February 2001 - March 2001

Well this certainly has been the WINTER FROM HELL hasn't it?

POSA Carb tuning session #1,130,211:

One of the things I have had difficulty with in adjusting the POSA is determining why the engine is running rough. Often the engine seems to run OK at mid throttle settings but will begin to run rough at the top end. Rex Taylor's video suggests that when the engine does this it is likely too lean and the needle should therefore be adjusted out. This makes sense, however I have noticed that when the needle is adjusted out past a certain point the engine will run at high RPM but appear to go lean in the last 1/4" of throttle travel (I had the needle adjusted out around 5 turns when I noticed this). Adjusting in to around 3 turns gave a better idle and a rough running full throttle which can be improved with leaning the mixture. In short it appears that as you adjust the needle out on the RevFlow carb the mixture does indeed become richer but only to a point after which it will suddenly go lean (Egad, no wonder I drink).

It is possible that a certain minimum amount of the needle must remain inside the orifice in order for the carb to supply an adequate amount of fuel for full power operations. Seems logical, however I was surprised at the amount. The picture shows the amount of needle remaining inside the orifice at full power and 3/4 turn (A) and 5 1/2 turns (B).

At around 3/4 turn the engine runs well at idle and top end with about a 150 deg rise in EGT when the mixture is leaned. There seems to be a rough spot between 80 and 90 % throttle however. This needs to be investigated.

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