heres the thing: Trail modded Scrambler with 39mm carb. Midrange meltdown last month. Rebuilt and piston seized up 10 minutes later. Dealer tells me that no matter what I do it will be too lean in midrange and must always be ridden at high throttle?!?!?!? That sounds like bull too me!!

Help me guys...I need to ride this week-end and this is really worrysome incase it melts down when casually riding the trails.

Could I put the stock carb back on??

Try lowering the clip position on the needle(this raises the needle to make it richer) if that dont work go to a richer needle.
good luck

2001 scrambler 2x4

that is the reason I put my scram back to stock. and got a lt250r. at crusing speeds the motor wants to pick up rpm and it is true you will always blow it. in 10 tanks of gas I went from stock to 60 over. no fun after that. sorry for the bad news

later
Xbdscram1

Drop the oil injection and go pre-mix. Simple way to make sure it gets the oil it needs.

it has nothing to do with the oil it has to do with the carb running very lean in midrange and not cooling down the piston

later
Xbdscram1

If it is jetted properly it should be fine. I know of a couple set up similar and they haven't had a problem. If you got your carb from HPD, AAEN, Hot Seat, or any of them they should be able to give you sizes of pilots, mains, and needle valve to get you real close.

when I got mine (38tmx) from aaen It burnd down in 3 miles. when a polaris gets ported it starts comming on the pipe about 5600 rpm. and starts to pull and revs up fast even when holding the throtle stedy at 1/4 to 1/2 throtle it will rev out and you are only giving 1/2 throtle worth of gas = lean mid range. there is no way around it. except having it wfo all the time to give it the gas = cooling that your piston needs to stay cool

later

There has to be something wrong. I can think of 3 other 400s that are ported. 2 of them have been ported for well over a year now and they do alot of trail riding where they sure as heck not running full throttle. Neither of these bikes have had a problem. The one, a very good customer of mine, has recently had a compression test (about a month ago) and it is well above service limits. Question - Are you at stock compression and if you are what octane fuel are you running?

I've got a ton of motor work done to my 400. The only midrange meltdowns occured with stock carb and the 38TMX. When I switched to the 39PWK; all was fine. It's plain and simple.... Your jetting is incorrect or you have an air leak in the crancase or combustion chamber. Most likely; it's jetting. I don't even run a radiator fan or antifreeze in the hot desert. What's your altitude, outside temp. and jetting? What needle do you have? Is your carb from HPD? Answer these questions completely and I can help.

Here's the deal. Trail ported (HPD), HPD Trail Pipe, 39mm Kehein Carb from HPD and a super modded airbox. I called Mike at HPD and the "normal" settings are with the needle clip at the middle position and I think a 38 pilot jet...not sure about the main. The dealer rebuit it but only leaft .03 piston clearance....Mike said this would cause instant piston seizure. It needs to be around .045 clearance.

Coming back to the lean midrange situation. When I put everything together three months ago it was fine. The plug "seemed" just a tad light. Two months later she melted on the side of the top of the piston. Mike at HPD told me that they make the snorkle bigger on there modded quads but do not remove the baffle. Too much air?? I had removed the baffle and added a third hole plus the bigger diameter snorkel.

Tonight I went and tested it and the plug seemed dry but not at all brown. Light as if nothing went over it... When I turned off the quad while rolling it seemed to take longer for it to shut down than ususal... air leak? The needle clip is on the before last to bottom space and i am running a one over-size pilot jet. I came back home and patched up the baffle in the airbox to try to cut down on air intake...wouldn't this make my mixture richer???

Mike told me I was the only one he has heard with these problems!

I am so pi$$ed off today!! Thanks for the replies and ideas...keeep em coming!