Gents

I bought this piece from someone who claimed they bought it with the intent of getting it running. They claimed they put a new piston and rings in it but can't get it to start because "it's back firing through the carb".

So I by it and start tearing it down. It does appear like the piston is new, and the cylinder is in very good shape. The piston size is stock, or 71.96mm. The cylinder is in very good shape as he said. The rings had 0.008" gap at the ends so they are very snug.

I put everything back together. I tried to start it and it did indeed backfire through the carb. Looking further I realized that the engine could be easily rotated by hand, virtually no compression. I am at a loss trying to understand why I have no compression. The rings and piston seem well matched to the clean bore. The crank seals should not have anything to do with compression??

I have not measured the compression, but you can turn the pulleys with almost no resistance. Also, when you put your finger over the plug hole there is NO compression resistance.

yet everything measured well??

Please help!

Thanks

 

When I tore it down, I put everything back together right. It had a new Namura piston in it, stock size (71.96). The ring end gaps were plenty tight 0.008" maybe even 0.007". The piston to cyl clearence is tight too, no more than 0.002". Cyl walls were freshly honed, no scars. Gaskets are good. The "ex" mark on the piston was facing towards the exhaust.

I can rotate the crank by hand with virtually no resistance. Free to spin.

When I tried to start it, it fired out of the carb! Other than that it sounded dead.

I'm at a loss why it's so loose.

 

Cranks seals could be the problem or even a leaking head or base gasket. Has to have a good top end compression along with the crank case seals holding lower end compression to allow the air/fuel/oil mixture to atomize properly and shoot up the transfer ports for ignition. First thing I'd do do is a compression check. Anything lower than 110 psi or so could be the problem. If compression is good enough, a leak down test can tell if seals ,gaskets are leaking. Does take a leak down tester if you have access to one. Kinda of a process of elimination. If you eliminate any mechanical,fuel or carb problem,could be an electrical problem. On the electrical end of it couple things you can do is to pull the reverse limiter black wire and see if that helps. They all have problems with limiters shorting out and you can just leave it unplugged any way. If that doesn't help,pull the black wire from the cdi(kill circuit) and try it. Plus how did you measure the clearance from the piston to the cylinder? Plus you said it was honed out,but were measurements taken at several points in the cylinder? If a cylinder is egg shaped, a new piston and rings may not help.http://www.cyclepartswarehouse.com/f...1998&fveh=5450

 

Yes, I am going to do a compression check this weekend.

I'll start there!

 

OPT, why would it backfire and shoot out the carb? Can timing be off? That far off? I've never had a 2-stroke do that!

 

Can be several reasons for back firing through the carb. Usually low compression is the main culprit plus seals can do the same thing. If leaking they lose lower end compression and what fuel/oil mixture that's able to make it to the top of the piston is too rich. Spark can ignite part of the mixture while the rest may be blasted back through the intake port into the carb. Another can be electrical. That's why I said to disconnect the reverse limiter black wire. If(when) the limiter shorts out,it can cause spark problems. Some can die when you place the machine in gear,forward or reverse or can cause popping back through the carb. Same thing if the internal kill wire on the cdi shorts out,but most of the time when this happens the atv will lose spark.A cdi can be the cause itself as it controls timing and advance along with the stator timing plate. My main concern is a std piston and rings in a 16 year old cylinder. Unless it was babied,barely ridden,it's hard to find a cylinder this old that really doesn't need to be bored and honed and an over size piston installed. That's why they have so many over size piston options. Product Search Engine For All Products - Wiseco Piston Inc. Plus here's a link to a pdf service manual that includes your model and explains mechanical and electrical functions and how to check each. http://gh-ftp.com/ORV%20Manuals/Pola...e%20Manual.pdf

 

Ok, zero compression. I already took the head off. It may appear that the cyl is out of round, but I'll get some measurements. I can see more of a gap around part of the piston than others. Hard to believe it's zero, and I mean zero!

 

Kinda what I figured using a std piston. If it's never been bored,first over from Wiseco 50mm(.020) should clean the cylinder up,but have it measured.Other brands of pistons can start at .25mm (.010) increments over stock bore,but I'd go with Wiseco.Plus don't forget to check for any up and down play in the lower rod bearing.If there is any,a new rod kit and crank bearings and seals will be needed.

 

Yeah, when I had to tore apart early, including both sides, there was very little to no play up and down on the crank. It seemed very tight. That's one of the first things that I checked to see what I was getting myself into. I don't know about the seals at this point, but let me start with the piston and cylinder. I'll have some measurements tomorrow! I'll take it to work, and have our technicians/inspectors measure it. Heck, I might throw it on the Zeiss CMM and scan it too..

 

Wow this is getting even crazier to me. So using my Starrett calipers, I measured the bottom and top of the cylinder bore, as well as 10mm up from the bottom of the piston. Man, using calipers it seems to confirm that the bore is right at 72.00mm, round all over. The piston is about 71.96 as marked, round all over. The ring gap is right at 0.008". No scoring or scratches on the bore. Using a straight edge, I checked out the cylinder and head for straighteness.. they both seemed very flat!

I will still measure using ID mics tomorrow, but man I am puzzled why I have NO NO compression? None! Not even enough to push air out of the spark plug hole!

Again, the piston appears to be new.. no scratches, burn marks, etc.. on it. Shiny shiny shiny.

This is wild to me. I would expect some compression?

Cranking the motor over with the head off, at BDC, the piston is just fully exposing the exhaust port and intake ports. All passages are clear. There are a few chips in the bottom of the crankcase, but the guy before me said he rebuilt the motor. Just a few chips.

No up/down side/side play on the crank shaft. None. All bearings feel tight.

To me, even if the crank seals were shot I would still get compression??

Head gasket is older, but still intact. Again, I would think if the head gasket was a little worn I would get some compression??

I will take it to work tomorrow and re-measure.

But man, I am at a loss! This is wierd? I know the gage work because I verified it on my air compressor. Also, the compression is so low that you could turn the motor over by hand with NO resistance!