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Old 10-11-2007, 01:16 PM   #1
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Will a two stroke engine run backwards?

Riding the dirt bike a few weekends ago with some quads. One guy parked his '87 Quadzilla on a hill, and it was slowly rolling backwards as the compressoin bled off.

Someone shouted that the thing might take off if it starts. . . . .

Well, is that possible?
Danger is one thing. But danger plus extreme discomfort for long periods is quite another.
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Old 10-11-2007, 01:20 PM   #2
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2 strokes

I never got a two stroke to run backwards, but I did get a old Ossa one time to run with the piston installed backwards. It popped and really did not want to run but it did, but not well. As soon as I put the piston in the right way all was fine, except the Ossa part.
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Old 10-11-2007, 01:27 PM   #3
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It was possible, the old bikes that had pionts could run back-wards.(never seen it)
I dont think anything newer than that will.
'cept for a dump truck I had (6/53 Detroit diesel).
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Old 10-11-2007, 01:39 PM   #4
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Have a Dyna III ignition on my old RD400. It would run backwards.
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Old 10-11-2007, 02:33 PM   #5
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I just saw a golf cart from the 60's, with a Harley-Davidson name tag, that had a two stroke motor and a starter that rolled it one way when you had the lever in forward, the other for reverse.

Every time you hit the gas pedal the starter kicked in until the engine took off.
Seemed to run just as well in either direction.
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Old 10-11-2007, 02:43 PM   #6
What's an estart?
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Ski-Doos and Polaris snowmobiles retard timing to fire before TDC when in reverse. Engine runs backwards.
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Old 10-11-2007, 02:46 PM   #7
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Had a 71' Suzuki 125 that accently bump started backwords once. I stalled it on a very small embankment in a friends front yard. Somehow bump started it as I backed down without knowing. Once at the bottom I let the clutch out to move forword and went backwords instead. Back then I also saw a friends old Yamaha mini enduro run backwords too. While running backwords he accidently touch the kick starter, not good. The engine locked up instantly.

As i recall two strokes don't sound quit right when there running backwords either.
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Old 10-11-2007, 03:03 PM   #8
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I think it was the detroit 60 series diesels that when you started them, you'd keep an eye on the oil pressure guage and if you didn't get oil pressure in 15 seconds or so you'd turn it off, as it meant the engine was running in reverse.

However, that was with diesels. There are many old applications for 2 strokes, especially marine engines, that would reverse rotation but the timing was always changed in order to do so.

I could see an old point igniton possibly firing in reverse off the backside of the cam, however if it were to do the the advance would retard the spark even more and prevent it from running. Anything made in the late 80's is going to have an electronic ignition and that will be impossible to run backwards. (someone chime in if they know anybody who still had points in 87)

I suspect the guy was thinking about diesels and talking out his ass at the same time.
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Old 10-11-2007, 03:20 PM   #9
What's an estart?
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Originally Posted by GreaseMonkey

I suspect the guy was thinking about diesels and talking out his ass at the same time.
No, he wasn't. If a two stroke engine gets bump started by rolling backwards, the engine can run in that direction.
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Old 10-11-2007, 03:32 PM   #10
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There was an old design of an outboard motor, that had to motor run in reverse for reverse. Had to kill the motor, and re-start it to go backwards..
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Old 10-11-2007, 04:11 PM   #11
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Yep. I've had a TS125 and a KTM495 take off backwards.

A mate of mine had a trials bike almost stall, cough and keep running at the bottom of a section. He grabbed a big handful, dumped the clutch and launched himself over the bars as the bike headed for the bottom of the hill...
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Old 10-11-2007, 05:54 PM   #12
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On a semi-modern bike it seems very unlikey (I've never seen it, been racing/riding two stroke mx bikes since I was 11yo), but theoretically it might be possible. The ignition timing would be seriously retarded and I suspect some bikes might be smart enough not to do it at all (most newish mx bikes have computers of sorts to vary ignition timing based on a few factors, don't know if it would notice the bike running backwards). Also, my CR250R has enough compression that it would be really hard to accidently start forwards, backwards would be near impossible even if it would do it.
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Old 10-11-2007, 06:16 PM   #13
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Back in the 70s my buddy had one. 65cc piece of shit. It would outrun my SL70 but it would miss shifts and I would pass him.

He started it one day, put it in gear and it went backwards!

It was funny as hell.

Harley Davidsons!
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Old 10-11-2007, 06:27 PM   #14
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Had a 60s Vespa 160cc 2 stroke brought into the shop one day. Customer said it was hard to start and would only go backward. I started it and sure enough it kicked back and ran backward. I tried to ride it that way but I'm just not that good of a rider.

Turns out the customer had taken off the point cam and installed it backward, thereby changing the timing from about 15 degrees BTDC to 15 degrees ATDC. Not all 2 strokes will run backward, depends on the engine design.

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Old 10-11-2007, 06:49 PM   #15
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They will run backwards

Had a very old suzuki 90, circa 1970'ish, points, dead loss ignition, flywheel removed for 'snappy acceleration' .. used to start it up a certain way, hand the bike over to my buds then watch them launch over the bars each and every time. Long time ago, pos bike.
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