Misfire mystery

[Fairlane514]

Put on the original (came with the car) "new" modern style distributor cap,rotor and wires. No change

Checked for ground continuity on the main studs from the terminal box. I had ground on both studs with battery connected, no continuity when battery ground disconnected.

Cant do much with the ignition switch, but it does seem to work, wiggled it while running, it never shut off.

Bypassed the main 30amp fuse .....no change, but may have gotten a little worse to be honest.

I did replace the ammeter when I first got the car, but it did have a misfire then as well.
I wired the new ammeter the same as the original and it read backwards , so I switched the wires. I also have the dash light wired in with the ammeter if that matters.

Coil looks to be wired correctly....red wire passenger side to the + on coil, black wire to - on the other primary stud drivers side.

[Fairlane514]

Also.....with battery ground disconnected, I do have continuity between the main studs on the terminal box

[Fairlane514]

No vacuum operated wipers, only manual.

Sprayed brake clean at manifold to head and carb to manifold.....no leaks (new gaskets everywhere)

I have a vacuum port, but my gauge is plugged in and shows the needle dip with each misfire.....which is random. not rhythmic.

My wiring is....

Drivers side of terminal box stud...Yellow/black tracer and coil black wire to neg side of coil.

Passenger side terminal box stud.....yellow wire

Ammeter (if looking at gauge installed in cluster and screwed to dash) .....passenger side yellow/black tracer. Drivers side.....yellow

From ignition switch one side to distributor cable, the other straight to positive side of coil (red wire)

Alternator (6 volt pos ground) has yellow/black tracer to drivers side of terminal box.

Only variables I have not switched out.......coil and carburetor. Coil ohmed at 1.5 primary, 7500 secondary

Carb is Zenith 1 which I rebuilt, checked jets for proper sizing and shape. float level good as well. I did not buy the Renner's jets...

[Bob C]

Possibly a valve is hanging up intermittently?

[Fairlane514]

Yeah, thought about that. I did a test with my compression gauge. I took the schreader (sp) valve out and then did 10 cycles of each cylinder watching the needle.

All four cylinders bumped the needle to 60psi every cycle quick and sharp. But thats only turning it at the speed of the starter, maybe at faster cycles one or two hangs up.

[Gene F]

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I know you said manual wipers, but does your intake have a plug in that port? I'm thinking remove the plus and idle it it. Tubing, and a dose of marvel mystery oil. If a valve is hanging up that might clear it.

[Fairlane514]

I have a video of my vacuum gauge while running, but Im not very good with computers and cant get it to upload

[Fairlane514]

Gene yes, I can try that, I have my gauge plugged into the intake, so I do have barbed port

[Fairlane514]

Ok,

Borrowed a carburetor and a new distributor cam.

Did each change one at a time, no difference....

Wasnt able to try the MMO in the intake vacuum port yet

[Jim/GA]

Originally Posted by Fairlane514

Coil looks to be wired correctly....red wire passenger side to the + on coil, black wire to - on the other primary stud drivers side.

You can't confirm correct coil wiring with above description.

You need to confirm that the black wire on the - post of the coil connects to one of the screw posts inside the black terminal box. That is the power feed to the coil.

The red wire connected to the + post of the coil needs to go to one of the connection screws on the key switch. The other connection screw on the key switch needs to connect to the wire that goes down to the distributor, to the points.

Just seeing a wire color on the coil is not enough. You have to know what's connected on the other end of those wires.

[Fairlane514]

Yes red wire is coming from the ignition switch to the + side of coil
The other side of the ignition switch goes to the armored wire to distributor.

Black wire to - side of coil goes to terminal box stud on drivers side which also has yellow/black tracer wire attached. Yellow/black is also attached to alternator.

Pretty sure I mentioned this in post #23

[Jim/GA]

OK, then try to adjust idle mixture a bit leaner. Make very small adjustments on a fully warmed up engine.

[devoreb]

In post 13 you said "no longer have pop out switch". Did you install new switch? If so the new switches are longer than original and just barely contact the gas tank. I had this problem, the cure was to glue an old inner tube to the gas tank. The inner tube insulated the switch from the tank. I found the problem by pushing on the dash panel while the engine was running. Good luck .
Bill

[Fairlane514]

It does have an aftermarket switch......I have run it with the dash hanging down and not mounted.....no change.

I have adjusted my idle mixture screw according to my vacuum gauge and about 1/4 turn either side of the most vacuum achieved, no difference.

I think at this point, it must be a sticky valve or two......I have Marvel in the fuel and oil, im just going to drive it a little bit hard and see if it finally frees up.

[katy]

Positive or negative ground? Check coil polarity to be sure that the + terminal is actually connected to +

[Jim Brierley]

One quart of MMO is either a mis-print or way too much!

[Fairlane514]

Positive ground as original....to the body and then again to transmission. Coil wired correctly.

One quart of MMO in the oil, about 2 ounces in 1/2 tank of fuel

[Big hammer]

Originally Posted by Fairlane514

It does have an aftermarket switch......I have run it with the dash hanging down and not mounted.....no change.

I have adjusted my idle mixture screw according to my vacuum gauge and about 1/4 turn either side of the most vacuum achieved, no difference.

I think at this point, it must be a sticky valve or two......I have Marvel in the fuel and oil, im just going to drive it a little bit hard and see if it finally frees up.

Sounds like a good plan ! Enjoy the drive !

[Anteek29]

Keep us posted on progress!

[Fairlane514]

Drove it some today, to a fellow Model A owner about 10 miles away.

It seems to have plenty of power and cruises well, I dont really notice the misfire when driving and underload. I come to a stop, it stays running. A little clutch chatter on take off (another issue to address), but upshifts fine.

No real difference as of yet.....

The other owner does have an aluminum head he wants to sell, I will post that separately. Cant ID it....Has the letter R with two circles around it cast right near the distributor hole.