What caused this piston failure? 340 fanner

[TurboSportTSi]

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?

 

[TurboSportTSi]

Forgot to mention: I pulled the old plugs and they were a nice chocolate brown color & dry. They were kind of caked up but nothing concerning. Could be normal for a 2-stroke, but the main point is that AFR seemed to be at least pretty close and both cyls were the same.

 

[whoisthatguy]

I had an identical failure from a plugged oil outlet into one air intake side of the carburetor. You already found the smoking gun. Now verify that you are consuming oil at 40:1 and that the oil level is dropping with each use. If you drain out the oil tank into the engine during the off season, time to get a new oil pump.

 

[TurboSportTSi]

Well, that is a bit of a relief. Bad news for both of us, but still good to have closure - thank you for sharing. Wonder what plugs those outlets...either way, I did order an inline oil filter, so hopefully that will help prevent this in the future. Not gonna keep junk from the pump out, but should keep everything else from getting stuck in there.

I also went through and found the oil pump arm at the correct mark at idle but way off at WOT, so I adjusted it to line up at WOT and be over-oiling at idle. Will monitor and adjust from there.

FWIW, the choke lever wouldn't go all the way to full choke, so I adjusted the choke cables after reading up on how to do it. The sled will now do partial choke and full choke. Carbs were already sync'd perfectly. Hoping all these tweaks end up being a good thing when I go to fire it back up! Rebuild kit and other parts should get here by next weekend so I should know something sooner rather than later.

 

[Gunit111]

Lots of oil on the wrist pin/bearing so I have doubts about the oil injection system being the problem.

Pics of the piston crown top and bottom would be helpful.

Is the ring locator pin on the lower ring still there?

Which side of the crankcase is the fuel pump vac line on?

Damage on the piston the full width the exhaust port with no marks on the intake side indicate overheating. Ring lands are not broken so detonation isn't likely to be the cause, but signs of detonation as a symptom would confirm overheating.

A lean condition is as likely to happen from an air leak as a carb problem. Something like a torn carb boot can be quick/sudden enough to melt a piston before the spark plug gets cleaned off (especially 'es' spark plugs). Do you have the means to pressure test the crankcase after reassembly and before first fire up?

 

[TurboSportTSi]

OK, I'm glad you said that because I just went out and looked at everything again. The PTO side piston does actually have a small band of scuffing that I missed when I was tearing it all apart - the jug has some slight scuffing in that location as well, but nothing too crazy. The garage door being open blocks the overhead light above the sled engine, so it was much easier to see this time with the door closed.

Pics of crown and underside of mag side piston attached. Locator pins are all present.

The impulse line for the fuel pump is on the PTO side. I inspected it and did a quick pressure test on it yesterday and the vac line did not leak. I didn't notice anything obvious with the carb boots - in fact, they looked better than the factory ones on our '08 Rev.

I do have the equipment to pressure test once it's all together...assuming the carb boot and y-pipe sizes are the same as our Rev's 600HO SDI. If not, I'll have to grab some adapters to make it work.

 

[Gunit111]

Carbon only sticks to really hot surfaces which is why a rich condition results in clean piston crowns and no color on spark plugs. Typically you expect the piston crown to be carboned up except smalls areas at the intake ports (piston wash). Both of your pistons have signs of detonation (which has knocked the carbon off), but the detonation is likely a symptom of overheating. I think you can rule out a lean condition as both sides have overheated; the mag side just went first.

The fan belt is intact and not glazed and spins when turning the engine over?

All the blades are still on the fan?

Fan has a good supply of fresh air? Use a parts diagram to determine if you have all the ducts and vents, including the correct hood.

Plugged/restricted exhaust/muffler? Mice like building nests in mufflers/cans and intake air boxes.

Timing? Usually too advanced timing cause pre-ignition and a hole in the piston crown directly below the spark plug, but definitely keeps heat in the cylinder even without burn through. Offset half-moon keys on the flywheel are a common mode on 2 strokes to help performance at the expense of heat. Worth a look. NOTE: Like a car, a 2 stroke is timed at a specific rpm but remember a twin 2 stroke fires both cylinders at the same time every half a revolution. So if your timing rpm is 4000 rpm, your automotive timing light is going to think that motor is turning at 16,000 rpm and may have convulsions. If it does work expect to see marks on one flash and no marks on the next flash. My Mac timing light will not work on my triples but a $60 disposable Canadian Tire timing light does work on my triples.... fair warning!

Temperature wise fanners are pretty robust. They require proper jetting, proper timing, adequate supply of fresh air and quick exiting of hot exhaust gases.

 

[TurboSportTSi]

First, I really appreciate your time and the detailed help!

It looks to me like the fan is attached to the crank directly - no belt. All fan blades are present.

I will look at diagrams (great idea!) To see if the proper ductwork is in place. The exhaust side looks pretty obvious that it's all there, but I need to figure out the intake side to see if anything is missing. Crazy that the fan pulls air from the muffler side...whose idea was that?

Speaking of the muffler, I blew through it to check for obvious restrictions...not sure how valid that test is. My wife, who was riding the sled the day it died, did say that the exhaust was really loud, to the point that she was thinking of having the kid wear ear plugs. Having never driven any other 340, I have no frame of reference. It didn't sound to me like there was a loose baffle or anything, but I suppose there could be something there.

I was thinking more about this last night and remembered that the sled wouldn't back off of our tilt trailer under its own weight like our other sleds do. It acts like the parking brake is set and the track is tough to spin by hand with the rear end in the air. High load for long periods could definitely make a fanner overheat! We probably went 3 miles (throttle pinned, about 25 MPH) on a trail to a park, then 3 miles back (throttle pinned), which would be plenty of time to overload and overheat, I would think. Should these things go faster than 25 with an adult rider? Sounds slow to me.

I pulled the secondary off, as well as the brake pads, last night and could rotate the jackshaft with no resistance, within the range of gear mesh. I then tried to spin the track by hand...still very stiff. Might be on to something here...

 

[Gunit111]

A 250 single cylinder bravo will carry me 30mph all day long. A 340 twin f/c should top out better than 60mph with an adult, but I haven't rode one so that is a guess. Acceleration won't be great but top speed shouldn't be bad.

Tight chain or track?

Track alignment?

Drive shaft bearings?

Wheel bearings?

Hyfax wear?

Track should turn with little resistance.

Most (if not all) f/c sleds have the fan near the exhaust. Hoods are vented to feed fresh air towards the fan inlet. Aftermarket vent screens should be avoided on fanners, Imo.

 

[retiredpop]

The edge chassis has had problems with cooling for the fan engines, especially the 550s. If you do a search on engine failures for the 550 in the edge chassis you'll find all sorts of information. As noted above the fan is blowing hot air that passes over the pipe. Crappy design and the edge chassis hood venting is not as good as the Gen 2 was.
You could raise the carb jet needle by lowering the clip to a lower slot to supply slightly more fuel in midrange cruising. That would let it run a bit cooler. A lot of engine failures occur at steady part throttle operation.

 

[TurboSportTSi]

The edge chassis has had problems with cooling for the fan engines, especially the 550s. If you do a search on engine failures for the 550 in the edge chassis you'll find all sorts of information. As noted above the fan is blowing hot air that passes over the pipe. Crappy design and the edge chassis hood venting is not as good as the Gen 2 was.
You could raise the carb jet needle by lowering the clip to a lower slot to supply slightly more fuel in midrange cruising. That would let it run a bit cooler. A lot of engine failures occur at steady part throttle operation

I have noticed that the Gen 2 has much larger and more vents in the hood.

I think I will richen up the mixture by one slot just for insurance. The sled is for my kid, so losing a little power and gaining a little cooling is a win-win. If it costs me a few spark plugs over the uears, so be it.

 

[TurboSportTSi]

A 250 single cylinder bravo will carry me 30mph all day long. A 340 twin f/c should top out better than 60mph with an adult, but I haven't rode one so that is a guess. Acceleration won't be great but top speed shouldn't be bad.

Tight chain or track?

Track alignment?

Drive shaft bearings?

Wheel bearings?

Hyfax wear?

Track should turn with little resistance.

Most (if not all) f/c sleds have the fan near the exhaust. Hoods are vented to feed fresh air towards the fan inlet. Aftermarket vent screens should be avoided on fanners, Imo.

I opened the chaincase and loosened the track all the way. Chaincase looks great and, after reading about it, will tension the chain to hand tight minus a quarter turn.

The track turns pretty stiff even when loose. I checked every wheel bearing and the hyfax, though I'm not sure how to tell if it's worn. It's certainly not worn through. I could not get the driveshaft out but will check it today for axial & radial play in the pto side bearing.

As far as I can tell, all the engine cooling ducting is in place.

I tore a base gasket trying to get the cylinder on, so it'll be a week or so until I can make much more progress.

Thinking of verifying the carb main jets and rebuilding the fuel pump while I wait... Also, my oil pump has the lines to the cylinders disconnected and has been leaving a small puddle in the bellypan...maybe a teaspoon every couple days. Is this indicative of a problem? Not sure how sealed the pump internals are, so a little bit of seepage might be normal, especially considering that the oil tank is full.

 

[Gunit111]

Some tracks are stiff but I would expect that a 340 would use a small lug single ply track, which should be light and reasonably flexible.

Most hyfax has two moulding lines along the sides. The first acts as a wear indicator the second line indicates the wear through point.

Oil pump has a single lip seal on the shaft. There are check valves where each output line connects to the pump; If the check valve leaks, the oil from the tank will gravity feed over time. If you want to use premix then get rid of the pump and tank; they make block off plates for the pump opening on the crankcase or make one yourself. Also make sure where the oils are disconnected from are sealed tight. If you want to use the pump then reconnect everything and set the throttle cable to line up the marks on the pump; Use premix until you can verify the oil consumption from the oil tank vs fuel used matches your desired mixture ratio (1 litre/quart to 10 gallons of gas).

 

[TurboSportTSi]

Yeah, I've already gotten the tank pre-mixed so I don't forget!

As for the hyfax, it looks like it's getting close to the first wear mark. Good summer project, I suppose. No idea if the track is single ply or not, but the lugs are around an inch.

Are you saying the oil pump has internal check valves on the two outputs? I assumed the injector nozzles in the cylinder had internal check valves, but I could be wrong. Just going by resistance felt when flushing with carb cleaner using the syringe. If the pump has check valves, then the mag side is definitely leaking. Might pull the input line to the pump and run some cleaner through it while it's easily accessible.


OK, quick update: I pulled the carbs apart to look around and adjust the clip position on the needle and noticed two things while I was in there; they were immaculate inside. First, the jets have "120" stamped on them...parts diagrams indicate that the factory jet is a 170 and is good for 0-2000' elevation. I live at ~4k and ride at 6-7k, so maybe the jet is close, assuming a 120 flows less fuel than a 170. Is the numbering the orifice size in thousandths of an inch? Anyone know of a good resource for jet sizing? I assumed it was jetted appropriately because I bought the sled from a local who rides where I do; however, he bought it for his kid and barely used it, so it might be set up for somewhere else. Looking at the registration history, the previous owner was in Utah, which is generally similar in elevation to central MT, but who knows what it was jetted for.

Second, the needle clip was set to full lean. I moved it to the middle position. Will monitor plug color and adjust as needed. Super simple to adjust!

 

[retiredpop]

All I could find was a jetting and clutching guide for a 340 touring. I expect your jetting to be the same. Clutching may differ slightly because the touring came with a 133.5" track.

 

[TurboSportTSi]

Thank you very much! Looks like my 120 jetting is just fine, though I may drop the needle clip from the #3 to the #2 position, especially with this year's higher than normal temps.

 

[Gunit111]

All the jets in your carb flow less with smaller numbers. 120 is about 35% smaller than 170... seems like a lot for 5K feet. If in doubt start big (fat0 and work your way down.

You mentioned you were running it at WOT. What rpm was it turning? Running it lower than the rated rpm means the fan is also turning slower.... along with thinner air at your riding elevation. At elevation you need to make clutching adjustments as well as jetting adjustments... both equally important.

While you are waiting for gaskets, I would suggest now is a good time to have a look at both clutches as well.

 

[TurboSportTSi]

Is there a way to determine which weights are installed without pulling the primary and disassembling? The manual lists a few options: 10, 10MB, 10MW, and 10MR. I didn't see anything stamped on the weights.

Springs are good to go.


So, I found a supplement to the manual for my sled. It's actually quite a bit different than the one retiredpop posted, evem though the motors are the same displacement. I ordered 145 jets based on the chart. We'll see what happens.

No idea what RPM it was turning as it doesn't have the optional tach.

 

[Gunit111]

Polaris weights are stamp on one side; should be able to see the markings with them installed. If not, just remove the outside cover (under some spring tension) and the belt; You can then move the sheave back and forth to see the markings on the weights. Springs are cheap and are a wear item. Check the weights and rollers for flat spots while you are there. Check the movable sheave and weight pin bushings for excessive play.

The jetting charts are pretty safe. But if these engine are prone to overheating like mentioned above, being a little fat will help with the overheating a lot.

Koso, and others make inexpensive digital tachs that are like more accurate than the factor analog ones. Two strokes are sensitive to rpm as the have a very narrow power band compare to the automotive world.

 

[retiredpop]

Is there a way to determine which weights are installed without pulling the primary and disassembling? The manual lists a few options: 10, 10MB, 10MW, and 10MR. I didn't see anything stamped on the weights.

Springs are good to go.


So, I found a supplement to the manual for my sled. It's actually quite a bit different than the one retiredpop posted, evem though the motors are the same displacement. I ordered 145 jets based on the chart. We'll see what happens.

No idea what RPM it was turning as it doesn't have the optional tach.

Good thing you found the supplement.
I just had another look at the chart I found and noticed it was for the Lite chassis. Sorry for the wrong information.