I'm new to this forum and have a question about my 1988 Polaris Trail Boss 250 2x4, just bought it last week. chassi w887527 carb Mikuni VM. i have a repair manual.
i read an earlier post from 8/25/2006 concerning hard starting and have gone through the carb completely. new jets, throttle stop screw, pilot air screw, jet needle and starter plunger. i disassembled the carb and soaked it in carb cleaner then blew it out with compressed air. am near possitive that all passages are clear. the compression is right at 100 psi. i reset the pilot air screw at one turn open and the idle stop so it just barely moved the throttle side upward. the floats were set with the arm parrelell to the bottom of the carb body with it inverted.
the pilot plunger has to be fully open and the engine cranks quite a while before firing and will slowly die. when i do get it running it runs great and performs well at full throttle. when warm it starts fine. haven't been able to do a bleed down test. not that mechanical yet. any suggestions would be appreciated.

 

open pilot screw to 2 -2.5 turns and maybe raise needle one notch, lower the clip, that will raise needle try it and let us know

 

These motors are also known for the crank seals going in them especially if they have sat for a length of time. If your sure the carb is set correctly you should be looking at diagnosing the seals. At 100 psi in the cylinder thats getting on the low side.

 

i tried opening the pilot screw with no difference. haven't been able to try opening the needle valve yet as it has been just too cold here. is supposed to warm up and will try it this weekend. thanks for the help so far.

 

optimum setting on the needle is setting 3 you shouldn`t ever have to change this inless you have other problems changing the needle position is like using a band-aid and will cause other problems down the road. Find the problem and fix that, Like I said earlier check your crank seals and it wouldn`t hurt to also do a top end rebuild 100psi compression is low and needs worked on.

 

well i'm going to do a bleed down test and check the seals. while i have it apart i'll hone the cylinder and put in news rings and piston if needed. do i need to pull the engine to do this.

 

No you can do the top end of the motor while is in the quad. when you take the jug off make sure to put some shop cloths around the connecting rod and wedged in the crank case those little clips holding the rispin in like to fall in then the job becomes a pain in the a--. personally if i was you I`d take the jug to a machine shop and have them check it for out of roundness if its to bad a simple honing and new rings will not fix your compression problem.

 

what about the seals? is that something i can do on the bike and will i need any special tools? i do plan on taking it to a machine shop. what kind of compression should i have? if i read it right my bore should be 72mm or 2.835 inches. i see where you can get over sized pistons. i'm not concerned about more power but at what point will i have to go to them?

 

well i pulled the head off today and there is enough play in the piston to move it back and forth about 1/16 inch. a rough measurement of the bore is 72 mm. i will be ordering a new piston and rings as soon as i get it back from the machine shop. so far i'm still wondering about the seals. i need to find out how to remove them.

 

The machine shop should before he does any work on the jug ask you for the new piston so he can make sure its matched. if you havent ordered the piston yet go for a half MM over bore 72,5mm. There won`t be any gains in HP and the machine shop can remove enough material from the jug to make the cylinder round again and not hamper the integrity of the engine.
The seals are fairly simple to do, the specialty tools you will need is a clutch puller about 25buck at Polaris and a harmonic balancer puller for the flywheel. search my posts I have explained how to do it recently.