Misfire mystery

[Fairlane514]

29 Tudor

Here's what I have done to chase the misfire.

Set points (.020) use NuRex wrench to set timing

New plugs Champion W18 gapped .035

Zenith carb rebuilt

Wiring all good

New condenser NAPA FA-82

Coil ohmed at 1.5 and 7500 secondary side

Alt is charging

Compression check, 60psi on all cylinders

Valve check by removing schraeder valve from compression gauge and do 10 cycles of each cylinder.......all four cylinders pumped up to 60psi each cycle without hesitation. Trying to test for a possible bad valve guide not letting valve return square to seat and cause issue.

It starts up with ease and idles but has a definite misfire that is not rhythmic. My vacuum gauge shows needle bounce down from 21 to 18 or so, randomly.

When a listen at tail pipe (new muffler) I can easily hear the misfire.

The distributor shaft is not loose or wobbly and it has the NuRex points plate and new style points/condenser.

Have not tried new coil

Fuel is fresh with some Marvel in it as well as the oil.....one quart of Marvel.

When driving really not a noticeable misfire by feel, but can hear it.

[wwirz]

Check and make sure you are not shorted somewhere in the distributor. Is the wire from the lower plate still OK? Is there a short in the terminal box?

[Fairlane514]

I went through the distributor all wires are good, reset points, filed them a little, reset timing. No change

The misfire is not consistently rhythmic, it seems more random. New wiring in terminal box seems good.

New intake/exhaust gaskets

[TMarsh]

Set timing using Les Andrews red book. Had a problem like you said, turned out that the NU REX wrench was off by a few degrees retard. When set with test light everything turned out OK. I think it has to do with modern points and NU Rex wrench. I think wrench is made ok, but some combination of new points and modern plate just did not jive in my situation.

[Fairlane514]

Thinking about possible weak valve spring. May return fast enough to seal when just using starter to check compression, but at running rpm not able to perform as it should.....

[Fairlane514]

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Ok, I will try with the light, thanks

[Big hammer]

Run it in darkness and look the distributor over, the flat plug wires to the body, coil wire to the cap. If you have an extra cap cut it down to have a good look inside.

[Fairlane514]

I have a clear cap, and can see spark at each terminal at night.

Also grounded each plug strap with a screwdriver, all cylinders react the same. No dead hole with this test

[mcgarrett]

Check to see if #3 spark plug strap is intermittently shorting out on the spark advance rod. With all the lights off in your garage, start it up and look for stray sparks.

[Fairlane514]

Yeah , did that as well, at night all I see is sparks through the clear cap, nowhere else.

Did the Les Andrews timing with a light bulb etc... No change

[Fairlane514]

I also notice (may be normal) anything more than halfway down on spark advance, it sounds like some detonation. Thats at idle, so like I said probably normal until its under load.

[Rob Doe]

Clean and tighten grounds and cable connections, if you have a safety fuse replace it and, if still failing, next bypass it, check the key switch by jiggling it up, down, side to side and that it has detentes at off and on. It should not shut down if not fully in the off detent. Tighten all connections behind the panel and in the terminal box, also at the coil.

Push the coil wire up at the coil and down at the distributor. You've had them apart.

If you have the the spark tester that Nurex sells, it can show you a misfire in the secondary circuit.

Grab the upper distributor plate by the spark lever and try to move it fore and aft etc. but in the plane parallel with the engine head. The center hole may be worn allowing the points to move around. Look at the points rub block / cam follower and make certain that it is not too worn allowing the occasional spark to jump to the cam.

Check that the condenser lower plate wires at the side of the upper plate are not sticking up too high and allowing a spark from the underside of the rotor to the connections. This would likely be consistent on #3 cylinder.

Look closely at the contact surfaces of the points. If they are pitted, burned or have high spots, suspect a condenser.

There are some voltage tests in chapter 2, pages ~30-32?? of the blue, Les Andrews, "Troubleshooting and Diagnosis" book that will identify faulty, high resistance connections, both power side and grounds.

That's all I can think of quickly.

[Fairlane514]

Thanks Rob, I went through the distributor, took it apart checked all suggested areas, they look good and almost new. No wobble in shaft, points look good, new condenser. Center is tight when moving the plate. Rub block looks good

Key switch may be in need of a closer look. I do notice when turning the ignition off, it seems to cut off very quickly maybe before the detent as you suggested.

Grounds from battery are good....one to the body at original location and another from there to the transmission with heavy 4 gauge wire. Both clean and tight

I do have a safety fuse wired to power cable at starter.

Mine is no longer pop out ignition switch.

I dont have the blue book by Les

[Synchro909]

Originally Posted by Fairlane514

Key switch may be in need of a closer look. I do notice when turning the ignition off, it seems to cut off very quickly maybe before the detent as you suggested.

I've had an aftermarket switch cause trouble. It was one with the terminals at the back protruding towards the radiator and parallel to each other. The terminals and the end of the switch are swaged n and had become lose. I use a light clamp to push them back into place, then pushed some epoxy putty in around the edge of the swaged end, left it clamped while the putty set and there has been no trouble since.
BTW, how long is it since you replaced the spark plugs?

[Fullraceflathead]

Try a different distributor cap to rule out internal arcing, this can cause a random misfire without any visual arcing.

[nkaminar]

Also check for a vacuum leak at the carburetor or intake manifold.

[Bob Bidonde]

There is a SNEAKEY SHORT CIRCUIT in the rotor that cannot be seen. This could be the cause. Also try a different distributor body.These slides might help you.

[Fairlane514]

New plugs Champion W18

I have a "modern style" distributor cap and wires I could try.

No vacuum leak, brand new intake/exhaust gaskets

Im also going to check out the ignition switch and the terminal box. Heard of the bolts touching the firewall......

Should have some results today

My current distributor body and cap are new

[Rob Doe]

Fairlane,


Thank you for the direct and clear responses to all the suggestions posted.

[Rob Doe]

When checking for manifold leaks, if your wiper is vacuum, plug the line. Check around the throttle lever at the throttle plate, too.