Hi all, I have a problem with 750 VS Holley (model 4160, P/N 80508S). It goes very lean on light acceleration in traffic and when going uphill. A/F gauge is showing 0-0,1V. When I back up throttle, it goes back to the 0.7-0.8V as while cruising at steady speed.

Before that issue, I changed primary jets from 70 to 66 because I was running very rich (fouled the plugs). After that I had off-idle hesistation. Fixed it by putting blue acceleration pump cam in hole #2. Now car is running OK, exept little off-idle hesistation sometimes and very lean condition at light acceleration. When I open the throttle more (until PV opens), car acceöerates OK and A/F gauge is showing 0.7-0.8V

I was thinking to increase primaries, but then I will go back to very rich (as I told, A/F gauge is showing now 0,8V at steady cruising speed) condition at steady cruising speed.

I was thinking also about using different power valve (right now I have 6.5 PV), but it couldn't be issue, because in my opinion PV shouldn't open at light acceleration or when going uphill.

Also it could not be issue of the accelerator pump, because accelerator pump works short time and doesn't help with steady light acceleration.

Please give me some ideas, what to try next and where I'm wrong.

I have cammed engine, with CC 224/230 cam, Edelbrock RPM intake and AFR 190 heads.

 

Is your idle mixture a little rich?

 

Check idle mixture screw setting, and fuel pressure (possible too high).

 

Idle is not so rich. It's little bit on rich side because of not dropping too much rpm, when shifting in gear. Otherwise engine will stall. Idle screws are opened something about 1.5 turns or so.

Haven't measured my fuel pressure because I have Carter electric fuel pump that should give 6-8psi fuel pressure. I will measure fuel pressure. How does it affect fuel mixture at light load? Higher pressure should give rich condition or what?

 

You are running into a situation where the idle mix screws and tansition slots are starting to phase out and and the main metering system is kicking in. Problem is you have too much carb or not enough engine. Try opening the idle mixture screws about a 1/4 turn more to give it a little more fuel. Short of that put a 1" 4 hole space under the carb to try to strengthen the pull on the venturi. A smaller carb would really be the ideal situation. Finally I am guessing you are probably getting about 13-15 in/hg of vacuum at idle. You could go to a 8# power valve to help out. The power valves purpose after all is for high load, low RPM operation.

http://www.hotrod.com/techarticles/588/

 

Increase the pri main jet size a few. a 66 is quite lean for a 750. lean random misfireing will show up on a AFR meter as very lean. 70's are not too rich on a 750.
Not near rich enough to foul plugs.
A 68-71 should be right.

Go back and check fuel level in the bowl, correct throttle blade, transition slot exposure ( on both pri and sec barrels)
Are u using a PCV system? Without it hooked up and functioning, the carbs idle and low speed circuit will be out of wack.
Any carb that has the 3/8" PCV port in the base is made to work right with a functioning PCV. If ya don't use it the idle and off idle is all out of wack. PCV is your friend....

 

Guys, thanks for your replys and advices!

Fast355, you are quite close about vacuum. I have considered changing PV, but I'm afraid, that it will open at idle, because my idle vacuum is quite low (about 9-10" Hg @700-800 rpm). But I'll try another PV.

F-BIRD'88, it really fouled the plugs with 70 jets, badly. I'll try to step up to 68. PCV system is not connected beacuse I didn't find pPCV fitting. According to the manual:
http://www.holley.com/data/Products/...199R9934-3.pdf
I have to connect it, but it isn't shown on any pictures, where PCV fitting is. Fuel level was correct, when I checked last time. May check it again. I haven't checked transition slots exposure, but I'm thinking that they are near limit, because I tried to open them as little as possible, when I was adjusting idle. Will check them.

One more detail. When fuel mixture went lean at light load, I have tried to shift from 3rd gear to 2nd and lean condition didn't go away despite of rising rpms.

 

See the pic PCV is your friend. This part of the reason why you think the 70's jets were responsable for the fouled plugs. A 70's jet in a 750 will not ever foul the plugs.

Be sure that when you hook up the PCv to the valve cover that you vent the other valve cover. It is supposed to breath through the crank case ( Positive ventalation) not create a vacuum in the crank case.
I would try a 4.5" power valve.

 

Woops

 

Connecting the brake booster and PCV to the same port probably isn't the best idea, if you can avoid it.