I just finished pulling the engine and tranny of my 1972 f100 with 360 FE engine to replace clutch and several leaking gaskets. I've been driving for 3 days, and now the RPMs while driving are way too high. When I am traveling 45mph the engine sounds like it is going 65mph and its burning through fuel at that same rate. At the end of my second gear around 20mph ish it gets really sluggish like its getting too much fuel or the timing is off. It does the same through 3rd gear until it gets to 4th gear. Then it sometimes and only sometimes starts to even out at the engine sounds match the speed of travel of the truck.

There are several other possibly related issues that could be affecting the rpm issue.
1. There is a random shudder at idle. It shudders for 3-4 seconds randomly when stopped at a light and then doesn't do it again for like 15 seconds. When it shudders it acts like its not getting enough fuel or that it is about to die. (fuel delivery issue?)
2. One of the first times we ran it after reinstalling the engine it randomly shut off and we could not get it started. It got all the way warmed up and we set the timing and it idled perfectly but 5 minutes later it just shut off. Me and my grandpa tried everything to get it to start again like new rotor, cap, and coil but couldn't get it to go. It was late so we gave up for the night and then randomly in the morning it started up just fine. Even with the same coil, rotor, and cap that was originally on it. We could not replicate the issue of it not starting again and it hasn't done that since.(Iginition system/electrical issues?) It has a pertronix kit in it so no condenser or points.
3. When trying to start it after it has warmed up you have to push the gas pedal all the way to the ground and let it crank for a second before it will fire. (vapor lock/hot gas?)

Everything but the engine has either been replace or rebuilt within the last two years. I just rebuilt the carb less than a week ago and the fuel pump 2 months ago. Since there is so many issues all at once I figured Id readch out to see if there was any good starting point instead of just replacing or testing things at random.

And just to be clear the truck ran perfectly beforehand. Amazing fuel milage (12-15mpg) No idle or rpm issues. It would do 65-70mph no problem. It was my first truck out of highschool and I have had it for almost 4 years so Ive had to learn it all as I go.

 

Any unusual smells like something is burning?

If you didn't swap in a completely different transmission or swap out rear end gears it should be running at same RPM at a given speed/gear it was before.

If the engine is revving higher than before with no gearing changes, I think your clutch might be slipping, perhaps out of adjustment or something else.

Regarding the hard to start, modern fuel, especially ethanol blend fuels boil and cause vapor lock a lot easier than fuel blends from years ago. When you have engine at operating temp and shut it off the fuel in carb and in the line between pump and carb can boil and vaporize. That can occur in as little as 5 or 10 minutes depending on how hot things are under the hood. It might take an hour or two to cool off enough to fire again.

Also, components such as the pertronix module and ignition coil can work fine when cold but stop working correctly when they get warm. Common symptom of a failing ignition module is engine starts fine, once it gets warm it shuts off, let it sit for awhile and it fires back up again. HEI, Duraspark, OEM and aftermarket can all fail in a similar manner.

 

You did remember to refill the transmission with gear oil and check fluids elsewhere?

Might jack the truck up and get the tires off the ground. Make sure they spin freely, if not might be an issue with wheel bearings, rear end gears or axle bearings, or brakes are sticking.

 

I figured out the problem was a weak spark because of a failing coil. I tested the resistance between the current coil and a spare in the secondary winding on the one in the truck is almost 2000 ohms over what it is supposed to be. I put in a half ok coil (damaged from failed voltage regulator) and it ran way better. It ran perfectly compared to how it did before. It is still hard to start after warm which could still be because the new old coil is semi-bad. When it got damaged by the voltage regulator it became harder and harder to start over time until I figured out it was getting 15.1 volts instead of 12v. Then I replaced the voltage regulator and swapped the coil to one that was new at the time and it started up easy every time. I bought a new pertronix coil that got here today so I will install it and see if that makes any difference.

The other question I have on the coil is does the mounting orientation of the coil matter? I currently have it mounted in the stock position (horizontal) ontop of the intake manifold. I read another thread saying that oil filled coils need to be mounted vertically with the terminals upright but people were giving mixed opions on that. Some said it didnt matter others said it will not work long without being installed correctly. My coil lasted 2 years on its side and I never had a problem with the pertronix system at all.

If it is vapor lock I read something about installing a phenolic spacer to help prevent heat transfer to the carb. Is that worth it? I do currently have a thick (1/4 inch) gasket between the carb and intake. But what you are saying makes me think you are talking about the metal line from the fuel pump to the carb. Would heat shield tubing or warp take care of the problem? I never had an issue with hot gas or vapor lock for the first couple years I had the truck. I also just put in a brand new radiator and thermostat (180 F) and the engine is supposedly (according to the dash gauge) running cooler than ever so I would think if it was a heat issue then it should have at least gotten somewhat better?

 

And yes I triple checked all fluids and made sure evrything moved freely. I also watched the clutch through the inspection cover as someone pressed the pedal and it seemed to engage and disengage properly.