carb jet size
Thread Starter
Drifting
Joined: Nov 2016
Posts: 1,961
Likes: 579
From: Pittsburgh PA
carb jet size
ok maybe this is a dumb question, but how do you determine jet size on a modified engine?
so here is my specific example- I've got an L89(factory aluminum head tri power setup) As part of the rebuild engine was bored and I upgraded to a roller cam slightly more aggressive.
so i know from reading factory jets in the center carb are 64s, but most seem to think that's already too lean.
should i set the carbs up exactly like factory to start? or should i jump up a couple jets knowing my engine is modified?
obviously the easiest answer would be an A/F ratio gauge, but I dont have a bung for one, and done really want to cut into my new side pipes.
so here is my specific example- I've got an L89(factory aluminum head tri power setup) As part of the rebuild engine was bored and I upgraded to a roller cam slightly more aggressive.
so i know from reading factory jets in the center carb are 64s, but most seem to think that's already too lean.
should i set the carbs up exactly like factory to start? or should i jump up a couple jets knowing my engine is modified?
obviously the easiest answer would be an A/F ratio gauge, but I dont have a bung for one, and done really want to cut into my new side pipes.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Welll..there is no tried and true formula......BUT....if it were here, I would have 72's in it for a base line......why? Well....lets assume 3% for ethanol fuel and 6 jets sizes for the decrease in vacuum....yes, becasue of the decrease.....it will not pull as hard on the booster and therefore will need a larger jet, plus the added airflow from the cam. From there....use an Innovate A/F meter.
Jebby
Jebby
Race Director
Joined: Jun 2020
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From: north of Chicago
2025 C2 of the Year ('64-'66) Finalist - Unmodified
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all I can say is look a a couple of your spark plugs (1) middle one and (1) on any of the four corners, put them back in and go take a 10 mile highway ride and re inspect the same (2) spark plugs, if they are
white: lean bigger jets needed
blacken: rich, smaller jets needed
Hard to determine in city driving, accelerator pump adds fuel to equation
white: lean bigger jets needed
blacken: rich, smaller jets needed
Hard to determine in city driving, accelerator pump adds fuel to equation
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Roller cams of even 8-10 degrees more intake duration over their flat tappet counter part increase vacuum. If you did NOT add headers and free flowing exhaust chances are that NO jetting increase is needed. You need to get out and drive it and without some kinds of A/F ratio sensor you need to look at the spark plugs.
Thread Starter
Drifting
Joined: Nov 2016
Posts: 1,961
Likes: 579
From: Pittsburgh PA
Roller cams of even 8-10 degrees more intake duration over their flat tappet counter part increase vacuum. If you did NOT add headers and free flowing exhaust chances are that NO jetting increase is needed. You need to get out and drive it and without some kinds of A/F ratio sensor you need to look at the spark plugs.
Exhaust is stock factory setup- manifolds to side pipes
random question- what is the factory jet size in the outer carbs? I can’t find it at the moment. Is it like 74?
