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AIRCRAFT;
All Rotax powered ultralight aircraft using Mikuni fuel pump.
Problem area;
Improper fuel pump mounting
Incident report;
Ultralight pilots flying various craft have reported their engines quitting for no
apparent reason. Most failures have been in hot humid conditions, after 10 or 15 minutes
of flying. Once down on the ground the engine starts and runs as if nothing was wrong, and
may go for days until it happens again.
Upon closer inspection it was found that the Mikuni fuel pump had been mounted directly to
the engine block below the carburetor, where two threaded holes that are just the right
distance apart to mount the pump are located. What happens is the heat from the engine is
transfer to the fuel pump causing vapor lock, the pump fails, thus not supplying any fuel
to the engine, resulting in an engine failure.
Once down on the ground the pump has cooled down enough to start working again, thus the
pilot in most cases is unable to locate the problem.
Suggestions ;
Change the mounting of the fuel pump so that it is in free air, no more than 11 inches
from the engine vacuum source, and away from any heat sources. Or when using the holes on
the engine block to mount the pump install insulating blocks between the pump and engine
block, so that heat can't be transferred from the engine to the pump.
One way of doing this is to drill out the fuel pump holes where the mounting bolts go
through, install a short piece of gas line, in the hole, install a bolt with a washer
against the head, into the gas line, through the newly drilled holes, install a washer and
then a nut, threading the nut up so that the gas line acts as an insulating block and
vibration damper, then tighten the bolt into the threaded holes directly below the carb.
When mounting the pump make sure that the little vent hole, on the face of the pump is
facing down to allow it to drain excess liquid from pumping chamber. Also make sure that
the vacuum line used is of the proper type and thickness, the use of regular gas line IS
NOT RECOMMENDED as the side walls tend to collapse causing fuel
pump failure!
Clamp all hoses with proper fuel line clamps. |
