Thermoquad problem-probably owner induced

[WesChapman]

1978 terra, stock, CA emissions 345 with the exception of the swapped on spreadbore manifold and thermoquad,

history: swapped on the thermoquad a couple years ago. I used the best looking carb out of a box of four thermoquads that I won on ebay for $25. Scout ran ok, idled smooth at about 19" vacuum, seemed to do ok on the highway and would see about 15 mpg with moderate driving. Over time, the engine was harder and harder to start after sitting. At first, if it sat for more than a month, I would have to dump fuel down the carb to get the engine running before the battery gave out. Then it was a week, then a few days, to finally, just over night. Figured the fuel wells were leaking. The smart thing would be to rebuild or procure a rebuilt carb, but I have neither the funds or the smarts to take the proper course of action. Keep in mind, once the engine was started, it ran smooth although vac reading was closer to 16 than 19" now.

Back to my box of carbs, which was minus two I had sent to mr. Mayben a while back for parts, or paper weights. I swapped on the other carb and hit the key. Engine fired right up after a couple revolutions, but once the choke opened and kicked down from high idle, the idle was terrible. Engine speed was 600rpm, vacuum was unsteady, between 9 and 12" and I could tell by the exhaust sound, the engine wasn't firing on all 8. Sprayed carb cleaner in the carb and everything immediately smoothed out for a second. Ok, vacuum leak. Some troubleshooting revealed a big vacuum leak at the egr valve gasket. Weird, I didn't touch the egr valve and the old thermoquad didn't have an issue with it. Fixed the vac leak, turned the idle screws all the way in, then back out 2.5 turns each. Fired her up again, and no improvement. However closing the choke or spraying carb cleaner down the throat would now cause the engine to die. Guess that was improvement. Decided to turn the idle screws open more. At about 5 turns each the motor smoothed out, the vacuum climbed to 16" and was rock solid steady. No misfire. Great! But....idle is 900 rpm and the idle set screw is not touching the throttle. Throttle verified closed. If I close the mixture screws and try to raise the idle with the idle set screw, it just doesn't want to run. At that point, I ran out of gas. All further testing halted.
Any suggestions what to check next? (once I get some gas)
ideally, I need to replace with a fresh verified good carb, but that's not an option for me now.

 

[Mark Pietz]

Some people love these carbs, others hate them. I ran them on a Scout and t'all, and I personally like them, but they do have their quirks. It's hard to know what's going on so far, but here are some ideas about where the problems lie with these things (learned after much frustration). From all you've said, I think the best thing to say is you need to go through the carburetor or have someone knowledgeable do it for you. You don't know its real history and it's set a while, now you have troubles with it. Getting it to run just ok after backing out the screws 5 turns isn't quite right.
All the vacuum and emissions hoses going into the tq must be supple and seal at their connections. That big hose at the front can harden and be a source of a vacuum leak.
I never liked the thick base gaskets for these things (or for the qjet, for that matter). They can be difficult to seal.
The phenolic bowls can warp over time and the little quad rings leak. I got pretty good at carefully surfacing the top of the plastic bowl using a glass plate and 220 grit sandpaper. There are a variety of air horn gaskets available made for a number of different applications (mostly mopar and a Ford app); when these dry out they also are sources of vacuum leaks. The air bleeds and emulsion tubes can be difficult to clean and carbs that sit gunk up. One can carefully dribble hydrochloric acid into these orifi and flush them well before it eats too much aluminum (toilet bowl cleaner), but it gets the crust out of them. I never trusted the nitrophyl floats and always bought the brass versions. Later on the rebuild kits provided a crappy flapper accelerator pump check valve, but some of the older carbs had a high quality brass disc with a check ball in it. They last forever. Oh, and the idle screws themselves. There were two versions - short taper and long taper. Setting the rod hanger height correctly is critical to getting the rods to transition correctly for mid-speed running.

And so on.

These can be great carbs, but have someone like michael go through one for you so you are certain the right parts are in it, or commit some time and effort to learn their quirks. They can be set up right and it is a rewarding experience get to that point.