Kx 80

Joined
May 8, 2000
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#1
On my '87 KX 80, i took the reed cage off and found out that the piston dosent go down far enough to uncover the port. Is it suppose to be like this? If so, how does it get gas into the cylinder? I'm finishing the rebuilding process and i need to get past this stepping stone. Any help is appreciated. Tanks.
 
Joined
May 21, 2001
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#2
i think i can help

when your looking into the reed cage . can u see any part of the top of the
piston...?
did you change pistons ? if so ,, you might outa get the numbers off the top of the piston call the place you got it from and double check for accuracy
hey these days ,,,,,,,,,,,,,,anything can happen
how bout the connecting rod did you have that changed /?
just check for accuracy on all the parts that was ordered or installed
and if everything checks out
id just put it back together turn the kicker without the plug in it or fuel in the cylinder by hand the motion should be smooth through and through
if ive forgotten anything
im sure someone else in DRN can help good luck

;)
 
Joined
May 8, 2000
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#3
I just found out from someone that it gets the gas/air mixture from the hole in the piston. He said that the actual piston dosent need to go down far enough to see the crown of the piston even at BDC. This seems a little odd to me. When i bout the bike for i let someone put a new crank in it but i'm using the same piston that came with the bike. The cylinder has definitely been bored out by the previous owner and maybe the wrong size piston is in there. I'll have to check it out to be sure.
 

nephron

Dr. Feel Good
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Jun 15, 2001
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#4
Answer: Start it. If it will start and run without shelling out the head :scream: , then it's ok.
 
Joined
Aug 18, 2001
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#5
Your engine is of the case reed design, where the air/gas mix is sucked into the case first then expelled into the cyl. through the transfers. Think about it, if the air/gas went directly into the top end wouldnt the bottom end bearigns and the wrist pin bearings have no luberication? (The answer is yes!):D
 

125 rider

Sponsoring Member
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Oct 1, 2000
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#6
When the piston is at TDC the spark plug fires igniting the mixture & driving the piston downward. The piston is pressurizing the crankcase as it travels down and will eventualy uncover the fuel transfer port which will allow fuel in. The fuel rushes in because it is in the crankcase which is under pressure from the piston. As the piston changes direction and travels upward, it will cover the fuel port and now create a vacuum in the crankcase and that will open the reed petals which will let more fuel/air into the crankcase. As the piston closes the exhaust port, it will compress the mixture and then in turn fire at TDC. I left out exhaust details to avoid further confusion. Think of the crankcase as a "holding tank" for the fuel before it enters the cylinder. The oil in the fuel also lubricates the bottem end while its there. This is probably more than you wanted to know but try to understand it.:)
 
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#7
Thanks for clearing this up for me. I had never known what a case-reed looked like before so i didnt know what was going on. I really appreciate the information guys.