Thermoquad low-speed fuel well not filling?

-

Bill Crowell

Well-Known Member
Joined
Oct 16, 2007
Messages
1,170
Reaction score
804
Location
Diamond Springs, CA
The car doesn't want to start; doesn't want to run at all on the low-speed circuit; when it does run it idles poorly; and the accelerator pump doesn't work. Does this sound like bad primary circuit O-rings to you?

The reason I ask is that it seems to me like the low-speed fuel well is not filling at all, and I thought maybe bad O-rings would cause this. The O-rings are about a year old, but we have really wicked gas out here in California.

I've squirted carb cleaner into the low-speed jets and they didn't seem plugged.

What else should I do to diagnose it while I have it apart?
 
Usually fuel pump problems will show up at low RPMs or starting, something else to check.
 
Sounds like the main jet wells are leaking. Those are the exact symptoms when they do. I use a product called Seal All to seal them when I build one. It's impervious to gasoline and works well.
 
Well if the Jets are leaking pass o rings it'll be really obvious because the manifold will be soaked inside , but that very well could be the problem.
 
Well if the Jets are leaking pass o rings it'll be really obvious because the manifold will be soaked inside , but that very well could be the problem.

I always grab the main jet wells with a pair of pliers and gently try to pop them off. If they come off, I clean them up and glue them back on with seal all. If they won't come off, I brush some seal all all the way around the perimeter of each joint and let it dry.
 
From the symptoms described.
- o rings leaking cause flooding/rich idle. Should not affect acc pump operation.
- no acc pump could be the pump plunger.
- if using E gas, it could react with the o rings & pump plunger, depending on the type of material...& cause the above problems. In my opinion, TQs should NOT be used with E fuel because it can corrode alum.
- I have had new pump plungers, where the plunger seal 'rolls over' because it is too soft. Result is no pump shot. The best plungers are leather ones if you can get one.
- leaking fuel wells under the pri jets. If the body-to-base gasket is not torn, there will be NO fuel leakage into the manifold from leaking wells. This because each well is sealed in it's own compartment in the carb base by the gasket. What a leaking well causes is a rich mixture because instead of the main jet controlling the amount of fuel drawn into the engine, the main system can draw on 'extra' fuel that has puddled in the reservoir.
 
Thanks a lot for all the great advice, guys!

Around here, high-quality gas like Chevron costs at least $.25 per gallon more than cheap mini-mart gas. I've been buying the cheap stuff (yes, it is E-gas), but I think I'd better open up my wallet and start buying the better gas.

Here's some more background: It's a 6461 S that was rebuilt about a year ago by someone who does a good job. It ran well until recently, and passed its California smog test last year with flying colors and a smooth idle. About a week ago it began idling poorly and, somewhat intermittently, not wanting to start; dying after starting while refusing to re-start; and when it was running fairly decently I wasn't getting any accelerator pump squirt whatsoever, so it sagged worse than granny's breasts when accelerating from a stop .

I doubt that the accelerator pump is bad because it's only a year old, unless California E-gas killed it. Since both the idle ports and the accelerator pump are fed from the idle well, I am suspecting that the idle well is not filling.

The fuel pump does deliver good volume into a jar while cranking (I guess it could still be low on pressure, though). The main jet wells were attached with panel seal at the time of the rebuild and look good. No fuel in the manifold to indicate main well leaks.

So I am thinking that the inlet passage which supplies the idle well is clogged, but I don't know where that passage is located because I'm really not all that smart about TQs. I would like to try cleaning that passage out. Can anyone tell me which passage(s) feed the low-speed circuit? If you need me to post a picture that you can refer to, I would be glad to do so, but I would need you to tell me what to post a picture of.

If that doesn't work, I think I'll send the damned thing out to a carb rebuilder for an autopsy and resurrection, and ask them to be sure to tell me what they find wrong with it. If I do, I will follow up and let you guys know what they say the problem is. In the meantime, I'm using a 6518 S (industrial) TQ that is freshly rebuilt, and the old camper van runs really good with that one. I can't leave it on there, though, because the 6518 has neither the EGR nor the same vacuum ports that the 6461 has, and which are required in order to smog it next year.

Thanks again, everybody!
 
Last edited:
These are easy carbs to work on. As I said in earlier post, I have had the acc pump plungers [ fairly new ones like yours ] double over; they then do not seal & you get no pump shot.
Unlikely to be a blocked idle jet [ IFR ] if you run a filter. The IFR will be about 0.031" or a little larger, so would need a large piece of debris to block...& get past the needle & seat.
Take the carb apart, easy to do!!! Unlike brand H [ & clones ] you not need any gaskets if you are careful. Especially if the carb was recently rebuilt.

Take pics &/or drawings of linkages/rods so that they can be correctly re-assembled.

Remove end covers over metering rods, then remove met rod hangar, rods & spring.

Remove all the airhorn screws, including the two under the choke blade.
Top [ airhorn {AH} ] can now be removed. Hanging down about 1.5" from the AH you will see the aluminium primary booster legs, one each side of the AH. Up end the AH & look inside the legs & you will see a small brass tube with a pinched end. That is the IFR & you can poke a piece of wire in it to make sure it is not clogged. Blow out with air if you have it.
Those booster legs sit on a small o ring in the black body.
These should be replaced. They are supposedly some sort of special design o ring [ x in cross section ]. I have always used standard 3/8" OD fuel resistant o rings, 1/16" wall thickness & NEVER had a problem. Readily available, no special kit reqd.

Acc pump. Remove the linkage screw on top of the AH & remove the S link. Tap lightly on the shaft & pump will come out the bottom along with the retaining valve.
If this valve is brass, shake it & you should hear/feel/see the small ball moving inside.
If it is pressed metal, you should see a small red pellet inside; this should move if you poke it gently with a paper clip.
Check the plunger. It has to seal in the bore. If it has shrunk/damaged, you get no pump shot.
 
Most excellent, Bewy! Thanks for all the good info. I don't have any problem assembling and disassembling my TQs because I have 4 old Mopars (LAs and RBs) that have TQs, and they're the only smog-legal carburetor for these vehicles, so I've taken them apart quite a few times but still not sure that I really know what I'm doing.

I'll try reassembling the 6461 after I've poked out the IFRs and installed some new fuel-resistant O-rings. Maybe that will help. Thanks again for your kind reply.
 
I bought some Viton x rings let me know if you want to try a pair.

Thanks for the offer, 66fs, but that's what I used before, and I'm thinking that maybe they can't stand up to our California E-gas, so I'm going to try what Bewy said, by going to my local auto parts store and seeing what they have in fuel-resistant O-rings.
 
I have used 87E10 in my TQ for decades with no problems whatsoever.
When the main well leaks, it fills the cavity beneath it, so nothing in the bowl is affected.
That cavity has a full perimeter gasket, so you can drive your car for years and not even know you have a problem, besides a slightly weepy gasket.. Theoretically it is possible for the gas in that cavity to drip into the plenum causing problems. When you take yours apart, look for evidence of fuel in the cavity, and in the plenum. If you find it, then you know. But just because you do find it, does not mean it is or was the ONLY source of your problems. Smoke may be a precursor to fire, but sometimes smoke is just smoke.
 
What has been said in post 13 about the main fuel wells, I covered in post 7. If the gasket seals, fuel cannot leak into the t/bores.
What it does cause is a rich mixture, but not all the time.
 
Just to add to the comment that some have used E fuel with zero problems. Do not doubt that. But the carb in question has had 'new' o rings fitted.
There are different brand o'haul kits available & who knows how good the quality is??
 
-
Back
Top