Hi All. I have an '03 340 XC with ~1k miles that I bought a week ago that started in 3 or 4 pulls consistently but ran better hot than cold (I was told this by the seller, saw it in person, and experienced it myself after I bought it). None of this was a red flag to me, as it's a 20 yr old carbed sled, so I figured the carbs could use a cleaning like they all seem to need, although it seemed to run well enough. First time out with it, it started like normal and ran around for a few hours on a short trail and in a meadow with no issue. Looking back, it was probably down on power, but I wasn't sure what to expect from a 340 fan and bought it specifically for the relatively low power as it's my kid's sled. Got back to the truck and it wouldn't re-start - changed spark plugs and no change. Got it back home and started digging in, figuring if it was something that just up and failed, it would be easy to diagnose. Verified spark and fuel, pulled carbs which looked surprisingly clean. Put it all back together and it wouldn't even start with starting fluid. Noticed that when I pulled plugs, the PTO side cyl would smoke and the fan side wouldn't. Checked compression and bingo...fan side was about 60 and PTO was ~110.
Tore the engine down and found the fan side piston scored all to hell on the exhaust side with stuck rings - intake side looked normal. I'm a car guy, and EFI-only at that, so 2-strokes and carbs are totally foreign to me, though I'm coming up to speed fast as I already had to rebuild the top end in my wife's Rev this season due to low compression in one cyl, but I could use some help troubleshooting so this doesn't happen again.
According to this video, the cause is overheating. My piston looks almost identical to that one, though I could see it maybe being a severe lean condition as well, based on the video. The trouble is, the failed piston is on the fan side and the PTO side looks great, so the entire engine wasn't overheating... The heat sinks on the jugs had zero debris accumulation either. Shroud was in place and in good shape. Hard to believe that one cylinder overheated.
On a hunch, I tried to verify that oil was actually being injected, but the pump moves things so slowly when pulling over by hand it was difficult to verify - might have to take it out and run it with a drill. I tried blowing through the little check valves/injectors that squirt oil into the intake port and could not, so I ran some carb cleaner through them with a syringe & tubing. The PTO side now flows freely and the fan side needed another course of treatment and still doesn't flow as well as the PTO one. Smoking gun? The failure doesn't look like a lubrication issue unless I'm reading it wrong. I have a new (used) jug coming, so the oil injector is out of the equation for now, but I would still like to understand what happened and make sure I don't burn down a fresh rebuild. I premixed the tank already, so I will verify that the oil level goes down as I ride, of course. What else should I check? Do the pics look like an obvious case of something?
Tore the engine down and found the fan side piston scored all to hell on the exhaust side with stuck rings - intake side looked normal. I'm a car guy, and EFI-only at that, so 2-strokes and carbs are totally foreign to me, though I'm coming up to speed fast as I already had to rebuild the top end in my wife's Rev this season due to low compression in one cyl, but I could use some help troubleshooting so this doesn't happen again.
According to this video, the cause is overheating. My piston looks almost identical to that one, though I could see it maybe being a severe lean condition as well, based on the video. The trouble is, the failed piston is on the fan side and the PTO side looks great, so the entire engine wasn't overheating... The heat sinks on the jugs had zero debris accumulation either. Shroud was in place and in good shape. Hard to believe that one cylinder overheated.
On a hunch, I tried to verify that oil was actually being injected, but the pump moves things so slowly when pulling over by hand it was difficult to verify - might have to take it out and run it with a drill. I tried blowing through the little check valves/injectors that squirt oil into the intake port and could not, so I ran some carb cleaner through them with a syringe & tubing. The PTO side now flows freely and the fan side needed another course of treatment and still doesn't flow as well as the PTO one. Smoking gun? The failure doesn't look like a lubrication issue unless I'm reading it wrong. I have a new (used) jug coming, so the oil injector is out of the equation for now, but I would still like to understand what happened and make sure I don't burn down a fresh rebuild. I premixed the tank already, so I will verify that the oil level goes down as I ride, of course. What else should I check? Do the pics look like an obvious case of something?