Hi All, I have been reviewing possible causes of poor idle (won't idle below 1,200) and my first diagnosis as overly rich condition on my 71 LT-1 with the stock Holley 4801 and 40709 fuel pump. This is the standard two line pump without a return line. The carb was professionally restored/rebuilt 3 years ago and not installed since. The fuel pump was purchased as a rebuilt original Delco 40709 and also just installed.
I have removed the carb and will go through it on the bench but I decided to run a pressure test on the fuel pump first. At the distribution block, I plugged one feed line and attached the pressure gauge to the second feed line. I was surprised to see 9 PSI when cranking the engine over expecting 5.5-6.5 PSI. The surprising thing to me is most all of the new 40709 replacement pumps I have checked specify 9 PSI as the rated maximum pressure (GM service manual specs it a 8.5 PSI). This seems strange that GM would have spec'd a 9 PSI pump with the Holley carb. Holley recommends 6.5 PSI maximum on their new double pumpers.
Do I really have a problem with the fuel pump or am I chasing my tail? Any thoughts would be appreciated.

 

That's the pressure reading with the float bowl needle and seat blocking all flow since the engine is not running, dead head pressure. You can't go by that.
If pressure was your problem you would see gasoline overpowering the needle and seat and dripping into throat while running.
Sounds like a vacuum leak, or fast idle cam not dropping, or throttle not closing for another reason.

 

After I ran the test I started to question if was going to give an accurate reading. I will rerun the test once the carb is reinstalled and running. It is still odd that the spec for the pump is that high factory matched to that Holley carb. I found gas pooled in the intake under the carb when I removed it so I still suspect an overly rich condition.

 

Did you adjust the float levels with the engine running?
I believe you said the carburetor was unused for a couple of years, did you tighten all the float bowl screws?

 

I did a visual inspection, checked the bowl bolts and checked the choke and choke pull down diaphragm. What I didn't do was to check the float level while running or properly set the idle screw which was turned in enough to expose all of the transfer slots in the primaries. From what I read is this nullifies the idle mixture screws from working which I experienced while attempting it to idle at 800 rpm. All of my past Corvettes have had Rochester carbs so the Holley setup is new to me. I ordered a carb rebuild kit and plan on going through it completely.

 

Gas pooled in the intake seems to indicate dripping. The extra air (throttle) is trying to compensate for that.
Set the fuel level, just barely dripping out of solid screw ports. Fuel inlet valve could have dirt in it. If it does the level won't stay there.
Set the transfer slots to square (.040') front & rear, and idle screws 1-1/2 turns out. Rear transfer slot is set with carb upside down.
Plug all your vac line ports to the manifold, including the PCV.
Then it should start & idle.
If still no, post a close up pic of your outer air bleeds. From both sides of the choke horn.

 

Don’t know if you ever read the posts from Lars on the Forum? He is by far the expert on everything Q-jets carbs, hands down. Last year I wanted him to rebuild my original Q-jet carb, that would be installed a year or so latter. Lars immediately responded that since he runs / test all his rebuilds on a live engine, leaving the carb sitting on the shelf with gas in it would be a disaster. Do you know if your carb rebuilder tested the carb with gas, if it has been sitting, there may be one of your problems.

 

Thanks for the comments guys. I went ahead and disassembled the carb for inspection while waiting on the rebuild kit. Overall very clean and all of the jets, power valves etc match the specs. There was just a small amount of a white power in the bottom of the bowls and on the needles. I ran a wire through all of the orifices and fount no issues. The primary throttle plate was open enough to expose all of the transfer slot and the secondary plate was completely closed so none of the secondary transfer slots are exposed.
One interesting observation about the air bleed holes you mentioned. I noticed there are no brass inserts on the outer air bleeds in the primary bore like all of the other carbs I see. However I found an old post on CF where a member observed the same on the 4801. Turns out for emissions specs, adjustment screws were installed then epoxy sealed at the factory. The screws are under the large round body to air cleaner gasket.
One question however; If both throttle plates are adjusted to expose .040 of the transfer slots and the idle mixture screws are then adjusted properly, then how is the idle speed adjusted? Seems like any change to the idle speed screw will mess up the transfer slot adjustments made earlier?

 

True. But that should get you close. If you have to make a significant change in the idle speed via the primary screw, you may need to flip the carb again and balance the slots front to rear. Since your rear slot was completely closed, and the front slot was adjusted past the slot, that was likely your problem.

And yes you have adjustable air bleeds! You have an OEM factory installed Holley for the emission era. If those idle air bleed screws have moved over the years, you will have he** to pay trying to get the carb balanced or run well. BTDT. Mine was all fubared. Try it the normal way above first, and we'll hope for the best. There is no spec anywhere for those idle air screws. They were adjusted by for each engine at the factory and then sealed "for life". If they are off and need adjusted you will likely need an expert with a flow bench and an AFR meter to get them set again. Adjusting them changes both the AFR of the transition slot and it's timing or length. Misadjustment can give you uneven mixture from side to side and/or some strange bogs/misses during transition slot driving or during the transition to main jets. Let us hope you don't have to go there. Mine took a long time to get close to correct. I even sent it to Holley. They didn't even know what I was talking about until they saw it and checked the original blue-prints. They did not get it perfect, but at least they got it closer. It takes a real expert to tune air bleeds, and these are not done in the normal way. I "fixed" mine some 40 years ago. It should be much easier today with all the AFR meter technology.

 

The Chevrolet overhaul manual states about a 1/2 turn from full closed for the secondary throttle. That transfer slot adjustment is a preliminary setting for the primary. You use the idle speed screw to set the idle and make sure to adjust float level with engine running before setting the final idle speed.