How are carbs different between a manual and automatic application? Can a carb intended for a manual trans be used or converted for an automatic?

 

I would be interested in the technical details of the answer to this question also. From what I can tell (by looking at the specs in the service manual) the differences are very subtle, and may have only to do with the differences in idle speed specs between auto and manual cars. I do know you can put an even numbered Qjet (for auto trans) on a 4 spd with no problems. MJ

 

I think the main differencs would be in the linkage connections. IE: the automatic equipped car might need provisions for connecting T.V. or kickdown cables and linkage. Due to the different tranny setups, throttle cables may not be exactly the same length. Tune up specs may call for a different idle speed, but that doesnt really change the basic setup or function of the carb. Choke mechinisms can be different from year to year and model to model.

 

I would think that due to the fact that automatics idle in gear (less vacuum) the power piston spring and idle air bleeds woul be different. Q-jets a very specifically calibrated .

 

I don't have an auto, but I was just looking at my vacuum ports on my Edelbrock (generic) carb. There is a port on the back at the top of the carb. The only thing I could think this would be for is fo "kickdown", as it only has vacuum when the secondary throttle blades are open (high throttle) and the air valve is closed (low RPM). I'm not sure of other differences, though.

 

Thanks for the replies. I've wanted to play around with a carb in an effort to learn something about them without messing up the one on the engine. I figure if it takes me a while to get it right I can always put my working carb back on with no down time. I've ordered the book on Q-jets by Doug Roe and I'm still looking for a carb. I thought it would be easy to pick up a 72/73 Q-jet at a junkyard reasonably priced but I'm not having much luck. I don't know what to look for in a used carb either. Is there any way to look at a used carb and know if it is junk or not able to be rebuilt?

 

Hi Pete,

That book is very good, deffinitely read through it. I've been playing with mine a little bit and it's working great currenty. As far as finding a good core to rebuild you may want to try and e-mail Lars, he could probably find you a good core, or if nothing else ask for one in the parts for sale section I'm sure somebody has one laying around. good luck, If i had a spare I'd give you first dibs.

:cheers:
Pat Kunz

 

Pete -
The differences between the auto and manual carbs are usually absolutely none other than that the auto carbs have a vacuum port on the forward, passenger side of the carb for hooking up the vacuum line to the tranny modulator. Some years and models had very slight jetting changes between the AT and MT carbs.

For example, in 1973, carb numbers 7043212 and 7043213 were used on the Hi Perf Vettes and Camaros for Auto and Manual respectively. There are no spec differences between these two carbs AT ALL. They both are jetted with #74 main jets, #44 main rods, 7046010 sec rods, and they both have the same part number float bowl, air horn, throttle body, and choke system. No difference other than the stamped-in part number.

Other years, take '69 for example, did minor metering changes between the auto and manual: the 7029202 ('69 350 300 hp auto) and the 7029203 (69 350 300 hp 4-spd) both use #67 main jets, but the auto carb has #42 rods (lean) with the 4-spd using #38 rods (rich). They also have different part number float bowls due to the vacuum fitting issue noted earlier.

An auto carb can always be used on a 4-spd car. Minor re-jetting may be required for best performance depending on year, and the vacuum port for the modulator will need to be capped off. Some 4-spd carbs cannot be used on auto cars due to the missing vacuum fitting on some years. However, the modulator vac line can alsways be connected to a different vacuum source, although not visually correct.

Hope this helps a little.

 

Lars,

Have you had any experience with the Edelbrock versions of the Qjets? I have rebuilt many Qjets for reliable service, but not specifically for performance. I purchased a 72 LS-5 Roadster a few months ago that has a brand new Edel on it, but it is not performing like it should yet. It currently has 73 jets and 50 metering rods. This Edel is an electric choke that I like, but looking up the application chart, it is for a Chevy truck. The covette shop that did all the work for the previous owner sure did take her to the cleaners. Any recommendations for the Edel at an altitude of 3300 ft above sea level?

Phlash 72 BB Roadster :seeya

 

Phlash -
I sent you an e-mail with some recommendations and suggestions on your setup - I can't post from my work computer. Since I can't remember what all I typed in that e-mail, it would be great if you could post it for people to see - there should be some good info in it regarding the Q-Jet main metering rod sizes and how it affects your actual WOT jetting....

 

Lars

I have a new edelbrock 1901 on my '73 SB (auto). Only mods are your re-curved distributor :cool: , a performer 3701 intake, 1.6 rockers, no EGR, and 2.5 in dual exhaust with flowmasters. Still looking for more "oomph"

Can you recommend a good setup for my 1901 and this application?

Thanks!!