Backfire

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We are overcoming problems one by one, but it seems that for each solved, another appears. With the rebuilt generator installed and charging, we’re stymied. Help, please!

We have tuned according to directions, using a borescope to get the No. 1 cylinder to TDC. The distributor has been cleaned and the points polished and set correctly, we think. The condenser is not shorted, the coil shows high resistance in the secondary. The plugs have been cleaned and gapped to 30/1000. The carburetor has been disassembled, cleaned and reassembled. The muffler is blown apart by an earlier backfire (see an earlier post about finding the right replacement).

Currently backfiring through the carburetor, and the bowl tends to fill with gas.

We’re overlooking or forgetting something or have something mis-adjusted. Sympathy appreciated, suggestions greatly welcomed, fixes out of this world.

Thanks for any help.

Bob & RJ

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Sounds like an intake valve must still be open as piston comes up,

forcing air into carb…..

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Couple of diagnosis questions.

Will it run at all or does it just backfire while cranking.

If it will run, is the back fire sporadic or have a cycle to it?

A compression check might help determine John’s point.

If that is not it, I would lean towards timing.

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If the carb bowl is filling with gas it sounds like you have a leaky float needle or you have a mechanical fuel pump that is pushing too much pressure and overwhelming the needle.

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I did some research and here is what one trouble shooting guide said about Backfiring at the carb.

1. Excessive Lean mixture

2. Late ignition timing

3. Incorrect valve timing

4. Improperly seating valves, particularly intake

5. Obstruction in fuel line

6. Dirt/water in sediment bowl

7. Intake manifold air leaks

8. Poor grade of fuel

9. Secondary wires crossed in the distributor cap

10. Badly worn or improper valve clearance

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Did you check the firing order and verify your cap is

wired properly?

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Well, it would appear that you have spark and fuel delivery and air, so that leaves something mechanical, which would include wiring, I guess.

To John’s point, if you have the metal wiring conduit, that is kind of like a maze and it is easy to get the leads wrong. I would test continuity from the distributor to the plug end to verify position.

Based on the Pierce-Arrow Wiring and Tune-up guide, which the PAS has assembled and I recommend getting if you don’t have one, Firing order is 1-5-3-6-2-4, with clockwise dizzy rotation. I don’t have an 81 but it looks like # 1 lead on the distributer is the lead on the inside, closest to the block or at 12 O:clock as viewed from the side of the car looking down on the distributor. Also, you haven’t mentioned flywheel timing marks.

On the potential that you have an air leak, you can spray carb cleaner around the joints in the intake system while cranking. If nothing changes, then probably no leaks.

It is probably something simple, it is just a process of elimination.

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It its NOT timed right.. I’ll explain:

But first: Peter, the saying is : 99% of ‘fuel or carburetor problems are the points and condenser’.

Before getting to the timing: The carb intake fills with gasoline because of a leaking needle and seat.. If you have an electric fuel pump instead of a Stewart-Warner vacuum fuel deliver system, the electric pump is 99% like the cause of the needle leaking.. The design of the needle in the ’20’s cars is designed to ONLY handle but 1.5-2# of fuel pressure. Any more and it WILL leak.

The floats in the carbs develop fine cracks and fill up with gasoline and sink, leaving the needle wide open.

OK timing: First I’m happy to hear that you used a bore scope to verify that #1 cylinder was on TDC.. BUT are you sure it was TDC on the compression stroke? The piston is at TDC on ‘cross-over’ stroke AND on compression stroke..

So just verify that when the piston is on it’s way up to TDC both valves are CLOSED. and they stay closed as the piston goes past TDC. Put the engine on TDC on the compression stroke for #1 cylinder.

Then look at the distributor cap, most S80/81 caps have cast-in numbers for each spark plug wire. Find #1 plug wire, note where it is on the cap. Lift off the distributor cap, and the rotor’s tip must be right under the #1 spark plug wire location.

AND the points should have just opened. or will open with the slightest bit more rotation of the engine.

I’m pretty sure you will find the rotor is NOT pointed at the #1 plug wire location.

The shaft that the rotor is resting on is NOT a one-piece shaft all the way to the bottom of the distributor drive gear. The shaft has a taper-sleeve fitting with the lower shaft, and this is how you set the location of where the rotor is pointing, and when the points open.

Remove the rotor, look in the end of the shaft, there will be a slotted head screw. Loosen the screw, it will unscrew out a turn or two then get stiff, continue to unscrew it and this will pull the upper shaft off the taper with the lower shaft, THEN the upper shaft and rotor can be turned to point at the right plug wire, and have the points open just as the piston goes past TDC.. And this should be with the timing lever on the steering wheel at full retard, which should be fully down, at around 7:O’clock.

There is a possibility that inside the wiring tube some wires have cracked insulation and are arcing to the wrong wires or spark plugs, but this is rare.

Make sure by using a ohm meter that the firing order is correct. The right wire is going to each spark plug.

With the above correct, your engine will start and run..

Feel free to email me for more discussion. : [email protected]

Greg Long

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Dear Greg,

You are very right about the timing it turns out that I had the distributor 180 out typical rookie mistake. I do appreciate all the help that you guys offer on a daily basis.

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