3 cylinder Model A

Jembow

Jembow

Senior Member
What have I missed? 28 RPU, just finished restoration. Model B motor, A manifold carb and distributor. Low mileage motor, I only put in new rings and did a valve job. Rebuilt carb & reset float level, works fine. Car runs down the road strongly but feels out of balance, clearly running on 3. Pulling plug wires, no 1 is the culprit. There is a fat 1” spark to each plug.

I have:
Checked compression, 70/80 psi each cylinder
Swapped plugs between 1 & 2, problem stays in no 1
Changed distributor cap, rotor, points and contact plate, new condenser.
Pulled manifolds, exhaust warped so new manifold fitted.

It has spark,fuel,compression - it should go Bang. 2,3 & 4 go bang, why not no 1?
 
Run the engine in the darkest place you can at night and look around the engine for stray sparks running to ground that should be going to #1 plug.

Look inside distributor body in strong light for carbon trails inside the body. Wipe out inside the body with a solvent, like isopropyl alcohol, to remove any grease film.

A 1 inch spark at any plug is a very hot (high voltage) spark that can then find a ground through other paths that are less resistance than through the spark plug. Are you using a super high output coil?
 
It’s just getting dark, I’ll go out and check for sparks. Didn’t think of cleaning out the cap. Thanks, I’ll report back
 
In a follow up to what Jim has said, the spark plug at atmospheric pressure will require less voltage to spark than it does under compression. So if there is a path for the high voltage to go besides the plug, it can reduce the voltage to the plug where it will not fire in the running engine.

The distributor housings, the plastic part at the top of the distributor with the two wings, are known to have internal faults. Measure the resistance between the internal "contacts" that the rotor sparks to and where the plug straps connect. Or just try a different housing.

Also check the gap between the rotor and the "contacts". It should be about 0.025 or less on all "contacts". And check the wire from the coil to the distributor. A friend of mine just cured an ignition fault by replacing that wire.
 
Last edited:
No sign of sparks. I’ll thoroughly clean both sets of caps and rotors tomorrow and try them both.
 
If you lift a spark plug strap an 1/8" and run the engine, you can see the spark clearly. Also possible, of course, a bad plug.
 
Take #1 plug and move it to another position. If the problem moves to the new location "bingo".
 
Jack, He said that he switched plugs between #2 and #1 and it did not follow the plug but stayed with #1 cylinder. I still think the problem is in the plastic housing, the one with the two wings. But I also think he should check the wire from the coil to the distributor. I would replace both, one at a time.
 
These slides may be helpful to see what is going on in the distributor.
 

Attachments

  • Distributor Body Hi-Voltage Leaks 151kb.jpg
    Distributor Body Hi-Voltage Leaks 151kb.jpg
    50.6 KB · Views: 142
  • Distributor Cap Mod 159kb.jpg
    Distributor Cap Mod 159kb.jpg
    52.3 KB · Views: 125
  • Rotor Gap 164kb.jpg
    Rotor Gap 164kb.jpg
    54.4 KB · Views: 107
Gentlemen, thank you so much for the excellent advice. So far I have thoroughly degreased the housing, cap and rotor, with a marked improvement on running. Next steps will be changing the coil (I have a couple of script coils to try), coil to distributor wire, and take the Dremel to a spare cap so I can see inside.

But we're off to a vintage hill climb this weekend so that won't happen till next week
 
Been slow to reply, we had a great long weekend at Kop Hill Climb, came back to find a tree down in front of my car storage, so I’ve been lumberjacking.

Model A: I’ve carried out most of your suggestions, cut down cap to see inside, no tracking, new ignition switch to distributor wire, degreased thoroughly, changed coil to original script one, spark not so fierce now. Sounds a lot better but still no change in engine note when you disconnect plug #1 and that plug is noticeably cooler to handle if you pull the plugs, so it’s not firing. Double checked compression, definitely 70lbs.

Wondered if both housings I have are faulty so I’ve ordered a modern style distributor cap to try.
 
Did you replaced the high tension wire from the coil to the distributor? My friend Ted chased a miss for a month before he fixed it by replacing that wire. The wire was defective.
 
You must log in or register to reply here.
Back
Top