OK, so I got most of the issues resolved, and the ATV runs fine ... but still have the following issues:
1) Takes a while to start - air bleed screw is set to 1.5 turns (tried 1.0, but 1.5 seems to work better)
2) If I adjust the idle to work fine in neutral, it bogs down under load. If I adjust it (via slide plate screw and/or air bleed screw) to run normal under load - it races up in neutral


.... that thing ROLLS... I'm scared to put a teenager behind the wheel, as at any curve or at any turn this thing leans as if it's about to flip over.... --- any suggestions? tried no pressure in tires, tried 30 psi - still scary...

Vlad

 

1.5-2 turns out was a normal setting on most.You can double choke,that is flip the choke lever all the way over and see if that helps on cold starts. Hard to get rid of all the bogging on this old style simple piston port engine as that's just the way it's designed. The bogging led the way to later model reed valve machines which solved most of that as they evolved. You can adjust the slide needle position up or down to see if that helps on the bogging also. Some of the bogging,erratic running could be from leaking crank seals also or if the belt and clutch parts are worn,this will affect how it runs.All these work hand in hand and have to be in good working condition.

 

Follow up question, I'm running a K&N style air filter (previous owner installed it) and no vent lines (nothing is blocked, but not connected to anything. Would that make any difference in how the ATV runs? If I need to change it, where should I route the vent lines? up towards handle bars? Should there be a check valve anywhere in the system?

Another thing that I noticed, but not sure if that's how it should be, when I depress the throttle lever, the amount of gas that's released is ridiculous - I can actually see gas seeping through the K&N filter - is that normal? related to the same question, after I give it a good pump with throttle lever, it takes awhile for the RPMs to come down.

 

There's your main problem if the gas is backing into the filter. Needle and seat are sticking or worn,possible the float level is off. If you haven't installed a carb kit,get a good one.Shindy Carburetor Rebuild Kit Polaris Trail Boss 250 4x2 4x4 1989-1999 Carb | eBay My two cents on filters is to use an oem paper filter or a good Uni foam filter. No tees are needed on the carb vents. Just run carb vent lines up,zip tie to the choke or throttle cable and let them terminate under the tank. Separate line to the drain/overflow tube on the carb bowl and let it extend past the bash plate. Get these fixed and you may see a big difference in the way it runs. Simple carb to rebuild. Look at these videos.

 

I'll try it! Thanks

 

Plus I never asked(just assumed) that you have the air box installed and not just the filter.If you don't there's another problem.

 

OPT, Thank you for your advise and help. Every time I look at that machine, I learn new things and find more issues - never ending fun! Maybe one day I'll even ride it!

I ordered the carb rebuilt kit, but meanwhile I took apart the carb, cleaned everything, checked the gaskets, checked the needle and seat - and now no gas is visible in the air filter .... still cannot keep it running smooth.

Instead of going piece by piece, I would like to list what I know about the carb and the engine:
1) Cylinder is bored to (I believe) 40 over, as I'm using 73mm Wiseco piston.
2) Oil Injection pump was removed, spacer inserted, throttle cable replaced (straight line)
3) K&N type of air filter
4) Carb details: Main Jet: 130; Pilot Jet: 40; needle seat: 2.5 Jet Needle Seat: 169 @ 0-4; Jet needle: forgot to check

On the slide, where it touches the idle adjustment screw - VERY VERY work out - like about 1mm deep, 6mm long very uneven grove

 

Jetting is stock for your 2003 model.Since you have the carb kit ordered,a couple other things in case that doesn't entirely solve the problem. One is check compression even though it's a new top end just to eliminate that. The other thing that most people over look is crank seals that could be leaking,especially the one behind the drive clutch.If it's leaking,you could adjust all day and night and it wouldn't idle or run right. The idle adjustment screw is simple and no set turns on this. Just turn it in or out until you have a slow steady idle. That from your posts is a problem area that the carb kit may help.If it doesn't then you can go to the trouble of a leak down test or simply replace the crank seals which are cheap.Solid rubber seal goes behind the drive clutch area,the metal one behind the stator plate. Polaris Trail Blazer 250, 1990-2006, Engine Crank Seals | eBay