can 2 stroke engines run backwards ok?

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steve.emma

Member

Oct 21, 2002

285

0

• Oct 7, 2007

• #1

ok first up this thread has nothing in particular to do with bikes or any type of bike engine in particular. the story is that i have an old racing go-kart frame in my garage that has been sitting around for ages. anyway i was thinking about repowering it with a spare 2stroke lawnmower engine that i also had lying under my bench. the problem is that the way that the engine would best mount to the frame means that the engine would spin backwards. now what im wondering is what would be the problem with this? i would not be using the pull start so therefore what else would be affected by the engine spinning in reverse? would the fact that the thrust side of the piston would now be against the intake side matter?
any light on this topic would be useful. i remember seeing a thread on here ages ago about a guys whose bike somehow ended up running backwards, and it scaring the crap out of him when he let the clutch out! :laugh:

 

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cujet

Member

Aug 13, 2000

826

5

• Oct 7, 2007

• #2

Yes, they will run backwards. Two stroke golf carts often run the engine backwards to reverse.

However piston pin offset along with cylinder offset and ign timing will make the engine have issues in reverse.

Chris

 

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CRazy250

Member

May 28, 2006

334

1

• Oct 7, 2007

• #3

what kind of kart/ motor is on it? ive got a old 73 bug stinger kart

 

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steve.emma

Member

Oct 21, 2002

285

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• Oct 7, 2007

• Thread starter
• #4

the frame is a complete kt100 type kart and the engine is a 4 horse tecumseh 2stroke motor. the motor is a vertical shaft type but other than the carb being turned around i can deal with that. i didnt think about the ignition timing issue though, wonder if there is a easy way round that?

 

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dpaxson

Member

Sep 30, 2006

54

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• Oct 7, 2007

• #5

it might run but not very well. Unless the ignition is set up for the spark to go at exactly TDC (which I doubt) the spark will go off at the wrong time in reverse. i've never tried this but if you could rotate the flywheel so it is the exact opposite of where it is now, then it would run fine.

 

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Richter415

Member

Jun 23, 2006

85

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• Oct 7, 2007

• #6

Ski-doo snowmobiles electronically reverse the engine for the Rotax Electronic Reverse program. Polaris now also has this system thanks to a law suit but they seem to run fine. Although rotax engineers are much smarter than I

 

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steve.emma

Member

Oct 21, 2002

285

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• Oct 8, 2007

• Thread starter
• #7

the ignition setup on the mower engine is pretty basic. looks like a magnet set into a allow flywheel which then passes next to the coil centre to generate spark as it moves around. havent pulled the motor to pieces yet but maybe the flywheel could be offset on the crank somehow to change the timing? will have a better look when its apart.
the info about snowmobiles and golf karts running their engines backwards to get a reverse gear is interesting. does that mean that their starter motors spin the engines in both directions as well? or do they have a extra gear on the starter for reverse direction (like a gearbox does)? i guess the issue of engine performance is not such a big deal in reverse for a golf kart in terms of ignition timing.

 

Jaybird

Apprentice Goon

LIFETIME SPONSOR

Mar 16, 2001

6,449

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Charlestown, IN

• Oct 8, 2007

• #8

The generator of a reversing 2T also serves as the starter.
The old engines with points have two timing marks on the flywheel, one for forward and one for reverse. Timing must be set using both marks. I think you can split the difference between the two timing marks, but I think I remember it being better to set in favor of the forward running mark. The reverse mark can be off a smidge and the motor will still run fine in reverse, due to the generator not functioning when in reverse (meaning the unit does not charge in reverse, so no electrical load is seen, therefore less resistance from the starter/generator).
I imagine mapped electronics takes care of the timing on newer models.

 

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stumanarama

Member

Aug 29, 2007

306

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• Oct 8, 2007

• #9

i was thinking about this long ago with my old motor scooter and always wondered....but now that im a little older and really think about it, i guess the ignition would be the only thing, i would say that if you could retard the flywheel/timing properly there shouldnt be any real problem(except maybe the cooling fins on the flywheel)however i could be wrong

 

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steve.emma

Member

Oct 21, 2002

285

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• Oct 9, 2007

• Thread starter
• #10

just thinking about the timing issue....if a engine would fire its spark say 6 degrees before top dead centre (for example). then if it was run in reverse direction the spark would be a long way retarded wouldnt it? so in that case if there was a way to massively advance the timing in the first place,then it would be right for the engine if it was used in the reverse direction. maybe in the case of a mower motor by remounting the coil in relation to the flywheel or something?

 

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dpaxson

Member

Sep 30, 2006

54

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• Oct 9, 2007

• #11

you could maybe remove the flywheel key and just rotate the flywheel to the right position. I don't know if it would stay though without the key.