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Unread 04-15-2017, 08:38 PM   #1
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Default Yahama g2 two stroke?!

So sometimes when I stop my engine will start running backwards, the only engines I know that can do this are two stroke diesels so what the heck is going on?
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Unread 04-15-2017, 10:35 PM   #2
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Default Re: Yahama g2 two stroke?!

I would pull the flywheel & see if the key is sheered off. that will make em run bakards.
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Unread 04-16-2017, 02:48 AM   #3
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Default Re: Yahama g2 two stroke?!

Quote:
Originally Posted by yamarancher View Post
I would pull the flywheel & see if the key is sheered off. that will make em run bakards.
Yeah, a 4-cycle could run backward if you put the carb on the exhaust. The cam is still spinning backwards, which will turn the exhaust into the intake.

The 2-stroke Yamahas did run the engines backward to achieve reverse "gear".
Only 2-stroke Yamahas were G1 & less common G3 were commonly sold in US. The 2-stroke has a big "horizontal log muffler" in front of the engine.

All G2a were 4-stroke, have a reverse gear in the rear axle, and don't actually run backward (it might backfire, but that's not "running backward").
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Unread 04-16-2017, 08:17 AM   #4
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Default Re: Yahama g2 two stroke?!

It does run backward, I figured out my power problem, it's got good spark but does not fire on every compression stroke. So unburned fuel is in the exhaust and that is the fuel for it to run backwards.
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Unread 04-16-2017, 01:32 PM   #5
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Default Re: Yahama g2 two stroke?!

Hmm, well that will be interesting. If spark is intermittent, where i s it getting spark while running backward? Spark would be something like 6+ degrees ATDC. Maybe there is something to that flywheel theory after all, since we are thinking there's plenty of fuel in the exhaust to run it somehow. But, I would put a timing light to it & see if it's "missing some sparks", or if timing is just "all over the place". While there are no timing markers on the engine, you can turn the crank over to TDC on the compression stroke & make some approximate TDC marks on the clutch pulley & the block with a paint pen or something.
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Unread 04-16-2017, 03:06 PM   #6
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Default Re: Yahama g2 two stroke?!

Mine did that when it got hot, it would sometimes do it starting hot. I took the shrouding off and found oil leaking from the rocker cover had plugged a lot of the cooling fins. Cleaned it all good and it was much better. However I think a bad ignitor can cause it because it will advance the timing at low rpm then its too far advanced. Idle timing is zero and 2500 and above is like 28 before iirc its in the manual.

It has advanced timing all the time and the ignitor retards it at low rpm, so if it fails it may not retard it.
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Unread 04-16-2017, 03:21 PM   #7
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Default Re: Yahama g2 two stroke?!

I have checked several, never yet seen one that was a super close match to the curve shown in the book.
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Unread 04-16-2017, 03:31 PM   #8
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Default Re: Yahama g2 two stroke?!

I have not checked them, but I know ones that fail show those signs of running poorly at low rpm. They like to do it when hot and some run fine cold some don't.

Yes a pretty typical advance curve. I thought about putting it on my clone because of the advance curve.
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Unread 04-16-2017, 03:32 PM   #9
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Default Re: Yahama g2 two stroke?!

Yep, I see the same symptoms on bad ignitor.
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Unread 04-16-2017, 03:35 PM   #10
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Default Re: Yahama g2 two stroke?!

I don't know how to make the pickup work on my clone. Looks like a ring with a window for the pickup to read, like a hall effect. Tough to research nobody seems to know. I'd have to mount a ring to the flywheel that would not fly off lol.
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