Hi guys, So I've noticed that the air flow sensor plate on my 81 GTSi sits a little high. Maybe a mm (but not 2) high on the far right side. I know there is a pin hiding under the fuel distributor inlet that I can tap down to adjust the plates resting setting (then redo the mixture adjustment and idle). My question is how sensitive is this? My car is running really well and a mechanic friend said if it seems to be running fine don't mess with it (wise words that I have never been able to follow). Other than it was designed to be flush with the narrow part of the cone what am I giving up by having it off a small amount? FWIW the car starts immediately when cold. Takes a bit of cranking when on a warm restart. Better now that I recently replaced the check valve on the fuel pump and a brand new fuel accumulator (there was a small leak in the diapram). There are no flat spots or hesitation when accelerating. Thanks, Jay
Are you measuring the position of the airflow metering plate with the fuel pump running as is specified? See pages D29 to D31 here: https://www.dropbox.com/s/z99soo68ny9n2qa/Mondial8qv_workshop_281-83.pdf?dl=0
As always Steve, you have quick replies with great info. So yes, went through the procedure in the pages you posted and the sensor plate is just about 1mm high. Mostly curious what exactly this affects. Seems to me that for a volume of air flow (how much the sensor plate is open) it has moved a tiny bit more than it would have compared to a perfect alignment. But... isn't that what the 3mm mixture screw is for to adjust to the correct fuel delivery based on sensor plate height? Intuitively it seems like if it were way off (high) and the mixture was set at idle then the more it opened the leaner it would get. Car runs great right now but how could it be better if I adjust the resting sensor height? Thanks
Yes, I'd agree that the mixture screw is adjusted to move the plunger to the correct position for where the airflow entering the engine at idle is deflecting the airflow plate so that should all be OK. It is a bit strange that for how "critical" Ferrari/Bosch seem to make this issue, they don't give any indication of what trouble might result if not perfect. My only guess is that if the airflow plate resting position is much too high = maybe the very low vacuum (low airflow) of starter motor cranking won't be enough to draw the airflow metering plate down at all (so never starts). If the cylindrical portion of the edge of the airflow metering plate still overlaps some of the cylindrical portion of the funnel, I'd think it would still work OKish.
I would agree that should be the symptom. The mixture may be set overly rich to compensate. We have no way of knowing. I do know from experience they idle and start better with a properly set plate if you are mindful of and have proper equipment to make all settings correct. There are few maladies with CIS that cannot be covered up or mitigated by some adjustment but it will effect some operation mode to some degree however slight. Its difficult to rate anything if you are not fully familiar with its operation when everything is as it should be. In the process of tuning any CIS equipped car, sensor plate height is first thing I do.
Just by observing the system from the outside this was my guess: If the plate sits a little high (1mm I'm guessing in my case, 0.5mm too much to spec) then at idle the system thinks that the plate is open just a bit more than it really is. The sensor has moved 3 mm but has only opened 2mm to air flow. It thinks more air is coming in than really is so to set it right at idle you have to lean it out a small bit from what it expects. Under acceleration when to sensor door is more open I assume that it is closer to the actual amount of air entering and the now the mixture is actually going a bit leaner.
