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Discussion Starter #1
so i bought a 98 300ex today and was told that there was no spark now but it was recently rebuilt and only 10 hours of ride time. i got it home and checked the spark and its fine. its turning over and has some compression, no guage yet so i dont know how much. i took the old gas out and removed the carb off the engine and pushed the throttle and it sprayed gas, so that seems to b good. I just started working on it today and just learning the basics but just confussed, im going to call the person i got it off of and c if he was making up a story and knows whats wrong with it but for now im just tryin to figure it out. Any help would b great.
 

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Welcome to the forums.. Put a plug in it and see if it will run?
 

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Discussion Starter #3
i did that and it just turns over. i took the head cover off and cheaked the timing and it all lines up. i dont have a feeler guage that i can check the gap right now but thats my next step. i dumped alil gas in the plug hole and still nothing. the guy keeps sayin the coil is bad but if its putting blue spark could it be?
 

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i did that and it just turns over. i took the head cover off and cheaked the timing and it all lines up. i dont have a feeler guage that i can check the gap right now but thats my next step. i dumped alil gas in the plug hole and still nothing. the guy keeps sayin the coil is bad but if its putting blue spark could it be?
It needs to be a good strong, bright blue spark that jumps from the electrode to the electrode ground. If it jumps off the edge of the electrode or around the edge of the plug, then it's too weak. When it's firing, you should be able to hear a distinct clicking sound.
 

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Also make sure you are not in reverse some 300s won't even turn over in reverse Im not sure about the older models. just a thought? I would deffinitely check the valve clearance it's supposed to be .004 but I would run the compression check first.
 

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Discussion Starter #6
i ordered a coil today but i checked the spark and it looks strong to me and it stays between the 2 parts of the plug and dont jump around. i need a different guage bc mine wont get in there at that angle to check the clearance of the valve. i hope i dont have to wait to long for my tools to ship. thanks for the help so far and i appreciate the quick responses.
 

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Like Moose said, I would do a compression test, first.
 

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Yea my 300ex wont start in reverse, it wont even crank over. My bros quad did this and it turned out to be 3 bent valves. Sometimes all it takes is a new spark plug, because sometimes they will show spark, but when under compression they are fouled. Ive seen this happen first hand. Check compression though like said!
 

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Discussion Starter #10
its not in reverse and i took the valve cover off and the rocker arms seemed to b fine. i removed the lil covers off and turned it over and they seemed to move right. ill try a new plug tomorrow but it looks brand new. i was thinking mayb the valves, if the compression is low thats prob what it ia right?
 

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its not in reverse and i took the valve cover off and the rocker arms seemed to b fine. i removed the lil covers off and turned it over and they seemed to move right. ill try a new plug tomorrow but it looks brand new. i was thinking mayb the valves, if the compression is low thats prob what it ia right?
If the compression is low, then it's either the rings or the valves.

If the compression reads low, pour about a teaspoon of oil down in the spark plug hole and check the compression again. If the compression reads higher, your rings are bad. If not, then it's probably the valves.
 

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Discussion Starter #12
ok i got my tester and the needle barley moves. does that mean anything certain or can that still b one or the other.
 

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ok i got my tester and the needle barley moves. does that mean anything certain or can that still b one or the other.
It could still be either one, but if it barely moves, then you should probably just go ahead and tear it apart and look for the damage.

I believe you said that you already checked the timing, so I would go ahead and pull the head and look for the damage.
 

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Discussion Starter #14
yea i guess i got nothing to loose if i remove the head bc its gotta come off either way to fix it, ill do that today and hope to find the problem.
 

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Discussion Starter #17
WOW so i removed the head and every thing and it seems like the guy might have rebuilt the motor but forgot to add oil b4 riding it. half of the piston was melted and some pieces were jaming the valves. its a mess so now i gotta rebuild the top end and hope nothing else is as bad as the piston. since i gotta get a new piston and the cylinder has some old piston melted to it whats the best fix that wont hurt the wallet. does the sleeve come out and if i buy a new 1 is it standard bore?
 

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depending on if the piston they put in it is stock bore you can have it bored like a small bit just to get the score marks or in your case burnt piston off lol. Sorry not a laughing matter. But yeah the sleeve will come out and you could resleeve it. Or you could bore it i have heard the 302 bore is a good kit for a 300ex.
 

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I would just have it bored 0.025 over. That's probably enough to clean up the aluminum from the old piston. Whoever you get to bore it should be able to measure the cylinder and tell you if it's ever been bored or not. I would just go 0.025 over whatever it is now, if that's enough to clean it up.
 

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Discussion Starter #20
so the cylinder wasent scratched just melted metal piston on it. i used a hone tool but im not an expert at using tools. i honed it till the metal was down and i attempted to use a digital gauge and its saying its 74.06. is that to much to use std size pistons (74)?
 
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