Q-Jet jet size
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Joined: Apr 2007
Posts: 55
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From: Rochester New York
Q-Jet jet size
Hello. Recently (100 miles) rebuild 350 in 1980 Corvette L-82. Bored 30 over, mild cam, auto tranny. Car ran very lean, pinging upon acceleration, so I had the q-jet rebuilt (professionally). They increased jet from 72 to 73. Metering rods were in the 40s (41?). Lessened the pinging, but still there when under a load. Should I further increase the jet size? To what? And what about the metering rods? Yes, I tried retarding the timing and using colder plugs, but it still pings. Any help is greatly appreciated! JG700
Le Mans Master
Joined: Apr 2005
Posts: 6,359
Likes: 428
From: Seattle Area WA
Back to basics, can you mention what the color of your plugs were to confirm that it would indicate a lean condition. Were the plugs white-ish in color, tan, black etc?
What are your two timing values, both centrifugal and total timing? You may want your mechanical to be 36 degrees at 3,000 RPMs and restrict your total to say 45 degrees and see how it runs. In Barry's sticky, the recommendation is to try driving it with the vacumn advance disconnected and see how it runs. Then adjust from there.
Those main jets sound pretty lean to me based on what the stock size jets are in my '76 Q-jet. However I just found Lar's paper and those are your stock jets (72). "Lars has posted or written what the standard jet sizes are for the various year Q-jets." Here is the link from Barry's sticky; http://www.lbfun.com/Corvette/Tech/v...ng%20Paper.pdf I'm currently running the stock main jets in my '76 which happend to be size 76 in my Q-jet in a warmed over 350 and they could probably be 77s. The advise given to me by Lars was don't mess with the secondary metering rods until the primary circuit is dialed in.
If you haven't already done so, read the sticky at the top of the page on timing info.
What are your two timing values, both centrifugal and total timing? You may want your mechanical to be 36 degrees at 3,000 RPMs and restrict your total to say 45 degrees and see how it runs. In Barry's sticky, the recommendation is to try driving it with the vacumn advance disconnected and see how it runs. Then adjust from there.
Those main jets sound pretty lean to me based on what the stock size jets are in my '76 Q-jet. However I just found Lar's paper and those are your stock jets (72). "Lars has posted or written what the standard jet sizes are for the various year Q-jets." Here is the link from Barry's sticky; http://www.lbfun.com/Corvette/Tech/v...ng%20Paper.pdf I'm currently running the stock main jets in my '76 which happend to be size 76 in my Q-jet in a warmed over 350 and they could probably be 77s. The advise given to me by Lars was don't mess with the secondary metering rods until the primary circuit is dialed in.
If you haven't already done so, read the sticky at the top of the page on timing info.
Last edited by Scott Marzahl; Jul 13, 2007 at 09:46 PM.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
I would start with a 75/45 jet/rod combo and work from there. Make sure the rods you use are NOT truck rods (.036" power tip). Regular passenger car rods have a .026" power tip.
Also get a book and learn about this stuff, it can be tricky.
BigBlockk
Later.....
Also get a book and learn about this stuff, it can be tricky.
BigBlockk
Later.....
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
You need to get the number off the side of the carb and look up the oem size it needs.
If it truly is for an 80 a couple sizes over stock should be ample and you should be close.
If the timing is right, then I would question your professional rebuild.
Your not using that old WWII potato gas, are you?
If it truly is for an 80 a couple sizes over stock should be ample and you should be close.
If the timing is right, then I would question your professional rebuild.
Your not using that old WWII potato gas, are you?
