Thermoquad Secondaries

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danielb927

danielb927

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So, I went out for a cruise today (first of the year finally!) and remembered how the secondaries on my carb don't work. I'm running a 360 with a thermoquad on it. I played around with the carb today, I don't think there is a vacuum leak anywhere (about 14-15 at idle, even with about 4 hoses plugged up with screws). When it's just sitting there, I can get the secondaries to pull and sound right. I looked in the engine with the filter off so I could see the carb. It has mechanical secondaries so I know when I get to the point of opening them. So, just sitting in park, I gave it full throttle, I could see the secondary air door flap down and it sounded good about 3 out of every 4 tries, when I did that though I had a thin smoke come out the exhaust and it smelled noticeably different. It stutters or almost dies when I get into the secondaries while driving though.

Other Engine characteristics: Hard to start (pump the gas when cold, floor it when warm) , Smells like gas while running (exhaust and engine compartment both, I think) , Really high idle while cold (I can drive 30-35 mph down the street with my foot off the gas before the car warms up).

My thoughts are:
Fuel Filter needs replacing?
Some other fuel system problem?
Carb needs a good cleaning, specifically the secondary jets?

Anyone have some better advice? I feel like I'm kind of shooting around in the dark without a real idea of what might be the problem.
 
I have been told that because of the gas on the market today that it eats up the rubber orings under the bowl that seal the jets. Not sure what they are called but your best bet is to get ahold of Dave at thermoquads.com I think is handle here is demonsizzler.
 
Was this carb the one that came with the engine from the factory, and is the engine modified at all? I tried to run a Thermoquad from an '83 360 on my 318 and the secondaries didn't work, no matter what I did to the carb. Eventually I just swapped to an AFB, but I'm going to try to run a Demonsizzler-tuned TQ some time in the future.
 
I seem to remember a so kinda lockout that wouldn't let the secondarys open till the primarys are fully or almost fully open. Does the primarys open all the way with the pedal?
 
This carb sounds so outta wack, it's crazy.
 
my TQ on th elil red i had used to always get dirt in the needle and seat. it would be hard starting and load up at idle. clean them out and it ran great.

could you have the secondaries opening too easy?
 
Thermoquads are really good carbs but are definitely more complicated than most. Make sure the linkage hooked up to the choke that keeps the secondaries from opening until the engine is warm is working properly. The airvalve door for the secondaries has a spring that is adjustable too. I have the SA Design's book on carter carbs and it has some good tuning and troubleshooting info in it. Go over all the basics before you start adjusting things.
 
Yep, way outta wack as rumble said. Unless you are very familiar with the internals and tuning of a Thermoquad. Its just hard to tell you how to do it on the net.

Sounds like you have fuel leaking problems (seats, epoxied caps on the bottom of the bowl, or bad o rings), the choke mis adjusted, air valve too loose, and secondary mechanical adjustments needed.

Pull it, rebuild it, start adjustments from scratch.
 
The phenolic fuel bowl's have a tendancy to warp and crack too but I have never had that problem on any of mine. I would definitely get a book and read up and dig in. Those carbs are really work well on the street and don't work bad on the track either. I had a stock '73 440 I pulled out of a station wagon in a '73 Cuda that ran 13's at 102 with nothing more than a little carb tuning.
 
So, it sounds like it needs a rebuild for sure. Should I go with a demonsizzler rebuild or just get a new carb since this is my first carb and the TQ is definitely not an easy one to work with for starters?

Answers to various questions:
- I don't know if the carb is original to the motor, it came with it when I bought it this summer and the guy said he did a rebuild. The quality of said rebuild is apparently questionable.
- Engine is a 360 with a comp cams 280H (or close) cam, everything else is pretty stock.
- I have the pedal to where the primaries open all the way and then the secondaries after that, so I don't think the linkage is a problem.

Other thing I remembered:
- When I did the timing, I'm pretty sure the thing was running with like 60 degrees of advance, which seems impossible. But, I tuned it back to 14 degrees, so whether the marks are off or not, the thing can run in what seems like a larger than normal timing range?

Questions:
1) Would some seafoam in the gas be a start to see if that does anything until I can rebuild/buy a new carb? Or would a good cleaning require taking it apart anyways?
2) How much could I be hurting this engine running it with an insane carb like this?
 
Unless the old guy down the street that used to tune Thermoquads back in the day's name is Seafoam. I don't think it will do anything for you.
Maybe a new carb would be the easiest thing to do in your situation.

A bad carb is never good for an engine. Whether its too rich, lean, or just plain not right for the engine.

I'd also verify your top dead center mark is correct.
 
With a big cam it would be a good idea to send it to demonsizzler and have it done by him. His TQ's hold drag racing class records and I have never heard any complaints about him. The cost of having your TQ set up properly is comparable to buying a new carb. Is the choke stove there and hooked up and does the choke pull off work? TQ's are great carbs but are sensitive to how much vacuum your motor has. They are great on the street because they don't heat the fuel in the bowl like an aluminum carb does. That means much easier hot start's with the pump gas they sell today.
 
I'd run through all the settings first and make sure everything is where it needs to be.

IF you have a big cam, deleate the step up springs if there stock and see how it does. You can use AFB/AVS springs if need be. Try without first.
 
Been a long time since I looked at a Thermoquad so this is just a shot in the dark. Is it possible the air flapper is hitting the lid on the air cleaner?
 
'64 Cuda said:
Been a long time since I looked at a Thermoquad so this is just a shot in the dark. Is it possible the air flapper is hitting the lid on the air cleaner?
Click to expand...

Don't think so, it flaps down and isn't close to the height of the choke so I don't think it could be hitting. I will check though, maybe take a quick drive around the block with the cleaner off.
 
Also, babyblue - how do I go about making sure my TDC is correct? I had that thought too but wasn't sure how to check it.
 
I'm kind of thinking about just getting a new carb that I will be able to tune since this is my first go around with this kind of thing. I want to learn the basics of carb tuning which is obviously going to be tough on this monster. Any suggestions?
 
I just looked and judging by the ports this thing has, it is an 80s thermoquad. I'm reading that those are the most complicated and least desirable. I'm thinking, dump it and get a Holly/Eddy?
 
Well, I ended up buying a Holley 670 Street Avenger. Sounded like it would be easy to tune and since I have never been to the track, I want something I can work on easily but that will be reliable for cruising. Looking for an ld340 but until then I got the eddy four-hole adapter to bolt it to my spread-bore manifold for now! Thanks for all your help guys, if anyone wants a T-Q let me know!
 
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