ok here goes. I just rebuilt the engine in my trailboss 250. I also replaced the ignition coil, rev boxes. and cdi box. I still had no spark. I put a new spark plug cap on and now I have spark. but it still wont fire. I completely drained, cleaned and filled the fuel system. cleaned and rebuilt the carb. I have excellent compression, spark, and fuel. WHY WONT IT START????

 

have you dont a leakdown test on the motor? that checks the engine for any air leaks. 2 stroke motors need to have an air tight crankcase. when you say you rebuilt it does that mean just the topend or did you seperate the crankcase and replace bearings and seals?

 

I rebuilt the top end. new piston and rings, and seals. from the cylinder up. I dont trust myself to do anything below the cylinder.....so i leave it alone

 

dont be afraid of the bottom end. there is absolutly nothing too hard in there. i was nervous the first time i did mine but once you get into it there is nothing too hard in there. to check for a sealed crankcase take the carb and exhaust off and plug the manifold holes. push the piston down so that its at the bottom of the stroke and remove the spark plug. make an adapter to screw into the spark plug hole and hook an air hose to it. feed about 100psi into the cylinder and listen for any leaks. listen carefully behind hte flywheel and the drive clutch.

a quick test would be to take the recoil housing off and try to wriggle the flywheel up and down. if there is any movement then your crank bearings are shot. with sloppy bearings the seals cant create a vacuum tight seal around the crankshaft.

 

Hi
How difficult is it to change the Crank Bearings and Seals on a 1986 Trail Boss 250 2x4 ?
Do you have to remove the Engine and can the Bearings be replaced from Outside the Engine ?
Thanks
Timmy

 

not sure, but i think you would need a press for the bearing to press on and off? the rest is fairly straight forward.

 

Did you remove the stator by chance? If so, you have to line up the mark on stator plate and case

 

If rod or crank bearings are bad You would notice if it ever cranks up! You dont have to split engine to replace seals,just remove the front and rear drive clutch and backing plate and the stator assy behind the recoil. Seals can be pulled out with a drywall screw and new ones just tapped in place using a DULL punch as to not damage the seals.If you are still planning to remove crank then would highly suggest having a new rod and bearing installed, and crank trued!Will save you headaches later! Oil pump gear and bearing on stator side MUST be pressed off OR heated and use a bearing separator to remove.
Must have a press and dial indicator and stand to true crank or let a shop do it.DONT know if this is your case,BUT I've seen a lot of top end jobs people have done go south because of neglecting the rod/crank assy bearings and seals! I bore cylinders all the time for carry out and first thing I ask is did you check the bottom end for play at the rod and do crank bearings feel/sound notchy! Sorry for the long post ,but I come across this a lot.
Hopefully its just the seals not allowing bottom end compression. Good luck OPT

 

Hi Old Polaris Tech
No problem on the "long Reply" - Keep'em coming,

Here is my other Postings, I'm sure you can see where I need to go.

I have an 86 Trail boss 250, I have a problem with it not wanting to Idle.
I have rebuilt the Top End (new Piston, Rings and Honed Cylinder), Compression is very good but once it is running, It goes from a Hi Idle - (the Pilot Screw has almost no effect where it is adjusted) to stalling when We switch riders or just come to a stop.
I too was told to check for an air/ Vacuum leak thru the crankcase but am finding it hard to find ways to block off the Intake and Exhaust ports properly. Should I just take it to the Dealer and "Bend over" and close my eyes.

And

Hi Guy's
Thanks for the Help, I have gone thru the Carb like 10 - 15 times, removed the Pilot Jet, complete disassemble, Set Float Level, Main Jet, Slider is seated all the way down, and reassemble of Carb.
Removed the Rubber Carb flange, I have a Spare engine (seized, Bad Piston) that I removed the Carb Flange, put a small amount of sealant and installed to the Intake side. My Exhaust flange and pipe are wet, I figured it was oil residue ?
I will have to remove the Pull starter and side cover to see if there's movement at the Crank.
I would assume the Crank Bearing's and seals are bad right now as it is a 1986.
How easy is it to change the Crank Bearings ? I have seen how the Seals could be pulled out using the "Sheet Metal Screw" trick but how are the bearings Removed ??
Should I just bring it to a Dealer at this point ?

as of Today 5/17/09

I just got the CLYMER REPAIR MANUAL POLARIS ATV 85-95 and POLARIS TRAILBOSS SCRAMBLER MAGNUM CLUTCH PULLER TOOL from eBay and will take the Engine off the Quad hopefully today, I bought this for my wife to drive like 1/4 mile to get our Horse and bring it back and forth to the Tac Barn so I'm more into it Starting and being able to Idle more than a Performance issue.

Thanks again for all your help !
Timmy from Connecticut

 

Well I did it, I Replaced Both Crank Shaft Seals and my 250 TrailBoss. It runs and Idles perfect. Starts right up and goes to Idle. I did it with the engine in the Frame.
I used the Clymer Repair Manual and Polaris TrailBoss Scrambler Magnum Clutch Puller tool I got off eBay ($ 58.00 Total), Had a Flywheel Puller, got 3 - 6mm x 3.5" bolts for that and used the old # 40 drill into the old Seals, screwed in a small sheet metal screw and pulled out the old ones.
I kind of took the long road around the real problem.
If you have worked the top end and it won't Idle - Change the Crank seals, Both of them.
Timmy from Connecticut