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Thermoquad cold start problems !

Kern Dog

Life is full of turns. Build your car to handle.
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I have a real crappy run of luck with these carburetors.
Every one that I have owned, old or rebuilt seem to give me trouble. The latest is the one in my 73 Dart Sport. It is an original 340 car that I got without an engine. I had a low mile 360 on a stand so I regasketed it and put it in.
The car can sit overnight and be impossible to start. No action from the accelerator pump. The fuel filter has gas clearly visible. I can spritz a quick shot of ether, carb cleaner or even brake cleaner and hit the key and it will start right up and idle fine. I can let it run a minute or so and the accelerator pump works great. An hour later, it starts fine. The next day, it requires another spray of something to start.
This dude runs great! It idles great, sounds nice and smooth. It makes plenty of power with no bogging or hesitation. It just fails to start when cold. I plan to sell this car so I really need it to run right.
This carburetor has been rebuilt by a local guy that I trust. I even took it back once and he got it to work while on the work bench. I am tempted to just buy a new Street Demon based on the ThermoQuad pattern but I'd rather not spend more money on the car.
Anyone have any tips on these carburetors?
 
The ThermoBogs are Prone, and well known for, leaking well plugs. When it is apart, it is prudent to seal them with epoxy. They will leak down overnight, and take a lot of cranking to get gas to fill the bowls. I have a friend that always plumbs in an electric fuel pump to aid in filling the bowls, minimizing the pumping required.
 
I recall hearing that about them leaking down. I wonder though if that is actually the problem here. I can crank and crank it, pump the pedal 100 times but ONE shot of starting fluid and it starts and idles on its own. It seems as if it is just the accelerator pump that fails to stay primed.
Choke not working ?:eek:
I do not have a choke on any of the other Holly/Demon equipped cars here and they all start even after sitting for weeks. Maybe these ThermoQuads are more sensitive ?
 
Doesn't that carb have a leather gaskit on the accelerator pump? I had problems like that on my 67 Polara
 
I had the same issue and when I looked at the pickup side of the fuel line from the tank to the carb the hose was rotted. Replaced that, no dice. Started looking and looking and finally found the steel line actually had a hole in it that didn't ever leak actual fuel but was sucking air enough that it gave me cold start issues or if the car sat longer than a few hours.

This car went from being a car I hated, avoided moving around and always had a can of b12 and jump box with me when I got in it, to the old reliable sit for a month and two pumps turn the key start anytime faithful ride.
 
Nothing wrong with those carbs IMO.

I had done a rebuild on my LRE and found the accelerator pump in the kit was junk, swelled up and didn't give a good squirt. Ended going back to the carb kit guy and he sent me a different accelerator pump and after that life was good.

Choke... we need them in PA. Make sure it's closed all the way and as soon as it starts it should pull off about 1/4".

If you had a good carb guy rebuild it he should have identified any bowl leaks pretty quick. If that's the problem the thing should look like a diesel when it starts.
 
I recall hearing that about them leaking down. I wonder though if that is actually the problem here. I can crank and crank it, pump the pedal 100 times but ONE shot of starting fluid and it starts and idles on its own. It seems as if it is just the accelerator pump that fails to stay primed.
Choke not working ?:eek:
I do not have a choke on any of the other Holly/Demon equipped cars here and they all start even after sitting for weeks. Maybe these ThermoQuads are more sensitive ?
They will start but not on the first turn with a mechanical pump after sitting for a few weeks...Spend some money and buy a real carb instead of messing with a thermoquad....Or send it to Scott harms and get a real rebuild...
 
I have a real crappy run of luck with these carburetors.
Every one that I have owned, old or rebuilt seem to give me trouble. The latest is the one in my 73 Dart Sport. It is an original 340 car that I got without an engine. I had a low mile 360 on a stand so I regasketed it and put it in.
The car can sit overnight and be impossible to start. No action from the accelerator pump. The fuel filter has gas clearly visible. I can spritz a quick shot of ether, carb cleaner or even brake cleaner and hit the key and it will start right up and idle fine. I can let it run a minute or so and the accelerator pump works great. An hour later, it starts fine. The next day, it requires another spray of something to start.
This dude runs great! It idles great, sounds nice and smooth. It makes plenty of power with no bogging or hesitation. It just fails to start when cold. I plan to sell this car so I really need it to run right.
This carburetor has been rebuilt by a local guy that I trust. I even took it back once and he got it to work while on the work bench. I am tempted to just buy a new Street Demon based on the ThermoQuad pattern but I'd rather not spend more money on the car.
Anyone have any tips on these carburetors?

Sounds like the well for the accelerator pump mechanism is running bone dry either from drain back or leakage at the check ball. Have you tried manipulating the linkage while holding the choke open to see if fuel squirts out the nozzels before starting. Choke butterfly working properly? It should be noted the ethanol based gas evaporates at a much slower rate than fuel without it. Don't believe me, put outside two containers with a small amount of fuel, one with and one without ethanol. After about 8 hours come back and check them out. Evaporation in your case would only be relevant if you had a severe "hot soak" issue after shutting down the engine or prolonged periods of time. That fuel's going somewhere and it ain't evaporating out the accelerator nozzels of the carb. One other thing that peaks my curiousity. Remember the days when you bought an accelerator pump that had a leather cup on the end? not anymore. Those leather cups swelled slightly with the presence of fuel and seemed to last for years. New cups are some kind of material called rubber (just like the band in my shorts) but unless specified, no one really knows what kind of rubber their made of. I do know some cups just don't like some types of fuels.
 
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Another thing I ran into, I installed the filter on the carb inlet. My carb rebuilder friend told me not to, but I didn't listen. After a ton of issues, I removed it, and they all went away. I still used an external fuel filter before the fuel pump.
 
Maybe I fixed it...
I dug through my parts stash and found a choke spring and lever assembly that fits. It keeps the choke door closed when cold so that engine vacuum pulls fuel through. It starts without even pumping the pedal!
Thanks for the suggestions.
 


This is how my month old accelerator pump from rebuild kit looked. It was really hard to start when cold. Ordered a new from quadrajetparts.com. New one has lasted longer :)
 
Maybe I fixed it...
I dug through my parts stash and found a choke spring and lever assembly that fits. It keeps the choke door closed when cold so that engine vacuum pulls fuel through. It starts without even pumping the pedal!
Thanks for the suggestions.

I was over thinking it and for some reason thought you were having issue when the engine is warm as well. My bad
 


This is how my month old accelerator pump from rebuild kit looked. It was really hard to start when cold. Ordered a new from quadrajetparts.com. New one has lasted longer :)
I always use the leather cup accelerator pump. Havent had any issues with those.
 
Where do you buy leather cups?
 
My 72 Charger is hard to start when it hasn't been run for a few days, but runs great when warm. 400 2bbl.
What would be the fix for this.
 
Ethanol raises hell here in NY, with carbs, eats seals,and what not.
If you go 70 miles out side of the city you can find pure gas, non ethanol blended, its same price as 93 blended fuel.

:thumbsup: on MIKES CARB PARTS, they are great!!
 
I just this summer switched back to a thermo quad . Had not had one since the 80s.
I had forgot what a great carb they are.
 
I just this summer switched back to a thermo quad . Had not had one since the 80s.
I had forgot what a great carb they are.
I had another one rebuilt a few months back. I may use it on THIS car:

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