I am troubleshooting a problem with an 1997 Polaris Scrambler 500 and I could use some help. It will start and idle with no problems. When I give it throttle, it sputters, cuts out, backfires, etc. (very similar to having water in the gas). I have done the following: cleaned the carb & checked settings/jets (it is perfect), adjusted fuel screw (no change), checked intake for leaks (all good), replaced & gapped spark plug, fresh fuel, new fuel filter, fuel pump holds pressure and appears to be pumping fuel, exhaust is not plugged, air filter is in good shape, cam chain tensioner & cam timing (good shape), pulled the black wire on the rev limited (no change), added a battery charger to see if voltage was causing problems (no change). I ran a leak down test and found that I am leaking approx. 10%. I could not detect air coming out of the intake or exhaust ports, so I am assuming that I am leaking past the rings into the bottom end. Another thing I found was that the valves were very loose. They should be at .006" and the clearance I measured ranged from .008" to .014". I believe the exhaust side was the higher measurement. Would the rings and potentially worn valvetrain cause this issue? Should I look elsewhere, like at the stator? Any help is appreciated.

 

Look close at the exhaust cam lobe. 1996 to 2000 models were bad on cam wear on the exhaust lobe. Especially if you have a lot of clearance on the exhaust rocker arm. Here's a worst case one. Would idle fine,but pop back under throttle.http://forums.atvconnection.com/pola...d-cam-pic.html

 

Especially a 1997 , the Scrambler did not get the H.O high duration cam until 1998 or 1999.
The good news is.
If you do need a cam they only offer the H.O cam now for the 500 and it really wakes up that motor in the mid and higher rpm power band.

 

Replacement for the 97 shows an updated #3086212 non HO cam,but you can use the #3085855 HO cam if you wish, that is if the cam is the problem. Only difference is the cam duration as the lobe heights are the same on both cams and the other head parts are the same.Main difference on the HO was the 40 mm larger carb. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

I found two problems. #1 The rocker shaft had shifted to the right far enough that it was rubbing on the back of the cam sprocket. The bolt that is supposed to prevent this on the left support was warn down to a sharp point and was loose. The left shaft support was only holding onto the end of the rocker shaft, so the shaft and support show excessive wear. #2 The exhaust lobe on the cam was warn down to nothing. See attached pics.

 

Typical cam that I've seen dozens of times.Think you need some parts.Listed the 98 model as it has an HO cam replacement # that you can use. Don't forget to replace the exhaust rocker arm and any other worn parts also. Gets a little expensive when this happens.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse
Plus there are cheaper aftermarket cams on Ebay,but I've never used one.http://www.ebay.com/itm/BRAND-NEW-POLARIS-SCRAMBLER-500-2X4-4X4-CAM-SHAFT-CAMSHAFT-1997-2012-/330983631495?pt=Motors_ATV_Parts_Accessories&hash=item4d10296a87&vxp=mtr