Carb jetting help
Thread Starter
Instructor
Joined: Aug 2012
Posts: 237
Likes: 3
Carb jetting help
1980 L48 Auto
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
The only true indication is a dyno test. You probably couldn't tell by the seat of your pants. If you decide to change the carb, first read Lars' excellent papers on tuning these carbs. Also, make very small changes at any one time. In any case, simply changing to a dual exhaust is not going to make much difference in performance. Just do it 'cause it's cool!
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
First, make sure your distributor is doing what it should. Lars Grimsrud's papers are a great source of information.
Then, get Lars' papers on the Quadrajet, read Doug Roe's book, and also Cliff Ruggles'. Then, get yourself a dash mounted air/fuel ratio gauge and see what the carb is doing.
THEN, you might fool around with some rod and/or jet changes.
Or just ship your distributor and carb to Lars and have him set it all up for you.
Then, get Lars' papers on the Quadrajet, read Doug Roe's book, and also Cliff Ruggles'. Then, get yourself a dash mounted air/fuel ratio gauge and see what the carb is doing.
THEN, you might fool around with some rod and/or jet changes.
Or just ship your distributor and carb to Lars and have him set it all up for you.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
1980 L48 Auto
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
By making jetting changes I was able to gain .5 sec @ 1/4 runs, but the change required me to go to smaller jets not bigger ? Original jetting was to rich for better performance. I measured the AFR (air fuel ratio) to know which way to go.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
1980 L48 Auto
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
I've put true 2" dual exhaust on my 80 Vette. Everything else on the car is original including the carb (Quadrajet 17080204). A mechanic suggested that I might want to change the jets and metering rods because of the increase of air movement due to the dual exhaust. Anyone have experience with this modification and a jet change?
Lars
Thread Starter
Instructor
Joined: Aug 2012
Posts: 237
Likes: 3
Once you upgrade the exhaust to move more air with less restriction on the exhaust side, you need to bump the jetting up a tad. This is more important when changing over to headers than when just going over to dual exhaust using the stock exhaust manifolds. Normally, when going from manifolds to headers, my recommendation is to bump the jetting up 2 jet sizes. With your stock manifolds and the dual exhaust, you can probably get by with simply raising your float level to .300 and getting the APT adjustment up to the height recommended in my Q-Jet Setup paper. Make sure your carb has the correct 71/43/CH jetting in it. You can bump the jet size up to a 72 to be on the safe side along with the other 2 mods suggested.
Lars
Lars
