Hi,

2 months ago I bought a used 1998 scrambler 400cc 2 stroke quad. I've been riding it pretty much every weekend or so since then and this last weekend I noticed a couple things.

1) Acceleration was a bit laggy when taking off. 2) 100% throttle felt like 60% throttle

It was obviously muddy so I took it out the next day to power wash/clean it. I covered the air box holes, gas cap, etc. and went to start it up just drive it around the block and i got 2 houses past mine and the thing shut off. I thought the fuel was off but it was on... I tried to start it up again and it started knocking, I recorded a little video below..

I've had a previous vehicle knock before (older honda, oil pump broke) so to me, it sounds the same. I pre-mixed it with am soil 2 stroke oil at a ratio of 36:1.

So is it time for a rebuild???? If so, I've read wiseco pistons are best. Should I do it myself or should I send it off to a professional company to rebuild? If not, what should I check for before taking the thing apart, water get near the spark plug or something?

 

Hell yes it needs to be torn down now,sounds like piston slap/rod noise! Don't expect to just get by with a new top end.This engine has a lot of parts from the rod,crank bearings,crank seals,water pump seals and bearing along with a new top end,etc that in all likely hood will need replacing also.The 350 and 400 can be a money pit real quick and if you don't have a manual hopefully you can download this one. And yes Wisesco is a good forged piston.Click on the 96-98 all models to get an idea of what has to be done.Index of /ORV Manuals/Polaris/1996-1998 - All Models

 

I bought the manual for the quad the week after I got it just in case. When I bought it the previous owner said the top end had just been done although it had been sitting for quite some time.

1) Did I cause this knock from excessive riding?
2) How does the engine go from being normal to piston slap/rod noise? I know it can't be low oil because I pre-mixed with gas, I really don't know so that's why I'm asking..
3) When you say the 400's are money pits, do you mean replacing the internal components are expensive or everything adds up when rebuilding (ex. bigger bore, machined head, bigger carb, new valves/reeds)? I'm just trying to understand why it becomes expensive.

Thank you for your reply, I appreciate it.

 

Anything can happen,but from what I've experienced over the years is don't believe what people claim unless it's your mama.. Main reason this is an expensive engine to repair is because of all the parts involved. I've said it before that I'd rather rebuild a 500 four stroke than a 400 2 stroke. This not to discourage you,but always seems once you tear this engine down,you'd find one thing after another that needs to be rebuilt. You've got the manual,but look a close at the break down on parts along with the three engine cases. Just my If I had to repair another 400 it would be rebuilt from the crank shaft up. This is the only way I'd ever guarantee one. I made the mistake years ago on just doing top ends that came back to haunt me.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehousehttp://www.cyclepartswarehouse.com/f...1998&fveh=5456
http://www.cyclepartswarehouse.com/f...1998&fveh=5456