383 with HEI

[67fury3kidd]

I am using a late model 400 distributor (77 i think) that has been converted to Chevy HEI on my (65) 383 with a 850 cfm thermoquad carb. Especially from a dead spot an sometimes when I am rolling when I punch the gas it chokes up, i think it may be because the carb is too high of a cfm rating or my timing is messed up some way. Possisbly my vacuum advance spring isn't the right tension i have very limited knowledge of the mechanical side of things so any advice would help

 

[The Horvaths]

The first thing I said was that the ThermoQuad was a poor choice.
If by "chokes up" you mean "backfires", this sounds like a lean condition upon acceleration. It is possible that your accelerator pump is not functioning correctly.



If you mean it "bogs", then that is quite possibly due to the carb being of such a large CFM. Try an Edelbrock 1406.

 

[commando1]

Send the TQ to the dump. Bolt on an Edelbrock 600 cfm.
Get back to us then.
Impossible to troubleshoot driveability problems when a TQ is hovering over everything.

 

[67fury3kidd]

The carb was rebuilt at a shop with a good reputation but i have a old carter afb that came off a 76 motorhome with a 413 I'll give that a try

 

[commando1]

The carb was rebuilt at a shop with a good reputation but i have a old carter afb that came off a 76 motorhome with a 413 I'll give that a try

I have a TQ restored by the foremost carburetor restorer ($$$) in the country. After 500 mi, It went back on the shelf because it ran like **** again.

 

[GJS]

Gotta agree with the fellas above, thats way too much carb. I'd try the Carter BUT don't be too surprised if it is a little touchy as it won't be calibrated to your motor.
Edelbrock 1406, from my experience is plug and play on a stock 383.. I can't see why an HEI conversion would effect that.

 

[thrashingcows]

I have to dis-agree.....the TQ is a great carb, I have been running them for many years on almost everything. They do require a lot more time to initially set up....it takes me a couple hours to get things set exact, then there is the tuning once on the motor. But once dialed in I have never had any problems.

The CFM rating really shouldn't make any difference, the motor only uses what it needs. I have run the big 1100cfm TQ's and got great MPG's and performance, from old worn out motors to boot. ;)

Even though a shop "rebuilt" the TQ I would still pull it apart to verify things. I have bought a couple reman'd TQ's and pulled them open almost right away....they were set-up so badly that the motor would never have run right.

Having said all that the TQ does require a certain finesse, and requires a lot of tinkering to get them right....most people either don't have the time or skill to dedicate to these carbs. The Eddy's and Holleys are a much more simple carb, with new parts readily available.

 

[Big_John]

I'm one of the guys that has had really good luck with the TQs also. In fact, I bought everything to convert my TQ 440 to the Edelbrock carb and I just can't bring myself to change a great running car.

Back to the OP.

It sounds like a carb problem... Accelerator pump or the secondary air valve is opening too fast. A lot of shops don't know how to set the secondary up right.

But.... as you mention timing, it only takes a minute to check timing with a timing light. Nothing wrong with doing the easy stuff first.... and you want to make sure all your ducks are in a row with the ignition. You know what they say, "90% of carb problems are electrical".

 

[The Horvaths]

The CFM rating really shouldn't make any difference, the motor only uses what it needs.

I guarantee you will notice a difference when you put the proper CFM carb on an engine. There is a science behind it.

BTW, I think the TQ is a fine carb, it is just way too big for this engine.

 

[Big_John]

I guarantee you will notice a difference when you put the proper CFM carb on an engine. There is a science behind it.

BTW, I think the TQ is a fine carb, it is just way too big for this engine.

Chrysler used 800 CFM TQ carbs for their 340. 850 on the 440. I've read there was a smaller size for the 318, but I've never seen one.

850 and 1000 were the aftermarket carbs.

The CFM rating sounds high, but it really is inline with the 383 engine if you are using a TQ. I've used an 850 on a 318 and was really pleased with how it ran.

 

[GJS]

Chrysler used 800 CFM TQ carbs for their 340. 850 on the 440. I've read there was a smaller size for the 318, but I've never seen one.

850 and 1000 were the aftermarket carbs.

The CFM rating sounds high, but it really is inline with the 383 engine if you are using a TQ. I've used an 850 on a 318 and was really pleased with how it ran.

I didn't know that.
All I do know is I never got the hang of setting a TQ up. TQs and Quadrapukes never much cared for me.

 

[The Horvaths]

Chrysler used 800 CFM TQ carbs for their 340. 850 on the 440. I've read there was a smaller size for the 318, but I've never seen one.

850 and 1000 were the aftermarket carbs.

The CFM rating sounds high, but it really is inline with the 383 engine if you are using a TQ. I've used an 850 on a 318 and was really pleased with how it ran.

Wow. I had no clue. Do you happen to know the reasoning behind this? I suppose it's the way the Venturi is set up?
Thanks for the info!

 

[Big_John]

Wow. I had no clue. Do you happen to know the reasoning behind this? I suppose it's the way the Venturi is set up?
Thanks for the info!

I really don't know, but I suppose it's because the secondary air valve works. It will only let as much air into the engine (if it's set up right) as the engine needs.

 

[67fury3kidd]

The reason why I chose the TQ originally was because from how i understood it. it opens and pumps the fuel in mechanically instead of using engine vacuum and thats why they would put the higer cfm ratings on smaller engines as well as bigger ones

 

[65sporty]

The reason why I chose the TQ originally was because from how i understood it. it opens and pumps the fuel in mechanically instead of using engine vacuum and thats why they would put the higer cfm ratings on smaller engines as well as bigger ones

The throttle linkage is mechanical but the air valve is vacuum controlled and adjustable. It works a lot like the Q-jet. This read will explain a lot.

http://www.allaboutvaliants.com/tec...72-1 (290) The '72 Thermo-Quad Carburetor.pdf