1978 Dodge 440 with a Thermoquad

Guys- I’m doing my best to renovate a 1978 Georgie Boy motorhome. It runs great, the only problem I have is that the Thermoquad carb leaks down over 2 days- I have to fill the bowl thru the vent stack to start it. MY QUESTION_can I just replace the o-rings causing the leaks and the other easier parts without having readjust this whole thing?I have rebuilt several carbs but not some as finicky as this one. Thanks-Al

I would rebuild it completely. That carb should not be finicky at all. I have rebuilt a few of them and not had any problems with them. I would make sure any o-rings and other rubber parts you replace are made for todays gas.

I agree. Or consider buying a rebuilt carb.

Check the plastic float bowl (the black part) for cracks too. It is sometimes a problem. Or don’t do that but buy a rebuilt one.

The best advice here is to buy a rebuilt Thermoquad. I rebuilt the one on my motorhome years ago a couple of times for various problems. I finally bought a rebuilt and it worked great for several years until I sold the motorhome. Sometimes it’s easier and cheaper just to bite the bullet.

Why doesn’t the carb fill up when you’re cranking it? Is the fuel pump 100%? Carbs often lose a fair amount of gas in the bowl after shut off because of engine heat, especially in motorhomes.

Good question on the fuel pump-I had thought about that myself-I chalked it up to the long upgrade from the RV tank to the pump. But ,I guess it should really do it no problem-probably is weak. What do you think-can I add an electric inline pump to help? Or should I replace the mech pump? Thanks-Al

If it was mine, I’d figure out how to put in an electric pump. That way when you go to start you first activate the pump, give it a few seconds to fill the carb, then start cranking. But you’ll need to find out how to do it safely and correctly, it needs to be hooked up in a way that’s safe in event of an accident.

I installed a electric fuel pump on my Dads motor home with a 318 in it. It started and ran a lot better after that. Just put it as close to the tank as you can. I went to salvage yard and got a fuel cut off switch from a Ford. So if it was in accident it would cut off the pump. I also put a switch on the dash as a back up.

Thanks guys- I think I will install an inline pump-kinda hard to get under this big ole gal-but we will find a way-Al

O-rings aren’t likely the problem. It’s more than likely leaking back down through the needle valve into the gas line. The float should keep the valve closed, but wear, float problems, and crud affect the system over time.

The best bet is a rebuilt carb. An inline pump would refill the bowl, but it doesn;t fix the actual problem.

TSMB, aren’t the needle valve and seat at the top of the bowl? How would the gas leak back down through the needle valve?

Texases, as usual you are correct. I did a bit of research and the fuel inlet fitting is in the float bowl cover assembly, which also includes the throottle plates.

While the fuel could pull down in a siphoning action, just as when siphoning a gas tank into a gas can, I stand corrected in my earlier post. I truely was thinking of the fuel draining down rather than siphoning out.

Didn’t the thermoquad have plastic or resin bowls? That is the reason I’d go with a reman or even change over to an Edlebrock replacement.

Still could be a carb problem, though. If you remember that car/boat thing my dad built, he had to battle vapor lock all the time (what with high rpms and very little air flow), so fixing hot carbs boiling out their gas became a sideline for him. RVs with carbs are some of the worst at this, with the heavily-loaded engines often under tight-fitting covers. Go through enough heat/boil cycles and there can be deposits in many bad locations.

Yes, it was a plastic body carburetor. They used plastic to keep the fuel cooler, less vapor lock, etc. Imagine a 30+ year old carburetor with plastic fuel bowls leaking.

I agree with a rebuilt carb. It just isn’t worth the time and aggravation to rebuild it yourself. I’ve rebuilt carbs many years ago when it was a normal thing to do, but in the case of this aged motor home it’s better to just get one that’s already fully done, tested, and has the initial adjustments already made.