'66 w/427@425HP Carburetor Jetting?

[Gene M.]

I have a shop manual that spec's staggered jetting as primary RH side as #74, primary LH side as #78, secondary RH as #82, secondary LH as #80. Are these correct? It was suggested to me to use #74 primary and #82 secondary on both. It would seem the #74 and #82 would be ok but isn't this all still too rich? What power valve no mention in shop manual. I assumed a 6.5 PV but you know what happens when you assume. Seems like everything spec'd and suggested is too big, maybe not.

[Jim D.]

Re: '66 w/427@425HP Carberator Jetting?

My source shows #70 pri. & #76 sec. with 8.5 power valves both pri. & sec. for a 3247 Holley. That's the same specs. for my 65 425HP 3124 Holley.

[Gene M.]

Re: '66 w/427@425HP Carberator Jetting?

My source shows #70 pri. & #76 sec. with 8.5 power valves both pri. & sec. for a 3247 Holley. That's the same specs. for my 65 425HP 3124 Holley.

Thank you Jim, that is more like what I would expect the jetting to be. The 8.5 PV, that is something I would have guessed wrong.

[Jack M.]

Re: '66 w/427@425HP Carberator Jetting?

Here are some good sources for Holley carb info:
Holley Information Table #1: Model, Kits, CFM, Venturis and Bores
Holley Information Table #2: Needle & Seat, Jets, Metering Blocks, Power Valve, Nozzle Size and Spring Colour

Jim's source was spot on... that data is confirmed in Table #2.
(NOTE: You can use 'CONTROL + F' to search by carb number.)

[Bob K.]

Re: '66 w/427@425HP Carberator Jetting?

The # 70 primary jet (.073) will not do the job using todays gas. Step up to a # 72 (.079). Keep the # 76 (.084) in the secondaries. Use a #25 (.025) pump discharge nozzle.This will cover up any off idle stumble. Also, check the metering block #. The 1st design stamped 4746 was changed to the 2nd design 4701 sometime before Jan 66, as indicated in the Chev parts book dated Jan 66.

[Donald A.]

Re: '66 w/427@425HP Carberator Jetting?

The # 70 primary jet (.073) will not do the job using todays gas. Step up to a # 72 (.079). Keep the # 76 (.084) in the secondaries. Use a #25 (.025) pump discharge nozzle.This will cover up any off idle stumble. Also, check the metering block #. The 1st design stamped 4746 was changed to the 2nd design 4701 sometime before Jan 66, as indicated in the Chev parts book dated Jan 66.

4901 is my understanding for front metering block, this is what is on my 1966, January, 2nd week original Holley 3247

[Bob K.]

Re: '66 w/427@425HP Carberator Jetting?

The 4901 Primary block would be on a Jan built carb. I have some notes on these carbs as we built them thru the years, but I could use any info of what Primary blocks are in carbs built prior to Jan 66.

[Samuel J.]

Re: '66 w/427@425HP Carberator Jetting?

Hi Bob,

Thanks for the great info! When you say the #70 primary jet won't do the job with today's fuel, I assume you're talking about pump gas. Will #70 primary do the job for those of us who stubbornly only put ethanol-free, high-octane (e.g. 104 Research or 98 R+M/2) fuel into their L-72s?

Sam

[Bob K.]

Re: '66 w/427@425HP Carberator Jetting?

I jet all of the 780 cfm carbs with this same combo, regardless of the fuel.

[Samuel J.]

Re: '66 w/427@425HP Carberator Jetting?

And speaking of jetting and air/fuel mixtures, what test method(s) do you recommend to determine if the carb is running optimally?

[Jim D.]

Re: '66 w/427@425HP Carberator Jetting?

And speaking of jetting and air/fuel mixtures, what test method(s) do you recommend to determine if the carb is running optimally?

There is only one way and that's what I do with every car I build - use a wide band reader and record your A/F ratios under all conditions. Anything less is nothing more than a SWAG. All the previous posts, including mine, are only a starting point in tuning the carb.

[Bob K.]

Re: '66 w/427@425HP Carberator Jetting?

If you can find someone in your area with an exhaust analyzer , that would be your best bet. Reading plugs if you know how. With over 40 years of being in the business, I have experienced most typical problems, and learned what works best for most applications. All my carbs are run on test engines. I have used exhaust analyzers, but know the way that a carb adjusts and engine sound work for me.

[Carl N.]

Re: '66 w/427@425HP Carberator Jetting?

Bob dialed in my 3124 about 10 years ago and it has been trouble free. Car is driven about 2,500 miles a year on pump gas.

[Samuel J.]

Re: '66 w/427@425HP Carberator Jetting?

Very helpful! In order to truly optimize the jetting using an exhaust analyzer, do you recommend that it be done on a chassis dyno (to simulate load) or can you get the same results revving the car in neutral?

Thanks in advance!

[Jim D.]

Re: '66 w/427@425HP Carberator Jetting?

Very helpful! In order to truly optimize the jetting using an exhaust analyzer, do you recommend that it be done on a chassis dyno (to simulate load) or can you get the same results revving the car in neutral?

Thanks in advance!

Neither. It needs to be done on a finished car driving it under all conditions. The readings should be recorded at idle, cruising in town and on the freeway, full throttle, moderate acceleration from cruising etc. Those readings will tell you if and where changes need to be done.