MFI Question

[denson]

Happy New Year to all:

My car is 1970 911 S. The MFI system is functioning rather well with two small issues ( I hope) When cold I get popping to the air filter housing. It seems to go away once the car is warm. The second issue is that every once in a while, I get an exhaust pop on deceleration.

Seems strange as I know that pops to the airbox mean it is running lean. I discounted that as it only happens when cold. Pops from the exhaust ussually mean the car is running rich. I do not think it is rich and have heard that MFI does sputter and pop from time to time.

The car otherwise runs fabulously. I fear having my mechanic work on it as I run the risk of other issues cropping up.

Any thoughts you may have are appreciated.

John

[304065]

Properly adjusted MFI does not sputter and pop, it screams like an angry chainsaw.

It's impossible to hazard a guess without an air-fuel ratio measurement, do you have access to an LM-1?

The whole procedure for diagnosing issues like this is laid out in CHECK MEASURE ADJUST, which is a systematic procedure for running down issues like popping, etc.

It can be downloaded here:

http://forums.pelicanparts.com/showt...+MFI+resources

Good luck! When the car warms up the thermostat is supposed to lean things out, which would only make the popping worse, which is why I can't recommend you start changing mixture settings without an analyzer. Also, most fuel problems are really ignition problems, so going through the CMA steps will help isolate the problem. The FINAL step in CMA is adjusting the pump.

[boxster03]

contact Grady. Or replace with carbs. Or do the check measure adjust. Then spend a lot of $

[hbrand]

I have done a lot of work on 911 MFI engines, and popping through the intake was almost always worn valve guides.
Remove the valve spring with piston on top, with air pressure in the cylinder and check valve to guide play. Takes only one hour to do.
Harold