Jet sizing

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63SplitWindow

63SplitWindow

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I'm in the process of rebuilding an 850 Holley with mechanical secondary's for my 340 motor. The current 770 Avenger carb has vacuum secondary's. I'm hoping to improve my 60' times with the mechanical setup. Anyway, this carb was off an L-88 427 chevy motor. It has 2 power valves (primary and secondary) and the jets are 80's for the primary and secondary. I'm thinking that's too big for the primary side. Any thoughts out there where I should start? This is primarily a track car so not looking for economy.
Thanks,
Bill
 
That’s probably not too big. If it is, it isn’t by much. I’d put it back to exactly stock and tune from there. IIRC that carb has down leg boosters so that jet size for that venturi/throttle blade size is probably pretty close.

Also, I think the MAB is pretty small on those, like .026 or .028 and that’s for a reason. If you go bigger on the MAB to try and correct a high speed rich condition you will screw up the bottom of the fuel curve.

If its fat at cruise, lean out the primary main jet and then if it happens to get lean at WOT tune that with your power valve channel restricters.
 
Blank off secondary PV since your going mechanical and hope you have an AFR to tune your jets on both sides.
 
Mims Gt said:
Blank off secondary PV since your going mechanical and hope you have an AFR to tune your jets on both sides.
Click to expand...

Holley used a PV back there to help clean up the transition from the T slot to the main jets. With a PV back there you can get fuel quicker than you can with just the main jets. It makes the tune up much cleaner.


Id think long and hard before I removed the secondary PV.
 
If you are at Sea Level = I would start with 78s and 6.5 Power
Valves or lower (less than your idle vacuum)
 
Yes Sir and be sure Power Valves are not blown especially
if your carb does not have a valve to prevent back fire protection.
 
63SplitWindow said:
For primary and secondary?
Click to expand...


Look it up in the Holley book if you can. My memory is that carb had a 10.5 PV in the secondary side.

Ahhhh, hold on. I’ll see if I can find my book and look it up for you. I would start at dead stock on everything and tune from there. I’m pretty sure the idle circuit will need some work because all those carbs were notoriously rich at idle but the rest of the calibration was pretty close.

Let me find my book.
 
Ok, I can’t find the book that has all the list numbers in it. I assumed you have the 4054 list carb. I did see the 4781 which is geometrically the same carb and it had 80’s front and rear and 6.5 power valves front and rear.

For some reason I have 10.5 in the rear for that carb but I can’t verify that exactly.

Also remember that the emulsion stack, main air bleed and nozzle (booster) type and position relative to venturi all affect when the nozzle gets fuel.

They are interrelated. Other than the idle circuit, those carbs had a pretty good tune up in them.
 
4781- what? There is 4781, and 4781-1 thru -8.
Standard 4781, stock jetting is 80 jet front and rear, 6.5 power valve f&r, 35 shooter primary, 25 shooter in the rear.
Other list number dashes had other jetting and shooters (25 rear shooter seems awfully small to me).
Later dash numbers had leaner secondary jet, and 31,31 shooters.
 
Just because the carb is going on a smaller engine....does not mean the main jets need to be smaller. They may...or they may not. A popular misconception.
 
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