I just bought a 98' 300ex. After riding four awhile the chain will loosen up, it keeps getting worse and worse i'll have to keep tightening it up over and over. I bought a new chain and sprocket kit do you think this is my only problem? I dunno anything about atv's really....i just wanna be able to climb some hills without it coming off on me again. Thanks

 

Could be you have a bad rear axle bearing. Put the frame on a block and see if you can wiggle the rear axle back & forth, up & down. If so, it's a bearing. Not too bad to replace.

 

I agree with bud. Check the axle bearings.

As long as you can get the axle out. Just had a old warrior that had been mudding. Water got in and my bro said he hammered on the thing for about 2 hours before finally knocking it loose! But, yeah after that part it's a cake walk.

 

300ex's do loosen if jumped. Mine does it all the time. I try not to jump big anymore w/it.

 

If it loostens when jumped, chances are the chain is adjusted too tight. If the chain is too tight, when you land after a jump, the suspension bottoms out, making the chain very tight, which in turn will either loosten the adjuster, break the chain, or break something else. Make sure you have enough slop in the chain so when the suspension is bottomed out, the chain still has a little play in it. Otherwise, your asking for trouble. The chain should not loosten up when jumping.

 

Key words..."should not" .. It says that on page 3..LOL
After having my boy race/ride one, me doing the same, countless others racing at a national level, I've seen em all get loose.
Never mattered who's swingarm or who's shocks from stock to wild.
Normal adjustment would be:
With rider on machine, 1" of up and down slack in the chain.
I set mine at 1 1/2" slack. I don't want too much.
What would you set it at budedm?

 

I compressed the bike all the way with a huge ratchet strap, then adjusted the chain to have a little slack while compressed.

 

I apologize if this is too simple...and not meaning to be a smartass...but does your sprocket kit include the countersprocket? You MUST replace all three components at the same time...countersprocket, chain and sprocket....or else you will have irregular and premature wear. It should also be a quality set...there are some crappy products out there. I have had great results with sidewinder products, and have always gotten o-ring chains.

If you did all 3, then I agree the bearings would be my suspicion after first checking the tightness using Budedm's technique.

 

If your chain tension ends up being OK, you might want to remove the bearing carrier from the swing arm and make sure that both are clean and free of any grease. (Outside the carrier, not inside... you want lots of grease on the inside).

 

This has always been a problem with 250X and 300EX quads if they are being jumped or overloaded.
The problem is the rear suspension geometry......

If you look at a 300EX from the side and imagine the plane of motion you will see what I'm getting at. The biggest problem is that Honda didn't really place the front sprocket in the right location in relation to the swingarm pivot. And it's far to close as well.

If you notch a mark on your carrier the next time you tighten it up, you will notice that the NEXT time you tighten it, it is STILL lined up. The chain is simply stretching.
I am not sure about the different swingarm and rear shock setups out there, but a stock swingarm and stock rear shock will ALWAYS stretch the chain on a 300EX when bottomed out.
If you want to experiment with what I'm talking about, place a jack under your engine and remove the rear shock. Play with the ride height,..you can have all the slack in the world with no weight on the quad, but the instant you get near the bottom out point,..it's banjo string tight.
There is nothing you can do about it because to allow as much slack that would be needed, would be TOO much slack and would derail under normal riding conditions.

Chalk it up to engineering.