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1963 Thunderbird intermittent idle issue

Engine is a 69 390, fresh rebuild by the PO… Aluminium intake manifold and new Edelbrock 4bbl Drop in Crane ignition.

Runs great… Except when its warm out, or even on cold days when its warmed up after a substantial drive…

AT that point here is what happens: No issue until I shut it off then it diesels badly, THEN… When I restart it will not idle, dies… If I keep my foot on the accelerator it will keep running, but not well… If I let it run for a good while, 5-10 mins, it smooths out… Consistently acts like this … Again… Not when the engine is cold, and not on a cold day until it really warms up…

I suspected a vacuum leak? Replaced any suspicious vac tubing… Cant get to the AT modulator easily, have not checked it yet?? Also suspect the intake manifold??

ANY ideas very much appreciated!!!

You have to check all the vacuum controlled devices for leaks. And do a vaccuum test with a vacuum guage both after the butterfly valve and before, at various rpms.

Another possibility is the throttle is not closing tightly enough at shut off. Do you have a throttle selenoid? It may be related to that. You could block off all airflow to the carb and see if that stops the problem. That would be a clue. Dieseling occurs more easily when the engine is hot. And when it diesels, it can suck out the gas from the carb bowl, making it run poorly when you start it up after this happens.

Do a complete vacuum check and check the throttle to make sure it is closing to spec at shut-down. One

Could the carb be set too lean? In the early 70’s they had a big problem with dieseling when they tried to lean the carbs way out for emissions. My 74 Olds had an anti-dieseling solenoid on it that when the key turned off, it would allow the throttle to close more completely to stop the dieseling and would pop the throttle back open again when the key was turned on.

Heres the problem… It runs beautifully most of the time… I can take short drives all the time, in the cooler weather and there are no symptoms at all?

The first question I’m going to ask is, are you using gasoline that contains ethanol?


Yep… How would that produce such an intermittent problem?

Just read this.


The carb will continue to deliver fuel if the motor is spinning, which it is when dieseling after shut off. Then the motor is full of extra fuel and gunk upon restarting, are you getting a whole lot of black smoke when this happens? Black smoke is too much gas, too rich a mixture.

One way to cut off the dieseling could be to put a low pressure electric fuel pump on the car, replacing the mechanical pump. Wire the fuel pump into the ignition circuit so the fuel pump electrical current cuts off when you turn off the key. That should cure the dieseling, by cutting off the flow of fuel. Then you can see what of the other problem(s) remain and troubleshoot from there. You should be able to source and install a low pressure electric fuel pump easily.

Sounds a lot like vapor lock (fuel boiling), related to the carb getting hot after long/warm drives. Ethanol in the gas makes it worse, I think, but fuel injected cars don’t usually have the problem. With a carb you do. So do like @Tester suggests and see if you can buy some ethanol-free gas, after a couple of tanks (to clear out the E10) you might notice an improvement.

I ask about vapor lock because my dad did a lot of work on it in the '60s, he ended up putting insulation on as much of the fuel system as he could reasonably access. Here’s a link to the type of insulation that worked well. It was amazing how badly some fuel systems were designed, with fuel lines running very close to exhaust manifolds, exhaust pipes, etc:

The insulation helped a great deal in most all cases (he actually had a small side business doing this for friends and acquaintances, I helped with some of the installations).

I’m going to say that you are running too rich, not too lean. Running a little rich can help when the engine is not fully warmed up, but once completely up to temp, you will have problems. But I don’t think that is the complete issue.

First thing to check is the choke, make sure it completely opens when the engine is up to temp. A lot of these problems are caused by that, in fact that alone could account for all your issues. Next is to set the idle air mixture screws when the engine is completely warmed up. They may have been set when the engine was still cold, or at least not fully warmed up.

If that doesn’t solve all the issues, then I would lower the float level a little. I always felt that the float levels were always set a little too high at the factory to begin with. With just a little erosion or compression of the rubber tip on the needle, you will get a spill over from the bowl and that will cause issues that you describe. I’d drop the float about 1/4" from its highest setting, or to the minimum setting.

If all that doesn’t solve the problem, then you will need to get the carburetor rejetted, and that is for a professional, which may be very hard to find these days.

One last thing, does your ignition still have points or is it a complete conversion to electronic?

The 390 engine used a hot idle compensator plumbed into the PCV valve hose and if it remains in the hose it may be sticking.

Is the aluminum intake a new after market part or a Ford part? Old Ford aluminum intakes are prone to crack at the crossover and if cracked may leak more when hot.

Also, is your 390 a solid lifter model? If solid lifters are too tight the valves often leak when hot.

I very much appreciate all the help… I am trying to make sense of all that is discussed, and the carburetor seems to be most likely…

To answer a couple of Qs… The intake manifold is Edelbrock, and new… Although Im thinking about putting the stock manifold back on…

The ignition is Crane electronic…

Im working on a plan here… What to do first?

Is the interior of the carb (the venturis) clean and shiny, or brown/black? Vapor lock leaves a fair amount of stuff in the venturis.

The dieseling could be due to an air leak, idle speed screw opened too much, too much timing advance, or something hanging in the carb linkage which is keeping the throttle plate open too far.

The loading up and dying when warm could be due to the idle mix screws being too far open. The excessively rich idle mixture will be compensated for a cold engine but on a warm one it will cause the engine to bog after idling a bit.

How many turns out do you have those mixture screws?

First test… Choke opens completely at temp…

Mixture screws were open 2 full turns… I turned in to 1 3/4? Thoughts?

What is the idle speed set at? Most old carbureted cars will run-on (diesel) if the idle is set too high.

Ill have to figure out how to hook up the old dwell/tach meeter that I have…

I set the mixture at 1 1/4 on each, and lowered the idle a bit, Ill test for a day or two…

If it’s a Performer manifold, I’d leave it on, but make sure it’s been installed correctly. They really are an improvement over the stock manifold but they need to be on there with no leaks. If the bolts weren’t tightened in the proper sequence and to the proper torque values, it’s entirely possible to have a leak that’s not readily apparent. Check also to see if Edelbrock says no manifold gaskets are to be used, as they directed with the manifold on my Olds big block.

Before messing with the idle speed and idle mixture screws you need to be certain the ignition timing is where you want it. Advancing the timing has a tendency to lean it out and retarding the timing does the opposite by causing the engine to richen up a bit.

A turn and a half or 2 turns out is usually about right in most cases unless there’s a lumpety-lump cam involved.