My son's 400 blew a piston due to supposedly an undersized jet in combinaton with his newly installed HP muffler. I differ in the professional diagnosis due to finding anti-freeze in the cylinder at tear down and the piston was greatly deformed at the surface. We purchased a replacement piston/ring from Dennis Kirk, he spent all day rebuilding, ran the motor for 20 minutes and it froze solid. I did notice that there was mist in the air agan unlike 2 stroke smoke. Can we have a possible crack inthe internal cylinder causing antifreeze to mix in the housing? He's beside himself at 14 yrs old and did the entire rebuild himself. We haven't checked the internal cylinder this AM because we were just too worn out from the previous days repair. Could it be time to take it to the professionals for mega $$$$'s or has anyone got a suggestion that may help us fix this problem with the new rebuild kit. I don't want to waste another full day without a proper diagnosis. Any help appreciated.

 

Man, I feel for your boy, what a disappointment.
Do you know that the antifreeze didn't spill in their when disassembling the motor?
Really, their are alot of different things that could be wrong here. Did the first piston look like it got hot and deformed or did it get beat up by something(debris in the motor)?
Honestly, you might want it checked out by a professional. If you don't know what all to look for when it is apart, you'll just end up wasting a bunch of time and money.
Besides, you really haven't given much info as far as what all has happened and what was done. Gives us some more info or take it to someone who knows.

 

Okie Dokie - Thanks for the response -

First go around >>> Same symptoms in that the machine started to stall ,backfired, no compression. Tore it down>>> The piston was deformed on the very top like severe pitting then jagged edges and one frozen ring. It didn't score the walls of the cylinder as we had that checked by Polaris Dealer. The troubles started when he converted to a high performance muffler and got some back firing. Just after the failure he was told by Polaris that he needed a larger jet to match the HP muffler as thjis may have caused the problem >> the engine lost compression and they said the failure could be due to fuel starvation or burning antifreeze that could have infiltrated the cylinder due to a blown gasket caused by severe backfire. I did not notice if antifreeze in cylinder was due to dismantling the motor or something worse.

Ok- He replaced the piston/rings from Dennis Kirk, had the 250 jet installed, did a very nice job installing components, and we buttoned it up last evening. He said the arrow on the piston was in the correct directon and that it simply wouldn't run if he had rreversed the direction. I believe this to be true. Anyway, after about 20 minutes and what I believe was vapor mixed with 2 stroke smoke the engine froze while he was riding a couple of miles from home at about 10mph. It started to stall, picked up, stalled, almost restarted, then simply had a very muted backfire and froze solid. I had to go pick up a very tired and frustrated puppy.

He can get a warranteed piston/ring replacement from Dennis Kirk and is just too whipped and dejected to tear it all down again and start over without more knowledge. We're thinking the Ploaris dealer here in MA.but will give it one more shot is we can nail down the cause.
Thanks for the response.

 

David,

Was the jetting problem fixed prior to running the ATV on the second rebuild?

Backfiring is deffinatly a sign of a lean condition, and with a pipe replacement some jetting adjustment is typically required.

You know how to do a plug reading right?

Also, this may be a shot in the dark, but is your oil injection system working alright?

-Josh

 

Can do a plug reading but not sure how to check if the oil injection system is working properly. Please advise.

 

Elroy and Piranha have already given you some great ideas and areas. As always, it's more difficult to diagnose something when you can't see it.
Along with just changing the main jet, there are a couple of other areas of the carb that control jetting throughout the throttle travel. There's an air mixture screw on the left side of the carb. Turning it out adds air (leans), but normally won't affect mixture too much past the 3 turns out. Obviously, turning in richens the mixture...this controls the idle-1/4 throttle travel.
Inside the carb is the slide/jet needle. There are different notches on the needle that will also change your mixture. Lowering the clip (moving towards the point), will richen the mixture, and moving it the other way will lean. This controls your mixture from the 1/4-3/4 throttle settings.
Start checking your jetting at the lower speeds first, then work your way up. Stabilize throttle setting for about 1/4 mile, hold throttle steady. Cut the engine using the electrical switch on the bars. Coast to a stop, and check your plug. White is lean, black is rich, cocoa brown is perfect.
When you say you had jagged edges, was the old piston in one piece...wondering if the piston skirt broke, and maybe some metal is loose.
To test your oil injection (first I would change the inexpensive inline filter on the LH side of the fuel tank, under the plastic, by the inline fuel filter)premix your fuel to 40:1 ratio gas/oil. You can remove the oil injection line by the carb. While operating the throttle, oil should pulse from the line. The higher the throttle setting, the more oil will be pumped. MAKE SURE TO PREMIX THE ADDED FUEL, ALONG WITH THE EXISTING FUEL IN THE TANK TO 40:1. The fuel and oil injection filters should be replaced at least annually.
Also, as long as things are torn down, did you change your upper cylinder (head gasket) also? Like Elroy said, it's hard to tell if coolant got in there during disassembly. Also make sure to retorque the base gasket after the engine has been warmed up good it's first time. Always make sure to hone the cylinder whenever it's removed.
I know you said this piston might be warranty, but in the future, HPD, Hot Seat, might be able to save you some ching on parts purchases.
HPD-inc.com, and Hotseatperformance.com. Also cycleworks.com has some informative pics on plug reading and engine FAQ.

You'll find lots of people here willing to help[img]i/expressions/face-icon-small-smile.gif[/img]

Good luck,
Ken

 

heh, cycle works is where i go to get all my Polaris stuff, fixing and etc...
more exact, Cycle works West is where i go.

 

That 250 main sounds a little lean to me.I ended up with a 265 in mine.Did you torque the head bolts to the right setting?Good luck.

 

Always break the piston in slow after a rebuild. I usually start the motor and let itget warm to the touch then kill it. Let it cool completely and do it again. I will do this 4 to 5 times before leaving the shop. After that I will ride it for about 5 minutes slowly then park it and let it cool. Do this a couple of timesthen start riding it a little harder each time. If your son didnot let it warmup good it could cause a seizure.

Ronnie

 

Good point Ronnie, lot's of people don't realize w/o the proper warmup, you can cold sieze a motor. Sooooo many times buddie have unloaded the quad, fired 'er up, and just ripped.....they've been really lucky. I almost always let my temp gauge get up to at least 100 before even climbing on. Then the first bit of riding, I take it easy to make sure the trans gets warmed up.

Ken