On Fri, 23 Oct 2015 12:27:13 -0500, Tim Wescott
wrote:

On Thu, 22 Oct 2015 09:03:57 -0400, Ed Huntress wrote:

On Thu, 22 Oct 2015 00:00:13 -0500, Tim Wescott
wrote:

On Thu, 22 Oct 2015 08:53:44 +0700, John B. wrote:

On Wed, 21 Oct 2015 14:01:25 -0700, wrote:

Because of the size and the way my bicycle frame is shaped it may be
more convenient, strictly from a mechanical point of view, to mount a
2 stroke bicycle engine upside down. Maybe upside down and backward.
If I did this the carb would need to be inverted because it is a float
bowl type carb. I suppose I could use a pumper type carb instead but I
don't know if I have one that is suitable as far as fuel and air flow
are concerned. Besides, small engine pumper carbs tend to be kind of
on/off devices in that they idle OK and and run wide open well but
don't do so well in the mid range throttle settings. I suppose I could
buy one but I'm cheap. So, if there is room to invert the carb what
else do I need to worry about? Will the crankcase tend to get too hot
since it will now be above the cylinder? And lets say that the engine
is not only inverted but also turned around. Now the engine will be
rotating the wrong way to drive the bike forward. Since the engine is
a two stroke it seems to me that I will only need to change the
ignition timing. I think this can be done simply by broaching a new
keyway in the spinning magnet flywheel. The ignition is a fully
electronic CDI type with no points. I assume the ignition works by
sensing the voltage rise in the magneto primary winding as there is no
other provision for detecting the position of the flywheel magnet.
Have I missed anything?
Thanks,
Eric

Model airplane 2 stroke motors are frequently mounted upside down and
run all right although if you flood one it might be a bit more
difficult to start and chainsaw run all right upside down.

As for running backward, I'm not sure of the efficiency as some modern
2 strokes use some pretty exotic porting that may be rotation
directional in nature. I'm leaning on model engine experience but some
glow plug designs of model engines seemed to run in either direction
with no problems and other, different in design, wouldn't seem to run
backwards at all.

The rotation direction thing is what Ed was referring to -- most model
airplane 2-strokes have intake ports that are timed by the crank, and
that lead the piston by a considerable amount. This makes the engine
prefer to run in just one direction. Cox reed-valve engines are
direction agnostic, as are the really old piston-timed engines.

I'd really like to see the engine the OP is talking about.

Me, too -- Eric, can you post a picture someplace, or a link to the
seller's website?

Chances are
that it's a piston-port engine, as most bike motors have been since the
beginning. My old O&R bike motor is a cross-scavenge, piston-port engine
-- the basic 2-stroke design that powered everything including
lawnmowers and ancient washing machines, and was used in all sorts of
applications where you're after low cost and smooth running, rather than
performance.


Weed-whacker and chainsaw motors are, to my knowledge, piston timed,
with the intake port to the crankshaft opened and closed by the skirt of
the piston rather than by the crank or a rotor attached to the crank.
So, they'll run pretty much the same in either direction, once you get
the spark timing sorted out.

I think that both piston-port and reed-valve intake have both been used
in chainsaws.

I could see that -- chainsaws have a bit more need to be high performance
in a small package than a lot of other 2-stroke applications. I suppose
I wouldn't even be surprised at a crank-timed one (I'm kind of surprised
that crank-timed 2-strokes only seem to be ubiquitous in model airplane
use, and even there the really big ones are piston-timed, either because
they've been repurposed from weed-whacker engines, or because they've
been re-designed from such engines).

I've actually had my hands on only two rotary-valve 2-strokes: my old
McCoy Red Head .35, which had a hollow crank with a window in one side
for a port (very strange), and a Yamaha 175 dirt bike. I put a "Git
Kit" in that bike, which included a new rotary-port disk, with
different timing.

--
Ed Huntress

Either one will allow an engine to run in either
direction, given the ignition timing issue discussed before.