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Old 12-26-2013, 05:54 PM   #1
BruceHarless
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Default piston backwards?

Have fuel, oiling, compression and very good spark on the plug, but no fire occurring in the combustion chamber. I am wondering if I put the piston in backwards and the ports are not lining up. (And if so, would that prevent firing? I think so, but this is beginning to mess with my mind).

I removed the exhaust pipe and looked into the cylinder from the bottom of the engine as it is mounted in the cart. With the piston at BOTTOM of its travel, I can see across the cylinder to the three port holes on the carb side. With the piston then moved to TDC, all I can see is the piston skirt, no ports visible on the piston.

Is this because the piston does not move high enough to reveal the holes on that side of the piston, or it it because the holes on the piston are all on the upper (carb) side of the piston? (Obviously I do not remember the location of the port holes in the piston skirt.) I had hoped to confirm or deny that the piston is in correctly, rather than remove and disassemble the engine unnecessarily.

Thanx in advance!
Bruce
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Old 12-26-2013, 06:00 PM   #2
1989Marathon
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Default Re: piston backwards?

If all you see from the exhaust side is the piston skirt, the piston is installed correctly. The 'windows' should be on the intake side. Have you tried squirting a little fuel into the spark plug hole before trying to start it? How much compression do you have?

BTW, you should let everyone know what type of cart/motor you are working on whenever you post a question. I just know it's a 1989 marathon because I responded to your last thread.
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Old 12-26-2013, 09:24 PM   #3
BruceHarless
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Default Re: piston backwards?

Hi again -- thank you for your help. Sorry I left out the cart ID, I knew better... I'm glad you understood what I tried to describe -- you literally have saved me from dismantling the engine just to make sure.

So now I think that it must have been severely flooded. Fuel in the exhaust pipe signaled that. Compression gauge read 115 lbs, and held there. However, today when trying to start it, I could hold my thumb over the end of the exhaust without it blowing by my thumb. Is that the degree to which that muffler restrains it? Sorta surprised me, I expected it to be stronger.

I'll have another go at it tomorrow, without the choke, and maybe a squirt of gas directly into the carb.

Again, thanks for the reply.
Bruce
1989 Marathon 2 cyc.
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Old 12-27-2013, 09:08 AM   #4
1989Marathon
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Default Re: piston backwards?

You won't feel much coming out of the muffler until the engine is running. The motor doesn't have a true "exhaust stroke" like a 4-cycle engine does.

If you are concerned that the muffler is plugged, or keeping the motor from starting, just loosen the two bolts(as far as they can go) that couple the muffler to the head pipe and allow it to leak at that point. You may have to loosen the muffler straps also. Don't run it very long like that. It is just for a test.

Be very careful with that muffler. It is EXTREMELY expensive. Probably more than you paid for the entire cart.
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Old 12-27-2013, 10:35 AM   #5
BruceHarless
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Default Re: piston backwards?

thanx again. I am not worried about the muffler being plugged, just checked it in the process of trying to get to run the first time. Glad you told me it won't be a strong exhaust pressure, though -- that is another worry dispelled.

Would you please tell me what the reed valve does in this engine? I did not do anything at all to that. If it is bad will it cause or keep it from firing? Is there a simple way to determine if it is not functioning properly?
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Old 12-27-2013, 11:04 AM   #6
1989Marathon
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Default Re: piston backwards?

The reeds are just a one-way valve that allow fuel to enter the engine, but don't allow anything back out into the carb/intake.

As the piston raises, it causes a low pressure inside the crankcase. Once the piston raises high enough, the window in the side of the piston passes in front of the reeds. Once it does, fuel is drawn from the carb, through the reeds, and into the lower part of the engine(crankcase). At the same time, the piston is compressing the fuel charge that is already inside the cylinder, and getting ready to fire. Once the cylinder fires, and the piston starts moving down, the top of the piston will eventually pass by the exhaust port and the high pressure inside the cylinder will be exhausted. Also, as the piston is moving down, it causes a high pressure inside the crankcase, and pushes the fuel mixture, that was previously drawn into the crankcase, through the transfer ports and up into the cylinder. This also helps expel the exhaust gas from the cylinder. This changing pressure inside the crankcase is why it is important to have good crank seals.

You can check the reeds a couple different ways. A quick check is to put your hand over the inlet of the carb, and crank the motor. You should only feel vacuum, never pressure, if the reeds are working correctly. The best way to check them is to remove the reed cage and check them visually.
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Old 12-27-2013, 11:47 AM   #7
BruceHarless
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Default Re: piston backwards?

You should write a book -- that is SO clear! Thanks very much. Bruce
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