July 30th 04, 12:50 AM
It has been my experience that autogas runs definitely leaner
carburated engines. I know autogas also has a higher specific gravity
- although I don't know the numbers off hand. But it does mean that
the calibration of a carb venturi will change - probably being leaner
for autogas. That you had the greatest problem in the winter, when
the air is most dense, also suggests that your engine was marginally
starving for fuel most of the time for whatever reason. It is
probably also still running lean on avgas.
An earlier post somewhere else accused autogas of causing carb ice
because his engine ran rough unless he used carb heat. I'd bet icing
isn't his issue, as carbs will naturally run richer with a higher carb
air inlet temp. He too was probably marginally lean even with avgas
under most conditions, but that it took carb heat application just got
him to a runable condition with autofuel.
I knew of a freshly overhauled O-300 powered 172 that then required
carb heat on climbout to run smoothly. Everyone assumed it was
somehow icing, but it went less than 100 hours before it burned a
piston from too-lean operation.
After complying with the one-piece venturi AD in our 172M, things were
so lean (especially in the winter) that we had to change the main jet
nozzle to get it to run right. I understand others have reported
problems in this area too.
When an engine is cold, idle operation with autofuel will behave
differently again, with liquid fuel accumulating on the walls of the
intake manifold. This liquid will go into the engine and cause
rich-induced stumbling on power application. The real question for
autogas vs avgas - is the engine operating at a reasonable mixture
over a wide range of air inlet temperatures when warm? What symptoms
can be expected when when the carb is out of a reasonable calibration?
Engine air intake systems are critical, and an engine/carb system that
runs well in one installation won't necessarily run even well in
another.
I'm an advocate of having an EGT (even if only temporary) for any
homebuilt, or any other installation that is showing mixture problems.
carburated engines. I know autogas also has a higher specific gravity
- although I don't know the numbers off hand. But it does mean that
the calibration of a carb venturi will change - probably being leaner
for autogas. That you had the greatest problem in the winter, when
the air is most dense, also suggests that your engine was marginally
starving for fuel most of the time for whatever reason. It is
probably also still running lean on avgas.
An earlier post somewhere else accused autogas of causing carb ice
because his engine ran rough unless he used carb heat. I'd bet icing
isn't his issue, as carbs will naturally run richer with a higher carb
air inlet temp. He too was probably marginally lean even with avgas
under most conditions, but that it took carb heat application just got
him to a runable condition with autofuel.
I knew of a freshly overhauled O-300 powered 172 that then required
carb heat on climbout to run smoothly. Everyone assumed it was
somehow icing, but it went less than 100 hours before it burned a
piston from too-lean operation.
After complying with the one-piece venturi AD in our 172M, things were
so lean (especially in the winter) that we had to change the main jet
nozzle to get it to run right. I understand others have reported
problems in this area too.
When an engine is cold, idle operation with autofuel will behave
differently again, with liquid fuel accumulating on the walls of the
intake manifold. This liquid will go into the engine and cause
rich-induced stumbling on power application. The real question for
autogas vs avgas - is the engine operating at a reasonable mixture
over a wide range of air inlet temperatures when warm? What symptoms
can be expected when when the carb is out of a reasonable calibration?
Engine air intake systems are critical, and an engine/carb system that
runs well in one installation won't necessarily run even well in
another.
I'm an advocate of having an EGT (even if only temporary) for any
homebuilt, or any other installation that is showing mixture problems.