SprintCarDriver

Rebuilt the Holley and put it on the car. I dry adjusted the carb according to the Holley specs. I readjusted once on the engine. Floats are just a tad low(lean on the fender and the level gets to the bottom of the threads. Timing is @ 10 at 1200 rpm with the vacum hose disconnected.

I still have a stumble off idle. Adjusted the acc pump until I was blue in the face. Removed the secondary acc pump arm to determine if this might be the cause. Nope.
Still fiddling around I discovered that if I cover the front emulsion tube the stumble went away. Anyone have a clue as to cause this? What am I missing?

I gave up when it got to be around midnight, but I need to get it running this evening so please point me in the right directions.

GWHITE75

Yesterday, I put a rebuilt 780 CFM vac sec. on my motor and might use it for a while. My primary carb is a 950 CFM HP Pro Series 4150 and the car runs good with it. The vacuum secondary 780 has #72 jets all around and an 8.5 power valve. I set the fuel levels lower than the site hole initially.

My motor (454) is set up with 17* initial advance (no vacuum advance) and idles at 850, but can go lower. I pull over 15hgs of vacuum at idle. My cam is a turbo baseline cam and idles like a stock motor.

Tonight, I will up the jets to see if it will take out the stumble, as I suspect it will. I will let you know.

Did you ever mention if the 750 was a double pumper or vac secondary. What are the specs on your cam? If you can only get down to 1200 rpms at idle, then vacuum is probably lower and will affect the carb. The air and fuel passages are different sizes in the metering blocks and body, depending on the type of carb you bought. If its vac secondary, the idle circuit and off-idle enrichment circuits are made for a mildly modified motors and will need bigger jets to allow better flow from idle through WOT. If its a double pumper, those circuits are ajusted differently and off-idle enrichment circuits are bigger.

Knowing where your vacuum is at idle and cruise, is necessary to set up any carb on a modified motor. If you could let us know what cam you are running and the type of 750: double pumper or vac. secondary, you might get a few more threads with input. Also, Holley has a tech line, at least they used to, and could offer suggestions for your particular engine.

SprintCarDriver

I have the following set up:
Comp cam, dont have the specs here.
HB 11.5 pistons
Manley 6.5 H rods
4150 Holley double pumper, 6.5 power valve, 62 primary jets.

I am running 1200 rpm showing 14in of vacumn. I can get it lower but that is in park.
I just remebered that autos are adjusted in drive. Opps :crazy: Change that tonight.
Your mention of jets perked my intrest as I changed my jets prior to rebuild as was running too rich. Went to 62. These may be too small.
Thanks for reminding me about the holley tech line. Keep forgetting that... For some reason, I feel that the information that I receive here is better than a tech line. I trust everyone here. Maybe that is worng :lol:
Thanks

SprintCarDriver

Here is what I found out last night.
Timing was dead on.
Could not get the idle below 800 in drive.
Vacumn was 8 - 10 inches.
Took it out on the road..
Crusing was 12 - 15 inches.
Mild accleration Vacumn droped to 5 - 7 inches.
Hard accleration vacumn droped to 0 Zero, Nada... (Can this be true?)

I have a 6.5 power valve in the carb. According to what I have read and been told, I need a power valve 1 - 1.5 inches lower than my crusing reading. This would mean that I need a 10.5 - 11.5 power valve.

This should explain my major stumble, Yes?

norvalwilhelm

I would stick with the 6.5 power valve. Your idle vacuum is only 8 so anything above this would cause the car to flood at idle. Main jets don't effect off idle. Transfer slots do. Could you have a plugged transfer slot? How about adjusting the idle a little rich to compensate for now? Your idle screws should be out 1 1/2 turns from bottomed. Screw them in a extra 1/2 turn richening the mixture and see if this helps the off idle. A larger squirter also gives a better off idle. If you open up the squirter after a certain size the screw through the middle needs to be special to pass more fuel. You can easily check the accelerator pump for proper adjustment by removing the air cleaner and watching it. A slight throttle movement should produce a squirt. No hesitation.
Good luck

SprintCarDriver

Tried adjusting the mixture to 1 1/2 and made no difference... Any less and the car just wouldnt run. Seems to like 2 1/2 turns.
Pulled the carb of and changed the acc pump and went to a 3.0 power valve.
No Change.
Pulled the carb off and changed the power valve to 10.0. The off idle stumble was gone, but the crusing stumble got worse. Seems it surges during crusing now.
What recommendations would you all have on timing. I am currently running 10 @ 800 rpm in drive. Cant remember total, will check that tomorrow as the Corvettee is getting a new exhaust system today...
Thanks

Chuck Harmon

SprintCarDriver,

Try setting your accelerator pump lever close to zero clearance, this made a big difference on mine when I went from zero to .020". I changed it right back!

Chuck


[Modified by Chuck Harmon, 11:29 AM 2/20/2002]

norvalwilhelm

Your carb should idle at 1 1/2 turns or something else is feeding it fuel. The 3 pound power valve will not work but for checking it is ok. The 10 pound just causes a very rich idle if you only have 8 pounds. Your throttles could be open too much causing the transferr slots to add fuel. If you close the throttle plates the idle goes too low so how about drill .100 holes in the throttle plates to admit air and at the same time alow you to close the throttles. Only throttles with air adjustment screws are to be drilled and the hole it drill from the bottom of course but in the side that causes the carb to open. It is easy if the carb is apart because you drill from the top and it closes the throttle. If you drill slowly you can hold the throttles closed while drillimg. All my carbs are drilled. Both my 850 double pumpers are drilled in all 4 throttle plates.
This helps bring the idle screws into control. If they still lack control a .015 piece of wire in the idle circuit solves this problem. This wire is a holley fix not something someone made up.
If you want to go with the .015 wire I can give you further instructions. The carb is designed to run at 1 1/2 out from bottoming the mixture screw and if you are out 2 1/2 turns it is out of adjustment.
Good luck

GWHITE75

Been away, but I see you have some good advice coming from the right people. While you have been trying to solve a problem that needs a hands on approach from people like Norval or Lars, you might be able to learn more about Holley's if you purchase a reference manual. You can get them from Summit or maybe even a local speed shop ($15). It covers all the shortcomings you might encounter, especially with the engine combination you have. If the motor was stock, you would not be on here talking about it, but it's not. The manual will explain the holley carb, include pictures and talk specifically about the problems encountered with various modified motors. You will probably find they recommend a smaller CFM carb for your motor, but they also include some fixes like the one Norval has recommended. I just found my old one last night: It's called Holley Carburetors by Dave Emanuel, Volume One. Excellent reference manual.

BTW what model of 4150? Is it a classic series 0-4779-? The number is located on the frontside of the primary air horn, drivers side. I agree with Norval, in that the power valve you had should be put back in. What the 10hg did was mask one problem and create another.

SprintCarDriver

Thanks for all this great info. Got the new exhaust on, So this is what I have planned for tonight:
Reset the timming. Currently it is set @ 10. I am going for 24 in @ around 3500 with +10 with vacumn.
Put the 6.5 Power valve back in.
Drill .100 hole in the throttle plates(primary only) and try that. Should I drill in both primary plates or just the one. Removal of the carb is not an issue as I have put a zipper in on it. :D
Thanks again.

89formula

62 size primary jets sounds way too lean for that setup. try 68's in the front. will probably get rid of that surge at cruise.

SprintCarDriver

Hate to keep being a pest, but I guess I just cant do this.
Set the timing.
Drilled out the primaries to .100
Put the 6.5 power valvle back in.

Now it takes 3.5 to 4 turns to get 10 - 15 in. of vacumn.
Once this is set, Can remove all the idle screw and the idle is 1500 or higher.
Set the timing to 8 deg. and hook up the vacumn line and the timing goes skyhigh.

Seems I have made it worse.. Guide this ole man.