HOLLEY FOUR CORNER IDLE - Chevelle Tech
2001 General Tech questions from 2001

 
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post #1 of 14 (permalink) Old Jun 16th, 01, 4:57 PM Thread Starter
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Just installed my Holley 750 DP and now trying to set the 4 corner idle. Out of the box the primaries were set to 1.5 turns out and the secondaries were 1/2 turn out. I've tried diff combinations of setting the idle mixture screws but the best seems to be pretty close to the way it came out of the box (surprise!).

Here's the problem, I've got an LS6 with a TH400 and 2500 stall. I can't get it to stay alive with less than a neutral idle speed of well above 1500 or once I put it in gear it stalls. So I'm sure my idle circuit on the carb is not even being used. The car also likes to hunt when in gear and not moving. I've got 10" of vacuum in neutral, and an inital timing of around 14. I do have a pretty healthy cam, Edelbrock "Air Gap" manifold, 10.5:1 pistons and 2.5" headers so getting a lower idle speed might be a problem. My plugs look good, but I'm worried I may start to foul them if I'm never really idling (as far as my carb is concerned).
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post #2 of 14 (permalink) Old Jun 16th, 01, 6:03 PM
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Sounds like a timing problem.

Do you have a vacuum advance distributor and is it connected to manifold vacuum?

How much timing do you have at idle? It so high you may be getting a mix of mechanical and vacuum.

I ran a roller cam, very nasty, no problems with idle.

Which carb are you using?
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post #3 of 14 (permalink) Old Jun 16th, 01, 6:08 PM Thread Starter
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The carb is a holley 750 DP with mech secondaries. Vacuum advance HEI connected to ported vacuum off the carb, but it does have the recurve kit installed.
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post #4 of 14 (permalink) Old Jun 16th, 01, 6:20 PM
 
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I'm with Wally. Sounds like you already have your advance coming in.

Check your timing and plot your advance curve with an adjustable timing light checking every 500 RPM starting at about 700 RPM.

If you need to idle at lets say 1000 RPM due to an agressive/lopy cam profile, then your advance should start 200 RPM above 1000 RPM or 1200 RPM. Meaning that if your initial is 10 degrees is stays at 10 degrees right up to 1200 RPM and then starts to advance climing as your RPM's climb until it peaks at full advance or total timing.

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[This message has been edited by racer1320 (edited 06-16-2001).]
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post #5 of 14 (permalink) Old Jun 17th, 01, 9:08 AM Thread Starter
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Is it possible that I don't have enough air flow through the primaries at idle and I'm idling on the primary circuit? I guess to check this I would turn in the mixture screws to see if my engine dies (with the vac advance connected). Also, you asked what my timing was at idle, are you asking with or without vac advance connected?

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post #6 of 14 (permalink) Old Jun 17th, 01, 10:56 AM
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Have you balanced the primary and secondary throttle plates. When I set my 4 corner up, I turned the carb over and made sure both plates were open about the same amount at idle and that the transfer slots were were covered.

Is this a factory 4 corner or a conversion. If it's a conversion there is a passage between the front and rear idle curcuits that needs to be blocked. Also, if your cam is pretty radical, you may need to drill holes in the throttle plates.

My 4 corner works great. With my Comp XE294 cam (250/256@.050) I idle in gear at 800 and out of gear at 1200. I have 6.5 inches of vacuum at idle in gear and have holes in all 4 throttle plates.

Look for vacuum leaks. They are the bane of the internal combustion engine.

------------------
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[This message has been edited by Pat Kelley (edited 06-17-2001).]
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post #7 of 14 (permalink) Old Jun 17th, 01, 11:26 AM Thread Starter
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OK, this carb is out of the box from the factory, so it's not a conversion.

I assume that by pulling the carb off and checking the throttle plates you mean making sure that the set screw makes both front and back open the same when they are not being pulled by the throttle cable. This may not be the case now since the best vacuum I got was when I set the primaries to about 1.5 turns out and the secondaries to about 1/2 turn out, so there does appear to be some difference here.
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post #8 of 14 (permalink) Old Jun 17th, 01, 11:59 AM
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What is the LIST number of this Holley??
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post #9 of 14 (permalink) Old Jun 17th, 01, 12:05 PM Thread Starter
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LIST number is 4779.
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post #10 of 14 (permalink) Old Jun 17th, 01, 12:40 PM
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Unless someone has done a conversion on that carb, it does not have an idle circuit on the secondary. Holley makes two off the shelf units, LIST 8156 and LIST 9379. If it was me, I would use the 9379, it is a annular discharge carb with 4 corner idle circuit.
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post #11 of 14 (permalink) Old Jun 17th, 01, 2:54 PM
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Get the engine to idle the best and lowest you can. Pull the carb. Back off the curb idle screw so the transfer slot just shows in the primary. Adjust the secondary blades so they are open the same as the primaries (transfer slot just showing). The adjustment screw is on the passenger side and is accessable from the bottom. Set the idle mix screws at 1 turn out, mount the carb, and see what happens. You'll likely need to adjust the mix and idle speed after starting. All 4 mixture screws should be very close to the same number of turns out when you finish. This worked for me. Of course, the float level must be correct or the carb will never dial in.


<BLOCKQUOTE>quote:</font><HR>Originally posted by JoenAnne:
OK, this carb is out of the box from the factory, so it's not a conversion.

I assume that by pulling the carb off and checking the throttle plates you mean making sure that the set screw makes both front and back open the same when they are not being pulled by the throttle cable. This may not be the case now since the best vacuum I got was when I set the primaries to about 1.5 turns out and the secondaries to about 1/2 turn out, so there does appear to be some difference here.
<HR></BLOCKQUOTE>

------------------
Pat Kelley
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67 El Camino, STRIP/street
Pat's Page (CR Calculator, Utilites, car specs)

[This message has been edited by Pat Kelley (edited 06-17-2001).]
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post #12 of 14 (permalink) Old Jun 17th, 01, 10:29 PM
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DO NOT drill holes in the butterflies on a mechanical secondary holley! There is a throttle stop screw for the secondaries to hold them open more if you need, but don't drill any holes!



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post #13 of 14 (permalink) Old Jun 18th, 01, 6:44 AM
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I know you already have a ton of suggestions but here is one more. Check to see how much vacuum you have at idle in gear. Then check to see what power valves are installed. If the two are very close numerically it may be that hen you drop it gear the vacuum drops and the power valve opens dumping excess fuel in the carb. Good Luck
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post #14 of 14 (permalink) Old Jun 18th, 01, 9:20 AM
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Gandolf, here is a quote, refering to idle curcuit modifications, from David Emanuel's book "Super Tuning and Modifying Holley Carburetors".

<BLOCKQUOTE>quote:</font><HR>This is one area where individual tailoring of the curcuits can make engine operations significantly smoother. If necessary, the first step is to drill holes approxomately 3/32-in. in diameter in all four throttle plates. The purpose of the holes is to increase airflow at idle while allowing the throttle plates to remain in the proper position relative to the transfer slot and idle discharce orfice.<HR></BLOCKQUOTE>

His is not the only source for this info. Both David Vizard, and The Holley Carburetor Manual recommend this also. In additions my carb guy, local racing guru Dave Lang, does this (he built my carb).

The key words above are "If necessary". This mod is only needed to correct idle problems cause by cams needing to idle at higher rpm, exposing too much of the transfer slot. Not every carb needs it, but some do.

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