carburetor Question

[gasoline56]

for an almost orig. 73 Rallye Charger with 400 cui with 4-Speed. No race ,only street. Weiand intake 4bbl. Currently works a 750cfm Holley, which seems to big. Have an eye on Edelbrock 600cfm. Does that work properly?

Thanks in advance for advice!

Dirk

[DynoDave]

Dirk,

Why do you think the 750 is too big? Is it running poorly? Just curious.

A 600 or 650 was the "std." for small block Chevys when I was growing up. You 400 would have come with a spreadbore Carter Thermoquad, rated at 850cfm.

[glr]

with DynoDave...I don't think the 750 CFM carburetor is too small....Just get the carburetor that you have calabrated/rejeted if it seams to be too rich.

[robertsrt]

If its a 750 vacuum secondary, i.e. a Holley 3310 (sometimes called a 780 vac sec) then its actually the factory carb for a 440 1970 and earlier. They used them factory on 383 magnums too.
Definitely not too much, as the carb will only deliver the CFM the engine demands. I have one on my stocker and its fine.
A 750 MECHANICAL secondary however is probably no good unless the car is a 4 speed. Its a heavy car, and with a high gear in the rear end, opening the secondary's might create a bog.

[gasoline56]

Did it this way, hope it works.

www.summitracing.com/int/parts/qft-ss-650-an/overview/


Thanks for your advice!

Dirk

[71se3834v]

Sept 26, 2014 17:06:16 GMT -5 robertsrt said:

If its a 750 vacuum secondary, I.e. a Holley 3310 (sometimes called a 780 vac sec) then its actually the factory carburetor for a 440 1970 and earlier. They used them factory on 383 magnums too.
Definitely not too much, as the carburetor will only deliver the CFM the engine demands. I have one on my stocker and its fine.
A 750 MECHANICAL secondary however is probably no good unless the car is a 4 speed. Its a heavy car, and with a high gear in the rear end, opening the secondary's might create a bog.

Why would a mechanical secondary be no good vs. a vac operated one? They did use the Carter AVS on 383's w/auto trans in '71.

[robertsrt]

Basically if the car is too heavy and if its got tall gears, combined with an automatic it will bog off the line. It will lose too much engine vacuum initially.
Now having said that, the OP did mention that his is a 4 speed car, which helps. But it is still heavy and has a stock engine. I assume it has stock gearing as well.
I would not buy a mechanical sec. carb for this application. The Carter AVS is a different animal than a Holley double-pumper.

[odzking]

Well, as long as we're on the subject of carbs, anyone here a ThermoQuad expert or close?
My question is this. If I go strictly by the set up manual for adjusting the carb, it just doesn't seem right. But I think it is still running to rich the way I have set it up.
Here is an abbreviated version of Service Manual for the carb and what I did:
Base idle mixture setting: screw both idle mixture screws in (clockwise) until they seat *lightly* in their holes. Back both screws out (counter-clockwise) 2.5 turns each. One manual said 1.5 turns and the other said 2, but that didn't seem right.
Then I continued until it seemed to peak the RPM's and I turned it back 1/4 turn.
I have had a couple people say that is fine, and some say it is not and to follow the Service Manual to the "T" or it will not be right.
I have 2 on my site and BOTH are very confusing to me.
I would very much like to keep it original but if anyone that can shed some light on this before I convert it back to square bore and forget about it, I'd appreciate it.
www.retrorarities.com/images/CarterThermoquadInformation.pdf
www.retrorarities.com/images/Thermo-QuadReferenceBook.pdf

[robertsrt]

I don't know very much about Thermoquads, but I've set up a few carbs, so here goes with what I know... !
You could refer to the manual as a starting point, i.e. to get the car running, but then I'd throw it out.
Every car and carburetor combo is different - even if they appear the same, they're not. Like I have a 440 and a 750 and so do you, but I'll bet they have to be set up slightly different to run the best.
So generally, on a hot engine, once the choke is off (out of the picture), set the timing. Give it too much to where it will ping under load then back it off until it doesn't. In other words Max timing. Type of fuel and carburetor setup will affect this, so you might have to circle-back to this again after the carburetor adjustment, but anyways, on to step two.
Set the base idle speed. Not too high obviously because it will cause the main circuit in the carburetor to begin to bleed in. But set it to a comfortable speed so when you put it into gear (if its an auto) it will survive, even if you turn on the A/C and be behave at stoplights etc.
The float level is critical too in this drivability case, so check that.
Using a vacuum guage, adjust one idle circuit screw, then the other. Back and forth to achieve the best vacuum reading.
Re-adjust the idle speed.
Re-adjust the timing.
Re-adjust the idle speed.
Done!
Re-jetting, power valves or metering rods, main circuit stuff, I'm not really good at. You should be able to read your plugs to see if you're too rich.
If its really wacky, that takes a bit more work. Simply takes time to drive and troubleshoot. Most carbs are pretty close right out of the box (if you pick the right size) unless your car is heavily modified or has something wrong like cam out of time, timing chain skipped a tooth, fouled plug or other ignition issues etc. Unfortunately, but rarely you are fighting a bad carburetor that will never come around. Thats a bad situation, Try a friends carburetor if you can, to try to eliminate that.
Hope this helps!

[Archie]

Hi Bob, like Robert said don't get too wrapped up with the manual. For a quick fix turn the idle screws in half a turn. You might not have optimum vacuum, but your air fuel ratio should be in the ball park. After you adjust them if you're still rich I would look at the float level. Carbs of this era were set to run lean so if you can't get there with regular adjustments something isn't right. Over the summer it was running really rich. I think we gave them two turns which was better, but think we needed to turn them in more.
My problem is the opposite. The TQ on my 72 ran extremely lean (factory jets/rods). The insulators on the plugs were bright white. I was able to make progress changing out the metering roads but I need to replace the jets to get it where it needs to be.

-Regarding the 750 holley you should be fine. For the street you will probably get better throttle response with a 600 but you will loose some top end. For streetability I would go with the 600, but if you want maximum horsepower stick with the 750. It's a trade off - all depends on what you want.