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 Post subject: Lean/surge at cruise
 Post Posted: Sat Jan 05, 2013 8:20 pm 
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Joined: Thu Feb 12, 2009 7:52 pm
Posts: 348
Hey All,

I changed to vp113 about mid last year. In crappy air it was fine. When I started getting into the better air, I had a hesitation off idle which I cured with a bigger pump cam. Well I was in some negative da air today and it didn't want to run at part throttle, and the hesitation is there. If you bring it up to around 1600 rpms, it will surge a bit.

I tried to decrease the size if the IAB, but it didn't seem to make a difference. So I am leaning toward not having enough IFR. Currently have a .033 in there. Am I on the right track with my line of thinking? How much bigger should I go to make a difference?

Thanks

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Best 9.87@131 on the rev limitor 1 Feb 2013
1/8   6.15@110 NEW BEST 27 Nov 11
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 Post subject: Re: Lean/surge at cruise
 Post Posted: Sat Jan 05, 2013 8:49 pm 
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Joined: Mon Oct 29, 2007 7:07 pm
Posts: 1422
Location: Florida
How much did you go down? Go at least .005, the .005 more, and if it's your 4150 you can try some .035's in the IJ and go back up on the IAB. Readjust your mixture after each change.


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 Post subject: Re: Lean/surge at cruise
 Post Posted: Sat Jan 05, 2013 10:44 pm 
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I went from a 74 down to a 67 on the iab and that is when the surging seem to start. Like there is not enough fuel to feed it on the t-slot. I will go up on the IFR to .035 and readjust. I need to get me some new o2 sensors, my sensors are dying.

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74 Nova
Best 9.87@131 on the rev limitor 1 Feb 2013
1/8   6.15@110 NEW BEST 27 Nov 11
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 Post subject: Re: Lean/surge at cruise
 Post Posted: Sat Jan 05, 2013 11:34 pm 
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Joined: Sun Nov 11, 2007 10:33 am
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Location: Montgomery, Alabama
Is there T Slot restrictors in there?


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 Post subject: Re: Lean/surge at cruise
 Post Posted: Sun Jan 06, 2013 8:53 am 
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No, this is on my 4150 style demon.

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74 Nova
Best 9.87@131 on the rev limitor 1 Feb 2013
1/8   6.15@110 NEW BEST 27 Nov 11
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 Post subject: Re: Lean/surge at cruise
 Post Posted: Sun Jan 06, 2013 9:09 am 
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Joined: Wed Apr 19, 2006 8:20 pm
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Location: N.J.
If you tune it for the good air there now am I thinking that it would only need a IAB and a mixture adjustment when the air returns to normal??

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running E85
Best ET 8.07
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 Post subject: Re: Lean/surge at cruise
 Post Posted: Mon Jan 07, 2013 7:51 am 
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Joined: Sat Feb 07, 2009 4:12 pm
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Location: Adelaide Australia
VP113 has MTBE as an oxygenate. It runs leaner than the fuel you used to use in the demon.


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 Post subject: Re: Lean/surge at cruise
 Post Posted: Mon Jan 07, 2013 12:44 pm 
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Joined: Thu Feb 12, 2009 7:52 pm
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I understand that, I just wanted to make sure I was thinking about this correctly.

Thanks

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74 Nova
Best 9.87@131 on the rev limitor 1 Feb 2013
1/8   6.15@110 NEW BEST 27 Nov 11
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 Post subject: Re: Lean/surge at cruise
 Post Posted: Mon Jan 07, 2013 5:42 pm 
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Joined: Sat Feb 07, 2009 4:12 pm
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Location: Adelaide Australia
Well yes, your thinking about it correctly, it needs more fuel and the best way to do that is the IFR. When you reduce the IAB to try to get more fuel you also reduce the amount of air flowing through the circuit drillings, that has a major effect upon the droplet size exiting the t-slot. Less air = bigger drops = greater AFR differences per cylinder. When a cylinder fires correctly it actually draws in the next intake charge more effectively so a poor combustion event from a lean cylinder (due to a missing large droplet maybe) will cause a stammer on the next event.


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