chain keeps falling off

Anyone have any ideas on how to keep the damn chain on ? I indicated my rear sprocket to run true and round. My chain is only 3/8 thick but the tensioner is about 3/4 wide. The engine sprocket and the rear sprocket line up good but the tensioner throws it of wack. Do they sell a not so wide guide ?
Maybey .400 thousands wide ? Thanks

Fakeplay,
Every chain tensioner I've seen needs to be twisted slightly to get it to align with the chain path. The bicycle chainstays typically are tapered toward the crank and therefore are not parallel with the wheel/sprocket/chain. Try giving the tensioner bracket a slight twist to get the pulley to align with the chain and exactly centered. Also you'll want to secure that tensioner bracket to the chainstay so it can not loosen and rotate into the spokes. I can't stress the importance of this step enough. Some use a self tapping screw through the bracket and bike frame and some drill all the way through and use a bolt and nut.Go no larger than a 10/32 bolt. Either way is good but make sure your alignment is right before drilling. The width of the pully groove is not important unless it is too narrow. A little wider than the chain will not cause any problems as long as the wheel is properly aligned.
Tom

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Thanks for the info, I'll give it a shot. I've been reading some of the forums and came across using a spare derailer. Any comments on that idea ? Thanks again.

Try the skateboard wheel mod, you will be pleased.

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If it ain't broke, and you mess with it long enough, it will be.

First thing, make sure the rear sprocket and the front sprocket line up. I had to put a bunch of washers to move the wheel to the left to make everything line up right. If everything does line up right you might not even need a tensioner if you get half links and play with the chain size a bit. Unfortunately on my bike I need a tensioner more to keep the chain up from hitting the bike. As 2door said, the first thing I had to do was bend the tensioner to get it to line up with the chain.
First I used the stock roller, but ditched it after a few miles because it is crap...
Then I use a derailer gear which actually worked pretty well, but for some reason it was eaten in about 100 miles. I think because the stock chain is really crappy and the rollers stick, and sticky rollers ate the gear for breakfast.
I have now settled on a skateboard wheel. I used a high speed drill as a lathe, and a file to cut the groove. And it works mint, couldn't be happier.
And I'll second what 2door said about the tensioner being wider than the chain not being an issue. The chain tensioner is not for guiding the chain onto the rear sprocket because it isn't aligned properly. In theory you should be able to use a infinitely wide tensioner sprocket, and still have the chain properly rain onto the sprocket.

I did actually try to use a derailer as a spring loaded tensioner, but I never got it to work right, so I gave up.
I'm also running a KMC420 chain. A bit wider than 415, but the same pitch and hasn't caused me any issues. And it only cost me $16 for a 6 foot bit.

there are post and mods everywhere on the forum, bottom line is the kit tensioners are pretty much junk, frank

Thanks for the info

you have to turn or twist the tensioner to line the chain up with the rear sprocket then tighten the bolts on tensioner.