Why does fuel flow go UP when leaning on the ground?

I observed a bizarre thing this morning. We were in a 200 HP Arrow-II
equipped with a JPI FS-450 fuel computer and were talking about leaning on
the ground to prevent plug fouling.

With the engine running at about 1000 RPM shortly after startup and the
brakes on, we were drawing 2.6 GPH. EGT was about 1000 F. As we leaned,
we got to the point where the engine started to run a little rough, the EGT
went up to about 1100 F, and (here's the strange part), the fuel flow rate
went UP to 3.2 GPH. Does this make any sense? Why would leaning make the
fuel flow rate go up?

We ran through a couple of rich-lean cycles on the mixture, and the
readings were repeatable. What's going on here?

On Sat, 18 Mar 2006 19:21:29 -0500, Roy Smith wrote:

I observed a bizarre thing this morning. We were in a 200 HP Arrow-II
equipped with a JPI FS-450 fuel computer and were talking about leaning on
the ground to prevent plug fouling.

With the engine running at about 1000 RPM shortly after startup and the
brakes on, we were drawing 2.6 GPH. EGT was about 1000 F. As we leaned,
we got to the point where the engine started to run a little rough, the EGT
went up to about 1100 F, and (here's the strange part), the fuel flow rate
went UP to 3.2 GPH. Does this make any sense? Why would leaning make the
fuel flow rate go up?

We ran through a couple of rich-lean cycles on the mixture, and the
readings were repeatable. What's going on here?

I suppose the reason for the 25% uplift in fuel consumption could have
been due to a 12% increase in revs?

If not, an artifact of the fuel computer, I'd think?

Brian Whatcot Altus OK

Probably due to hysteresis in the mixture cable. It would be impossible for the fuel flow to go up if the mixture was working properly and there were no hysteresis in the system.

Make sure you only go one direction with the turning of the mixture control. You can't be going back and forth twisting the mixture control. One way only and log your data.

Karl
ATP CFI ETC

"
wrote in message .com...

Probably due to hysteresis in the mixture cable. It would be impossible
for the fuel flow to go up if the mixture was working properly and
there were no hysteresis in the system.

Make sure you only go one direction with the turning of the mixture
control. You can't be going back and forth twisting the mixture
control. One way only and log your data.

Karl
ATP CFI ETC


--


You'll have to excuse my ignorance. Are you saying it could be related to
magnetism?

A hysteresis loop in this context refers to the idea of a lag between
cause and effect. The mixture control cable is not going to accurately
100% of the time precisely represent your mixture knob movements. Slack
and stretching in the system can make it such that, for instance, when
you change the direction you are turning the knob, the actual mixture
setting on the carb (or equiv for FI systems) might not move exactly
the same at the same time. Imagine if the cable was very sticky for
instance. You're not going to get ideal response from your inputs.

As for whether or not that's your reason for increased fuel flow, I'm
on the fence. I like the increased revs theory too...

-Ferdi