Racing Fuel Systems

Holley 870 Avenger set up

I have high lighted the differences in orafice sizing between this 870 Vacuum and a typical 4781 850.

870 is 2 corner, 850 is 4 corner idle

Why the big swings in IFR, HSB? Pvcr, And power valves.

I have read that this carb has many issues and I would like to put it together so that it works for the next owner

Do you know if this is the original configuration on the Avenger ?

This is the new age of the holley junk configured by some whizbang kids working at Holley because the corporation does not care anymore what they are selling to the public. Poorly configured large cfm carb to sell cheap to unexpecting buyers.
I believe they are Straight Boosters in a 1.56 venturi. Not gonna work well in a street car.

0n the 850 4781 what series are we talking about, dash -1 all the way to -8 , and now 4781C.
I believe the earlier models were 2 corner idle with 2 power valves. Newer 4781C being 4 corner idle.

According to Tuner, the 80870 and its sister, 80531(850) vac sec carbs were derived from the 3418-1 855 used originally on the early L88 BBC. He suggested returning the calibration of those carbs to that of the 3418-1. I did this several years ago with great results. Changes were: Pmain air bleed .028, Prim and Sec PVCR to .070, Piab to .076, white cam, plain color vac spring. In my application, I used a 9.5 primary and 7.5 secondary power valves, and added a .077 Tslot restriction. Interesting, that the calibration is close to that of 4781. Hope it helps.

The numbers for the 80780 are the stock numbers. The carb has downleg boosters and is the 80780-1.

I had one of the L88 carbs and have all of the stock numbers, the funny thing is I was thinking of going this route.

I am now going to convert it to 4 corner idle and adjust the bleeds based on recommendations .

What are the thoughts of keeping the IFR at .036? Is that too much fuel? I was thinking of making them 33-34? Is it worth it??

I ran IFR of .035, but 2 corner idle. I've never ran a 4 corner idle carb.

The IFR is NOT sized for idle fuel. There is always enough fuel for idle. The IFR is sized for the T-slot, which covers the step to the main system & low speed cruise.

Aftermarket Holley carbs suffer the following dilemma as opposed to factory Quadrajet, Carter (both factory and aftermarket) and some of the factory Holley carbs.
Some of the Chevy OEM Holley carbs had a lower air bleed in the main venturi . Some of the Chrysler OEM Holley carbs used a separate idle jet and transition slot jet, referred to as the first 3 circuit Holley carb.


On an aftermarket Holley carb.
Delivery of fuel and air mixture thru the idle jet, at say 8” of vacuum idle speed, is considerably less than delivery of fuel and air mixture at cruise when vacuum is say 14” .
This is why idle mixture screws can become insensitive at idle with big cams and or tight LSA.

When we increase the idle fuel restrictor the cruise fuel becomes too rich thru the t-slot.

Sometimes the easiest fix is to increase idle timing to about 20 degrees or so, to increase idle vacuum.
This helps pull more fuel and air mixture thru the idle jet at idle speed and balance towards the mixture at cruise speed. “GTO Geoff” knows all about this. Of course you will have to restrict the mechanical advance then. More valve timing (duration) more idle timing.

Another solution is install a transition slot restrictor in the main body to balance the fuel delivery from idle to cruise speed.
The idle air bleed comes into play here also.
Remember that when we adjust idle mixture screws out richer it will effect both the idle and cruise AFR.

Using the smallest IFR ( idle jet ) you can get away with is always preferable but not always possible with a big cam and or tight LSA.

Can anyone add to this?

New spreadsheet shows original, revised and the L88 vacuum

Should I add a TSR ?

Rg,
I think you covered it very well.

The only thing I could add is about idle mixture screw sensitivity with low vac/ tight LSA cams. Both the result of more cam overlap, which in turn requires a higher idle rpm to keep the engine running & some will just screw in the idle speed screw to raise the rpm. Now the engine is getting all the idle fuel it needs fuel from the exposed T slot; mixture screws make little or no difference. Correct fix is to [a] ensure the engine has max ign timing at idle [ up to 55 deg. yes 55*!!! ]. [ b] Then add bypass air to reset the pri t/blades so that a 'square' [ or less ] of T slot shows below the blades at idle. The mixture screws will now be responsive! Must be done in this ^ order.