My son has a 1963 Mercury Comet with a 170 straight 6 and automatic trans. Originally it would die when he made a sharp left. We fixed that by rebuilding the carb and putting in a new float. Then we found that the back brakes weren’t working and had to replace a clogged brake line. Now for the weird problem. Whenever he brakes it pulls slightly left and the motor almost dies. If he anticipates the pull and steers right the car keeps running beautifully. I am assuming that the pull is probably caused by not getting all of the air out of the brake lines, but why does the car almost die unless he steers right when braking? The car does not die anymore when he makes left turns. These are non power hydraulic single cylinder brakes so there is no vacuum connection involved. The fuel filter and pump have been replaced.
The drum brakes are not adjusted correctly. That is why it pulls. Check the right front first as it is not working as hard as it should.
It is likely that the “left turn stall” has only been made better by rebuilding the carb, not solved. It is very likely this is normal for this car if it tugs a bit left under braking.
Fix the brakes and the stall will likely go away. If not, adjust the idle speed up a bit.
Oh, and you should really consider swapping out for a dual master cylinder for that single pot. It is waaaay safer and restoration guys don’t consider it a problem.
Have you gone over the brakes with a fine tooth comb? Have you worked on drum brakes before? EVERYTHING needs to be in great shape on an old car like this for them to work at their best, which was none too good. I had a '65 Mustang with the 170 cid 6. Thank goodness it was slow, because the brakes weren’t very good. Are all wheel cylinders dry? All drums and shoes in great shape? Any rusted brake lines? Are the flexible brake lines new, or near new?
Mustangman suggested the drum brakes were not adjusted properly. I am going to go through them and check that as soon as I can.
I am familiar with drum brakes. Let’s just say I have a few years on me. I have owned several vintage model cars when they weren’t considered vintage so have dealt with drum brakes before. In certain cases, I actually like them better than the disc brakes of today. Yes, they were more fiddly, but they did the job. I can’t imagine my 47 tractor with disc brakes.
All physical parts of the braking system are in good shape, now.
OK, good to know you’ve worked on drums, some folks have no idea what they’re getting into. But know these are not as good as discs on a modern car, your son will need to be prepared in case somebody jams on their brakes in front of him.
I wonder if, for some reason, the ignition is grounding out under certain conditions when the engine shifts on its mounts. Check the primary wires going into the coil and distributor. Look for bare spots where the primary wires might be touching the engine.
I suspect the pulling brakes could polluted brake shoes, some brake fluid on the shoes. I think the stalling is a float issue still. The level could be too high or too low. If you get black smoke on restart, too high. No smoke, too low.
As to the dying, maybe the float level adjustment needs to be revisited and go out another 1/3 turn on the carburetor mixture screw.
The suspension should also be carefully checked as a bad ball joint or control arm bushing can cause a brake pull. One does not want a ball joint to fail as that can be lethal.
Try driving it yourself. Your son’s reports of how it feels are difficult to use as a way to diagnose a problem like this one, in an old car with old technology. You seem to know your way around the machine, so you might be far more qualified to test out some theories.
I would look at the left rear brake. If it’s trying to lock up it might cause a swerve and could also cause the engine to bog down a bit with the drag going back through the drive shaft to the transmission.
All the flexible hoses lines are suspect. They can deteriorate from the inside out and become clogged or create what functions like check valves that hold pressure in the wheel cylinders. If they aren’t pretty new, you might consider replacing them all.
If the OP can buy no ethanol gas, use it in this old car. If not, treat the ethanol gas with marine fuel stabilizer and ethanol treatment. Stabil Marine is one brand.
Thanks for all the suggestions. I will try all of them. As to the ethanol gas, we have places here in SC that have non-ethanol gas and that is what we use exclusively in our old engines. That white rust and deteriorated fuel lines bother me.