Engine stumble, Any thoughts?

My 172M doesn't have any stumble for some reason. It is the rpm range
generally less than 1500 rpm though, where the carb has to evolve from
the idle jet system to the main jet. Carbs have a problem in this area
because this transistion simply involves a gradual change of the jet
system design, yet has to keep the mixture within a burnable range
(about a 9:1 to 18:1 air/fuel ratio) or there will be a stumble. I
don't know how carb designers (in the 30s I suppose) got it to work as
well as they do.

The main jet system is simple. The idle jet system is always a wierd
collection of ports etc.

Is it a lean stumble? or is it maybe a rich stumble?

When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off. My guess is a rich
stumble. But that's just a guess, eh?

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

"nrp" wrote in message
oups.com...
My 172M doesn't have any stumble for some reason. It is the rpm range
generally less than 1500 rpm though, where the carb has to evolve from
the idle jet system to the main jet. Carbs have a problem in this area
because this transistion simply involves a gradual change of the jet
system design, yet has to keep the mixture within a burnable range
(about a 9:1 to 18:1 air/fuel ratio) or there will be a stumble. I
don't know how carb designers (in the 30s I suppose) got it to work as
well as they do.

The main jet system is simple. The idle jet system is always a wierd
collection of ports etc.

Is it a lean stumble? or is it maybe a rich stumble?

When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off.

That's very true for a cold engine and especially with autofuel (which
I use in the 172M). In this case he was using 100LL in a warm engine,
so it wouldn't seem so likely.

I have never heard before of the kind of stumble given by the OP. It
could be a rich flash afterall.

"nrp" wrote in message
oups.com...
When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off.

That's very true for a cold engine and especially with autofuel (which
I use in the 172M). In this case he was using 100LL in a warm engine,
so it wouldn't seem so likely.

I have never heard before of the kind of stumble given by the OP. It
could be a rich flash afterall.


I think you would find it to be true for warm engines also. There is a good
film of fuel on just about the whole surface of the intake manifold in just
about any carbureted engine.

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.