NAPA 1969 GS Carb Dashpot Number

Al the dashpot does is give the throttle a cushion so the engine doesn't stall if the car decelerates too fast. I seriously doubt that's your issue

 

The original carb number should be 7029242 according to the service manual. We're going to need some more information. When you say backfiring, is it through the exhaust or through the carb? Was it doing this before the carb was rebuilt or is it a new issue after the carb was installed?

 

Probably not backfiring per se, but more of a popping through the exhaust. Sounds like the carb is too lean. So just to clarify-

This is a NEW condition AFTER the carb was rebuilt?
Was this carb on the car before?

 

And just out of curiosity, what kind of vacuum reading are you getting at idle?

 

A 1970 Q-jet is a 750 CFM carburetor. .076 and 42B rods would make it super rich. The 800 CFM carburetors used mid 70's jets stock, so a 750 set up like that would be very rich, especially with those primary rods. There are 2 guys on V8 who can properly set up your carburetor, you should have used one of them.

Get the part number off the carburetor, should start with 704

 

We're assuming the numbers Mike posted are jetting numbers. They could be anything.

Its a possibility that the engine could have something as simple as a vacuum leak. Unfortunately, there are more questions than answers here

 

Could be, I read it as 76 jets, 42 primary rods and DA secondary rods. A vacuum leak would make it lean though, not rich. This is an unknown carburetor at this point.

 

Should be 7029241. I could not find that number in my sources, but it would certainly be in the 69 Chassis manual. I found a 7029242, which would be the automatic transmission carburetor for the 69 GS400. That has .070 jets, 45B rods, and AY secondary rods. The 7029241 would be the manual transmission carburetor for a 69 GS400? Might have different stock jetting. Got a 1969 Buick Chassis Manual Mike? Here's a cheap one,

http://www.ebay.com/itm/Original-19...ash=item25e1496ddc:g:t2oAAOSwNZtZ0Q3I&vxp=mtr

 

OK, so then we have to think that the stock jetting would be the same, .070 jets, 45B primary rods, and AY secondary rods.

You have .076 jets, 42B primary rods, and DA secondary rods.

The rule of thumb for Q-jet jetting is if you go up more than 3 jet sizes, you go UP one rod number to re-establish your cruise air/fuel ratio . Given that, I don't understand why your guy did what he did. The only reason for going up in jetting is if you are running headers and a hot cam. He went up 6 jets sizes, and down 3 primary rod sizes. The AY secondary rods have .0567 tips, the DA rods have .0440 tips, so they are richer as well. That carburetor must be pig rich.

 

Mike, it really helps if we know more about your combination. Look at my signature. Every time I post, you see all the details of my engine build and car. Are you running headers, cam?

 

The only reason for going up 6 jet sizes is if you are running open headers, a big cam, and single plane intake. The new gas is 10% ethanol. While it makes some difference, I doubt it needs re-jetting in most cases, certainly not yours. I would go back to stock jetting for that carburetor except for the secondary rods, I would leave that.