1997 Polaris Sportsman 500 Top End Problems

[Arctic Thunder]

Lets try this new section out and see what you guys know about these darn things.

1997 sportsman 500 ATV.

Runs pretty good, starts good, idles good.

Now the problem.

When pulling it hard, uphill, engine good and hot, it will start to miss on the top end. Not a bog but a miss.

Did some messing around and noticed if I turned the fuel off when holding it to the bar, the miss would clean up and pull hard after 15 seconds or so. Of course it would run out of fuel if I left the fuel off but it cleaned it up.

Also if you let off and stop for a second or so then nail it, it will pull hard again for 5-10 seconds.

So here is my thoughts.

Float level is to high?
Main jet is to big?
Possible Coil problem?
Exhaust lobe on cam is flat? I doubt this because it can run good.
Exhaust is plugged up?

What are your ideas?

I have checked the carb and I am running a 155 main jet. The vac boot in the carb appears to be good. (still stuck to the cap and doesn't appear to leak). Intake to air box is clear, air filter is clean, from filter to carb is clear.

Thanks

Thunder

 

[Arctic Thunder]

Think I figured it out.

In 1997 the 500 ran a stock 142.5 main jet. It has the 155 main which was stock in the 2000 and later HO's. So it ran just ok at lower elevations, under 1500', but at higher elevations the pilot and needle were fine but the main was to fat. So it would load up when in the main.

So I installed the 142.5 and it is a lot crisper here at home around 1200'. Should be night and day in the mountains.

Thunder