[markzepp820]

I have a 79' chevy truck with a 350. The motor is pretty much stock, it has an aftermarket performer intake, headers and a holley 600. I think the stock jet size in this holley is 66. What smaller jet size would be good for my 350?
Also, what are the benefits of reducing the jet size?
thanks

[aaronls41]

why are you wanting to reduce jet size???running rich,,dead spot,,what???going to smaller jetting sometimes make things worse...take a look at the plugs and tail pipes that should tell you which way to start...A 600 on a 350 isnot over kill so you might be close....doc41

[F-BIRD'88]

On common holley 600cfm vac sec carbs (eg:#1850) the useable primary jetting range is from #60 to #70 jets. You can experiment with smaller primary jets. Try a 60 jet. If the truck surges or pings at part throttle at 30 to 70MPH, the jetting is too lean.
Move up in jetting till the surgeing at part throttle just dissapears. (driveability returns) Usually a 62-64 jet is good. Then you can restore the WOT max power AFR by upping the secondary jetting.
Usually you can rob the sec metering plate #21 out of a holley #3310 to good effect. Equals a #76sec jet. Try a lighter, purple sec vac spring.
If after rejetting the primary side the throttle response is worse,yet it doesn't surge at steady cruise (30 to 70mph), up the accelerator pump shooter to a .028 or .031" shooter.
The primary jet size has little or no effect on the idle and low speed cruise (below 25MPH) This is controled by the idle circuit and is not easily adjusted.
If you find that the thing seems "too rich" overall during the summer you can block off or restrict the exhaust heat riser passages in the heads to cool the intake plenum. I find the Performer manifold runs too hot overall in the summer when the exhaust risers are not restricted or blocked. The hotter the plenum is heated the richer the effective jetting will be at cruise and idle. Too hot causes vapourlock, hard hot starting and fuel perculation. A cooler plenum makes the jet size effectively leaner and is a little more volumetricly efficient but a warmer manifold is better for vapourizing fuel. You have to find the best happy medium of jetting and plenum heat for best over all power and driveability.
I'd try blocking off one side of the heat risers and restrict the other side. (1/4 to 3/8" hole) then get a Holley carb heat shield to protect the fuel bowls from radiated heat from the manifold runners. It works. this will effectively lean the jetting and add some power. Then, start working with the jets. A 62 to 64 primary jet and #21sec plate may help things.

[markzepp820]

thank you for the info.