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Discussion Starter · #1 ·
I have posted a bit on this before, but here is the situation:

My 496 continues to run rich at idle with an Avenger 870. This is a 10:1 496, Lunati HR 231/239 @ 0.05, .600, Brodix RR-Os, air gap intake, all installed in the car in January. Ingnition is pro-billet MSD with 6AL, 18 degrees initial, 36 total, all in by 3K. So far I have fouled 2 sets of plugs and can't seem to get the idle circuit set correctly...(otherwise, the car rips off idle).

It has a steady 9 inches of vaccum in park, 7 in gear, mostly due to my 5000 ft altitude. I am running a Carter 172 with no regulator; float level is set at the bottom of the sight glass.

The carb currently has a 3.5 PV in front, and with the transfer slots squared front and rear, the car does not want to idle.

I can get it to idle at 750 RPM or so with way too much transfer slot exposed in front, but at that point the idle screws have little effect on the idle vacuum (unless they are ~1/4 turn from seated).

I am wary of increasing the rear blade opening much further as other posts have suggested - won't this bring the rears onto the main circuit and keep the carb rich at idle?

Before I drill the front blades (thinking 1/16th bit to start), does anyone with more experience have other ideas?

Thanks,
Rob
 

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IMO, your setup is a little too radical for the Avenger series of Holley carbs. The HP line would do a lot better.
That said, can't hurt to drill the throttle plates but start out small...a 1/16" is fine to begin with to see if there's an improvement in idle quality.
 

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Discussion Starter · #4 ·
Thanks for the PM Chris - just sent you a reply.

Bill, I hear what you are saying and in the back of my mind I have had a nagging thought that I should have gone with a different carb. That said...I guess at this point I will see what I can do with the Avenger. If it ends up that it won't work...well, I guess there is always ebay.

Rob
 

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Sure, could always replace the drilled plates with new ones.
 

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Vacuum advance hooked up? 18* initial may not be enough IMO. I ran 20* intial + 12* vacuum = 32* IDLE timing on my 454 with a 211/230 cam. Without the vac advance seem to run like @ss.

Try this: turn the distributor to ~30* at idle just to see how the addtional timing would impact idle/vacuum. Of course don't drive it this way, but for testing purposes its great.

Your cam isn't that radical to warrant drill bits just yet IMO.

You can also pull a vacuum cap off to simulate drilled blades BTW. You don't wanna square the rear blades - their higher in the bores than the primaries

FYI, others here with that cam (I have it to) are idling at about 850 rpm and getting 12* of vacuum, but I do understand you altitiude has an effect on this.
 

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Discussion Starter · #11 ·
Thanks for the responses so far guys - the timing figures I posted are with no vacuum advance...

Also, I am running a PCV on this motor, and I am running the NGK -8 plugs originally spec'd with the heads..

Bob, are you running -7s? I was thinking about this as well and wondering if guys were running the next step hotter plug because, aside from the idle issue which is a major player, no doubt, it is a plug fouling machine at this point.

Frankly, I'd be thrilled with a solid idle mixture at 900 RPM at this point...drilled primaries or idle air bleeds is looking like the best option, short of swapping carbs.
 

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Vacuum advance hooked up? 18* initial may not be enough IMO. I ran 20* intial + 12* vacuum = 32* IDLE timing on my 454 with a 211/230 cam. Without the vac advance seem to run like @ss.

Try this: turn the distributor to ~30* at idle just to see how the addtional timing would impact idle/vacuum. Of course don't drive it this way, but for testing purposes its great.

Your cam isn't that radical to warrant drill bits just yet IMO.

You can also pull a vacuum cap off to simulate drilled blades BTW. You don't wanna square the rear blades - their higher in the bores than the primaries

FYI, others here with that cam (I have it to) are idling at about 850 rpm and getting 12* of vacuum, but I do understand you altitiude has an effect on this.

20 intial + 12 vacume = 32 at idle, seems a but much. also u realy shouldnt have 12* worth of vacume advance at idle. how much mechanical advace do u have. 20 plus another 229if u have light springs) is a ton of advance.

also how does high altidude afect vacume. what needs to be understood is vacume is just less atmospheric presure. the less atmosferic presure u have the less "vacume" u have. 16 iches of vacume is actuly 7 psi in your intake. 30 inches would be 0psi and water would boil at room tempature in 30" of vacume. and this is a sea level. if u go up the ambiant air pressure is less therefor the presure " or vacume" in the intake would be less
 

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Discussion Starter · #14 ·
I think the very general rule of thumb is 1 inch of vacuum lost per 1000 ft ASL, which would put the idle vacuum I have at 5000 feet right in line with what Vince has at sea level...
 

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I had the same issues you had ...I drilled mine 3/32's and now idles great and mixture screws finally work!
 

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Here is my 0.02 worth...

I run a HR296 236/242 @ 050 and .610 lift on a 402 and Edelbrock Performer carb. I would say I am very much in the same neighborhood as you. Biggest difference is I am running a Muncie rather than an auto and you have a few more CI.

With 18* initial I ran about 7" vacuum at around 850 idle and had issues with the throttle blades needing to be open too far. I used to run an adjustable vacuum leak in the PCV line to set my idle speed. Transitioning off idle was painful.

I spent an evening and A) swapped my 10-15" sorta adjustable vacuum advance can for a 3-9" fixed vacuum advance can and B) limited vacuum advance travel for a total of 12* instead of 20*. I hooked this up to manifold vacuum. I effectively start up at 18* and then idle at 30*.

I no longer have issues with throttle blades being open too far. My off idle stumbles are greatly reduced. And my vacuum at idle has gone up to 11".

Bottom line: big long duration cams absolutely need lots of ignition advance at idle to work right. Always always make sure the ignition really is setup correctly before hacking on the carb. You want a vacuum advance that fully activates at 1-2" less vacuum than you have at idle in park and you want it connected to manifold vacuum. NOT ported vacuum.

Somebody mentioned drilling out the air bleeds... DO NOT do this! Yes it will lean out the idle circuit but it will also raise the vacuum level at which this circuit turns off. This is very incompatible with a low vacuum cam setup and you will wind up with off idle stumbles in a serious way. In fact the right course here is usually to reduce the air bleed size AND the idle restrictor jet size both.
 

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Exactly w/the above posts....I say turn the dizzy before turning a drill!
 

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Discussion Starter · #20 ·
Steve and Vince, thanks for the input. It is certainly easier to turn the dizzy a few degrees and see how it responds before I drill...and turning up the timing is free. I hear what you are saying about the vacuum can.

On the plus side, tho, I checked and replacement throttle blades are about 10 bucks...really no more expensive than swapping power valves.
 
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