Polaris ATV Forum banner

1 - 4 of 4 Posts

·
Registered
Joined
·
2 Posts
Discussion Starter #1
Recently picked up a 2000 Scrambler 400 2x4 that seemed to be running great. I wanted to go through it and do some general maintenance and while I’m a pretty lousy mechanic, the interwebs had me feeling confident enough to
- swap clutch (old one felt like it took too long to start moving)
-drain radiator and fill with new coolant
-change counterbalancer oil
-change “transmission” fluid
After doing all these things, I try starting it up only to hear it turning over for longer than it normally does (using electric start which has not yet failed me) After attempting to turn over for maybe 5-10 seconds, it makes a huge explosion and smoke comes out the exhaust. I was working in my garage and the sound scared the **** out of me. Scared to try it again, I thought I’d do a compression test and got a reading around 60 (very low, right?) Spark plug also looked pretty black when I pulled it out.
Did I do something to blow the engine up? I can’t figure out what I might have changed to do something catastrophic. Any help would be appreciated. I read through forums for about an hour before giving up and posting, but if there’s a post out there that covers this, please point me in that direction.
 

·
Registered
Joined
·
9,525 Posts
Difficult call - I'll guess the bang was a backfire caused by a late spark perhaps caused by a fouled spark plug and the plug may have fouled for one of many reasons, but low compression and an unclean burn could cause plug fouling. Since the engine still turns over, did you check the compression with the throttle wide open?
 

·
Registered
Joined
·
2 Posts
Discussion Starter #3
I retested compression this morning, making sure to hold throttle wide open and got a reading of 100psi. The manual I’m working off of says compression should be Std +\- 15%. I don’t know what that means but 100 still seems low to me . I’ve read that squirting a little oil into the spark plug hole and retesting will tell you if it’s worn rings vs valves but I’m not sure if that holds true for two-stroke motors. Is that something I should try?
 

·
Registered
Joined
·
9,525 Posts
Two strokes run with oil in the fuel - squirting in oil does nothing to raise the compression - the compression is low - normal is about 120 - at 100 psi, it might start, but will be low on power - I suggest taking the top end apart to check the condition of the piston and rings.
 
1 - 4 of 4 Posts
Top