high speed miss

[Standing Elk]

Just curious what you guys think before I start changing parts. Got the A all serviced after its long winters nap. Checked and set the point gap. Two of the lobes have always read .018" and the other two have always read .022". Nothing new there. I set my point gap with a dial indicator with the distributor removed from the car so I know exactly what the gap is. I then set the timing to be correct using a test light just like I have done forever. Yesterday I took the coupe out for the first time this year. Started and ran very well at lower speeds like 20-30 mph. But when I got up to my normal road speed at 40-45 mph I could detect a miss. Sometimes it would get worse and then get a little better for awhile. I always use non ethanol gas and it had fresh fuel in it yesterday. It did not act like a fuel issue. I do have the newer modern type upper plate in the distributor. So any ideas?? Maybe a bad plug?? What do ya think? Thanks

[Chuck Dempsey]

Long shot, but I've seen the spring attached to the points weaken, and not return quickly enough.....

[WHN]

Have looked at your spark plug?

[Standing Elk]

WHN I have not looked at the plugs yet but plan to. Might just put a new set in to be safe.
Chuck, the spring on the points seems to be what I would call normal but a good suggestion. Thanks to you both

[HDowse]

I had a similar problem with my ‘29 coupe, bone stock motor, Zenith carb, and it was pretty consistent at higher RPMs. I just had the car out and it tested well at higher RPMs. I had been having trouble keeping the GAV knob from turning by itself so I pulled the carb, removed the GAV needle and holder but left the seat in the carb. I found that the needle wasn’t thru the end of the holder enough so the tiny round spring clip wasn’t holding the needle from turning. I then reinstalled the carburetor and, to complicate things here, checked the point gap at .020. For whatever reason, one of these operations solved the high speed miss. I think it may have been as simple as making sure the carb gasket was good and the carb to manifold nuts were tight so as to eliminate what may have been a slight vacuum leak. Also, I saw a good article in the Restorer on setting the points with a dial gauge so you are probably OK there but I would think a new cam w/o .004 variation would help keep the point setting more consistent. The article said most folks using a feeler gauge set their points about .003 too tight and then the block wears and you get points that are too tight. I’m going to try setting mine at .022 and see how the motor behaves. Good luck with your diagnosis....you’ve checked for good gas flow, blue spark etc.?

[jb-ob]

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Elk,

First correct your distributor issue by replacing the upper shaft. Most often out of round causing your dist. cam to give you inconsistent gaps. ( more likely than the cam, the next 'suspect'.)

Next flush the carb jets & passages with carb cleaner after a winter snooze.

It's still just a Model A, start with cheap solutions working toward expensive.

$.02

[Mulletwagon]

Had a similar experience which turned out to be insufficient valve lash on one lifter. Worth checking if all else does not resolve the problem. Hopefully you have adjustable lifters.
Good Luck

[eagle]

Before I'd look too close, I'd put a heavy dose of Marvel Mystery Oil or Sea Foam in the gas and run a tank through. See if it changes anything.

[Standing Elk]

Thanks fellas I will take a look at each suggestion.

[Big hammer]

Clean or replace fuel fillers, check gas tank cap vent, gav set to lean or to rich