Holley 9380 850 race carb idle setting - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-30-2008, 03:02 PM
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Holley 9380 850 race carb idle setting

About two months ago I fired up the engine for my race car. I had two camshafts the smaller being a solid roller 600 lift, 242 and 254 dur at 50, lobe sep 110. My compression was encroaching 300 and the engine sounded fairly mild but idled down to 1200 easy. I had another cam that I thought was to big but given the results of the smaller cam I swapped them out. The second cam is a solid roller, 617/630 lift, 274 and 284 dur at 50, lobe sept 106. With the second cam I can not get it to idle below 2200 to 2400. At an idle it does not sound like thump thump thump or chug chug chug it sounds like rap, rap, rap and it hits real hard when it idles. Now the problem: I have the idle set screw set all the way in and it still wont stay idling. It will set there and hammer if I open the throttle a little, but that doesn't do me any good. How do I set this carb so it will idle?

Bob

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Old 04-30-2008, 03:45 PM
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850 Idle

Have you made any adjustments to the idle mixture screws?Sounds like you may need to richen it up,or you have a vacuum leak somewhere.
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Old 04-30-2008, 04:07 PM
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Holley 9380

Done the spray bottle around the carb and checked all vacuum plugs. Removed the carb and checked the foot print on the gasket and it is sealed. If I crack the throttle the engine responds with an explosion of rpm. No hesitation, no flog or stutter. I have the idle mixture screws on the side of the front and rear bowls set at front 1.5 turns out and rear 1 turn out.

Bob
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Old 04-30-2008, 08:14 PM
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850 Idle

Quote:
Originally Posted by Quarter Flash
Done the spray bottle around the carb and checked all vacuum plugs. Removed the carb and checked the foot print on the gasket and it is sealed. If I crack the throttle the engine responds with an explosion of rpm. No hesitation, no flog or stutter. I have the idle mixture screws on the side of the front and rear bowls set at front 1.5 turns out and rear 1 turn out.

Bob
Okay, I've been thinking about this one for awhile.The problem may be the powervalve,you changed to an even bigger cam so your making less vacuum at idle.If your powervalve is lets say set at 8.5hg and your only pulling 6 or 7 hg at idle the powervalve is open and your dumping a bunch of fuel. I don't know, just a thought.I had a similar problem with a blown small block.
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Old 04-30-2008, 08:41 PM
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Take the power valve out and throw it away if you have one block it off and jet it up about 4 sizes, I like to square jet when you do not use a PV. they are of no use on a race engine they are used so at cruse and you step into the throttle you have a source of enrichment. You are WOT in a drag car from start to finish in most cases the PV is out of the loop and if it is a footbrake car the acelerator pumps take care of you when rolling into the throttle. I would try to back out the mixture screws a little more than you have them and keep them even 2.5-3 turns out. Where is the base timing? with that much duration you need to lock the timing or it is not going to idle. I would start with about 32-34 degrees and see how it works,

Mike
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Old 05-01-2008, 07:31 AM
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Carb Setting

Okay, I can figure out how to plug the power valve if I have one. I have heard people talk about locking the distributer but don't know how. I have a MSD Pro Billet distributer and a MSD programmable 7530 ignition box. If I lock the distributer at 34 do I then program out some timing with my ignition box under say 1000 rpm to make it start easier?

PS
My compression is 254 to 257 with a warm engine and throttle wide open. I am burning Rocket 111 race gas. I assume my vacuum is good but will check it on Saturday.

Bob
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Old 05-01-2008, 10:08 AM
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Quote:
Originally Posted by Quarter Flash
Okay, I can figure out how to plug the power valve if I have one. I have heard people talk about locking the distributer but don't know how. I have a MSD Pro Billet distributer and a MSD programmable 7530 ignition box. If I lock the distributer at 34 do I then program out some timing with my ignition box under say 1000 rpm to make it start easier?

PS
My compression is 254 to 257 with a warm engine and throttle wide open. I am burning Rocket 111 race gas. I assume my vacuum is good but will check it on Saturday.

Bob

Bob, you have all the tools to do it with. First pull the distributor and remove the gear, take the nuts off that hold the advance and the shaft together spin the shaft 180 degrees and put it back together the timing is now locked. Your box has a start retard in it set it at about 10 degrees and it should fire right up. MSD has all their instructions on their web site so if you have any problems go on there and print them off they will walk you through it.

Hope this helps, Mike
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Old 05-01-2008, 10:12 AM
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Timing

Thank you I will do on Saturday

B
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