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May 23, 2013, 9:55am

ScootDawg Scooter Forum :: The Garage :: 2 Stroke Scooters :: 580 miles, 0 psi compression

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Topic: 580 miles, 0 psi compression (Read 412 times)

litninrod
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580 miles, 0 psi compression
« Thread Started on Jun 19, 2012, 2:31pm »

I picked up a Meitian 49cc 2 stroke as a project. It wouldn't fire so I began running checks. Good battery, good spark, getting fuel, but when I ran the compression test it came up ZERO.

The scoot only has 580 miles on it. Even if it had 5000 miles and worn out rings I would expect 30 psi or something.

I took the head off. The cylinder looks shiny and smooth, and the piston goes up and down. No hole in the top or anything obvious like that.

So how is it possible to get zero compression?

Any ideas?

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Pony66
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Re: 580 miles, 0 psi compression
« Reply #1 on Jun 19, 2012, 3:28pm »

You have reed valves behind the carb. If they are stuck closed, no air can get in.

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litninrod
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Re: 580 miles, 0 psi compression
« Reply #2 on Jun 19, 2012, 4:42pm »

Good to know. I wasn't aware reed valves might stick. I took the carb and reed cage off. Reeds were oily but didn't appear stuck. I cleaned them up with carb cleaner and reassembled. Still no compression.

I took the head and cylinder off to inspect the piston and rings. Rings were slightly larger than the piston diameter and had to be compressed slightly to get it back together, so I 'should' be getting some kind of compression reading.

I am getting little puffs of air out of the exhaust when I crank but not very strong.

Any other thoughts?

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wile
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Re: 580 miles, 0 psi compression
« Reply #3 on Jun 19, 2012, 6:53pm »

Are the piston rings GAP opposite each other? Are you sure you do not have air leaks between the head. How about your plug hole check if air is coming out of there.
Wile

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litninrod
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Re: 580 miles, 0 psi compression
« Reply #4 on Jun 19, 2012, 8:10pm »

Plug hole was good. I couldn't feel any air escaping anywhere around the head.

However, upon further review I think someone put a wrong set of rings in it. The ends of one ring are like 1/4 to 3/8 inch away from touching each other and the other ring seems fully collapsed like it wouldn't really make contact with the cylinder.

So if it's not reeds sticking or air leaking out of the head, it's gotta be rings right? Not much else to go wrong is there?

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wile
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Re: 580 miles, 0 psi compression
« Reply #5 on Jun 19, 2012, 8:15pm »

That would do it, Sounds like you found the problem wrong rings or they messed them up.
Wile

« Last Edit: Jun 19, 2012, 8:15pm by wile »

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gregw
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Re: 580 miles, 0 psi compression
« Reply #6 on Jun 19, 2012, 8:16pm »

Right. unless crank case has big leak in it... even then you should get some compression...

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Re: 580 miles, 0 psi compression
« Reply #7 on Jun 20, 2012, 1:35am »

When you did a compression test did you have the throttle at WOT to allow the most air the engine could draw in with the butterfly valve open ? A compression test with the throttle at idle will read much less . Even with bad rings it should have some compression . Is the head cracked ? You can push down the kick starter by hand to see if it has compression .

New rings are cheap I would try new ones . If the cylinder looks worn I would opt for a BBK instead of a new 49cc rebuild kit as its not much more

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litninrod
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Re: 580 miles, 0 psi compression
« Reply #8 on Jun 20, 2012, 3:42pm »

Odd ball piston. 41mm instead of the usual 44mm. But my Suzuki deal found a set of rings to fit it.

But now it has compression and runs, so problem solved. Thanks everyone!

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wile
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Re: 580 miles, 0 psi compression
« Reply #9 on Jun 20, 2012, 4:15pm »

Great!! that was a strange one to say the least. Good job bro ride safe.
Wile

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Re: 580 miles, 0 psi compression
« Reply #10 on Jun 20, 2012, 6:20pm »

Jun 19, 2012, 8:10pm, litninrod wrote:

Bingo....that is your problem...you found it. Here is the deal.....2 strokes use an oil pump...from what I have seen well over 50% of them that come though our shop....the oil pump stops..or never did work. The engine either seizes up...or rings wear down to nothing. Typically a whole top end is not needed as the cylinder does not wear as much.....due to the fact it has more surface area than the thin piston rings do. I would start with just a new set of rings..and pull the oil pump line from the carb with engine for a few seconds. If no oil comes out.....you can A..buy a new pump.....or B premix as most 2 stroke riders here do for that very reason. The Chinese oil pumps are a junk copy of the Mikuni oil pump...some of them work...some of them don't.They cost 15-20 bucks if you chose to replace it....the real Mikuni is 50 bucks most places...and I have yet to hear of one failing..... 8-)

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