Differences in carbs

[matthud]

Can someone tell me what the differences between 2 Qjets is? For example, is there anything mechanically different between 7029215 and 7028209, they are both for 427/390 with M/T, one is for 68 the other 69.

 

[wishuwerehere82]

http://www.eaglesclassicparts.com/techpage.html

7029215 1969 quadrajet chevy manual transnission

7028209 1968 quadrajet chevy manual transmission

 

[matthud]

I know they are for different years, I was asking if there is something internally different between the two. Is it possible to swap the two without affecting performance? Are they actually the same carb?

 

[wishuwerehere82]

They are actually the same carb. made in different years.

This being said, there could be drastic differences in jetting and adjustments on the two carbs. The only way to know what the jetting is would be to pull them apart and read the numbers on the jets and measure the diameter of the metering rods.

 

[MMM]

I have the Rochchester books at home that specify what metering rods and jets the carbs originally came with. I will try to research the information tonight. Just one other difference came to mind. From 1967 until 1974, the idle circuits in Q-jets got leaner to meet the continually tightening federal standards. Richened up some after 1975, when cat converters were installed. Really got lean in the early '70's. Doug Roe's book on Rochester Carbs (at least the original version) had a very good section about the problems with the lean idle circuit and some suggested fixes for off-road use. Lean idle circuits and off-idle transition circuits are the main reason for off-idle lean stumbles with Q-Jets. Stock 1970 Q-jets for 442 W-30's had bleed holes drilled thru the primary throttle plates to help solve this problem.

Mike

 

[motorman]

Can someone tell me what the differences between 2 Qjets is? For example, is there anything mechanically different between 7029215 and 7028209, they are both for 427/390 with M/T, one is for 68 the other 69.

both use a 45 pri metering rod,71 pri jet, secondary metering rod "AX"

 

[matthud]

both use a 45 pri metering rod,71 pri jet, secondary metering rod "AX"

So that means that in orinigal condition the carbs are exactly the same?

 

[MMM]

My books agree with what Motorman published for jet and rod sizes. However that does not mean the carbs are identical. The references I used are Rochester service bulletins, both dated November 1971, Bulletin 9C-3032 for the 7028209 and Bulletin 9C-3040 for the 7029215. The following major parts are shown to differ between the two carbs: Throttle body, Fuel bowl (main body) and the Air Horn plus numerous smaller parts. This is what I expected based on my earlier remark. These parts contain the idle circuits (tubes and bleeds) and the off-idle transition circuits. I suspect these circuits are leaner in the '69 carb. Will it make a difference? Probably not, but the carbs are not identical.

Good luck,
Mike