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Discussion in 'Moto Club' started by muscles, Feb 23, 2007.
The easiest way to tell if you need a new top end is to remove the exhaust header from the cylinder head and look in there with a flashlight while you slowly turn the motor. If the piston skirts are scored, new stuff is in order. HH
When you pulled the carburetor out of the boot connecting it to the intake, the reeds should have been visible down inside that boot.
How does the plug read? You didnt mention your elevation. With sea level jetting, Id expect it to run like crap >6000 (way too rich).
I had a problem with a YZ 125 like you described. No power until very high rpm. The prob with my bike was the needle/seat that controls the flow of fuel into the fuel bowl. If the surfaces are pitted, corroded, or even a speck of dirt, etc, too much fuel will get into the bowl and cause it to run rough or flood and not run at all. My bike dripped gas out the overflow tubes that hang under the back of the engine. I only noticed this when the bike was stopped and someone else was riding it. $25 part and it ran fine. The needle is connected to the float when you took the thing apart. The seat is the brass part screwed in the carb bottom where the needle came out from. Hope this helps you.
Get a manual with a jetting guide. It'll give you a good idea. The pilot/slow jet has the biggest influence on low end.
Then do a compression test. Pull the spark plug, intake and exhaust and view the reeds and piston skirt (you can see the skirt by working the kickstarter by hand and looking through the exhaust and intake holes).
If it were me, I'd spend $150 on a new top end (piston, rings, needle bearing). It's likely that the top end is clapped-out. The cylinder bore should be lined, so it's cheap to rebuild. If you're real cheap, try just installing new rings. PS, you'll need new head & base gaskets once you crack the top end.
My 2005 KX250 is on it's 3rd top-end. I rebuild it every 12-20 rides. There is an instant difference.
If you really wanna make it go like new, put in new crank main bearings & seals, and a new con-rod big-end bearing too. Old clapped-out main bearings vibrate and rob power and make the bike rev slow. Leaky internal seals make carburetor jetting impossible.
PM me if you want a phone number. I can help you decide what stuff you wanna try. For about $400 you can have an engine that's as good as new. For $40 you can "freshen" it up a bit and that may be all you want.