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121
Joined
12/24/2015
Location
Kingwood, TX
US
I’ve noticed my 04 kx125 has been really tough to start lately (only starts with starter fluid). Cleaned the carb and jets, new gas, new plug. Bought a compression tester and it had about 120 psi.
Threw in a new top end, gaskets and vforce reeds and it’s only improved to 125psi.
What else could be going on?
Throttle must be held wide open when taking compression test.
Did it run fine before you messed with it?
I feel like it hasn’t run “well” in years. I ended up doing another compression test this afternoon and got it up to about 142 psi which made me feel better. I was about to send the cylinder out to be replated!
Seat the rings and take another compression test.
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What were the ring gap(s) at when you put the new top end in? You could put a new piston/ring assembly into a worn cylinder and it still won't have compression if it's worn out. For a fresh ring package you'll want .010" or close to that. Compression can also bleed out by the power valve assembly.
Honestly have never checked the ring gap. I’m going to send this sucker out to Millennium Tech or down the road to JM racing to get replated.
You should always check it. You put the brand new ring into the cylinder and then push down with the piston to make sure it's perfectly square. Then use a feeler gauge. If the cylinder is worn too much the gap can be .017"+ and it won't run properly. The could be the case here.
Hold up, JM looked me right in the eye and told me to run a piston that had no clearance in my cylinder and it would break-in fine! He just honed it, bc it only went in halfway. Turns out the cylinder was re-plated to a Wiseco A piston spec. My guy in Conroe simply saw the re-plate stamp on the side of the cylinder, asked what piston came out and told me the Vertex would seize, if it would start at all. BTW, hot small bore set up is to run the STIC PWK AS carb. Result is a full race built engine with loads of low end as well.
Hey I'm next door in Atascocita with a freshly rebuilt '04 KX125 engine. Has a new crank/rod. I just need to paint it's frame and good to go.
Know anybody that needs a backup '04 KX125? or maybe a '22 YZ250F(45hrs). Perhaps a '24 300SX or '06 200SX? Just sayin' neighbor. LOL
Welp, had the cylinder replated and slightly bored by a well known shop. Installed a new 56mm piston kit. Compression is somehow lower now (120psi) than it was before all of this.
Will compression get better once I break everything in?
Not going to buy another bike, so I’m about to give up and be done with moto altogether.
Compression will go up 20-30psi once it runs for 30+seconds and the rings expand.
If yours is still low after all of that work then compression is likely bleeding out somewhere other than the piston rings. Power valve could be the culprit.
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