I have a small problem with my Holley HP 950.

The carb runs fine; good power, no stumbles. The issue that I am having is the idle is not consistant. The idle is set about 950rpm. After driving the car around and coming to a stop, the idle has increase about 150-200 rpm. After about 30-60 seconds it will slowly idle back down to the original 950 rpm. I have experimented with the mixture screws, secondary thottle blades adjustment with no changes. This carb is only about a year old so there appears to be no "slop" in the throttle shafts. I'm out of ideas. :confused:

Thanks in advance.

Could be a small vacuum leak.

Originally posted by Pat Kelley:
Could be a small vacuum leak. My vote exactly. When the motor heats up, poorly sealed/unsealed area expands allowing for vacuum leak. Check plenum and intake gaskets. Might want to pressue test coolant system.

are you sure its not distributor weights sticking?

Not enough throttle return spring tension?

I had a similar problem once. Except it would idle down if I reved it a little. Half throttle or so.
Turned out to be slightly too tight mounting bolts.
I had a few gaskets and a thick paper isolator. Made the throttle stick.

Thanks for the replys guys. smile.gif

I don't think its a vacuum leak. I have had the carb off numerous times, install new gaskets, have had the intake off and back on; no difference.

It's not the return springs. Have two of them installed, and you can "pull" forward on the carb linkage and it does not make a difference. Don't think its anything mechanical.

I'll check the distubtor weights, but I doubt thats it. Installed an new billet MSD dist last year.

The reason I think it's carb related is the 850 I had on before did not do this.

Is this a brand new carburetor or did you buy it used?

New.

So its been doing this for a year? You've stated that the carb is a year old and the 850 didnt do it,when did it start doing this? Where do you have the vacuum advance hooked up?

No vacuum advance.

Its pretty much done it from day one. I have just kind of gotten use to it. Did not drive the car alot last year and I was fighting some other problems.

There are some days where it does not ack up as bad as others. It seems to be temperature related. The warmer it is outside the less it does it. I am running a 1/2" NOS plate on top of a 1/2" open wooden spacer. Unless its pretty warm outside (up into the 80's) the carb stays actual cool to the touch. I just noticed this the last few times I've had the car out. I may try running it without the insulating spacer and see if that has any effect.

Check your throttle blades at idle position. Are the primary and secondary blade position equal?

If the secondary blades are flat, and the primary blades open to far to you may uncover the idle transfer slot on the primary. This will cause lazy throttle response, and inconstant idle speed.

Try backing out the primary idle screw until the blades are flat. Now open the primary and secondary blades an equal amount until you reach your idle rpm. The secondry idle set screw is on the secondary linkage side. Its a set screw in the base plate.

You didn't find a vacuum leak, your advance is not sticking, and throttle returns properly it has to be pulling full at the transfer slot due to blade position

UPDATE:

I found out what the idle problem was. The advance weights in the distributor was not being "pulled" back in at idle.

I am running an MSD 85551 distributor with the lightest springs that was provided. Appearently the 2 lightest springs (I think the silver ones) did not have enough tension to pull the weights back in at idle.

Removed one of the springs, installed a blue one (next stiffer) and one silver spring. Problem solved. Set the intial timimg at about 21-22 deg, with a total of about 40. Idles consistant at 1000rpm and runs like a champ! graemlins/thumbsup.gif graemlins/hurray.gif

graemlins/thumbsup.gif
That is exactly what I was going to suggest you check.I have had the same problem with the lightest springs in any kit I have come across.

For that reason you will never hear me suggest anyone ever use the lightest springs.
After a while even the medium springs will lose a bit of tension.
I commonly use one heavy and one medium.

Bob West calls it!