Thermoquad idling issues

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toypar

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Recently started and broke in the cam on my 340 with a 750dp. I have a rebuilt Tomco Thermoquad 6138s from the 70s that has never been on a vehicle. I’ve wanted to try out a Thermoquad for awhile.
I don’t have the divorced Choke figured out or hooked up yet. I was able to get the car up to temp by holding the throttle around 1500-2000 rpm.
Choke wide open, and I let of the throttle the car dies. I have to go almost 4 turns out in the idle air screws to get it to somewhat idle. Idle screw maxed. When I slightly close the choke the car revs up and runs clean.
The carb is very responsive when I snap the throttle, but will not idle.

Where should I start? I didn’t find a vacuum leak around the base gasket or anything.

E78AE5D5-7D21-4E9E-A467-783D90025E2D.jpeg
 
Have you adjusted the metering rod tower?
 
I have not. I haven’t touched anything , but were would I start with the adjustment?
 
I followed the procedure that you had posted before just now. I have to have the idle speed screw maxed out and idling of the fast idle screw for it to somewhat idle around 1000.
If I lay a finger over the primary choke area, it revs up.
 
In this order:

[1] Engine idling, look in the primary throttle bores & see if fuel is dripping from the boosters. If yes, that is the problem. Could be poorly seating needle & seats, wrong float level, internal rubber o rings missing or leaking.

[2] Check the spacer/gasket under the carb. Check that it covers ALL the passages in the carb base that cause an air leak. You need to REMOVE it to check it properly.

[3]What are the cam specs?

[4] Report back.

[5] Congrats on using a great carb.
 
Bewy said:
In this order:

[1] Engine idling, look in the primary throttle bores & see if fuel is dripping from the boosters. If yes, that is the problem. Could be poorly seating needle & seats, wrong float level, internal rubber o rings missing or leaking.

[2] Check the spacer/gasket under the carb. Check that it covers ALL the passages in the carb base that cause an air leak. You need to REMOVE it to check it properly.

[3]What are the cam specs?

[4] Report back.

[5] Congrats on using a great carb.
Click to expand...
I will check those things tomorrow. A quick glance earlier in the evening didn’t show any signs of boosters leaking at idle. In order for it to idle, the primaries have to be opened way up.
I am running the Felpro thick 60-273 gasket. When I had it off tonight, it looked like it covered everything, I will check it again tomorrow.

Cam is a comp xe275hl. 231/237 at .050 .525 lift. With the Holley it had between 9.5-10.5 inches of vacuum at an idle. It would jump up to 17-18 at cruise.

Now I have the Carter M6902 fuel pump. On the Holley it had between 6.5-7.5 psi at idle.
 
what's your idle-timing?
I have a Hughes cam; 230/237/110 @.050 tappet, with a lil more lift than the Comp. I can idle my 367 down to 550 in gear (manual trans) and pulling, with just 5*advance. On a good day, I can get her down to 500. The advertised on this cam is 276/286 @ .008 tappet. My normal idle-timing is ~12>14.

The point is, you'll get it figured out, just stick to it.
Make sure the metering rods are staying down at idle, altho, I don't think that is your problem.

I think I have the same mechanical fuel pump.
My secondaries are closed up tight.
I have a lil bypass air, thru the primaries, one hole in each throttle blade, I forget how big, but guessing ~3/32 (.094?) This was required to be able to sync the transfer slots to the mixture screws. Some TQs have an internal bypass, but you gotta go look for it, and IIRC it is not adjustable. It's been awhile since I looked at one.
Happy HotRodding
 
My timing currently is 19 degrees at idle. 34 all in. Vacuum advance is not hooked up at this point. LD340 intake, 10.3:1 compression with RHS heads. Stock stroke.
 
try 12 degrees, reset the mixture screws to 3/3.5; decrease the idle speed to 750, then try adding some bypass air.
I used to do this by drilling a small hole in the top of the PCV valve, then adjusting the speed screw as may be required for idle speed and quality. When I got it right, I calculated the area of the hole, devided it by two, back-calculated for the new pair of holes, then drilled the plates. Finally, I closed the hole in the PCV valve.
But I don't do that anymore.
Nowadays
I take the carb off, flip it upside down, then using the speed-screw, I set the Transfer slot exposure to from dead square to a lil taller than wide, both the same. Then I make sure the secondaries are closed up tight but not sticking, and finally, I bolt the thing back on.
From this point on, I do not touch the speed screw.
I then reset the mixture screws to 3/3.5, and fire it up. I will set the idle-rpm with timing; 12>14 is usually close.
And finally, I will fine-tune the idle quality with Idle-Air bypass as described above.
That takes care of the idle.
The Power-Timing will have to be re-engineered.

The above assumes that the Fuel-level is correct and is stable, and that the only air the engine is seeing is coming in past and/or thru the primary throttles, plus the PCV, and that the metering rods are staying down..
 
This is a manual disc brake car. The metering rods looked like they were “bouncing” a little bit when I did get it to idle.
I will pull the carb and check the gasket and set the transfer slots.
If I currently set transfer slots like you say and back the timing off, this thing will not idle at all unless the Choke is manipulated partially closed. Something has to be plugged or way off on just the idle circuit.
I feel like I should probably buy a gasket kit and pull the carb apart and check floats and blow out all the passages.
 
Is it normal for these carburetors to make a slight whistle when feathering the throttle to keep it running?
 
No. A whistle could be a gasket in the air path down the primary side. (Or secondary side but the air moving on the secondary should be zero at idle.)
 
I flipped the carb upside down and the primaries have the transfer slot hidden with the idle screw at about three turns from seated.

The second photo is with the idle screw maxed out. Also one transfer port is cut with a taper and isn’t square.

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B57BA401-172D-4155-8716-91B911B0B6F4.jpeg


71229FCB-0BDC-4410-8CDB-D67B238C3462.jpeg
 
I pulled the carb apart and found the floats set about 3/16 low. Reset floats to 29/32, and cleaned out all passages. Found the idle screw about 3/16 of an inch shorter than another Thermoquad I had laying around.

I was able to get the car running with the idle mixtures about 4 turns out. Car idles around 850rpm. Is this excessive for the idle screws? They sure seem long winded. The tree is set around 1.75. Seems to be rich at an idle, but that could be my cam.
 
TQs will whistle under some conditions & is quite normal.

With THAT cam in a 340, you are going to need at least 30* of idle timing. It is futile trying to tune the idle cct UNTIL the engine has the correct idle timing.
To get the correct idle timing, you do this 5 min test [ vac adv disconnected ]: engine warmed up, idling [ in gear if auto ], turn dist slowly to advance timing. Idle rpm WILL increase. Once you have the highest rpm, check what the timing is. If it is 32* [ as an example ], then that is what it needs for best idle quality & HIGHEST VACUUM. Use vac adv connected to manifold vac to add the additional timing. You will need an adj VA unit with Allen key fully CW. You may need to fabricate a stop to limit the total amount of VA added.

This is not a Holley [ or clone ], so you don't need a gasket kit if you are careful.

Mixture screws 4 turns out + closing the choke to get the engine to idle means the idle cct is lean.....or you have an air leak. Wouldn't worry about the length of the mixture screws as long as they make a difference when you turn them.

img307.jpg
 
You might try going clockwise with the tree adjustment one turn and see if that helps. What you likely have going on is metering rod bounce due to the duration on that camshaft.
 
Rusty,
The tree adjustment has no bearing on the idle cct. Met rods are bouncing because of low vacuum. More idle timing will help & more than likely improve vac enough to stop the bouncing.
 
Bewy said:
TQs will whistle under some conditions & is quite normal.

With THAT cam in a 340, you are going to need at least 30* of idle timing. It is futile trying to tune the idle cct UNTIL the engine has the correct idle timing.
To get the correct idle timing, you do this 5 min test [ vac adv disconnected ]: engine warmed up, idling [ in gear if auto ], turn dist slowly to advance timing. Idle rpm WILL increase. Once you have the highest rpm, check what the timing is. If it is 32* [ as an example ], then that is what it needs for best idle quality & HIGHEST VACUUM. Use vac adv connected to manifold vac to add the additional timing. You will need an adj VA unit with Allen key fully CW. You may need to fabricate a stop to limit the total amount of VA added.

This is not a Holley [ or clone ], so you don't need a gasket kit if you are careful.

Mixture screws 4 turns out + closing the choke to get the engine to idle means the idle cct is lean.....or you have an air leak. Wouldn't worry about the length of the mixture screws as long as they make a difference when you turn them.

View attachment 1716181502
Click to expand...
After I got the carb adjusted to run 850-900 rpm at idle and about 4 turns out in the mixture screws, I pulled it to look at the transfer slot. (Very very rich idle, and wanting to stall when starting in first and reverse. This is a stick car with 3.55s)

The transfer slot is barely visible. Probably just a square. I noticed when I was unbolting the carb that on the pass front of the carb, under the vase it was wet with fuel and a small puddle had formed on the intake. Same for the drivers rear of the carb.
When I pulled it off the base of the carb was wet and there was pooling in the intake. I may have bumped the squirters when I was unhooking the linkage, but this amount of fuel seemed excessive.
All the shafts feel tight, wouldn’t think a primary shaft is leaking fuel, it was wet below it in the passenger side.
After the car sits for ten minutes, it’s kind of hard to start. Have to pump or open the throttle for it to fire up. Before it starts it kind of wants to kick back on the starter like there is too much timing. I verified 20 deg at idle

This must be my rich condition. The carb ran quite well when rolling into the throttle from about 2,000 rpm in third. It just spun the tires when the secondaries opened. The road was wet, so couldn’t test too much.
 
Bewy said:
Rusty,
The tree adjustment has no bearing on the idle cct. Met rods are bouncing because of low vacuum. More idle timing will help & more than likely improve vac enough to stop the bouncing.
Click to expand...
After messing with the carb today and getting a good look at the metering rod tree, it wasn’t bouncing up and down, but just rattling back and forth.
 
I would remove the carb top & verify that the two o rings are in place. Take measurements of the airhorn projection [ the part that contacts the o rings ] & measure the same area [ depth ] in the body. Subtract the measurements to ensure that the o rings are being compressed.

I had a Summit TQ that looked like the airhorn projection had been scraped on concrete! It wasn't even flat, at a slight angle & the o rings didn't seal. This will cause a rich idle/leakage.
 
i know the o-rings are there. they were a x ring style. they were still pliable. I will take some measurements tomorrow. The fuel wells looked intact, but i didn't test those for leakage.
 
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