TheSamba.com :: View topic - setting and resetting the AFM...wont stay put!!

Mon Jul 14, 2014 11:35 am

I have been working on adjusting the air/fuel mixture on my 81 Westy (aircooled) the get head temps down. I have followed the procedure as outlined by 'amskeptic'. Standing over the motor, I get it dialed in quite nicely. I go for a quick drive, recheck and none of the arms have moved/loosened, but it reverts back to running lean, and therefore hot. Any idea how/why this is happening?

Mon Jul 14, 2014 12:40 pm

I am not familiar with the documentation, but Bosch never intended for the air flow sensor to be adjusted other than the volume screw. In other words, not prying the cap off and tweaking the points or spring.

There are a number of things that can cause the engine to lean out. You did not mention if this engine has an oxygen sensor. If it does, it will fight you all day long. Engine temp also affects mixture. The hotter the engine vs a cold engine the less fuel it needs. The ECU richens the mixture for cold running. The fuel pressure regulator also controls mixture. By controlling the amount of fuel returning to the tank, it determines the system pressure. More pressure, yields more fuel per injector pulse, lower system pressure, less fuel per injector pulse. If there is no Oxygen sensor, then the easiest way to run the engine richer is by adding resistance with resistors into the head temp sensor. It tricks the ECU into thinking it is cold out. This saves tweeking on the air flow sensor. By an assortment of resistors from radio shack. 100K ohm to 500K ohm. One should give you the desired richness.

Mon Jul 14, 2014 1:08 pm

I say to just drive it and keep an eye on the CHTs.

Mon Jul 14, 2014 2:54 pm

Its all sorted out now...a bit of a brain fart on my part... :oops:

Mon Jul 14, 2014 5:19 pm

So what happened? Spare the next compleat idiot with your enlightenment.