I recently acquired a 97 Trail Boss 250 2wd. 2 days ago it started briefly and then wouldn't start. It would just crank and crank. I thought the plug must be fouled. Yesterday I took the plug out and could see spark although the electrode was very black. I replaced the plug with a new one and it still wouldn't start. I then put a shot of starting fluid down the plug hole and it started right up. I rode it around the block and got it going pretty fast. As I pulled into the driveway and stopped the engine slowed down and stopped and it again wouldn't start. I put another shot of fluid down the airbox intake and it did start. It seems like if I can get it past a slow idle it runs good.

A friend that has owned several Polarises thought it could be the coil or CDI since it was having trouble going from idle to moving and wanted to die at idle. To me this sound more like a carb issue than an electronic issue. But the ATV does run badly in damp weather and does better in dry weather. Yesterday was damp but dried up later.

I've rebuilt carbs on a Honda 90cc engine. This carb looks similar but slightly larger. How hard is the carb to remove and disassemble? Any different than the Honda?

Also when it doesn't want to start the electric starter won't run. Just gives a click with a new battery. But when it eventually does get to starting the next time it will crank just fine with the starter. The battery acts dead sometimes but is just fine other times.

 

Check compression first before you replace the carb kit. Might be the starting problem is because of a compression problem,piston, bearings starting to seize OR could be a starter problem itself.Eliminate one thing at a time.

 

I will check it tonight. As long as my compression tester fits the spark plug hole. On my 90cc the plug hole is smaller than my tester. But I think this plug is the same diameter as the ones that I've used on 4 cylinder cars.

 

One other thing I forgot to mention is to make sure you don't have too much slack at the thumb throttle. About a 1/16" to 1/8" only so the etc contact points in the throttle housing don't touch. This can cause it to die when you come to an idle. Compression needs to be at least 110 psi or more.

 

I rode it quite a bit yesterday. It is running pretty well. The only issue is what it is hard to get moving from a stop if you let the revs go down. Once it is moving it has no issues. When it bogs from a stop the exhaust is pretty smoky. Does that say it is too rich when starting? When moving the exhaust is not noticeable. I am premixing as the oil injection was disabled by previous owner.

I did not do the compression check because I couldn't get the tester in without having to remove the gas tank. A buddy didn't think it had a compression issue since it started from cold easily and had plenty of power once moving.

 

If you're mixing it richer than 40-1 to 50-1 (say 32-1) then this could be the reason for the rich running and bogging,plug fouling.

 

Mix is 40:1 My first tank was with 93 Octane gas. The last was 89. It seemed to run better on the 89 which is what Polaris calls for, I believe.

 

I'd still do a compression check when you get the time to see where you stand on the top end.Plus may just be needing the float level checked or could be time for a carb kit.If the needle and seat are worn,could be letting too much fuel into the crank case. Smoking is not always oil related on these 2 strokes,but can smoke from excess fuel also which can cause the bogging at low rpms. Here's couple videos on the Mikuni carb similar to yours.

 

I did a compression check yesterday. It was 115. From what I've read that is a little on the low end but still pretty good. Is this correct?

 

On the low end! This is just about bare minimum compression for most to be able to run.I know I'm sounding like a broken record as I say this over and over to people,but I wouldn't do just a top end only especially on a 16 year old engine.I've seen too many that people have done only to have the bottom end crank/rod bearings fail. I'd bore cylinders at the shop for carry out and would ask,but most times they'd say "the bottom end is fine". You just take a chance on a partial engine rebuild.