Mount a 2 stroke upside down?

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
Reply to
etpm
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It sounds goods. Most 2-strokes don't care if they run forward or backward, as long as the ignition timing is adjusted.
If you showed us your engine, I didn't see it. One exception about running backwards is if the engine has a rotary intake valve mounted on the crankshaft. That's been used on some old off-road motorcycle engines and some other high-performance types, but it's very unlikely on most other applications. I'm guessing your engine doesn't have pump/squirt lubrication, which is another limitation on running backwards.
Lots of 2-strokes run upside-down. As you say, it's a matter of whether the carb has a float bowl and which way it thinks is "up."
Reply to
Ed Huntress
Worry about plug fouling. I don't know if it'll happen or not, but oil drains down, and oil fouls plugs.
Reply to
Tim Wescott
2 strokes have been mounted "upside-rown" in ultralight aircraft for many years. Mounted that way they do tend to foul plugs and flood easily when starting, but they generally run fine after starting.
Reply to
clare
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.
But I don't think that you will do any mechanical damage by trying it.
Reply to
John B.
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.
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.
Reply to
Tim Wescott
Just a thought -- if you end up reversing rotation, make sure that there's not some feature already on the flywheel to accommodate that. I could see an engine company making it so you could just flip the flywheel over, or move the magnet to a different spot to reverse rotation, just to keep BOM costs down. You probably won't be so lucky, but keep your brain engaged when you open it up.
Reply to
Tim Wescott
I'd really like to see the engine the OP is talking about. 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.
I think that both piston-port and reed-valve intake have both been used in chainsaws. Either one will allow an engine to run in either direction, given the ignition timing issue discussed before.
Reply to
Ed Huntress
Is rebuilding an engine really the easier option here? I just can't imagine that it is.
I have a bike with an engine strapped on. It's a decade old Golden Eagle kit with the drive ring and belt and a 25cc Redmax weed wacker engine. The only engine problems have been gas tank leaks from that ethanol shit in the gas. It starts in negative temps, or with old gas. Very solid little engine.
It sounds like you're going for the engine mounted inside the triangle of the frame and not something strapped over the rear wheel though. The fake motorcycle style is all I see these days. Never come across another belt drive bike like I have yet.
Reply to
Cydrome Leader
The "Whizzer" motor bicycle, and motor kits for bicycles, was made from around 1939 until about 2009 and there are some NOS still available.
Reply to
John B.
That's the style of kit I see in Chicago. I still prefer the leafblower type as you just put a plastic bag over it and nobody knows it's a bike with an engine.
Reply to
Cydrome Leader
Me, too -- Eric, can you post a picture someplace, or a link to the seller's website?
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).
Reply to
Tim Wescott
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.
Reply to
Ed Huntress
ED-the engine is piston ported. These engines are ubiquitous online. I know they are not all made in the same factories but they are all similar. See the link:
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I had, years and years ago, a Yamaha 80 that used the rotarty valve setup. It was interesting because the carb was inside the engine case on the right side. I think Kawasaki made a similar engine. Also interesting was the fact that there was a Honda motorcycle that not only looked a whole lot like the Yamaha but had some parts that were interchangeable. Like the front forks. These weren't the telescoping type forks so to swap forks the whole front end needed to be changed. Eric
Reply to
etpm
Aha. Yes, your basic piston-port 2-stroke.
There sure were a lot of oddities in those early Japanese bikes. They were pretty clever.
Reply to
Ed Huntress
Don't your "plastic bag" make a funny noise when you are driving?
Reply to
John B.
On Wed, 21 Oct 2015 14:01:25 -0700, snipped-for-privacy@whidbey.com wrote as underneath :
I just wonder if the ignition in a point-less system will work so well if the flywheel rotation is reversed? With the points type magneto it makes no difference but with the pointless type the system is optimised with a mapped timed firing by potted electronic components built into the coil assembly (hence the modern easy start 2-stroke systems) and the coil can be much smaller and cheaper as the UHV is generated by electronic circuitry rather than directly by the magneto. So- maybe the mapping will not make the corrections in the right direction etc.? I would anyway try running it in reverse with an electric drill etc. to see how the ignition might work if you do this?! Just a thaught - I havnt tried so absolutely no actual experience with running this type of ignition in reverse, someone else here might have?! C+
Reply to
Charlie+
The bag is for bringing it inside buildings where bikes are ok but not other "vehicles".
Reply to
Cydrome Leader
Thanks for the reply Charlie. It would be a good idea to see when the spark occurs. If, for example, it happens at 12 degrees before TDC then I need to make sure it happens on the other side of TDC since the crankshaft will be spinning the other way. Using a drill to spin the engine so I can watch the spark timing is a good idea. The electronics for the ignition are located away from the engine, it appears that the magneto coil is only a coil and has no device for sensing crankshaft position, so all the timing is done in the black box that is mounted away from the engine. But I still need to see what the timing is to make sure I get it right when reversing the engine. Eric
Reply to
etpm
Many of the very simple old 2-strokes just relied on the magnet *leaving* the field to time the spark. Sort of time, that is. As the motor speeds up, the spark timing actually can advance a few degrees that way.
You may be able to tell from the relative position of the magnet in the flywheel, to the magneto coil, where the timing is. In any case, it will give you guidance about setting it on the opposite side of TDC if you want to run the motor in reverse.
Reply to
Ed Huntress

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