1979 Town Coupe 400 Engine Questions

Hi Bill!
My name is Justin Lindberg. I own a beautiful 79 Town Coupe with 35K miles on it. I just cannot get it to run the way it should. When the engine is relatively cold (after warming up a 2-3 min) – the car drives wonderful. Accelerates nice and smooth, and shifts gears beautifully. There is no hesitation or anything to complain about. However – once the engine warms up (after a few minutes tops) – it becomes a different experience. It idles ok (could be a little better) but mainly – it does not accelerate with the same confidence as it does when “cold.” It also hesitates and does not accelerate smoothly. It shifts harder into 2nd and 3rd. Not nice and smooth like it does “cold.” I know that vacuum leaks can be an issue. But, this particular vehicle was purchased from the original owner and you can tell that the hoses are still all in the right places. Also – yes I know that the hoses are 40 years old but they truly look brand new. No cracks or anything in them, even when bent or twisted to check. So, maybe there is a leak someplace but I wouldn’t suspect it yet. The fuel pump is new, and I had a guy clean up the carb and the accelerator pump was replaced. fuel coming out of the pump is clean. Fuel filter replaced and rubber fuel lines. What I want to know is – is this normal for the car to run more sluggish when warmed up? Maybe due to all of the emission control stuff and the small cam? I am thinking of replacing the cam/upgrading to new manifold and 4bbl carb. But I really don’t want to do that. I would say maybe it is ignition related if the car ran bad when cold as well, but it doesn’t. I have a suspicion that it is the carb not acting right. It is the stock autolite. My thoughts are replacing the autolite with a new 2bbl and keeping everything else stock. Or, going all out and doing the cam, manifold, and upgrade to 4bbl. But again, I don’t really want to do that. Maybe you can recommend someone near Chicago that knows these old 400 engines on a 79?
Thanks for reading!
Justin
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Hello Justin –
It is not normal for an engine to become sluggish as it warms to operating temperature. Not being there to see what you are experiencing I can however offer the following.
Ensure that the engine is actually reaching operating temperature by checking the thermostat. Make sure that all of the tune up items are in good shape. This will include ignition timing, carburetor adjustments, spark plugs, high tension wiring, distributor cap etc. The vacuum advance and choke operation should also be tested for correct operation. All of the above along with their adjustments are explained in the factory Shop Manuals. Any failures or combination of failures of the above can cause poor hot engine performance therefore they should be addressed first.
In my opinion upgrading to performance equipment is not a good choice as it most likely will produce no difference as these engines performed perfectly with their original factory installed parts. Checking the original parts to expose problems is the best way to go. Good luck on an easy repair.
Sincerely,
Bill