Hi -
My 1999 Scrambler 400 (2-stroke) has seized twice recently - both times when I was running it wide open. Most of the piston looked spotless - but scarred on one side, which lined up perfectly with the exhaust port. Since I'm running with an HPD pipe and a stock silencer, is it possible that the stock silencer is too restrictive, causing a build-up of hot exhaust gasses by the exhaust port?? Otherwise, everthing on the bike is perfect.....a local mechanic suggested a less restrictive silencer might permit the hot gasses to exit properlly. Only happens when I am running at full throttle for extended periods (like racing a friend down a dirt road)....Any ideas?

 

are you on a stock bore or have you rebored?

lots of people run the stock silencer so I don;t think thats your problem sounds like oiling, or if you rebored the camfer may not have been done correct afterwards.

 

I used to run an HPD Trail Pipe with the stock Polaris Silencer with no problems whatsoever. I recently upgraded to an HPD Silencer, but I still have the stock silencer for trail use...

 

Thanks for the note....

By the way, did you pick up any increase in performance with the HPD silencer?

 

I'd have to say it freed up a couple of additional horsepower, but I've only got the butt-o-meter measurement. It is certainly louder than stock and probably is above the legal limits for public trails here in Minnesota.

 

Too lean on the jetting, the piston melts at the exhaust port due to excessive cylinder temperature. You might consider installing an egt gauge which will aid in jetting the carb.

 

Thanks - I am beginning to believe that is true, especially since I've leaned it out for high altitudes....I may increase the mix and add a bit of oil to the gas tank each ride.

And now, to sound really stupid, while I've rejetted a number of times and thought I knew the bike pretty well.....what the heck is an egt gauge? When jetting, I've had to guess by keeping it from sputtering at low rpms.......would be great to actually know what the right mix is - I ride at 10,000 feet, 3500 feet, and sea level, and rejet by guesswork.....obviously, I've gone too lean on occasion and lost a piston by guessing incorrectly.

Thanks.

 

Adding oil to the fuel tank will lean it out further as the oil displaces gas as it flows through the carb fuel circuits. You can add the oil but increase your carb settings also. EGT is short for exhaust gas temperature gauge, you can monitor the exhaust temps and use it as a tool to adjust carb settings. 1250 degrees is about the max you want to see at wot or any other throttle setting. 1000 at idle 1150 at half throttle is a good place to shoot for. The airscrew/pilot circuit affects primarily idle to 1/4 throttle position then the needle adds fuel up to 3/4 throttle, then the main jet adds more fuel from 3/4 to full throttle. Pay attention to the throttle position if you have performance issues, that will tell you what circuit needs to be adjusted. The plug color also indicates air/fuel mixture, paper bag brown is a good color to look for down inside the plug.

 

Listen to powerslider. He's tellin you the truth. Bad jetting will kill these motors quicker than anything. Jet it right or leave it stock. Put a bigger mainjet in it till it runs like crap and then go down one. If in the heat of the summer with no good air if it boggs off the line go down one more. The oil thing he's telling you about, I have been preachin about for years. If you add more oil you thicken your fuel/oil mix and that will lean your motor out. More fuel/oil mix going into the cylinder means a cooler cylinder/piston. Adding more oil only kills mosquitos it does nothing for your motor .

 

davein,there a lot of variations from 3500 feet to 10000 feet,the piston melted cxauze of lean conditions at the main jet jet,you might consider get yourself a dial-a-jet on your carb allowing you to lean or richen the main jet lines
right on the fields.