bacelaw

If highway speeds are a main concern, just replacing the rear gears can make a significant difference. Do you know what the current rear gear ratio is?

This forum has taught me not to panic until you have all the vital signs. A compression test is absolutely worthwhile. Even if you had a valve issue, it could be fixed without pulling the engine. Move methodically from compression test to the other tests mentioned - you may find something simple.

I use a small sledge to rap on the puller (if you have the proper puller). A couple good shots and it will pop.

I think the conventional wisdom is a larger gap cold, especially the exhaust valves. Add a couple hundredths from the manual specs. I set them on a non-running, but HOT engine, then double-checked while running - worked well for me.

I've used both. I have both. To answer your question, you touch one of the shoes to the drum (while it's on), so it's just touching the drum...then you use that as a reference for the drum diameter. I've used both in sequence, and I prefer the homemade tool for ease of use. But hey, you do what you like 👍

I use the method @Sam Buchanan describes. Works great and gives you more control. The Aamco tool has the gap built-in so to speak. You have fumble with the diameter of the drum, then make sure the dial thing is properly set and at the end pray that the built-in setting is correct - there's no way to really double check it. The homemade tool allows for more flexibility.

For what it's worth, I have the Aamco tool, and I find my homemade tool much more effective and easier to use with just about the same results...

It's frustrating to chase an issue like this, and easy to get pulled in 10 different directions. I agree before you pull the cylinder head, a compression test and a leak down test will give you more information. With good oil pressure, good compression, no leaky valves, you'll have ruled out any major trouble.

I think some have had luck sliding a small o-ring over the throttle shaft to cure any air leaks there.

This is really interesting and could help lots of people with the same upgrade. Do you have some pics and more details of what you did and how you did it?

If you cannot get the idle to adjust down lower than 700, you must have a vacuum leak or the throttle plate isn't closing (or both). Most likely areas are intake manifold gasket, carb base, and the throttle arm inside the carb itself... With no air leaks, she should idle way down under 600 without issue.

What is your idle rpm? You should be able to idle her down to 600rpms or less no problem. You may want to revisit vacuum leaks. The idle screw should change rpms with very little movement of the screw- unless you have vacuum leaks at the carb throttle plate arm, at the base of the carb, and/or intake manifold.

The 41 plymouth master cylinder doesn't have a hoop on it for the pedal mechanism...on the 41, the master is totally separate from the pedal assembly....fyi

When you adjusted the valves, was the engine hot?

If she idles down at 600rpm smoothly, I think you can rule out any major vacuum leaks. So, it's got to be fuel delivery (fuel pump or carb) or ignition related. I bet Keith would suggest testing fuel pump pressure. If that checks out, and you said the carb is new, and the gas is fresh, it must be ignition related or timing. Does the vacuum advance and mechanical advance on the distributor function as they should? It's possible you have a bad/new condenser out of the box.