For the most part , this Holley 390 has done wonders for my truck. Fine tuning it has been a learning experience since it is nothing like the old carb the truck came with. After adjusting the fuel / air mixture on the Holley 390, performance improved remarkably. However, there are 2 symptoms that have me scratching my head.

Symptom #1 When I start the vehicle, the RPM's remain high for quiet some time. I have to "kick" the accelerator several times to finally get the truck back down to curb idle. Is this the nature of an electric choke?

Symptom #2 During acceleration immediately after upshifting, the truck hesitates and then picks up. I've adjusted the fuel/air mixture up and down and all around in an attempt to correct the problem with no luck. Can anyone offer any advice?

 

Try adjusting the accelerator pump screw or changing the position of the a/p cam. You can also get an accelerator pump cam assortment. Most Holley manuals suggest that heavier vehicles repond (better) to a longer "pump shot" which can be achieved by reducing the size of the pump "shooter" - in tech terms the "accelerator pump discharge nozzle" - from which gas is shot into the venturis as the accel. pump/ linkage is actuated.

The cam determines the overall size and timing of the shot, while a smaller nozzle will extend the duration of the shot.

 

Jerry,
I'm by no means an expert on this but:

1. There is a adjustment for the high speed idle. It is accesible from the rear of the carb and is a pain to get at. It is behind the electric choke. Mine was set for 2k rpms or something close to that.

2. I have the same stumble. It cleared up a bit by changing the squirter size up to a 31. I may go to a even larger size and see if it goes away. I dont think changing the position on the pump cam will help because it is an immediate hesitation.

Hope this helps some. Good luck!

Ron

 

The high idle screw is a pain to get to. Have fun with that one.

Can we get some more info on symptom number 2? You say it hesitates after upshifting. Does it fix itself or do you have to pump the throttle to make it go? How does it respond when you floor it while idling along in gear? There are lots of things that can cause a tip in stumble. The thing most often adjusted is the acelerator pump, but sometimes all that does is cover up a problem caused somewhere else.

Before you can fix it you have to determine if it hesitates because of too much pump, or not enough, or if it's even a carburetion problem. The sudden vacuum drop when you get back in the throttle could be allowing the power valve to open and make it rich for a second or two when it really doesn't need to be.

If the primaries are open too far at idle the carb won't transition smoothly into main metering. This is fixed by adjusting the secondary opening at idle, which is a PITA just like the high idle adjustment, or by drilling small holes in the primary blades. Drilling holes in the blades should be a last resort because it can't be easily reversed if it doesn't help.

If the secondaries open too soon they will cause a stumble, so make sure they don't start to open unless you want them open. The 390 is a pretty small carb, so it doesn't take much airflow to open up the secondaries. If the spring is too light it is very possible that they will open when you're just revving the engine while stopped, but you can't feel it until there is a significant load on the engine.

When I set up a carb the accelerator pump is usually the last thing I adjust because I want everything else to function properly without needing the accelerator pump to crutch it.

 

The hesitation after upshifting is momentary and picks up on it's own without me having to pump the throttle to make it go. When cruising on the highway, the accelerator response is great. No hesitation at all.....only when upshifting. The Holley tech told me the Holly 4160 (390 cfm) comes stock with a size .025" accelerator discharge nozzle (shooter) and recommended I increase the shooter size to a .031"
When I adjusted the primaries at idle I did not adjust the secondary idle screw because it wasn't readily accessible at the time. This adjustment requires removing the carburetor from the intake manifold to access the adjustment screw on the bottom of the carb. Too much of a PITA at the time and I was too eager to get my truck up and running. But if this adjustment will solve my hesitation problem, I'd better go back and do it right.
I would like to delay the secondaries opening. Changing the springs in the vacuum secondary diaphragm should do the trick. Which spring should I use so that the secondaries open between 70 and 75 mph?
I'm not sure I like the electric choke on this carb. Do you run your vehicle with an electric choke or have you switched to a manual.

 

Your description of the problem sounds like a problem with the transition to main metering. The accelerator pump would likely cause a problem any time you hit the gas, not just in one gear. Also, when a hesitation fixes itself it often indicates that the hesitation is due to a rich condition because it fixes itself when the extra fuel is gone. When it's lean the engine just lays down unless you pump the throttle a few times to get fuel in the engine, although a slight lean condition could also fix itself.

 

I increased the accelerator pump discharge nozzle (shooter) from a size .025 to a .031 with no luck. The hesitation is still there so the solution to the problem lies elsewhere. I pulled a spark plug out and it was black and sooty so I am running rich. Occasionally when I turn the engine off it will "diesel" or "run on" but not always.

Can anyone tell me the symptoms of a blown power valve?

 

I swapped back down to the .25 squirter just for *****s & giggles. The hesitation was way worse. I'm going to go up a notch from the .31 and see what happens. I'm running 48 jets in the primaries. On the secondary adjustment get a coat hanger and fabricate a screwdriver for the adjustment. Really it should be a half turn past seated. I dont think this is your problem I still think it has to do with the pump shot or maybe timing????

 

Ok, went from a .31 squirter to a .35 and the problem is gone but.....check your float level. Mine was low and I changed that at the same time so I'm not sure which cleared up the problem. Runs fine now Later!!

Ron

 

I increased the accelerator discharge nozzle (shooter) again from a stock .025 to a .031 and now to a .035 shooter. The hesitation is still there but not as bad. I advanced the ignition timing from 6 btdc to 10 btdc and it seems to run better. Air/fuel mixture screws are approximately 1 turn out from seated at highest intake manifold vacuum of 17 hg. Spark plugs are no longer fouled and look good. The hesitation is almost gone but not completely. The Holly Tech rep suggested I reduce the jet size from a #51 to #50, so I did. The altitude here is 1300 ft. Still, there is a very slight hesitation. This Holly 390 was new out of the box when installed. Everything looks good internally. To be on the safe side, I installed a new 6.5 power valve with no improvement. This hesitation is more like a flat spot upon acceleration. When I accelerate very gradually it's almost unnoticeable but when I accelerate more aggressively the hesitation is there. This is driving me nuts! If I don't resolve this soon I may need professional counseling. Help!

 

I had that same problem with my 390 carb. I had the primary throttle blades open to far so it was uncovering to much of the transition circuit. Check how much your primary idle screw is turned in from first contact. It shouldn't be more than 5/8 of a turn from first contact. In one of my Holley manuals it says to start with the secondary and primary idle screws at 1/2 turn after first contact then start the motor, if it isn't idling fast enough give the secondary idle screw another 1/8 of a turn if it still isn't going fast enough do it again then adjust the primary screw if it still isn't going fast enough. To reach the secondary screw without taking off the carb you can use a set of gunsmith screwdrivers that come in a little kit. They have a little ratchet that fits the various bits and hold them at a right angle so it makes it easy to turn that screw without taking the carb off. I also went to a .035 discharge nozzle and that worked well. Also look for that little check ball that is supposed to be in your vacuum secondary housing. Holley leaves it out on the 390's because they were made with light cars in mind. Without the ball the secondaries open way to soon. It's a 3/16" ball bearing.

 

Inliner......thanks for the advice. You are correct, the check ball is missing from the vacuum secondary housing. It came from the Holley factory this way. Can I get one of these check ***** from a auto parts store or must I obtain one directly from Holley?

 

Jerry, I just went to the junkyard and found a truck that had a Holley carb and took the check ball out of it. You might be able to find one at a bicycle shop, just tell them it's a 3/16 ball bearing. I didn't have much luck at the parts store the only way to get the check ball was to purchase the big carb repair kit that cost about $75. You might also want to purchase the Holley quick change secondary spring housing since you have the housing off anyway. That kit and the spring kit make it real easy to tune the secondary opening rate.