467-Ratman

My motor has been running rich and I really noticed it when we took off the old exhaust pipes. At the connection on the header it was very black. My setup in the engine has about 10 to 11 inches of vacuum at idle. My carb has 75 and 76 jets with two powervalves both at 5.5. The motor really sucks gas and my plugs are very black, even with the MSD 6AL box. Do I need the power valve in the back, and how about the front one, should I change it to a lower number like a 3.5 or 4.5? My four corner idle screws are set at 3/4 of a turn out on all four. Looking for suggestions! :confused: :confused:

Chuck Harmon

I don't think the ower valve is your problem. If you have vacuum at least 1-2" above the power valve number, going to a lower PV# won't help. This will only give a bit of hesitation as you move from the main stage to the power stage of the carb. I am running 70's in front and 74's in back on my 950 Holley and getting a 14.5:1 a/f. But I have removable air bleeds which may not be the same size as what you are running. The larger the air blead, the leaner the mixture.

for what it's worth on a side note, I still have a bit of roughness in the engine. Measuring the temp of the header at the flange, my #1 pipe was about 300 degree F less than the others, the plug was wet with fuel. I am hoping a new plug wire will make a difference. I will be posting about this under a new topic "Jacobs Wires are Weak".

Chuck

467-Ratman

After my Dyno run last fall, the guy changed my jets to 73 and 74. Seemed way too small for the 850 and even though the A/F was better my motor just did not like it. I am not sure what else to check, maybe its just the joy of having a wild cam and very low vacuum........

lars

467 -
Do you have the secondary throttle screw cracked open? If not, bump the secondary idle adjustment up to flow more air through the secondary side - this will lean you out and clean you up a bit.

467-Ratman

Very dumb question for you Lars, but are you talking about the back two idle screws for the adjustment or is there another one to adjust?

Chuck Harmon

467,

You have to turn the carb over to se it, but there is a small screw that adjusts the stop for the secondary throttle plates. If it is opned slightly this will lean out the mixture. I can get to mine with a self modified tool, a small alan wrench ground to be a screw driver.

To test to see if this would help, try disconnecting the pvc hose from the back of the carb. This will also create a leaner mixture by causing a vacuum leak. If the engine runs better you are probably on the right track.

If the engine speeds up with a vacuum leak it is a pretty good indication that its running a bit too rich. But it is far better that all the air entering the engine does so past the throttle plates to mix with the fuel.

Chuck

467-Ratman

Thanks for the help, I will pull the carb tonight and check it out......

lars

Chuck is right - the sec throttle adjustment screw is underneath the carb on the passenger side right by the sec throttle shaft. Crack the secondary throttles open so that the throttle blades just barely go to the lower edge of the transition slot in the throttle bore - you can even expose just a tad of the transition slot but this may get your idle speed too high. It usually takes me 3-4 times of installing, testing and removin the carb to get the idle on the secondary side set just right: If you do it correctly, both primary and secondarfy throttles should be open exactly the same amount at idle.

427Hotrod

I usually just take the screw out and insert it from the other side. that way I can play with it "real time".


JIM

The Dude

:withstupid:
Yup. Just reverse the screw, baby.

big632

I would make sure your power valves arent blown. They might be dumping fuel by them.

norvalwilhelm

I believe most of your richness is coming from your idle and transfer slots and not your jetting. I like drilling the 1/8th holes in the throttle plates to give more even air flow and close the transfer slots. If you open the back throttles and it is 4 corner idle the transfer slots in the rear are just upen allowing fuel to be sucked out of them.
I would drill the 4 holes and also install approximately .015 wire in each of the idle circuits to lean the idle mixture out and make the idle mixture screws more responsive.
Both of these modifications are recommended by HOlley in the event of a rich carb.
I don't believe it is the power valves and it is rare for a power vavle to blow with the modern check valves that holley installs.
I run 2 850 double pumpers with the above modifications and still run 80 and 82 jetting and mine is lean with the power valves.
I also get 15 mpg.

467-Ratman

I talked to the guy who sold me the carb , custom rebuilt carbs from New Jersey, last year. He did not think I needed the holes or the wires, but the more I talked to him he was a believer in carbs running richer. Sounds to me your solution really works in that you have TWO 850 carbs! Thanks for the information and help!

Brettmc

Ratman - Didnt you have someone put the wires in the idle circuit last fall?

Brett :thumbs:

norvalwilhelm

I know how to make a carb work and my tradings at the deals gets me full price for them after a year or two. I do not like motors running rich. It wastes gas, horsepower and wears engines.
Holley even recommends the modifications I outlined with wilder cams.

467-Ratman

Yes, but the guy who works on my car said I did not need them and pulled them out! Oh, well maybe its time to find a new mechanic.......

Brettmc

Or rely on yourself for your carb mods/decisions!!

Brett :thumbs:

Chuck Harmon

467,

But I thought you said you already had 5.5 PV's? If your mechanic removed the power valves, you have to increase the jets about 4-6#'s to have enough fuel for WOT. This would make the car run quite rich for normal operation. This is why the power valves are such a great idea. The manifold vacuum holds the gate closed until you floor it. Only then does it opens the gate to allow even more fuel. Extra fuel that you didn't need until you did floored it.

Comparing jet sizes from one carb to another is a bit "out to lunch". Air bleeds can be different sizes from carb to carb, so can power valve restrictions, idle feed restrictions, venturi sizes, air cleaner and exhaust restrictions, etc. But re-installing power valves would be my first change to get the car running at the proper a/f for normal driving while still maining the proper a/f for WOT.

Chuck

Fevre

Think he meant his mech took out the wires he put in the idle passeges, not the PV.

467-Ratman

Sorry for the confusion, power valves are still in and are 5.5. What he took out the wire restrictions that someone else put in! I think the first thing to try is drilling the 1/8" hole in the throttle plates. After that I will see how it works then maybe put the wires back in. Thanks for all of the help as always!!!!