Another Thermoquad problem

Fuel and Air Systems

  1. taguy16

    taguy16 Active Member

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    I can't get the Thermoquad on my '73 400 to quit running too rich. I put a carb kit in it making sure to install the primary well seals correctly, but this only helped a little and I still have black exhaust coming out of the tailpipes - I have the idle mixture screws turned all the way in, too. What are the next things to look at being the culprit? The only thing I can think of is it might have a cracked fuel bowl since its made out of plastic and over 46 years old (so it's got to be getting brittle by now). I'm wondering if this carb is even worth investing any more money and time in and whether I would be better off getting a replacement one.
     
  2. Russ69Runner

    Russ69Runner Well-Known Member

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    Ok you rebuilt it did you replace the float's or check to see if they have fuel in them. Have you tried to go down one jet size. Another thing is your choke plate sticking like half way. Just somethings to look at. My Holley had nitrate floats and they where old and to heavy to adjust them The car keep flooding. Could not adjust it at all. So got new floats and all is good. Got to be something simple. Russ.
     
  3. taguy16

    taguy16 Active Member

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    The choke seems to be working properly as it's open all the way when the engine gets warmed up. The floats are new but they are the plastic kind because that's what the local parts store could get. I was thinking of going with brass floats eventually if I could get the carb to work well because I've read the plastic ones can take on fuel as yours did as they age. Of course with a TQ switching to brass floats is an investment since there are two of them.
     
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  4. Rich B

    Rich B Well-Known Member

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    Roger that with floats, brass will leak also and sink. Yup, learned the hard way. Thermoquad on my ‘Lil Red Express ran great, did add water injection. Used humidistat to shut off on rainy days lol.
     
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  5. 69hemibeep

    69hemibeep Sponge Bob Square Wheels

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  6. taguy16

    taguy16 Active Member

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    I took the carb back off of the engine and disassembled it again. The fuel bowl doesn't seem to be leaking anywhere and the new floats I installed aren't taking on fuel. The metering rods are part # 2001 which according to this Hot Rod article - https://www.hotrod.com/articles/mopp-0501-tuning-the-carter-thermoquad-carburetor/ - measure .065/.052/.040; the primary jets appear to be part # 4095 (.095"). Could someone please tell me if this combination could be giving me the rich idle I have, and if so how much smaller should I go in jet size to lean out the idle? I'm beginning to think that there's nothing structurally wrong with this carb unless someone has any other ideas of things I could look for while it's apart.

    Thanks for all the posts!
     
  7. Rich B

    Rich B Well-Known Member

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    I’d look for a choke problem, not completely opening up or releasing. My engine was a 360CID and carb matched. Maybe somebody rejetted also. As I recall those had the tamperproof fuel set screws.
     
  8. Russ69Runner

    Russ69Runner Well-Known Member

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    This one may need to go to a carb man. My buddy had a carb and his motor was running really bad. We took it to a race guy I know and they did the sonic cleaning and adjusted the carb. We put it on and the motor never ran so good my buddy said. Some thing's are better to let an expert work on that dose carb's all the time. Just a though then you don't have any more worries. :thumbsup2:
     
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  9. 69hemibeep

    69hemibeep Sponge Bob Square Wheels

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    Jet size have nothing to do with idle, how was the float level? Trouble shooting a carb online is a problem of its own get some new eyes on it that works on them.
     
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  10. taguy16

    taguy16 Active Member

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    The choke blade is completely vertical (open) when the engine is warmed up, plus the choke coil is new, so I doubt it's the choke. I'm thinking this carb must have gotten jetted too rich at some point, maybe by a hot rodder looking for more power.
     
  11. Russ69Runner

    Russ69Runner Well-Known Member

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    Maybe Summit racing has jet's for it. They have a tech line also. Those guys are pretty good. The jet should have a number on it. If you can find out what it came with then maybe you can order a set. Russ. :thumbsup2:
     
  12. 69hemibeep

    69hemibeep Sponge Bob Square Wheels

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    You have the idle screws all the way in and it runs, jets are the least of your problems.
     
  13. Russ69Runner

    Russ69Runner Well-Known Member

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    I still would take it to a carb man and let him look at it buddy. Sometimes it is better to do that than racking your brain over it. I try to fix every thing I can my self but sometimes I will brake down and take things to a shop. That is my last result of course. Good luck and tell us what you find out. :thumbsup2:
     
  14. taguy16

    taguy16 Active Member

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    I did some more research online and the consensus seems to be that the float level should be 29/32" even though my Plymouth manual says 1 (&) 1/16", which is what I set it at. I've read that too high of a float level can cause a rich idle, so maybe this is the problem. I reset the float level to 29/32" and am going to give it another whirl.

    Another thing I thought of is to take the idle mixture screws out and measure them because maybe they are too short. Maybe the right length ones got replaced at some point. There's a good chance that I'm not the first one that has rebuilt the carb: I've read where carbs that are rebuilt by carb rebuilders will often end up with a mismatched set of parts in them because these places don't just rebuild one carb at a time - they are doing large quantities of carbs all at once and putting them back together with parts out of large bins of parts they get back from their parts cleaners and/or platers.
     
  15. Russ69Runner

    Russ69Runner Well-Known Member

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    The man I use only dose one carb at a time. No switching of parts ever. I don't know who you are talking to but that must be if you send it off. Hope the resting of the float work's out. If not find a local speed shop that dose carb rebuilding. Russ.
     
  16. taguy16

    taguy16 Active Member

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    I'm talking the places chain parts stores get their rebuilt carbs from, not a one man operation like you're describing. My feeling is if you buy a 45 year old carb off of eBay or a slavage yard there's a good chance it's been thru one of the mass rebuilder places at some point in its life.
     
  17. Russ69Runner

    Russ69Runner Well-Known Member

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    I know what your saying and feel the same way. That is the reason I take my stuff to a guy that has had a race shop and was a racer back in the day. It is Bobby Watt's in lake land Fla. I know he has had his business for over 30 year's and I trust him. Carb's can get bored out is what we call it and they are not worth the trouble to rebuild. At that point it is better to get a brand new one. I don't know where you are but would look at talking to some people in your area and as who they use. I rebuilt my Holley and they are fairly simple carb's. Found my float's where to heavy so got new one's and that fixed my problem. I would not adjust on the flow into the bowl. But you say you have new float's. I think we are a like in trying to fix it our selves. But sometimes out side help is a good thing. It is like pulling teeth to get me to spend the money for someone else to do it but sometimes have too. Well hope you get her fixed buddy. Russ. :steering:
     
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  18. Rich B

    Rich B Well-Known Member

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    Yes, the thermoquad was one impressive piece sitting atop a Plymouth or Dodge engine, and ram air...... The phenolic body was supposed to keep fuel much cooler than anything previous. Nobody using a cool can to lower fuel temps? Guess theory was gas with a greater expansion base would yield better performance vs hot gas.

    Carbs were complicated as hell anyways compared with fuel injection. How they ran so well is beyond myself.
     
  19. taguy16

    taguy16 Active Member

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    Just to put a period on this thread, I finally got the engine running about as I think it should. I ended up taking the carb off again and disassembling it to lower the float height from 1 (&) 1/16" down to 19/32". I also noticed the throttle shaft had a tiny amount of play in it so I went ahead and took it out and bushed its bores with brass bushings I got online. Now there is no smoke at all coming out of the tailpipe when the engine is idling, and the mixture screws on the carb are about 1/2-1 turns out.

    I'm going to call this good for now. One more problem solved on this very challenging project.
     
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  20. 69hemibeep

    69hemibeep Sponge Bob Square Wheels

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    Good now on to the brakes
     

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