Thermoquad help questions

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Your welcome. Call me Rob.

You did say you rebuilt this unit right?

The Vannth guide lists nothing for your carb. No rods sizes, no jet sizes, primary or secondary.

In the base/bottom section, (AKA the throttle body) in the middle slightly towards the front in between the primaries on the primary shaft is an egg shape lobe. This is the cam. It moves a level that will mechanically give rise to the tree. Those parts are there right?

Your vacuum reading will be low unlike a stock cammed engine. The tree spring is probably stronger than the 10-12 hg your pulling. It was fine for when it was stock but now with the performance cam & a lower vacuum level, it’s not. The mechanical lifting of the tree won’t be timed very well for enrichment but it’ll happen. The cam is a one size fits all.
This spring removal just eliminates the tree early rise and early rich conditions your getting. Let’s see where the heck it is when the throttle opens and the rods are down in the jet like there supposed to be.

The vacuum port in the rear should be full time vacuum so taking a reading from there should do the trick.

When you get closer to it being dialed in with what you have (current rods & jets) you’ll know better under driving conditions where you stand and what direction to go in to adjust that AF gauge in the right direction.
 
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Thanks Rob!
Yes, unit was rebuilt, and yes the cam is there and functioning. I did print out the Vannth guide. When I saw there was no info on this carb I started comparing my rod and jets to other models. I find that my jets are the same as in a ‘73 340 AT #6319S carb so figured they can’t be too far off. This has .098/.137 jets and the rods are 67/57/45 vs the 340 rods at 67/52/45 so same but slightly leaner mid lift.
And my vacuum at 11-ish is lower than stock so I get why the spring could be too much.
In the 30’s here with possible first bit of snow, so may be a day or two till I’m back on this!!
 
Sorry, small correction, the mid rod is richer being smaller.
Your in the 30’s! Freakin YIKES! Ouch! For sure I’d wait just a little bit for a better temperature. I’m not a fan of the cold much less the snow. I don’t think it helps much working outside doing mechanical inspections and repairs on heavy equipment much.

I’m Looking forward to the tests and results.
As long as the carb is tight, no air leaks, bind free, in spec with the small slew of measurements & adjustments, this thing should rock for you. I think you’ll like the carb in it’s flexibility and performance.
 
update:
I removed the main rod tree spring, and since I had several left over Edelbrock springs I cut a "pink" to the length of the stock one, and could tell that it was less strong. Installed that, went for a test drive. Idle adjusted to about 14:1 A/F, I could hold a steady in town lower rpm cruise in the 14-15 A/F range, but any throttle at all and it now goes to around 17:1, and obviously not running nearly as strong as the previous 10:1.

If this is due to the spring change only then I guess I would need something in between. But I am at this point working with too many variables. The carb would not slow fully to idle, due to the accelerator pump rod not dropping enough. (even though set to instructions.) So I did a slight bend to that and brought idle down to 800 or so.

Previously (old carb) I was running my advance off full manifold vacuum so tried "T"-ing off the rear port. Seemed I could not get idle screws to do much. Changed advance hose back to the front timed port and then got the idle mix OK. (not sure why this would matter....)

And, my final frustration of the day, I stopped along the road and tried to change the secondary spring slightly. It unwound on me, sounded like it came loose, or broke? Anyway, no matter how many turns I could not longer get any spring to the air valve, so it stayed wide open to drive home.

Had to quit for the day. 32* with snow forecast, so a bowl of hot chili seemed more appropriate. :icon_fU:
 
You over wound the spring inside. Let me know if you need another.
 
OP, I would run a vacuum guage inside the car, and record where the needle is when the metering tree finally rises. Then mic. the spring length, and stretch it .100. Then run and record where it raises the tree. And so on and so forth.
 
I lost hold of it and it unwound and seemed be unconnected.
I pulled the top and apparently when it spun loose it backed the adjuster out too far, even though the lock nut was still in. Live ‘n’ learn!! Spring is fine, just fell off.
 
Dana said:
OP, I would run a vacuum guage inside the car, and record where the needle is when the metering tree finally rises. Then mic. the spring length, and stretch it .100. Then run and record where it raises the tree. And so on and so forth.
Click to expand...

This was an “Industrial 440” carb, probably from a heavy motor home. I found one of my parts carbs was from a 440 truck. Compared springs and the truck spring was shorter and possibly lighter. I installed it for my next testing. I do have vacuum gauge to watch also. Thanks.
 
lemondana said:
-midnight340- Your adapter that you show is for a Quadrajet. The Thermoquad adapter has more meat between the rear flange bolts. Hold that adapter up to the bottom of your TQ and you will see the potential for vacuum leaks on the TQ base.
Ask me how I know. Edelbrock has two different part numbers, one for a TQ and another for a QJet. Although they show the same picture for both.
Click to expand...

I appreciated you noticing this. So far the thick stock type gasket above the adapter is sealing off where the TQ overhangs the QJ adapter,
...BUT I do have a vacuum leak which was hard to find. (My vacuum gauge routed inside is showing 5” vs my normal 10-11”.) With a small hose to my ear I discovered a pinhole leak inside one of the adapter’s cast side indentations. Must have “modified” a bit much with the Dremel!!
:D
I’ll do some JB Weld today and that should fix it.
 
So I epoxy'd the spot on the adapter where I heard the hiss when I was listening through a piece of hose. Fixed the hiss. Helped a bit, vacuum is up from 5" to 6" now, but that's not right. (and I checked my vacuum gauge reading against my Mighty Vac, so it's accurate.)

I sprayed some parts cleaner all around the base and heard no change in the idle. I also checked the connections on the vacuum line to my brake booster. Everything seems tight. I've run a small hose from my ear to the carb, followed every gasket edge, carb connections, etc.

Where could I be losing vacuum? I had 10-11" when running the Edelbrock 750 so why am I now at 5" in/hg.??
 
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