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Holley Idle 2 schools of thought

696 Views 15 Replies 5 Participants Last post by  PRNDL
So which school of thought do you guys recommend as far as idle is concerned?

Secondaries closed with just a small 1/8 turn to prevent binding. Then set the idle via the primaries only?

or…

Secondaries adjusted to match the exact same level as the primaries. Then make idle adjustments by adjust both fronts and rears the exact same?

Both methods assume the primary transitiin

AED Holley 750 HOHP DP 4 corner idle. Came with 1/8 holes in primaries only. and PCV hooked up. One inch open spacer.

The engine is a freshly rebuilt 383 stroker. All machined, brand new stroker internals, 30 over, zero deck, 6 inch rod Skip White unit with 10cc dished pistons. AFR 195 heads, Performer RPM AIR GAP, dual plane, and Comp Cams XR282HR 230/[email protected] 110LSA CAM. HeadersBreak in has been completed.


I’m noticing that if I set my idle to 900 rpm via the first method, primary adjustment only, my rear idle mixture screws stop responding. I can screw them in all the way and no change.
When I set the idle via the second method, primary and secondary matching, to 900 rpm, the front mixture screws stop responding.

I’m thinking of lowering my idle to 800 rpm.

Any ideas?
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No if three idle screws are feeding fuel the engine is unlikely to stall. Even closing two are unlikely to stall the engine.

The idle issue is more one of getting air which is why your carb has the drillings in the primary throttles. This to a huge extent negates the need for the secondary idle circuit being there at all.

The open spacer while being an aid to WOT redline power messes everything up else where. Unless you drive this competitively only on a track and nowhere else I’d get rid of it for a 4 or 3 hole, but that said any spacer does not insure any power improvement and can be detrimental, this is the kind of thing you experiment with on a dyno or with timing slips. The latter is also skeptical data because track times get diluted with human and mechanical variations one run to the next so it can take a number of timing slips averaged to see if there is a trend and that trend is repeatable.

My baseline setup on four corner idle is set the primaries to the general rules of primary transition slot exposure then crack the secondary side to supply sufficient air to establish consistent idle. Depending usually on the cam and how much air it requires to get the needed idle speed I might not even open the rear idle mixture screws. I generally only fiddle the rear mixture screws if I think I’m getting the primary screws too open where the air bleed circuit is getting unstable. This is something you just develop a feel and hearing for over the years of doing it.


Bogie
 

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My preference on spacers are insulating. It’s usually a good thing on carbs to reduce heat transfer into the carb especially these days with pump gas being blended for high pressure fuel injection rather than sloshing about at atmospheric pressure in a float bowl.

Bogie
 

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If you open the idle mixture screws too far the fuel flow overwhelms the air bleeds and they fail at their job and at this point get yourself caught up in the air bleed dance where it’s easy to get lost.

I find it much easier to set the primary side as you would to a stable idle with a smaller cam then crack in the secondary side just enough to clean the idle to the proper and needed RPM. Rebalance the mixture screws based on the thoughtful position of the throttle blades primary to secondary. Unless you’re running a huge cam it generally doesn’t take much secondary opening of either the throttle blades or the mixture screws.

If there are already air holes drilled in the primary throttle blades it might not even require opening the secondary blades nor the mixture screws at all. But here you need to be cautious not the crank the primary mixture screws so far open to overwhelm the air bleeds.

Bogie
 
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