Thermoquad idle issue

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crewchief

A & P Mechanic
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Trying to smooth out my idle, only to find that if I begin to close the primary butterfly slightly the idle begins to run better. I have all new vacuum lines and thick carb gasket.
A vacuum leak cannot be found anywhere. Any suggestions?
 
The big downfall of the Thermoquad is the plastic body. It warps at the base. Need to take apart and sand flat again. Probably your problem .
 
From All par
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.
 
Trying to smooth out my idle, only to find that if I begin to close the primary butterfly slightly the idle begins to run better. I have all new vacuum lines and thick carb gasket.
A vacuum leak cannot be found anywhere. Any suggestions?
I don't see how Tim's reply works. But .

How did you check leaks? And did you check the carb gasket? The gaskets on the carb itself? What are the build specs and engine size? Once the butterly closed a bit, it introduced a restriction that speed up the air. What is the vacuum reading with the slow idle/fast idle?
 
the bottom of the plastic bowl has a slight raised Ridge to ensure positive seal by biting into the gasket. By filing the plastic body bottom you're probably introducing a leak. I've rebuilt a hundred TQ's and have never seen a warped bowl. I think that's an urban legend.
I agree there's nowhere near enough info to diagnose.
 
Trying to smooth out my idle, only to find that if I begin to close the primary butterfly slightly the idle begins to run better. I have all new vacuum lines and thick carb gasket.
A vacuum leak cannot be found anywhere. Any suggestions?
you have thin gaskets on both sides of thick 1/4 gasket? the 1/4 " gasket leaked on mine, I use one thin gasket only.
 
340 built with mild cam (because of power brakes). The TQ was rebuilt by National Carb, Fla. Thick gasket is used, vac at idle is 11 in. and increases to 15-18 when primary is closed slightly and runs smooth.
 
340 built with mild cam (because of power brakes). The TQ was rebuilt by National Carb, Fla. Thick gasket is used, vac at idle is 11 in. and increases to 15-18 when primary is closed slightly and runs smooth.
And what is the rpm doing during this time?
And what is your idle-timing?
And what are the mixture screws set to?
And do the mixture screws stall the engine when screwed all the way in?
And have you checked the float level?
And where is the vacuum advance hooked up to?

Sometimes too much air is really, not enough fuel.......
 
RPM fluctuating 800-1000
Timing about 22
Both screws set out 1.75 turns
Yes, turning them in will cause a stall
No, to the float level
Dist vac attached to OSAC valve and then to the port next to PVC hose and mixture screw front of carb
Again, slightly closing the butterflies smooths out idle and increases rpm

IMG_0384.JPG
 
340, mild cam, idles at 11" @800rpm. Idle is hunting. float level has not been checked.Mixture screws are working and are set at 1.75 turns, with idle-timing of 22 *. No apparent vacuum leak.

If it was mine, this is what I would do;
I would verify that all hoses are in good shape and properly routed. I would , plug the supply hose to the booster, or pinch it closed hard. Next
I would go to post #11,I would remove the vacuum advance hose for now,and if the hunting persists, I would go to post #13. If the pistons are both stuck down all the way and not dancing, then I would make sure the secondary butterfly is fully closed but not sticking.
If the hunting persists;
I would back the idle timing down to 12/14. I would return the idle speed to 750. I would adjust the mixture screws to 2.5 turns. I would flip out the PCV,seal the engine, and put a vacuum gauge on the dipstick tube.By now the engine should be fully warmed up. Readjust the mixture screws as may be required.And the hunting should be gone.
But if the hunting persists;
As to the vacuum gauge on the dipstick tube. There better not be any vacuum in the crankcase with the pcv valve out and flipped out of the way, and the engine sealed. Cuz, that would indicate that the intake is sucking air from the valley. That would have to be fixed. But if there is no vacuum under the intake, then
If the hunting persists;
I would pinch closed a rubber supply line between the pump and the carb. All the way closed. Then I would wait. And Wait. And Wait.
As the fuel level in the float bowls falls, one of three things is gonna happen;
1) The idle speed will stay the same for a while then slow down and soon stall.
2) The idle speed will rise for a while,plateau for a while then continue as in 1)
3) The idle speed will immediately begin to slow down and soon stall.

1) is what you want.
2) says the fuel level is too high, pull-over is too easy and the engine is idling rich.
3) says the fuel level is low, and the low-speed circuit is struggling to get fuel.
If you get a 2 or a 3, you will have to check the float level setting. If the float level setting is correct, then I would change it up or down as required up to 1/16 to 3/32 and retry. If this doesn't improve a 2 or a 3 to a 1, then the low-speed circuit will need to be looked at more in-depth.
If the hunting persists;
If the hunting persists, the engine may be possessed of the devil; I would call a priest who is well versed in removing demons. There may be prayer and fasting involved. I am 212 pounds so, this may actually do me some good.
 
Just asking here. What do T quads like for fuel pressure? Just got a buddies 440 dialed in. It came off the dyno which should be more plug and play than it was. The mechanical fuel pump put out 8# at idle and the Edelbrock didn't like it. It was eye burning rich. A regulator set to 4# did the trick.
 
Just asking here. What do T quads like for fuel pressure? Just got a buddies 440 dialed in. It came off the dyno which should be more plug and play than it was. The mechanical fuel pump put out 8# at idle and the Edelbrock didn't like it. It was eye burning rich. A regulator set to 4# did the trick.
Mine was OK with what the HO pump P4007040 put out. I think it was 7 or 8 psi. My Holleys were ok with that too. And the AVS/AFBs too. I run that pump on everything.
 
OE fuel pumps were low and IIRC, were between 4 & 6 psi. Increasing pressure requires a float setting change. Floats struggle to seal past 8 psi is what I have noted. Not impossible but not advisable.
 
I have had quite a few eye-burning-rich come to me. I always found it in the T-port sync......So far.
You know, I reread and edit every post I make, and I realize I harp on the sync an awful lot. I get that. But I get a lot of cars that other mechanics gave up on. It seems there is a group of mechanics in my area who have forgotten how to tune carbs.
The first thing I do when these cars come to me, is to check the sync. I never received one yet, that was reasonably close. Maybe I just get the crap jobs now, cuz I priced myself out of the market, IDK. I'm ok with that.
But I just gotta harp on the sync again. Eye-burning rich is almost always in the sync.And yes if the float-valves cannot handle the pressure, then a guy might crank up the curb-idle to compensate, Or he might crank up the timing to increase the idle speed. But the root problem,for me, has always been the T-Port sync.
And once I get that set, pinching the fuel line gets me to the right fuel level. Changing the float-setting to get the right liquid level clinches the deal.If the float valves cannot handle the pressure then a regulator would be the final answer. But I've just rarely had to go that far.
But then, I don't travel in racing circles, so the biggest tunes I get are around 250@050. And they are always streeters. That is all I accept. Streeters are a 2-gear easy pull. No big power requirement.
Altho I did work on this old crate Hemi once.....in a 71 Cuda. Beautiful car. And Hyup, you guessed, it was the T-port sync.
So Again, I know I harp a lot on that crummy T-Port sync.
But there must be a reason..........
 
Don't know about float levels or what type are inside. It was rebuilt by National Carb and does run great down the road. Haven't checked the rod hangers as of yet, been too busy to get back under the hood today to check things out.
 
AJ, here are some answers:
Post #11, By passed OSAC-no change
Post #13, slight up and down movement during rough idle. Held it up and idle increased slightly.
Secondary butterfly fully closed
PCV system operating as required
No vacuum at oil dipstick tube with both valve cover inlets plugged, slight pressure observed
Cannot pinch fuel line between pump and carb due to steel line being used
Noticed no vacuum where dashpot is plugged into carb. Dashpot inoperative.
 
Did you say you can find no vacuum leaks? How did you check for vacuum leaks? I'm assuming the engine has been rebuilt: has the deck been cut? the heads been cut? how much? has the intake been cut? how many miles since rebuild? does the engine use oil, especially at freeway speeds? is there an oil film on the back of the car after freeway speed cruising? Was the cam dialed-in? Did you take a picture of the timing gears/chain when at TDC, before you buttoned it up? You mentioned "mild" cam, cam specs?
What I'm getting at is that the vacuum seems low. Is the vacuum steady or fluctuating? If steady, but low: either a vacuum leak above the intake, or below the intake, or late valve timing.
If fluctuating: sticking valve, bad valve spring, valve(s)not seating.
 
This 1972 340 block and heads sat in the basement since the mid 80's. I built it from the bottom up with all new internals, crank, rods and pistons last year. Block was bored 40 over standard machine work was done. No deck cutting and heads were machined for new valves 2.02. Stock intake was not cut either just checked for cracks and squared.
Hydraulic Flat Tappet Comp Cam 260H, duration 260/260, Lift .440/.440 with 901-16 spring kit.
Cam break in when perfectly, high zinc, drained after 30 min run. Engine ran better during break in then now. Rings should be set with 500 mi on the motor.
 
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