Ford 312 missing some once warmed up runs good cold

Ford 312 ci Thunderbird

Nice car, what is your question?

Car runs great cold …once fully warmed up (stop for quick lunch) come out start car will sporadically miss until driven a little then starts running normal again. Plugs good. Wires good, engine clean and in good shape. Kinda acts like gets hot while sitting with no air flow or water pump running …then cools once running?? Maybe a carb adj need? Any ideas?

Sounds like it is running too rich. If the factory carb is in place, that means is is leaking a little extra fuel past the needle seat or an air bleed is clogged. I think a carb rebuild is in order.

The problem might be vapor lock.

The next time you stop and let the car sit, pop the hood open to the safety latch to let the heat out from under the hood.

When you come back, shut the hood and see if the engine starts/runs normally. If so, that’s vapor lock.


Might be that…used to run normal fuel with ethanol but switched to recreational a year ago. It has a stock Holly 4 barrel on it.

Increases my opinion that it needs a carb rebuild. Clogged air bleed, I’d guess. Be sure to test for vapor lock as @Tester suggests first before spending time on the carb.

I have been thinking that might be it, vapor lock,…good idea…I will try it this week…then if no success on to the carb idea…

Carry a water bottle in the car on a warm day and pour it over the fuel line before you try to start the car after lunch. If it starts, you had vapor lock.

Is this a new problem? If so, it could be a carb problem. Is the choke adjusted correctly?

Not really new problem been dealing with it for awhile since it is not a daily driver. I think the choke is good …starts up properly and runs with choke on at slightly higher rpm until I knock it down by giving it a slight throttle punch . I drive it about once a week. I am going to check out the vapor lock thing then a carb choke/rebuild project second. Thanks for your help.

Good idea thanks.

If it hasn’t been done recently, probably needs a carb rebuild job. There’s an off chance it is something simpler though. I had a similar symptom on a Ford 302 w/ Autolite 2100 carb, turned out to be the power valve was leaking gasoline into the intake manifold when the engine was off. Made it run way too rich to the point it would barely start when shut off warm, after sitting 10-15 minutes. Worked great cold though. Replacing the power valve (verify the new one isn’t leaking while you got it on the bench), that completely fixed the problem.

Is the choke fully open (you have to take off the air cleaner and look) when you start it when it’s hot? Just dropping off high idle isn’t enough.

I will check it out. Thanks