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Old 01-27-2009, 04:04 PM
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Holley Idle mixture screw help

I have a Holley 600 on my mild 400 smallblock chevy. I'm also running a descent size cam and that is making the tuning difficult. I couldn't get the idle mixture screws to adjust and I figured out the transfer slots were exposed. I opened the rear secondaries a little and some adjustability has come back. However it is in only one idle mixture screw. If you srew one side in all the way the engine will die. When you screw the other side in it will keep running. I just rebuilt this carb and blew out all of the passages. Would opening the secondaries anymore help? What about drilling the front throttle plates?
Thanks for your time and help.

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Old 01-27-2009, 04:12 PM
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a 600 cfm on a 400 is quite small on a oem 400 and one with a cam is really too small.
I seen this problem before and its caused by using the wrong metering block to carb body gasket.
your blocking a hole that should be open.
my guess is on the side that has no change.
get the right gasket for your carb and it should fix it.
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Old 01-27-2009, 04:50 PM
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Drilling the throttle blades is not nessessary. What camshaft?
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Old 01-27-2009, 07:03 PM
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holley

Was it working before? Try removing the idle screw and spray carb. cleaner in it and blow it out alittle w/ lower pressure air. Or the air bleeds down in the air hone may be dirty also. Just a suggestion..
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Old 01-28-2009, 12:05 AM
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It has a Comp Cams XE284.
Thanks again
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Old 01-28-2009, 02:08 AM
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Big duration cams like the XE284 need a lot more base idle timing.

The cam needs at least 24deg timing at idle or the throttle blades end up too far open at idle exposing too much idle transfer slot.

I run that XE284 cam and run my timing locked out 34 to 36deg locked timing at idle.
The increased idle timing will allow you to reset the throttle blade position with proper idle fuel transfer slow exposure. Once you do this the idle screws will be responsive again. The idle will be clean and throttle response much better. Needs a power valve swap to a 3.5 or 4.5 rated.
I suggest you removethe carb, flip it over and look at the throttle blades.
Reset the transfer slot exposure (.020-.030) both primary and secondary and reinstall the carb.
Lock out the mechanical advance timing and set the timing to 34-36deg at idle.
It will idle like its supposed to @850-900rpm after you make these changes.
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Old 01-28-2009, 07:30 AM
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yes, more timing will help a lot.

I would run as much initial timing as you can (right before the starter starts to complain when turning over a hot engine) then use the vacuum advance on the manifold source. should be between 16 and 20 degrees initial then another 10 to 14 from the vacuum advance. So and idle timing in the mid 30's will make the idle rpm increase a lot which will allow the idle screws to be should down so the blades are in the slot.
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Old 01-29-2009, 01:47 AM
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Thanks for the replies. I will bump up my initial timing and see if that helps. I did replace my 6.5 powervalve with a 5.5. I guess I should have went lower. In the past I have had hot restart problems with this engine. it doesn't seem to like alot of initial timing. However my 11-1 CR doesn't help.
Thanks again guys for your help.
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Old 01-29-2009, 09:10 AM
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Quote:
Originally Posted by 4speed57
I have a Holley 600 on my mild 400 smallblock chevy. I'm also running a descent size cam and that is making the tuning difficult. I couldn't get the idle mixture screws to adjust and I figured out the transfer slots were exposed. I opened the rear secondaries a little and some adjustability has come back. However it is in only one idle mixture screw. If you srew one side in all the way the engine will die. When you screw the other side in it will keep running. I just rebuilt this carb and blew out all of the passages. Would opening the secondaries anymore help? What about drilling the front throttle plates?
Thanks for your time and help.
As some of the other guys mentioned get as much base timing in it as possible and make sure the transfer slots are open no more then .020. If after doing both of these you find it still will not idle with them at .020 or less then you will probably have to drill the holes in the butterflies to get them back down. Starting with our Speed Demon we added the idle-eze feature for cams with durations of .220 and above which allows you to open a valve in the baseplate to get the butterflies back down but with the Holleys you still have to drill. More then likey what you will find when you pull the carb off is that you have gone past the .020 on the transfers slots and when you do this you start to pull that fuel out as well. At that point the mixture screws become ineffective for the most part. Hope this helps.
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Old 01-29-2009, 12:03 PM
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Quote:
Originally Posted by 4speed57
In the past I have had hot restart problems with this engine.
HoT restart: Install a remote Ford starter solenoild on the fender and a jump strap on the GM starter solenoid. (if the GM solenoid is pooched, replace it)
Install a starter heat shield and wrap. Headers are very hard on starters.
Check and clean the + battery/starter cable and the ground cable.
If the starter motor is pooched, replace it.
Install a dash mounted ignition power interupt switch (big red wire on the HEI) to kill the spark while hot cranking.
Crank it over with the spark off, them throw the spark switch and it will fire easily.
Make sure you have the OEM rear starter brace/bracket installed on the starter motor.
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Old 01-29-2009, 12:42 PM
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Thanks for the replies and help. I pulled the carb off and I noticed since I opened up the rear secondaries the front transfer slots are where they are suppose to be. It still seems to run really rich at idle and like I mentioned before, one idle mixture screw has no effect when you adjust it. I'm going to pull the front bowl off and make sure the gasket is the right one. I'll also blow thru the passage again to make sure its not plugged. I drove my car down the road with the one mixture screw turned all the way in. It ran ok, but hesitated when you snapped open the throttle. The engine should have died when I had the one side turned all the way in.
My combination is a 406sbc, 215 Dart Iron Eagle heads, 11-1CR, Weiand X-celerator intake, Comp Cams XE284 cam, and a Holley 600. Most would agree I need a good dual plane like a RPM airgap and a larger carb. The intake and carb are off a 350 and are practically new. I didn't have the money to buy a larger carb and better intake, so I used what I already had.
It seems this engine has always run rich. The throttle response is sluggish and the spark plugs are usually wet with gas. I have been using Autolite 25 spark plugs. With my 11-1CR I thought I should use a colder plug, but maybe thats why they foul kind of easy. It also seems bumping up the initial timing causes hot restart problems. I have thought about doing the ignition power interupt switch that F-BIRB'88 mentioned. I'm getting closer to fixing the problem. I do think its a combination of timing and carb.
Thanks to all for your help
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Old 01-29-2009, 01:06 PM
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Because of the very high compression ratio you will have to compromise the spark timing a bit to avoid detonation on pump gas. The motor really requires 100+ octane gas. Lock out the timing at set it at 28deg BTDC to start with.
Blow out the carb air bleeds with compressed air. The plugs need to be colder than "stock" but not too cold. 2 to 4 heat ranges good.
A intake manifold leak can really mess up any tuning efforts.
A dual plane hi rise will drive a lot nicer than the Weiand accelorator manifold.
Weiand has a nice new Stealth dual plane hi rise out.
The Pro Products Crosswind/Typhoon is a good one too.
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Old 01-29-2009, 01:14 PM
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The autolite #25 is not a cold plug. It is in fact a "hot plug" used on a stock like low compression daily driver type motor.
Is that the correct reach for that cylinder head?
If it is the correct reach/type spark plug I suggest you run a Autolite #144 (.035" gap) with locked timing around 28 to 32deg.
Get a 3.5" power valve.
What is the list number on the carb air horn? What jets?
The correct slightly cool spark plug with a non projected tip (autolite 144 Champion RV8C AC R42T NGK BR6FS) will allow more timing without detonation.
Yet once the idle spark timing is corrected, will not foul at idle and low speed.
Fattening up the secondary jetting a bit will also help supress detonation some.
Drilling out the throttle blates on the carb is a last ditch thing on a motor with a very very radical full race cam. You don't need to do this. Just get the timing sorted out and the carb throttles/ transfer slot right and the power valve corrected and it will idle like a taxi cab. Make sure the PCV is hooked up correctly and functional.
Other than needing the the ignition cut off switch, you'll find that locked advance works very well with the XE284 cam. Do not eliminate the vacuum advance but it will definatly need to be fine tuned (amount limit and rate in and out) to avoid detonation at cruise with the excessive cr. Crane makes a nice adjustable vac advance.
You need to get friendly with your local race gas dealer and blend your gas for a net 98-100 octane to run full spark timing and get the max power out of it with 11:1.

Last edited by F-BIRD'88; 01-29-2009 at 02:05 PM.
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Old 01-29-2009, 07:15 PM
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Thanks for the reply and help F-BIRD88. My carb is a cheap model 1850 Holley. The jets are stock, I haven't changed them out. I need to check with Dart and see what plugs they recomend. I have an adjustable vacuum in my garage. I have never locked out my timing before. I've heard people talk about that before, how would you do it? I have a stock HEI.
Thanks again for your time
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Old 01-29-2009, 08:13 PM
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Here is a simple way to lock the advance on a HEI.
I do it a little differently by simply removing the advance weights and springs. Then move the advance mechanism to fully advanced travel. Then simply bind/wrap the mechanism locked fully advanced so it cannot move, using two plastic electrical cable ties. Orient the knots of the cable ties down so tha the rotor will go back on.
Start it up and reset the timing with a timing light.
Normally 34 to 36deg is good but your cr is so high you probabily will need to keep the timing around 28 to 32 deg.
I suggest 66 to 67 primary jetting and swapping the secondary metering plate out for a richer #21 metering plate on your 600 carb. This will fatten up the jetting a bit for ya. You can rob the #21 sec metering plate from a 3310 carb.
Alternatively if you have a selection of small numbered pin drills, you can drill out the main metering jet orrifice of your stock # 39 metering plate by .002" or .003" to fatten it up.
Just don't get carried away. The slightly richer secondary jetting will help.
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Last edited by F-BIRD'88; 01-29-2009 at 08:25 PM.
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