i just recently picked up a 98 polaris scrambler 400 and it had no compression so i tore the motor down and noticed the exhaust side of the piston was burnt from the top to the upper ring and the side skirt was all scraped up! now one question i have is what direction is the arrow on the piston sapose to be facing or what way are the ports on the piston facing? cause the piston and gaskets were all pretty much brand new so they must of done something wrong when they rebuilt it! i did notice they put the c-clips in wrong. another thing i noticed was they installed a 38mm flat slide mikuni tm carb and the exhaust pipe is not stock! the carb was completely gummed up when i got it! does any one know where to get a rebuild kit for it?? i am sending the cylinder to ritters cycles and im going to have them port the intake and exhaust, bore the cylinder to the next size up, cylinder head mod, and a new silencer! now a couple more questions sorry, what size jetting should i get or am i going to have to experiment, and what about clutch! the bike is going to be on 28 inch mud zillas or bigger if i buy new ones and its going to be snorkled

 

Correct me if I am wrong, but I don't think Scramblers have a low range on the tranny, sounds like you want to build a mud bogger, that quad was meant to be a sport ute, more toward sport. Even if you mod the motor out, it is still going to be hard to turn 28" or bigger tires without a low range, you will probably smoke a lot of belts or wreck the clutch trying to spin huge mudders in a mud pit. I'm not saying it can't be done, but you will have to beef it up and mod the heck out of it to make it into a real mud pit runner. You would be much better off building up a Sportsman for deep mud, and use the Scrambler for fast trail riding and 4x4 playing.

 

Jut remember the ring stops on a two stroke ALWAYS face toward the intake then you won't have to worry about the arrows! Have the cylinder measured and checked for deep gouges for the correct oversize piston. Use Wiseco pistons!Best available piston for the 400! You can get the complete kit with gaskets OR use Cometic gaskets which are cheap. If engine was installed properly and jetted properly(with the flat slide who knows?) then as a rule of thumb, seizure on exhaust side are caused by lean jetting or overheating conditions or air leaks,if seizure is on intake side of piston it's usually lack of lubrication! If piston circlips are still in place,not the cause of problem even if they are facing wrong! Be sure to check for ANY up and down movement on the rod(side to side ok) or the money you spend will go down the drain!!If mine I would try to find at least the Bst38 or Bs40 Mikuni carb unless you can have the flat slide jetted properly!Jetting is something you just have to experiment with! Not carved in stone on jetting specs and depends on altitude,temperature, engine modifications,etc!
Another thing I've seen many times over the years is for people forgetting to bleed the head to remove trapped air pockets when coolant is installed which can cause one to seize in a hurry! Good luck OPT

 

ya i know it can be done i have a 97 scrambler 500 i did the trans swap for low range and i have been running my 28" mudzillas for awhile now no problems spinning them even when i got it pinned in rocks and its still running the stock clutching i do plan on changing the clutching just havent got to it yet!

 

also another question is the piston i pulled out say 150 on it! is that a over sized piston already??

 

yes it must be 84.50, stock 83 one size of wiseco left. after that then you need either a big-bore piston 87mm, may still be a few out there. you can still resleeve after a 87mm.
The tm carb, just a straight tm. no x So that has round main jets? not sure on that.

with a stock carb or tmx piped ported and rechambered head. just a guess but a 300 would be on the rich side, to start your plug checks.

Stock s-55 weights might be heavy enough. get rid of the stock red sec spring. go for a epi orange maybe a place to start. In the primary i'm trying a polaris brown next.
wanted a red/white but couldn't find one.

 

Thanks I just found the carb is a tmx

 

thank you , and ya i just figred out that the carb is a tmx 38mm
im sending out my cylinder and head today to have them worked
over. with the jetting is there a tuning kit that i coud get or do i
have to buy the jets one by one?

 

do you think it is an oem tmx from like a 01 cr250. what shape is the slide? does it have two thin flat peices, with a rounded hump in the middle (old style tmx)? or is rectangle (approx.98-02 oem motocross carb)? or does it have rounded ends with a consistant thickness and a hump in the middle(newest design)? I believe the second two use a different pilot jet than the old ones. I have few other things to say but got to go now.

 

its flat on both sides except for on the top of the valve were the throttle screw hits it! i took pictures but i cant figure a way to get them on here. its kinda hard to explain! but ya both sides are flat and has a rounded center! my cylinder and cylinder head are a ritter cycles waiting to have the trail porting, cylinder head mod and girdle kit done should be a week or 2 but i would like to get everything else in order before i get the parts back! rick from ritter cycles said i would be better off buying a stock 400 carb and having him bore it it out,then useing the tmx would you guys agree or do you think ther tmx would be better?