Spark Plug Firing
 

We have a '29 roadster with a stock engine, including the old distributor. I have a problem that I cannot resolve. The engine is running on only 3 cylinders. And it is an ignition problem. While running, I am able to short our each plug with a std Mark 1 Mod 0 screwdriver, see a spark and the engine slow down. I get this response for all cylinders, except the fourth. I have disconnected the strip from spark plug and tested to the lug on the boat....no joy. I have tested for continuity from that lug to the post inside the boat and get continuity. I have tried three boats with same reaction. And, yes, I even replaced the rotor. What's up? Any suggestions?

Re: Spark Plug Firing
 

Have you tried a different distributor body? Rare but they do fail

Re: Spark Plug Firing
 

Yes I have....that is what I am referring to as a "boat". Thanks...

Re: Spark Plug Firing
 

I have seen in the past a problem with the #3 spark plug wire arcing to the #4 spark plug wire imbedded in the distributor cap due to a very small gap between the #3 spark plug wire and the distributor cap. Take a look at the engine running at night so you can see if any arcing is occurring.

Re: Spark Plug Firing
 

Under normal circumstances I would suggest switching out plugs, 3 to 4 and 4 to 3, and see if the problem follows the plug. However you say that you do not get a spark at #4. The only thing that I can think of is the rotor is not centered in the housing (boat) or that the cam is not concentric.

Remove the top from the housing and turn the engine over by hand, with the hand crank. Observe the point opening at all 4 locations. The opening should be the same at all 4 within 0.002 inch. Also observe the gap between the rotor and the bosses inside the housing. The gap should also be the same at all 4 locations within a few thousands. The gap between the rotor and bosses should be about 0.020.

While you are in there, measure the points gap. It should be about 0.018.

A handy thing to have is the clear housing top. That way you can observe the sparks at each rotor location while the engine is running. The clear tops are available from the usual places.

Re: Spark Plug Firing
 

Sorry, I had never heard the distributor body referred to as a boat. Learn something new everyday.

Re: Spark Plug Firing
 

I've noticed with recent distrubutor bodies and rotors, the rotor to contact gap is much larger than the specified .025". Bending the rotor tip can help, but it is still larger - .050" or larger. Check it carefully.

Re: Spark Plug Firing
 

Confusion occurs when different names for a part are used. I see distributor body call "Boat", Distributor Cap and Distributor Body in this one thread alone.

I would start by trying what was suggested in post#4. Sound like you may have some carbon tracking going on.

A clear cap aides in seeing arcing due to carbon tracking inside the body.

Re: Spark Plug Firing
 

He has changed "boats" (housing, distributor body) so it is unlikely that all three were bad.

One other thing to check is the wear in the distributor. Try moving the cam back and forth. There should be no perceptible play. While you are at it, oil the distributor.

Regarding the gap between the rotor and bosses inside the housing: This should be as small as practical without hitting. The spec is 0.025, but less is better if possible. Some housings are loose on the distributor and can move causing the rotor gap to change. Try to find a housing that fits the distributor with very little or no perceptible play.

If the ignition system is marginal, a little thing, like the rotor gap, can cause one cylinder to not fire. Check the health of the ignition system in general by observing the spark between the coil wire and ground. It should be a hot blue and easily gap 1/4 inch or more. Check the electrical connections, points, etc.

Re: Spark Plug Firing
 

I assume you have checked compression on all the cylinders since you say it is a ignition problem? If compression checks good, first thing I would do is change the plugs around, and check again to see if the problem moves.

Re: Spark Plug Firing
 

These slides may help you observe / diagnose what is going on:

Re: Spark Plug Firing
 

Thanks guys for all you inputs and suggestions. I will be using a clear cap today and do further diagnostics. Will let you know how it turns out.