Help with Holley adjustment

[Tom Sarno]

I just installed a Holley universal 650 vacuum secondary carb and have two very basic questions.

Which way do you turn the needles to lean out the mixture? Second, what is the purpose of the vacuum fitting that is on the metering block?

Thank you in advance for your help.
Tom

[Clem Z.]

Re: Help with Holley adjustment

most but not all holleys turning the screw in leans the idle mixture and the fitting is for the distributor vacuum advance.

[Gary Schisler]

Re: Help with Holley adjustment

IIRC, "in" would be clockwise. However, my friend has a later, aftermarket replacement that I think is just the opposite. In actually richens the mixture. There is a very small blurb about this in a Holley tuning book I have somewhere in the garage. I will look this up and post the information.
Gary

[Tom Sarno]

Re: Help with Holley adjustment

Thanks Clem.

The fitting above the mixture screw does not seem to have enough vacuum for the advance?

The car seems to be running to rich at idle, would more or less timing fix this?

Thanks
Tom

[Clem Z.]

Re: Help with Holley adjustment

it may be ported vacuum and only have full vacuum after you crack the throttle plates. most of the holleys with the reverse idle mixture screws have large knurled heads on the screws and have a decal on the metering block show which way to turn the needle to rich or lean

[Mark #28455]

Re: Help with Holley adjustment

If you turn the screws clockwise on standard Holley carbs it leans the mixture. The smog carbs sometimes richen the mixture. One thing to watch with Holley carbs is the off-idle transfer slots. If you have to turn the idle up on your engine (large engine or high idle RPM) you may expose more than about 0.030 of the transfer slot in which case the idle mixture screws will no longer be able to control the mixture and it will be too rich. You would know this as you could screw the idle mixture screws almost all the way in with little effect. Usually the screws are about one turn out from closed.

What to do if you require a higher idle speed than the transfer slots will allow? Bubba drills a hole in the primary throttle blades! The right way is to look at the underside of the carb and find the set screw for the secondary throttle shaft (just like an idle screw on the primary but goes in through the base plate from the bottom and looks like a set screw). Turn this clockwise with a jewler's screwdriver until the secondaries are just barely cracked open, then reset your primary throttle idle screw.

On Holley carbs, the manifold vacuum source is usually on the base plate, the ported vacuum is usually on the metering blocks. Your engine will likely run better with manifold vacuum to the advance mechanism.

Hope this helps,
Mark