Whoah nelly! I think I found it. I put the rings back in the bore and held it up to sunlight. All the way around the ring, with the exception of one single spot, there is a huge gap? On both rings! The rings are jacked up, out of round, or something. there is literally a huge gap all around!

Not sure what is up with that? The single contact point moves when you move the ring, so that tells me that the cylinder is not the issue. I think I need to order new rings.

 

In your earlier post that you said you had more gap between the cylinder and piston in certain areas.Usually this happens on the exhaust and intake side of the cylinder from the force of the rod. This alone can allow blow by on the rings down past the piston crown. Air /fuel mix enters as the intake port is uncovered,as the piston fires and forced down ward this mixture compresses in the lower crank case and shoots up the transfer ports to the top of the piston. I've seen seals pop completely out(behind drive clutch) and this could explain no compression also. Basically has to have a good top end and bottom end compression.

 

Trying to think if my philosophy is correct. I understand what you are saying. When I install the ring(s) they are only making contact in two spots. By the gap and almost opposing it. At first I thought the cylinder was out of round. So I rotated the ring 90 degrees. The same contact spots followed the ring, not the cylinder location.

If the cylinder was out of round, and the rings ok, wouldn't the contact areas remain in the same location?

I'll need to measure the cylinder with mics.

 

Ok.. the verdict is in. The Namura piston in it is a 71.96mm, probably made for a 72mm bore. The piston, which appears new, measured correctly.

Using a Zeiss, we scanned the top, middle, and bottom (about the bottom of the stroke of the rings) for size and roundness. All were oversize, or approximately 72.22X. Service clearence of 0.006 would say that my largest bore should be 72.112mm. So it is oversize, or about 0.010 piston/cyl clearence.

Worse yet, the roundness at the top was 0.0006" (good), middle 0.0025" (borderline out), and 0.006" (yes, six thou) bottom. It is a very funny shape, which odd opposing lobes. Probably as you said, the front and back of the cylinder.

So my theory is that the stock size piston and rings were worn out. The previous person honed a already oversize jug out, and couldn't correct the out of roundness either.

I am going to a 73.50mm piston, just to make sure it cleans up good.

Now, do I assume the seals are ok? Do I automatically replace those to? Not knowing why the original piston wore out (time, debris, broken skirt, lack of oil, bad crank seals, etc..) I don't know if I should go further?

This thing is not a pretty beast, and it will get minimal easy riding. So I don't want to invest a TON of money in it.

Do I just do the top end, maintenance, and go about my business? How do I know if the crank seals are bad? In the past, if there is good compression but no "sucking" from the carb that tells me the seals are bad.

Taking both the clutch and stator covers off I don't see any oil, exhaust residue. It's pretty clean. The crank had little no up/down or side/side play so the bearings seem good. Pulling up/down on the rod shows no movement either.

Oh wise one, please advise!

Thanks!

 

What I figured if the rings were worn down on a new piston. Egg shaped cylinders can do this quickly. And yes most of the wear is on the intake and exhaust side because of the friction/rotation of the rod and piston.Had several people try to get away with this same thing as the PO of your machine did. Only way to test if seals are bad is through a leak down test. Seals are cheap. Just takes a little labor to get to them. Need a drive clutch puller and the seal is right there. Harmonic balancer puller needed on the flywheel,then pull the stator plate. Once these are off it's simple as popping old seal out then installing the new one.On the new piston try for a max of .002 final clearance between the bottom of the piston skirt and the bottom of the cylinder.

 

OPT, thanks for the help. So I went 0.060 over, it took nearly 0.015 a side to get cleanup. It was that bad. I ended up getting a Wiseco.. all that work I wanted to get the better piston for $15 more bucks!

The machinist actually used an ID grinder to get the final size, or about 0.001" clearence. We figured that honing would take that to 0.0012" or 0.0024" total. That is a good snug fit and what Wiseco recommended.

The CMM showed it was on size and round within 0.00002" (yes, 20 millionths!), so he did an outstanding job! Surely better than factory.

Put everything together and she fired up in one pull.. purrs like a kitten.

There are some other bugs I need to fix, minor stuff. I need a new front strut, pads all around, and eventually a new petcock.

Other than that, it rides out well. Carbs pretty good too!

Thanks again, I'm sure I will have more questions later!

One actually, is there a "fuse box" panel cover? Seems odd that the wires are uncovered under the plastic.

 

Makes a big difference if the cylinder is bored and honed right..Hopefully it'll last for years now.Only circuit breaker is a 20 amp thermal breaker in a sealed pouch on the harness up from the battery. They can get corroded over the years. If you ever lose power to the lights or ignition switch,etc, suspect this breaker. Can get it at any auto supply. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

Sigh.. this thing is giving me more problems than any other project I've had in the past.

So I wasn't able to ride it much since I did the top end, it's been too cold. I did start it 4-5 times to ride it a little or move it around. In all instances, it started first pull and ran awesome, like it should.

When I last put it up, I noticed a small (about a half of a dollar bill) gas spot under the atv. It appeared to be coming from the overflow. My petcock does not like to shut off all the way, so I am relying on the needle to stop the flow. I know I know..

Then the puddle disappeared.

Then it came back.

So yesterday I go to start it.. pull..pull..pull, nothing. Electric start.. nothing.

Spark is strong, compression seems tight. Plug was soaked. Removed the air filter and there was gas in the air box. Seemed flooded.

If the carb overflows is it possible to soak the engine so much that it won't light? I tried full throttle, choke off, and it started to stumble, but would not light.

So the carb is off, the needle and seat actually look really nice, no taper or funky shape on the rubber tip or debris in the seat. I replaced the petcock, so now the gas won't run when it is shut off.

I removed, what I believe to be, the engine crankcase drain and about 1-3 thimble full of gas came out. I blew some light air into the engine to help dry it out.

I am praying that it was just very flooded. If it is something else, this thing is getting tossed.

Thoughts?

 

Ok, drained the crankcase, cleaned carb. All is good. Started first pull again. Now the brakes are jacked. Time for a new thread.

 

Over flow gas can wash down the rings and flood the crank case as you've found out.If this happens too much can damage that new top end. Doesn't take that long to just put a new carb kit in to solve the fuel problem along with installing a new tank valve. Gas needs to be shut off on these old 2 strokes when not in use.Items #17,18. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse http://www.ebay.com/itm/Polaris-Trail-Boss-1989-1999-Carb-Carburetor-Kit-/380717279696?pt=Motors_ATV_Parts_Accessories&hash=item58a484a9d0&vxp=mtr