Proper 930 mixture adjustment techniques?
 

I hooked up a A/F meter to my 930 the other night and at idle I got a reading of about 13.5/1. When holding it at higher rpms it would stay there but after letting off it would go off the charts over 15/1(only while coming back down to idle speed). I would assume I should do this test under load and on boost to see where I am at but how do my numbers at idle look? I get an occasional hunting idle and alot of decel popping so I thought I might be lean. But now I am not so sure. Any thoughts, Thanks guys

Well Len, at idle my open-loop A/F definitely registers on the "Rich" side of the scale, but without a wideband sensor that's about all the information you're going to get. I think the only way to really set it up is going to be using a CO meter. I set mine to about 3.5% CO at idle, and the A/F gauge shows rich in all cases except when letting off the throttle and decelerating when in gear. As long as I see "rich" under load I'm happy.

It's almost impossible to measure A/F ratio with ordinary smallband lambda under load. It's only there to indicate on which side of stochaistic mixture you are and most eninge managment systems bypass lambda under WOT, beacuse it's unreliable when it gets hot.

If you really want to know your mixture then you need to shell out for wideband lambda or get yourself EGT-sender.

So in essence the Bosch O2 sensor and Edelbrock A/F ratio meter I am using is useless? Or are you saying it is fine for checking at idle only? If it is fine for at idle, what do you think of my 13.5/1 ratio? Thanks.

It is not useless because it will let you know you're not lean under load. But I don't think you will be able to set your mix up with it. I used my o2 (same setup as you have, O2 sensor and Autometer A/F gauge) to get my motor running while I was waiting for my Gunson CO tester to arrive. Using the O2, I set it to be "just a bit rich" at idle. The Gunson said I was running at 13% CO using this method. Wayyy rich.

how much does a Gunson CO tester cost?

Interesting topic here as I am fighting a similar issue.

My CO reading (Gunsen Gas Tester) at idle is 6.2 (WAY RICH).

However, under load on a dyno, I am lean above 5200 RPM. Considering I track my car alot, this is bad.

Any suggestions.

Brian

Gunson costs about $160.

Brian, is your boost line running to the WUR hooked up and functioning? It's supposed to richen the mixture (via dropping the control pressure) when you are on boost. Easy enough to check in an unscientific way-with the car idling, pressurize the line or WUR port to 11psi and see if it bogs due to too much fuel. Also might want to test your main and control fuel pressures.

Yes dump the CIS and go with EFI. Or get a Andial or Rice fueling unit for the CIS. Set it up on the dyno. My car is rich at idle and goes lean with boost.

Colin:

How do I test the control pressures?

Brian

OEM 930 CIS is pretty well tuned for the car and usually runs a tad rich on boost until approx. 5000 RPM when it starts leaning out somewhat, but still on the rich side.

You can use smallband lambda to check mixture under part load and short pulls but it's just too non-linear to give you good indication of mixture.

Also, your idle-mixture has very little to do with your on-boost mixture. Fuel quantities are much bigger when on boost and you can tweak idle-CO up and down the A/F range a lot, without getting any discernible influence on full-boost A/F.

So yes, your gadget is kinda useful for telling you if you are leaning out...at least before it gets too hot. For exact numbers, get the wideband.