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Old 03-25-2009, 07:30 AM   #1
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Default O.S. 21 VZB High Idle Issue???

Quick question. I have an O.S. .21 VZB I have about 3 gallons through it. It was running awesome temps in the 240, then it started to not want to idle. I took the carb apart and cleaned it and the needles out. Now I have it back together have it tuned up good and again it's running awesome no complaints there. The issue I have is when I start it up and set the idle it idles fine but as soon as I give it gas the idle stays high for about 5 6 seconds then it drops back down to a normal idle. It now will stay idleing but it's odd that when given gas it stays idleing hihg for 6 seconds. I alread checked and the slide is moving free and returns home so that's not the issue. Any ideas?
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Old 03-25-2009, 07:52 AM   #2
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Normally that is caused by the ldle speed being set to high and the LSN set to rich to compensate for a high idle.
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Old 03-25-2009, 07:54 AM   #3
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Quick question. I have an O.S. .21 VZB I have about 3 gallons through it. It was running awesome temps in the 240, then it started to not want to idle. I took the carb apart and cleaned it and the needles out. Now I have it back together have it tuned up good and again it's running awesome no complaints there. The issue I have is when I start it up and set the idle it idles fine but as soon as I give it gas the idle stays high for about 5 6 seconds then it drops back down to a normal idle. It now will stay idleing but it's odd that when given gas it stays idleing hihg for 6 seconds. I alread checked and the slide is moving free and returns home so that's not the issue. Any ideas?
did you check your needle settings from the looks of it ..its not tuned properly
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Old 03-25-2009, 07:59 AM   #4
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Normally that is caused by the ldle speed being set to high and the LSN set to rich to compensate for a high idle.
+1 set your idle first, then tune your LSN to stabilize the idle. Idle should be 1 full turn from closed w/ the velocity stack out.
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Old 03-25-2009, 08:20 AM   #5
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+1 set your idle first, then tune your LSN to stabilize the idle. Idle should be 1 full turn from closed w/ the velocity stack out.
Thanks for the input guys. I guess I have been lucky with this O.S. it's been running really good with no issues just some tuning. Looks like after it started to idle bad I was tuning it to adjust for the idle issue. I will look into this tonight. So 1 turn out. With it not running do I just give it full gas then tighten the idle screw in then back it out one full turn start it up and tune the LSN from there?
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Old 03-25-2009, 08:51 AM   #6
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Your LSN is too rich.


Bring your engine up to temp.

If you are at the track make sure you tune the HSN first and be sure its not too lean.

Personal experience with these engines has tought me that the spray bar setting should be at 2'oclock from flush for an 8mm restrictor. (This position works well with restrictors down to 6.5)

Set your car on the wall or starter box and make a full throttle pull to clean it out. If your engine does not come imediately back to an idle BUT does drop down after a few seconds, its too rich on the LSN.

Adjust your LSN one hour leaner and then do another clean out. It should be less time to come back to idle at this point. If that time is less, you are going in the right direction.

Keep sneaking up on the tune going leaner hour by hour, doing a clean out in between each, until you do a clean out and it comes imediately back to idle. You may notice during this process that your idle it comes to gets higher, just set the idle screw accordingly until the LSN is at the proper tune, THEN SET YOUR IDLE.

You should be able to go to full throttle and let off having the engine drop imediately back to idle.

If your track is slick, or it's too punchy for you to drive, you can back of the idle just an hour or two.

My OS engines through the years have only ever needed tuning on the HSN from track to track once the bottom is set and the idle is right.

I hope that helps.
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Old 03-25-2009, 08:54 AM   #7
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+1 set your idle first, then tune your LSN to stabilize the idle. Idle should be 1 full turn from closed w/ the velocity stack out.

I think you got that backward. (???)


Never "tune" your engine to idle....


One idle setting is not the end all.... other wise they would be preset.
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Old 03-25-2009, 12:07 PM   #8
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I think hes got it right. The 1 full turn gets the idle gap set right. And you got to get the idle to work or u cant drive your car. If it dont come back to idle you blow out turns and it jumps nose high. Its common to see a v spec set up with the idle gap too wide and the lsn set rich to lower the idle.
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Old 03-25-2009, 12:14 PM   #9
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do you set your idle gap with the restrictor in or do you take it out then set?
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Old 03-25-2009, 12:17 PM   #10
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I think hes got it right. The 1 full turn gets the idle gap set right. And you got to get the idle to work or u cant drive your car. If it dont come back to idle you blow out turns and it jumps nose high. Its common to see a v spec set up with the idle gap too wide and the lsn set rich to lower the idle.
Just pull out the Velocity stack, pushing the slide closed back out the idle screw till the gap is JUST completely closed, then go back in one full turn. This is a close to perfect setting, it may need 1 hour here or there to achieve the optimum tune. Idles should come pre-set from manufacture, but they don't. All EB Mods engines do..
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