Changing MFI AFR's

BSNMOE · 08-14-2009, 04:40 PM

My 2.4ltr S was set up at a local shop to a AFR of 13.1. I as wondering, if I wanted to experiment a little, how many clicks rich would I have to go to get in the 12.8 range?

356RS · 356RS's Garage

Are you talking about the idle mixture? If so, you can try 2 clicks clockwise on the MFI pump idle mixture control (hex headed/spring loaded) thats in the center of that small metal cover. Do you have a AFR meter?

BSNMOE · 08-15-2009, 12:10 PM

Mark, I don't have an AFR meter, that's why I asked. I have MSD and was going to experiment with plug gaps, maybe a hotter plug so I was going to enrich the mixture. When the shop had the car I was running #8 NGK plugs gapped at .040. I want to go a little more to the rich side on the main and open up the plug gap and maybe #7 NGK's. I've read on some posts these cars like it in the 12.8 range. Without a meter I would have to run it and pull a plug so I was wondering if anyone would know about how many clicks to go from 13.1 to the 12.8 range? Then I'll adjust the idle.

tobluforu · tobluforu's Garage

Damn, that's almost like throwing darts in a sense?? I wish I could help, and the only in put I have is to get a wideband, add a bung on each header/HE and adjust that way. These cars are so different from one another in regards to timing, stacks, cams, etc. that it's hard to throw a number out there.
Good luck

356RS · 356RS's Garage

toblufore is right. But if you want to experiment, go 3 clicks counter-clockwise on the main rack and take a long test drive. Remember the main rack is going to change the whole range from idle to WOT.

BSNMOE · 08-16-2009, 04:22 AM

I went 1 click rich on the main and 2 clicks lean on the idle,cleaned a plug after warming up, drove at 3-4000RPM's and up to 7000RPM for 1/2 hour. No real change on the plug or tail pipe! Then I went 1 more click rich on main, 2 clicks lean on idle and repeated and the plug and tail pipe was black & sooty. That 2nd click rich appears to be too rich.

356RS · 356RS's Garage

Somethings not quite right. I wouldn't experiment anymore with the adjustment screws. I'd go back and start over with the "Check Measure Adjust" on your engine to make sure everything is set and performing correctly.

BSNMOE · 08-18-2009, 03:52 AM

I believe I've found the answer on Early 911S site. A guy stated that the reading on his AFR meter changed from 13.8 to 13.4 with one click on the main. Therefore: mine was set at 13.1 so one click rich would be 12.7 which is were I wanted. One more click from there would go to the 12.3 range which is too rich as my plugs and tail pipe indicated.

356RS · 356RS's Garage

Great news. Glad that it worked for your engine. An AFR meter is a must for tuning an MFI engine. The 1 click 2 click method is not the same for all engines and is not really set in stone that it will work the same for a 2.0S and 2.4S. Anyway, I'm glad it worked for you.