467-Ratman

I originally got my rebuilt carb with 82 in the primarys and 82 secondary, then my friend who helped with the new MSD stuff install said the motor was running way to fat, burning eyes and all. He put in a set of 76 up front and the motor fell on its face. We then put in a set of 78 and seems better, but it still seems a little down on power. He told me with it running rich I will be fouling plugs? It seems to me with the MSD 6AL box it should burn all the fuel it can get? Thanks for the help and input on what other BB's are running.

norvalwilhelm

The 850 double pumper comes stock with 80's in both. The richness comes from the idle circuit and the transfer slots. If you have a decent cam you need to work on both these circuit's and the power valve.
At cruising speed the idle circuit, the transfer slots and the main jet are all contributing to the mixture not just the jets.

467-Ratman

Is it as simple as just adjusting in the four idle circut screws? My Crane cam has 550 and 570 for lift and 236 and 246 for duration at .050, its got a pretty good lump to the idle. How do you adjust the transfer slots?

clem zahrobsky

the first thing to try is to reduce the area of the idle fuel restrictions. then next increase the area of the power valve restrictions while reducing the area of the jets. if you have more than .020 of the idle trasfer slot showing at idle you need to drill the butterflys. these are the reasons that double pumpers are not made for the street. there are several good books out there that tells how to modify holley carbs.:chevy


[Modified by clem zahrobsky, 9:44 PM 5/14/2003]

norvalwilhelm

I always drill the butterflies. With the 4 corner idle circuits drill all 4 with about .100 hole. I also put .015 wires in the idle circuit. The idle screws should be out 1 1/2 turns. This modification of the butterflies allows you to close the throttles and cover up the transfer slots.
These modifications are recommended by holley and they even sell drilled throttle plates but you can do your own.

chewy

Make sure it isn't something simple like the idle vacuum is too low for the power valves. Most dual plane manifolds with 850s like 78 or so jets a single plane will need 82s or bigger.

Brettmc

I have an 850dp on a 468. I don't think my cam is quite as big as yours. I'm pullin down bout 15" of vacuum at idle. This is why I didn't mess with the power valve since all Holleys come with a 65. I didn't mess with my jets either since after I got the idle right, it seemed to run just fine. The idle took a while to get right tho. I adjusted ALL FOUR idle transition slots so that there was .020" exposed. When I fired her up, she idled at like 1800 rpm. So I gradually cranked down the idle by exposing less and less of ALL FOUR of the transition slots. Now, I have it idling at 650. At this idle, I have about .008" of each of the FOUR idle transition slots exposed. All this was done with the idle mixture screws out 1.5 turns. After I got the slots right, I adjusted the idle mixture by adjusting them for max vacuum at idle. When I was finished, I had picked up an extra 1.5" of vacuum at idle by adjusting the idle mixture. Now, it no longer stumbles off idle, putt-putt when I take off from a stop, nor have any more stumble when I punch it from mid-rpms (no eye-burning either). After you get the idle right is the time to work on jetting and power valve. Lars helped me a great deal via email and also his demon and holley tuning papers were a big help.


[Modified by Brettmc, 9:31 AM 5/15/2003]

gkull

Other than the above suggestions. try running about 6-8 jet sizes different front to back. So if you went 76 put in a 82 or 83 rear. That way you have a lean cruise for the best gas mileage and clean plugs and at WOT your still in the richer max power air fuel ratio.

I have drilled throttle plates also. Clem is right about getting a holley hop up book.