MFI Mixture Adjustment Stops

Wasatch 911S · 09-06-2016, 10:53 AM

I did what I thought was a pretty thorough search through all the MFI threads and could not find an answer regarding the two mixture adjustment screws.

I realize the idle mixture screw has 6 settings, but when you reach the last one in either direction does it have a hard stop or just returns to the first (1) or last (6) detent depending on which direction you are turning it? Mine seems to just keep rotating after 6 clicks. Which is not good because I really can't figure out where the middle setting is now. I am using an AFR meter so I do have a hope of getting it right.

Same question on the load mixture screw that has 12 settings? Although I have not had to turn that one as far, but would like to know if I do keep turning it will it just stop or rotate back to the first detent? Thanks.

jluetjen · jluetjen's Garage

I'm pretty sure that they are actually screws. When you reach the limit at one end you'll bottom out. I don't remember what happens if you back it out too far.

motomiata · 09-06-2016, 02:05 PM

Both screws are running on a finite threads.

if you back out the idle screw to far it will eventually hang on spring loaded adjustment screw.

The part load screw may unscrew from the rack if you keep turning.

But don't go beyond a few clicks on either. If you can not dial in you engine within a few clicks your problem lies elsewhere. Follow the CMA, adjusting the pump is the very last thing to touch. You may get totally lost if you short cut the CMA procedures. Ask me how I know.....

What AFR values are you seeing?

Wasatch 911S · 04:59 PM

Thanks for help. Breathing easier now because I got back to my starting point. For me regarding the idle screw, the spring loaded driver just started slipping when I got to the end. I was able to back it out once I got the main back to my starting point and obtain an idle
AFR of 13.7 which had the car idling and starting fine. Had no idea that adjusting the main had such an effect on the idle AFR. Should have done more homework before I got the tools out. Pretty obvious now, but getting the Main set first and then fine tuning the idle is the way to go.

I stated down this path because my idle AFR was fine but my cruise/load AFR was too rich. Between 10.8 and 11.5. I will just turn the Main one click lean, reset the idle mixture if needed, and drive the car, get new numbers, and walk it in one turn at a time until I can lean out the cruise mixture. Probably will only take two clicks of the main to get there. Thinking once I get around 13.8-14.0 on cruise and 12.5-12.8 on acceleration I should be good.