After all the mods it seems like i have a knock in the engine. It is not a ping from fuel. It definately sounds like a knock at idle. What could this be? If i rev it, it goes away. If i ride consistantly at any rpm it is gone. Should i be worried? Could it be a wrist pin or a rollar bearring on the end of the rod? Or is it normal with the 250's. I have not noticed it as much before the mods.

 

Both the 250 and 400 motors are prone to a piston "rattle" at idle. Not being there in person, it's hard to say that's exactly what it is, but I would put money on it being that. By the way, if it is, it's nothing to worry about.


Ken

 

I had this happen on my 425 turned out to be worn out drive clutch buttons.Take the belt off and start it and see if it is where the noise is.

 

Yes I had the very same problem as you, air cooled engines and Wiseco pistons aren't good beacuse they are soo rigid and won't expend with the engine as it warms up. Wiesco pistons as I see you are running I have ran them too, they are strong PERIOD. They can't do anything but be strong a cast piston in a 2 stroke engine that doesn't run high RPM is the way to go . Polaris 250cc don't rev like other 2 stroke engines, so why would you need such of a strong piston like Wiseco?? I changed to a cast piston and the problem was gone, after I did some research I sandblasted both forged and cast pistons to get an easier view of them then I dipped them in 10w30 and on the forged piston it just wiped right off where on the cast piston it soaks in the metal making for very good natural lubrication this is good because it's how a 2 stoke is lubed by the fuel and if the piston absorbes the oil in the fuel it will lube your clyinder walls and make your engine last twice as long and run quietly. I ran a Wiseco piston and my Trail Boss didn't hold compression and my clyinder wall wore out and it started like crap, I ran it for a year with Wiseco I was forced to switch because I had 60PSI of compression. When I switched to a WSM piston It ran much better and It was quiet, now it's had a WSM for 4 years still with 112PSI of compression. That's cast VS forged, now I can see why you would put a forged piston in somthing like a 250r but you don't need somthing that strong in an engine that only runs 8000RPM

 

Mojave, the stock Polaris engines have had a long standing problem of breaking off skirts, that's why it is advantageous to change out to an aftermarket unit. Search the archives, there's lot's "O" posts about it.


Ken

 

Also the compression is low on your machine if the head is milled. Time to put some rings in it. A replacement cast piston may be a little stronger than the OEM, but if it's modded you wouldnt see me running one. By the way, a fully modded 400 runs only around 7500rpms. Are you going to trust a cast in something like that? Dont think so. I'm glad you havent had any problems, but eventually it will happen if it's modded. The good ole piston rattle lets me know all is ok and loose. The big bores run an extra 1.25-1.5 thousandths in clearance and make some noise. 2many, tear the top end down, send me the cylinder, that way you can look everything over as well. By the way all, got the updated pics in my file with the nerfs on. I really like them.Bubba

 

Bubba297,
I am going to take you up on your past offer. I have an extra head that i will send you. I went to the polaris dealer to drive a newer one that was broken in and i heard the same knock. It was not as bad but the mechanic said that it was normal. He said that mine was very broken in!! I need your info on where to send the head. After you do the porting, I might get some other goodies done to it. Not sure yet. I want to see what gains the porting will give me. Hopefully i will be satisfied with just that. About the knock i guess i was getting paranoid. Ya know with all the mods!! I didnt want to cook an engine.

 

For general use like trail riding, cast is the way to go I have proved that with dirtbikes and ATV's they last more than twice the time without re-ringing. If you are are wild and like to ride wide open all the time go forged. When I had my Wiseco piston I would get sick of buying rings and gaskets.
Yes I will trust a cast piston at 7500 RPM. Beacause 125 and 250cc dirtbikes run 12,000+ RPM and they trust cast pistons.

 

The biggest thing in this case is coming down to piston size. Those 125's and 250's are pretty small in comparison to a 400 piston. You definitely do not see a car race motor running cast pistons at 13:1 or 14:1 compression, and lots of them never see 8000 rpms. Run what you want, but I had a meltdown on my wife's wheeler just trail riding one saturday from the middle of the day until dark. The temp dropped about 25 degrees and we didnt make it back up the long hill to the truck. It had a dime sized hole right in the middle of the crown. A forged would not have done that running just a little lean. By the way, looks like you have all the nice riding area you want looking at those pics. Bubba

 

Bubba, does this mean you don't need nerf pics? [img]i/expressions/face-icon-small-smile.gif[/img] Hope all went well.


Ken