L78 Carb Jetting

[James W.]

Can someone give me information on what sizes the primary and secondary jets used in the Holley carb that was installed on a 1965 Corvette with the L78 engine?

Are there better jetting sizes that can be installed for "normal" city/highway driving?



Thanks,

James West
Omaha, NE.

[Wayne M.]

Re: L78 Carb Jetting

James --- Bob Kunz gave me this info in 1992 for the List 3124. Primary jets #70; secondaries #76; power valves, both 85's. Now the '65 Chev interim shop manual calls for the primaries @ #72 (not 70).

Can't offer advice on those best for "normal" driving.

[Carl N.]

Re: L78 Carb Jetting

James,

In the Austin Tx area the elevation is about 500' above sea level - I run #72's front and #76 rear - this combination may prove to be a bit on the rich side - but with todays gas I found this combo works best for my driving style. This is with stock L78 heads, cam, TI Ignition, intake and exhaust manifolds. Only deviation from stock spec is the valve train (studs, arms, push rods, keepers, and seats)

I would suggest starting with what Wayne shows or even #68's in front and #74's on the secondaries and work up (or down) from there.


Always replace the power valve when you tear it down and be sure you have 0.015" clearance between the actuating lever and the fuel pump rocker lever at WOT.


I also leave the small filters out of the fuel bowls - IMO with todays gas they tend to "load" up if you are only going thru a tank of gas every 6 weeks or so.

[Ray K.]

Re: L78 Carb Jetting

James

The original Holley #3124 jet sizes were as Wayne has stated -- Primary #70 and Secondary #76. The 1965 Interim Shop Manual does indeed reference a #72 primary jet and #76 secondary. Carl has a lot of experience with the L78 engine and so his selection of #72 / #76 should work very well. Higher altitudes may respond to smaller jet sizes as he indicated.

Ray

[Michael H.]

Re: L78 Carb Jetting

James --- Bob Kunz gave me this info in 1992 for the List 3124. Primary jets #70; secondaries #76; power valves, both 85's. Now the '65 Chev interim shop manual calls for the primaries @ #72 (not 70).

Can't offer advice on those best for "normal" driving.

Wayne,

I think the interim shop manual refers to the 425 HP passenger car engine which was to have #72 primary jets. Pretty sure all #3124 carburetors for Corvette are supposed to have #70 primary jets.

For non ethanol fuel, #70's would be best but #72's would be ok for fuel with ethanol.

[Clem Z.]

Re: L78 Carb Jetting

Wayne,

I think the interim shop manual refers to the 425 HP passenger car engine which was to have #72 primary jets. Pretty sure all #3124 carburetors for Corvette are supposed to have #70 primary jets.

For non ethanol fuel, #70's would be best but #72's would be ok for fuel with ethanol.

the holley parts manual says 70 so they were shipped from holley with 70

[Michael H.]

Re: L78 Carb Jetting

the holley parts manual says 70 so they were shipped from holley with 70

Yup, I just looked it up to be sure also. The #3124 carburetor for Corvette is #70 primary and #76 secondary.

The #3130 carburetor for the same 425 HP passenger car engine is #72 primary and #76 secondary.

[Wayne M.]

Re: L78 Carb Jetting

....I think the interim shop manual refers to the 425 HP passenger car engine which was to have #72 primary jets. Pretty sure all #3124 carburetors for Corvette are supposed to have #70 primary jets. ....

Here's a pic of what shows in my '65 Interim Manual. Seems they were calling for #72 jets for both passenger (LIST 3130) and Corvette (LIST 3124). Maybe it was a typo.

I notice that in the 1966 Chassis Chev Service Manual, the 427/425hp versions take #70 jets for Corvette, yet #73's for Chevrolet.

[Michael H.]

Re: L78 Carb Jetting

Here's a pic of what shows in my '65 Interim Manual. Seems they were calling for #72 jets for both passenger (LIST 3130) and Corvette (LIST 3124). Maybe it was a typo.

I notice that in the 1966 Chassis Chev Service Manual, the 427/425hp versions take #70 jets for Corvette, yet #73's for Chevrolet.

Wayne,

My mid 1966 printing of the parts book agrees with the Holley book.

Corvette primary jet #70 for both 65 with #3124 and 66 with #3247.

Passenger primary jet #72 for 65 with #3130 and #73 for 66 with #3246. (?)

I wonder if the 65 printing of the parts book agrees for 65?

[Wayne M.]

Re: L78 Carb Jetting

.....I wonder if the 65 printing of the parts book agrees for 65?

My '65 P&A30 (sheet revised July 1, 1965) shows Gr 3.792 '65 Corvette (396) primary (w/3868826 Holley 22R-40-70) And the passenger car does show 72. My Oct '65 printing of the '66 P&A30 agrees with what you state, ie. #73 for the '66 Pass w/Sp./H.Per (427). But no mention of the '66 Corvette.

Strangely, I can't find where the secondary jets are listed.

So maybe the printing of the '65 Interim Manual was pre-production.

Then there's that revision to early '66 Corvette Holley jetting (on the L36 ?). Tech bulletin (blue) on 2/15/66 says to correct engine surge by changing #63 jets for #65's; effective in production on carbs dated 594 or later.