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Jim,
I had the same problem with my IO360 (non-turbo). In addition to the
symptoms you mentioned, I also experienced low fuel pressures in cruise
when the boost pump was off. Fuel pressures were running at 17-18 psi.
when it should be 24 -26 psi. After replacing the mechanical fuel pump
twice, flow checking the throttle servo, replacing the fuel dist.
spider, and cleaning, then replacing the injector bodies, I finally
discovered that air was being injected into the fuel by the Dukes boost
pump. This only happens when it's off. Hence, when taxiing and the
engine stumbles, and you turn on the boost pump the problem goes away.
Try this little test after you've warmed up the engine and are
experiencing the rough running engine. Remove the cowls and connect a
clear plastic line from the throttle servo to the fuel distribution
spider. Use some sort of clamp to temporarily hold this line in place.
You may need to run the boost pump to fill the line with fuel. Try to
get most of the air out of this line as possible. Restart the engine
and have a buddy watch the clear fuel line for bubbles coming from the
throttle servo - there should be none. I think you'll find bubbles
coming through the line. If so, next, connect a fuel line from the
gascolator to the mechanical fuel pump bypassing the the boost pump.
(This assumes that the boost pump is in series between the gascolator
and the mech. fuel pump). Restart the engine. I think you'll find that
the bubbles are gone and the fuel pressure will be higher and steady and
the engine will run smoothly.
If this solves the problem you need to replace the Dukes pump. Don't
fly the airplane with the temporary lines in place or boost pump shunted
out! Take great care not to run your new Dukes pump dry. They need a
constant presence of fuel to operate. I shut off the fuel after
shutting down the engine so that the fuel doesn't drain back into the
tanks - I don't have a header tank.
Good luck. I hope this solves your problem.
Hal Woodruff
N360HW
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