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Lean/surge at cruise
http://motorsportsvillage.com/forum/viewtopic.php?f=6&t=9853
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Author:  fourtenposi [ Sat Jan 05, 2013 8:20 pm ]
Post subject:  Lean/surge at cruise

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

Author:  jmarkaudio [ Sat Jan 05, 2013 8:49 pm ]
Post subject:  Re: Lean/surge at cruise

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.

Author:  fourtenposi [ Sat Jan 05, 2013 10:44 pm ]
Post subject:  Re: Lean/surge at cruise

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.

Author:  Scott Smith [ Sat Jan 05, 2013 11:34 pm ]
Post subject:  Re: Lean/surge at cruise

Is there T Slot restrictors in there?

Author:  fourtenposi [ Sun Jan 06, 2013 8:53 am ]
Post subject:  Re: Lean/surge at cruise

No, this is on my 4150 style demon.

Author:  Beretta [ Sun Jan 06, 2013 9:09 am ]
Post subject:  Re: Lean/surge at cruise

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??

Author:  shrinker [ Mon Jan 07, 2013 7:51 am ]
Post subject:  Re: Lean/surge at cruise

VP113 has MTBE as an oxygenate. It runs leaner than the fuel you used to use in the demon.

Author:  fourtenposi [ Mon Jan 07, 2013 12:44 pm ]
Post subject:  Re: Lean/surge at cruise

I understand that, I just wanted to make sure I was thinking about this correctly.

Thanks

Author:  shrinker [ Mon Jan 07, 2013 5:42 pm ]
Post subject:  Re: Lean/surge at cruise

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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