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 Post subject: Is this even possible...
 Post Posted: Thu Jan 17, 2013 1:34 am 
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I have a small block Ford that I shift at 8000 rpm and go through the traps at 8700 rpm. I recently started using a LM-2 to monitor AFR and to get a better idea of what was really going on...

The carb is a "pro built" 750 cfm 4150.

The idle AFR is "off the chart" lean. Changing the four corner idle screws makes no difference in the AFR reading. I do, however, have the O2 sensor in the collector and the collectors are "slip fit" so there may be some air coming in throwing off the idle readings.

As soon as I hit the throttle the AFR goes to ~9.5 - 10:1. As I go through the run the carb will lean out to ~12:1 at the stripe. It is a steady curve from ~9.5-10:1 to 12:1 through the run...

Is it even possible for the engine to idle at 20+ AFR? I do have to "tap" the throttle to keep it running.

Timing is locked at 28*, the cam has ~270 @ .050 duration and I am using a transbrake to launch the car at 5400 rpm.

Prior to getting the LM-2 I would just jet the carb for the best ET not really knowing what was going on with the fuel curve.

The carb currently has .076 idle air bleeds and .036 high speed air bleeds with no power valve and Holley 84 jets at all four corners.

I was thinking I would go down on the idle air bleeds and up on the high speed air bleeds.

Can any of you guys give me any suggestions as to how to flatten out the fuel curve?

Thanks in advance. Any and all suggestions will be greatly appreciated.


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 Post Posted: Thu Jan 17, 2013 9:40 am 
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webermaniac wrote:
Idle shows so lean because of air back flowing into exhaust. You can try and place some temporary restriction downstream from the sensor, or extend the exhaust some, then the real AFR will be displayed.

I put a glove in the pipe :)) to see AFR at idle.



Thanks for the reply... However, I should have mentioned, the car does have a full 3" exhaust...


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 Post Posted: Thu Jan 17, 2013 10:16 am 
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By partially covering the exhaust outlet you will be creating pressure in the system which will keep the fresh air from coming into the system from any possible leaks and giving you a false reading. You don't want to completely block the exhaust, just create enough back pressure to keep fresh air out.

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 Post Posted: Thu Jan 17, 2013 10:54 am 
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Slowpoke wrote:
By partially covering the exhaust outlet you will be creating pressure in the system which will keep the fresh air from coming into the system from any possible leaks and giving you a false reading. You don't want to completely block the exhaust, just create enough back pressure to keep fresh air out.


I understand now... Thanks and I will give it a try when we get it back together.

Any ideas of how flatten out the fuel curve?


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 Post Posted: Thu Jan 17, 2013 10:57 am 
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Try .070 in the idle bleeds, readjust the mixture for best idle at your normal operating temp.

As far as the MAB going larger will make it worse. Put 28's in them, jet to 83's and try it. Jet the car from there for best MPH, not for O2 readings.


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 Post Posted: Thu Jan 17, 2013 1:52 pm 
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Only 28* timing on a small block? O:) Do you know that this makes the best MPH? :-k

Reason I ask is because not enough timing will show funky AFRs also.

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 Post Posted: Thu Jan 17, 2013 5:05 pm 
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How do the plugs look?

Hysteric


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 Post Posted: Thu Jan 17, 2013 5:54 pm 
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Mark... Thanks for the response. I will try the bleeds/jets you suggested and see how it looks.

Ken... Thanks for the suggestion for the timing. We have tried timing up to 32* and several combinations with the 7530 and the car will ET the best with 28*. I have to use a flat top piston and the chambers are VERY small to get right at 13.8:1.

hysteric... The plugs take about 3 runs to get any color on them. I run a C4 transmission so I can't kill it when I cross the stripe. I usually throw it in neutral then idle it to the scales and tow back to the pits. I use NGK -8 racing plugs and they look VERY lean, to me anyway...

Another piece of information I failed to mention and I am not sure it makes any difference.... I was using VP C12 fuel. We can run C12, C14, C14+, or C16. I may try some C14 or C14+ and see if raising the timing gives a better ET.


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 Post Posted: Thu Jan 17, 2013 8:30 pm 
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Don't take this the wrong way, but you really should be looking at MPH for tuning more so than ET. Granted, the quickest "might" also be the fastest but with traction as a variable that can effect ET, most of us use MPH for tuning.

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 Post Posted: Thu Jan 17, 2013 9:13 pm 
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Ken0069 wrote:
Don't take this the wrong way, but you really should be looking at MPH for tuning more so than ET. Granted, the quickest "might" also be the fastest but with traction as a variable that can effect ET, most of us use MPH for tuning.


Ken... Will do on the mph!! With our car usually the lowest ET produces the highest MPH. In past years we ran a BFG 275/50/15 radial due to rules. With the BFG radial it either hooked or it didn't - there was no in between - it would either be a good pass or an aborted pass right off the hit. We are now permitted a 26x10 slick so I presume tuning for the highest mph will now be our preferred method.

I really appreciate all of you guys giving advice and suggestions. This gives me a good place to start once we get everything back together.


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