Holly 600CFM Adjustments , 72' 390

[DGrant09]

We recently rebuilt our 390 and the carb hesitates on initial acceleration. It also did this before the engine work. The carb is 1 year old.
I've ran the timing from listed 6 degrees to 12 without improvement. Now since I have been tweaking the mixture screws, I'd like to take the carb back to back to the initial settings. What is the initial mixture setting and would anyone have an idea on the hesitation issue (if a carb adjustment, how-to)?
Thanks!!
Dan

[spartman]

Sounds like your accel pump isnt working or sending enough of a squirt of fuel out.

Not sure how to adjust it on a Holley though.

[DGrant09]

Sounds like your accel pump isnt working or sending enough of a squirt of fuel out.

Not sure how to adjust it on a Holley though.

Thanks
That's sort of what we were thinking.. Hopefully someone will chime in..

[spartman]

Only other thing I could think of would be the distributor is a little slow and not kicking in when its supposed to.

I would of course check the acc pump first.

Keeping it under the KISS principle ya know.

[DGrant09]

Only other thing I could think of would be the distributor is a little slow and not kicking in when its supposed to.

I would of course check the acc pump first.

Keeping it under the KISS principle ya know.

Before the rebuild on the engine, we went to a Duraspark ignition with a new distributor (old one had cracked gears). Pretty much everything except the carb and non-moving items are new on the motor.. $$ Ouch!

[Thunderfoot]

On the idle adjustment screws start around 2 turns out... but adjusting these will not change a hesitation problem... so adjust them in to where the motor starts to slow down and then open them up 1/4 - 1/2 turn...

Spartman is correct on the accelerator being the problem... There are two different ways you can adjust a Holley... The first is there is a plastic cam that is around the shaft and it has a screw that holds it in place. There are 2 holes in the cam that the screw will mount in and changes how fast the pump arm moves, you can try the other mounting hole and then there are other cam profiles that you can get to try also.
The other is the squirter in the top center of the carb is mounted with a Phillips screw and the squirter will have a number on it like 25... This number relates to the size of it and how much fuel it will give... 25 is small (lean) and 34 is big (rich) and there is number in-between you can get to try there as well.

Hope this helps...

[DGrant09]

On the idle adjustment screws start around 2 turns out... but adjusting these will not change a hesitation problem... so adjust them in to where the motor starts to slow down and then open them up 1/4 - 1/2 turn...

Spartman is correct on the accelerator being the problem... There are two different ways you can adjust a Holley... The first is there is a plastic cam that is around the shaft and it has a screw that holds it in place. There are 2 holes in the cam that the screw will mount in and changes how fast the pump arm moves, you can try the other mounting hole and then there are other cam profiles that you can get to try also.
The other is the squirter in the top center of the carb is mounted with a Phillips screw and the squirter will have a number on it like 25... This number relates to the size of it and how much fuel it will give... 25 is small (lean) and 34 is big (rich) and there is number in-between you can get to try there as well.

Hope this helps...

We'll give it a try tonight When I run the mixture screws in (clockwise) the motor accelerates all the way to the stop. I'll try this after we adjust the cam you mentioned and see how it does.

[Thunderfoot]

When I run the mixture screws in (clockwise) the motor accelerates all the way to the stop

That is a sign that you have a fuel leak somewhere in the carb... fix that first... It is most likely the power valve is blown, leaking or the wrong size...
The Power Valve is mounted on the metering plate, you have to take the 4 bolts out that hold the bowl on and the metering plate is between it and the main body of the carb. The Power Valve and metering plate look like the photo below...

These mixture screws work like this... the screw is a needle valve that adjust the amount of fuel that is given to the motor in the idle circuit, so as you screw the screw in (clockwise) you are leaning out the fuel in this circuit. So if the motor speed up with the screws all the way in then it is getting fuel from some other place...

NOTE: for the mixture screws to work (adjust) the motor has to be idling, that is the butterflies have to be shut... if the motor is idling at 1,000 rpm with the butterflies cracked open a bit then the mixture adjustment will not work correctly... just an FYI...

[fitzwell]

i second the power valve. If it was running ok before, pump cams don't just "go bad". On a stock 390, think i'd start with a 8.5 valve & see how the motor likes it.

[DGrant09]

Thanks for the input and ideas!!
Well, working with the accelerator cam today and no change.. We'll have a look at insides tomorrow. Frustrating for the carb is only a year old and it has done this since installed, could have been a factory issue, now mine..
We'll get-r!
Dan

[td]

i had the very same problem with my new holly and went thru the very same steps you are now trying to find the prob. when i finally figured it out (by watching the dvd that came with it) it was a simple secondary spring swap. according to holly all carbs are shipped with the lightest secondary spring and its up to you to find the right one for your application. (according to the weight of your vehicle, heavier the vehicle heavier the spring needed.) mine even came with 3 extra springs.may not be your problem but i thought it would be worth mentioning. p.s here's the spring kit part # incase you didn't get any extra springs with yours and you do need them-HLY-20-13 .

[fitzwell]

Hold the phone on going into the carb. The secondary spring is usually the middle of the range from the factory. Shouldn't be any reason to change it on your application at this time. Check the basics first. Contrary to popular belief, Holley carbs are pretty reliable, and don't need constant attention, if the initial setup is correct.
check this out..it might offer some insight.

http://www.holley.com/data/TechService/ ... 20Info.pdf


and, as always...

[Thunderfoot]

Good info there fizwell I like how they talk about the backfire protection for the (power valve) but it is only on their "performance carbs"

Dan, one other question for you... If you turn the idle mixture screws in and then it speeds up, with it like this can you adjust the idle screw (return stop screw) to slow the engine idle back down?

If not check that the butterflies are closed all the way and that the throttle linkage is not keeping it from closing all the way...

If you can adjust the idle screw to slow the rpm down then set the idle mixture screws out 1-1/2 turns and adjust the idle screw (return stop screw) to get the rpm down, then go back and try to adjust the idle mixture screws again...

Just one other easy thing to check...

[DGrant09]

The power valve appears to have fixed the accel issue, however, cylinders 1-4-6-7 are now getting fuel soaked. I pulled them to do a compression check and fuel was literately being sprayed against the wheel well from the drivers side head (probably the 6-7 cylinders). I was extremely fortunate to have picked up a fire ext 6 months or so ago!! Yep, the engine on the driver's side caught fire with the fuel spray!! We were fortunate to have the ext in the truck and had the fire out quite quickly. Only one melted wire. I suspect one of the plug wires set the gas off. Next time I'll pull the rotor first..
Back to the carb. On Monday it is going to a carb shop for a check. Something is definitely wrong inside and we have to much invested in the truck to loose it now.. We'll let you know what they find..

Dan

I also picked up two new extinguishers today!! Best $$$ I've spent in years!! $15.00 saved $3,500 plus!

[Thunderfoot]

FIRE!!! That's a scary thing...
Glad you got it put out ok and no one got hurt and everything is mostly ok... It is good to be prepared.

Weird on the carb, they are a pretty simple carb, not sure what would cause it to do that... be interesting what your carb shop finds out.