[91 polaris 2 stroke]

I decided to start another project. I have an old Polaris 6x6 that hasn't run in five years or so, due to a knocking in the engine. I pulled it out of "storage" and cleaned it up. I messed around for a while and got it running. The knocking was still there, it is not a super violent knocking, but you can definitely feel it. Decided to pull the engine and take it apart to see what was wrong. Aside from some bad gaskets and carbon build up, it was in great shape. I figured the knocking would be from a sloppy bearing, but both ends of the piston were nice and tight. Even the crankshaft was tight. I am somewhat stumped, and I forgot to test compression before tearing it all apart. I did not check timing either. Could premature spark cause a knocking? Any help would be appreciated, thanks.

 

[latebird]

Now you are into a different realm of engines - two strokes are inherently noisy engines due to their design - the crankshaft doe not run in a bath of oil like a four stroke and there is very little lubricant on the cylinder walls - the knock can be caused by a variety of components including the primary clutch. It's always a good idea to remove the clutch and start the engine to determine if the knock is in the engine or the drive line.

Since you are beyond that, 90% of knocking in a comes from a loose fitting piston (and they are loose to begin with by design), the crankshaft (either rod bearings or main bearings) or a gear driven by the crankshaft. The 400 engine drives a counterbalance through a gear drive system. Rusty gears, a chip of metal between two teeth of the gears, a bad counter balance bearing or the balancer not being timed correctly could all result in a knock.

Since the spark timing is set by the flywheel, it highly unlikely that spark timing has anything to do with a knock.

Measure the clearance between the piston skirt and the cylinder at the point at which the cylinder has it's largest dimension from front to back (usually just above exhaust and rear transfer ports. If it exceeds .0025", the cylinder should be bored and the next largest size piston installed.

 

[91 polaris 2 stroke]

Ok I will measure the cylinder later in the day. Also I had the wrong year wrote down, it is a 1997 425 magnum if that makes any difference. I also ordered a full gasket set for the engine, is there anything else I should check before putting the engine back together?

 

[latebird]

OK - I was confused by your used name and the machine description - being a 425 four stroke makes all the information I previously provided VOID.

The knocking you were hearing prior to taking it apart would most likely have been the primary clutch, but could be the main bearings, rod bearings, a scored piston, the cam chain slapping a guide, the auto decompressor in the camshaft, piston to cylinder clearance, a loose exhaust pipe or something out of the norm such as bad fuel, loose flywheel, bad magnet on the flywheel or the counter balance not timed correctly.

 

[91 polaris 2 stroke]

Sorry for the delay, its hard to find anytime now days. I put new rings and gaskets in the engine. I got it set back in and hooked up to everything. It no longer knocks (even after I put the PVT system back together). Everything engine related is running great. I see a chain is all stretched out and shot so I ordered a replacement. The gear on the center axle that the chan drives is also bad. I could use some advise on replacing said gear because I am not sure how to get the axle out and taken apart. On the rear axle the bearing on the left side is bad. Do I need to replace the bearing on the right side as well? or would that be a waste of money? I also have some brake work to do, but I can manage that on my own. Thanks!

 

[latebird]

Sorry for the delay, its hard to find anytime now days. I put new rings and gaskets in the engine. I got it set back in and hooked up to everything. It no longer knocks (even after I put the PVT system back together). Everything engine related is running great. I see a chain is all stretched out and shot so I ordered a replacement. The gear on the center axle that the chan drives is also bad. I could use some advise on replacing said gear because I am not sure how to get the axle out and taken apart. On the rear axle the bearing on the left side is bad. Do I need to replace the bearing on the right side as well? or would that be a waste of money? I also have some brake work to do, but I can manage that on my own. Thanks!

I can send you a service manual, but I cannot advise on the procedure for replacing the sprocket - as to replacing the bearings, always replace both or you will be duplicating the job in a short time to do the other side.

 

[91 polaris 2 stroke]

I will look in my service manual, I will also replace both bearings. Thanks for the advice.