Suggestions welcome- ‘46 runs rough when warmed up

[Bruce_MO]

Recently restored ‘46 Mercury with a stock 59A motor. It has about 300 miles on it since it was finished. New issue has shown up recently. When cold, it runs very well. When it gets fully warmed up (temp gauge to the right of center, but nowhere near hot), it starts to miss or stumble a bit. Seems to do it on level roads at 25-30mph, and when I just start to decelerate. It usually seems to pull under load just fine with no stumbles. The stumbling is generally pretty faint; doesn’t feel like the motor is cutting out with a “hard” loss of spark. It idles smoothly hot or cold.
The distributor was rebuilt by Michael Driskell, and the coil was rebuilt by Skip Haney. I rebuilt the carb and fuel pump, and I just replaced the condenser with a new NAPA Standard one, thinking that condensers were a likely problem if the issue only occurs when hot. I just snugged up intake bolts and carb mount studs, making sure no vacuum leaks were causing the problem.

Before I start throwing time and $$$ at it, I thought I’d get some opinions. My next step is to swap a friend’s coil to rule that out. Thanks in advance for your suggestions and assistance.

[tubman]

I think you're on the right track. Could be the condenser as well. Just don't change them out at the same time.

[Mart]

Could there be something causing the carb to run rich? I had a similar stumbling on my 41 pickup and the power valve was not seating properly. Do you need choke to start it? Does it run fine from cold without any choke? This could indicate over rich. The only part of your description that would not point to over rich is that it idles fine hot or cold.

[Bruce_MO]

I was thinking I’d go thru the carb again if the coil checks out. It doesn’t seem to be running rich per se; starts very easily hot or cold. Just a quick push of the starter button. I will choke it if it hasn’t run for several days, but I can push the choke mostly in after 8-10 seconds.

I can’t recall the specific power valve I installed in it 2 yrs ago, but I’m very familiar with all the issues and remedies for Holley 94 PV’s. I bought another 7.5 PV from Vintage Speed to put in it if/when I start chasing the problem as a carb issue.

Still not sure what carb/fuel issue is heat related… other than vapor lock!

Your comments are appreciated!

[hueyhoolihan]

gosh! where do we start?

there are literally hundreds of realistic possibilities. including electrical, mechanical, and fueling. a systematic approach involving a good grasp of the nuances in the symptoms is essential. and that would involve first-hand observation i'm afraid. an expensive replacement of major parts, many of which are probably not the cause can be effective if not on a budget.

i've gone through a similar experience...after purchasing a non-running car, i eventually got it running, but not running "right" once warmed up. i ended up taking it to a long-time professional. he is in his 80's. the first time in my life i've had to do that, and after installing a rebuilt non-original distributor and a rebuilt carb (both were rebuilt by the gentleman) and finding a vacuum leak (my guess as to the root cause, and something that i didn't have the resources to track down) he had it running better than ever.

i plan on re-installing, some day, the distributor just to see if it was truly part of the problem. and i might do the same with the carb too. ...mostly to satisfy my own curiosity.

[JayChicago]

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From original post: "It usually seems to pull under load just fine with no stumbles." That seems to me to rule out a weak spark problem.

I had a sticky valve issue, and the intermittent miss came in after warming up. Marvel Mystery Oil in the gas tank worked a miracle, cured it. But different than your symptoms, the miss occurred both cruising or accelerating.

[FortyNiner]

When my '46 Tudor was first on the road after being rebuilt, it would behave in a similar fashion. In my case, the generator was weak. One rebuilt generator later,all better.

Your mileage may vary.

[custom_lettering]

Pull a few plugs. They tell a story.