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Thermoquad troubleshooting...Help!

10 replies to this topic

#1 chubcityracing

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Posted January 13, 2014 at 11:26 pm

Ok, here's the skinny....I'm not an advanced mechanic by any means, but I know how to turn wrenches...However, I know nearly nothing about carbs, so hear is my issue....


'85 Ramcharger, motor is a '75 360 just put in, runs like a gazelle, has a new Thermoquad on top of it...600 or 650 cfm if I remember right....Here is my problem... When the truck sits for a couple days, the carb has to be primed to start the truck.  I pour a little gas down the front of the carb and she starts right up.  It also starts fine when it's warmed up, just not when cold.  Brand new fuel pump, and I can see the gas spray in the front chamber of the carb when I flick the accelerator, so I don't think it's an issue of the fuel draining down.  I also disconnected the fuel line before I started it, and there was still gas in it, so it's getting fuel I believe, it just doesn't want to fire up.  Any help from you guys is greatly appreciated...I'd love to get that old beast back on the road!

Edited by Bob Lincoln, January 14, 2014 at 09:05 am.
Modified language for G-rating.

#2 dana44

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Posted January 13, 2014 at 11:36 pm

Thermoquads are known for leaking at the base of the body from cracks in the base or gasket leaking. What you are going to have to do to check this to make sure it is the problem is to start the engine and as the engine is being shut off, open the fuel full bore and leave it this way, With the engine shut off, fuel should start dripping inside the bottom of the the butterfly within a few minutes at the least. If so, time to take the carb apart, regasket it, and during the rebuild, check to ensure there are not any hairline cracks that are allowing the carb to leak. Good luck and keep us advised of what you find.


Added thought:


You may luck out and find that the base screws are loose, and that is where it is leaking, which can happen, so if you do find the leaking at the gasket, check the base screws that point upwards, so underneath the carb, are tight, and not loose, which could help fix the issue that much faster.

Edited by dana44, January 13, 2014 at 11:38 pm.

#3 chubcityracing

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Posted January 13, 2014 at 11:43 pm

When my Dad and installed the carb, we looked over it with a fine tooth comb for cracks.  We ordered it new from Summit a couple months ago, and didn't want to go to the trouble of installing it and having to take it back off.  My Dad is much more "seasoned" mechanic than I am...lol, and this has him thrown for a loop.  I will check the gasket out, but I'm pretty sure it's good too.  Thanks for the reply!


Just read your edit, and that's exactly what I was going to do in the a.m....lol

Edited by chubcityracing, January 13, 2014 at 11:44 pm.

#4 ImperialCrown

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Posted January 14, 2014 at 04:12 am

 If you can see the fuel spray out of the accelerator pump nozzles when opening the throttle on a cold engine, then you probably have some gas in the float bowl. Is the bowl properly vented to the charcoal canister and the bowl vent adjusted properly?

Many rebuilt carburetors aren't plug n'play. They have to be set up and adjusted properly after installation.

 If you pump the gas a few times before cranking, does it then fire up?

Does the choke plate snap shut on a cold engine? 

Edited by ImperialCrown, January 14, 2014 at 04:14 am.

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#5 chubcityracing

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Posted January 15, 2014 at 01:18 am

Tightened the nuts down on the carb, they were a little loose. Negative on pumping the gas, it floods....and yes, when it's cold it does close up as it should. Still perplexed....lol

#6 mopar56

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Posted January 15, 2014 at 10:44 am

if I remember correctly from my thermoquad rebuilding days there are two small o-rings in the bowl that must seat exactly upon re-assembly or they will not operate correctly, is there warranty on this rebuild?, if not then might be time to look inside
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#7 Moon

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Posted April 9, 2014 at 10:21 pm

Sorry for late reply; I just saw this.


The Thermo-Quads (some at least) had a plastic middle section that was subject to warping and vacuum leaks.  How to fix it is an open question (double gasket?); stopping any vacuum leaks would help if they exist.

#8 David Eidell

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Posted April 14, 2014 at 05:43 pm

Would not a vacuum leaking Thermo Quad run like crap once the engine got started.


Try jamming the CHOKE PLATE FULLY CLOSED when starting the engine. I've owned engines that refused to start without the choke plate being fully closed. Pump the gas 20 times and nothing with the choke wide open. Just a thought ---

#9 dartndodge

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Posted April 14, 2014 at 07:22 pm

Yes I agree, Thermys sometimes develop a hairline crack in the Phenolic body, usually a result of tightening the 10-12 thru bolts too tight....this comes from inexperienced "Holley" specialists.


There is no such thing as a "New" thermoquad. They havent been made since the midish 1980s. Carter died then too. That is why Mopar used Rochesters in that period on V8s.


They also have 2 glue on sections in the body of the carb, with age I guess that can come loose.


No real solution, but a good site to look up is Dave at "Demonsizzler"...He builds Carters for concourse and race applications eg the Fast class 340 stroker Dusters etc.


Myself and All my buddys still use Thermys, the Holley guys cant believe it when we use less gas and still go faster. Not to mention that glorious intake noise.

And yes, they all leak fuel into the plenum, when sitting around for a few days. Why dont you get a cheap eletric fuel pump, not a race one, just a cheapy asian one (or second hand), and turn it on b4 you start the car....simple. 


Great Underrated carb, of course fuel injection has taken over nowdays.



#10 racerhog

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Posted Today, 05:58 pm

If the squitter is good? Just mass the pedal a few time and hold it open for 15 sec. Take your foot off the gas, Turn the engine over one turn. Mash the pedal 2 or 3 more times, then hold the throttle open for about 3 more sec. Take your fott off the throttle and light it off.. Todays fuel is really lean for the older stuff.... :)
I still run a T.Q. Great Carb...

Keep us posted if you can....
Just my 2 cents....

Edited by racerhog, Today, 05:59 pm.

#11 KOG



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Posted Today, 09:02 pm

Second the electric pump idea.  This is also the solution to the Holley 2bbl on 360s which leak through the float bowl to throttle body gasket.  Just wire it to the ignition circuit.  Switch on and wait until you hear the pump stop before attempting to start.  Then you've got a full float bowl.


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