ive got my 300ex motor back together ...but it wont run ..it has fuel and fire ,but nothing.....I bought it used and it had work done ...when i took it apart I noticed it was bored with a HC piston and the cam had been upgraded ....iv'e no clue as to what brand or stage and i think this is my problem .
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.I belive the after market cam requires a advanced timing setting??????
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anyone who has built a 300ex using aftermarket cams please help....maby even how to identify the cam so I can ask the manufacture
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Im wanting to assume it is a hottcam ,because thats the brand im always hearing about....

 

The timing gear can only go on the cam one way. The timing marks on the gear should be the 2 flat with the head with the middle mark up. If that is not the case, you have something off. If the marks are off a little (less than a tooth) your chain is worn out.

 

When cranking the engine over, does it fackfire, back thru the carb? That is the easy sign your timing is off.

 

I'd have to agree with tractor.. take off the valve cover, align the mark in the eyehole on the side of the motor to the mark on the flywheel.. should be a T on it.. walk around the bike and see if the lines on the cam gear are lined up with the flat machine part of the head.. If those line up.. Take out the tensioner and check to see it has enough tension.. If all that is ok.. replace the cam chain, doesn't take much for them to wear out.. or what may not seem like a lot.. it truly is.

 

ive got the timing marks as you guys suggested ...and its only backfired once ...
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check this out ......as i said it had a bored high comp piston....and a aftermarket stage? cam
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now it has the same cam but a stock bore piston...maby its juist over loading???
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I can buy a new hc piston for the price of a stock cam ...so it would be like it was before the engine failure ( id rather have it faster than slower)

 

Shouldn't have any affect on it not starting.. I'd say that you are probably going to have to replace the camchain. Buddies z400 did that a while back.. thought it was the valves.. and timing and everything he tore into it.. same problem.. everything was correct.. brought it to me.. put a new camchain in.. wahlah.. runs like a champ. They are fairly cheap from what I can remember.. I just looked them up at honda at they are 100 bucks.. ebay has them like 40 - 50.. That would be my best guess though.