Running Rich Holley 750 DP

[Mark8117]

Need some advice please.

Running a 408 stoker with 340 X heads and a Air Gap intake.

Vacuum 9-10 psi while in gear just put in a new 4.5 power valve, there is not a PV in the secondary

If I turn in the mixture screws it does not stumble.  It will change the sound of the car but will not kill the motor.  It seems like I am getting gas from somewhere.  I do not have mixture screws on the rear secondaries.  (Mech secondary)
Fuel bowls are set below the windows
At idle it will have black smoke and at full throttle. 

[Strawdawg]

are the primary throttleblades set properly in relation to the transfer slots?

[Mark8117]

Steve. I have the showing just over 1/8" on the slots.

[cudabob496]

closing all mixtue screws and still running,
is a sign of a leaking power valve.

[Strawdawg]

About 0.040" is the vicinity you should be in.  If it won't idle in that range, you may have to resort to the old hole drilling trick. It will be hard to get it to respond to the needles open that far

[Mark8117]

Could I be pulling from the secondaries?

Steve- hole drilling trick?  Sounds non reversible what impact does that have both positive and negative?

[Chryco Psycho]

could be a gasket leak or a bad power valve , there has to be extra fule getting in there , the powervalve are front & rear si the first place I would look

[Strawdawg]

Could I be pulling from the secondaries?

Steve- hole drilling trick?  Sounds non reversible what impact does that have both positive and negative?

If the throttle blades are open too far by the idle speed screw, then the engine will be running more off the main circuit than the idle circuit and the carb will not respond much to the adjustment of the idle screws.   So it is always good to check both the primary and secondary  throttle blades relationship to the transfer slots.  I usually try to set them so that about 0.040" of the slot is showing below the blades altho 0.030 to 0.060" should be fine.  See the attached drawing.

If the idle speed screw must be turned in so far to get the desired idle speed that more than say 0.060" is showing on the slot, then the trick is to drill a 3/32" hole in each blade so that air goes thru these holes instead of having to open the blades too far to get the idle up.

If you set the blades in range and the idle speed is good, then no holes are needed.  The needles should then be responsive.  If not, it sounds like a leak as Chryco suggested.  Double pumpers often run richer at idle than they may need to because they are aimed at the performance group that have bigger cams, etc in the engine which makes for a weak low speed signal.  If everything is good with no problem with the power valve or gaskets and the blades are set right, then you may have to stick a piece of fine wire into the idle feeds to restrict flow.

I have never seen one that needed an 1/8" exposed so something does not smell right

[Strawdawg]

another pic

[Mark8117]

Wow thanks for the great detail, I will see what I comeup with.   

[cudabob496]

what jet sizes you running?

when I had black smoke, it was a bad power valve, which
was damaged after a backfire.

Maybe yours is just loose, and not tightened down enough?

If pull power valve, can put it in your mouth (assuming not soaked in gasoline) and suck or blow on it.
May be able to hear air pissing through, if there is a leak. Or,
it may even be visible.  Sometimes the diaphragms look wrinkled, but
may still not be leaking.

[Strawdawg]

Mark, I am not sure I am interpreting what you wrote correctly.  did this problem start after you installed the new power valve, or did you have the problem before?  I took it to mean a new pv did not fix the problem.

If it started after the new power valve, then it is almost certain to  have something to do with that act.  Take the valve out, suck on the back of it and the plunger on the other side should retract.  If it does not, it would seem to have a hole in the diaphragm.   If the valve is okay, look at the gasket and be sure  it is not broken/cracked.

If that is all okay, make sure the gaskets are sealing on each side of the metering block.  Also be sure your float is not stuck and the carb is simply flooding from too much gas.

While the throttle blades should be set properly, it does not sound like this is your main problem.

[Mark8117]

Strawdawg-  Yes I still had this issue after I put in a new PV.  But, I did have a couple backfires up throught he carb and maybe popped the new PV.  I did not get to it this weekend but hope to tonight.  I believe all the gaskets Metering block and bowl are in good shape.

[Strawdawg]

yeah, it sounds like it might have popped it...newer carbs seem to have backfire protection and I think I saw someone selling a mode that can be installed on older carbs but I may have dreamed that part

[Chryco Psycho]

Most new carbs have a check ball to protect the PV & they really do not pop easily .