminal on the side of the distrubutor case to the points - some times this wire either breaks or the insulation becomes chafed, so that when the vacuum advance moves the breaker plate, this jumper either gets grounded or looses continuity - this could also be part of your problem.

Could also be that some of your intake valves aren't closing completely...have you done a compression test ?

Let me know how you're making out...

[Guest Hal Davis (MODEL A HAL)]

I won't go into detail about the stupid way I found out, BUT... when you rewired the car, did you make certain you didn't get the coil primary wires reversed? I can tell you from first hand experience, you wire the coil backwards and it WILL backfire.

[Bob Kinker]

Here is another one to try. Verify your plug wires are going to the proper plugs. It is real easy to reverse a couple of them. I know this from experience!

[Guest]

Do you use a lead substitute? If not, you might want to give it a try. I don't have anything as old as your plymouth (I'm more of a classic car guy), but my '75 used to backfire because the engine is designed to run with leaded fuel.

[ply33]

All Plymouth "L" head (flat head) 6 engines came from the factory with hardened exhaust value seat inserts. For that matter all Chrysler built cars from the early 1930s into the 1950s did too.

Lead and/or lead substitutes are not needed on these cars. Maybe if you have a 1960s muscle car, but not on a L-6 engine from Chrysler Corp.

I'd second looking at the items that Frank mentioned in earlier posts.

[The Old Guy]

If the car sat as long as you say, it may be an intake valve stuck open. Check the compression. If you have a low cylinder, that will tell you.