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marjanboo

Bad crankcase seal cause?

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Ok so this is what  I think I got a replate new topend on my kx125 03 this caused more preassure in my topend making my crankseals leak sending fuel into my transmission an burning oil at the same time giving me a rich condition bike still grunt in upper gears not lower so making me think jetting but also in the process pressurizing my transmission an leaking out my countershaft seal wich must be the most prone to blowing out in the transmission due to the output shaft constantly going around an actually on my bike I think the countershaft seal is the same as the crankshaft seal so if you get a new topend an your bike runs bad like this maybe on older bikes I dunno depends on seal then get a countershaft seal leak while chasing smoking issues an jetting with no lose of oil or water the crankshaft seal seals bad. Plausible? I will get the crank seals done next week what do you guys think?

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Then do the crank bearings too. But the compression from the cylinder does not pressurize the low-end, or the gear case. Atleast not enough to cause the problems like said.

Edited by Diverdown

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38 minutes ago, Diverdown said:

You'd know if the left crank seal was leaking (sucking air). It would run very lean, and may very well have blown up already.

I'm just ganna do the left one at the same time I suspect the right one  

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51 minutes ago, Diverdown said:

Then do the crank bearings too. But the compression from the cylinder does not pressurize the low-end, or the gear case. Atleast not enough to cause the problems like said.

The simplest two-stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. ... As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase

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2 minutes ago, marjanboo said:

The simplest two-stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. ... As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase

An I also saw a post where a guy had a brand new yz250 with a bad right crankseal an a bad countershaft seal 

Edited by marjanboo

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6 minutes ago, marjanboo said:

I'm just ganna do the left one at the same time I suspect the right one  

Yep l always do both, was just my way of saying atleast it's not the left one.

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11 minutes ago, SteveThe Snakes said:

Yup and might throw you down then spin you around backwards. Ask me how I know. 😳

9D5208BE-3A47-486D-BFAE-8313C1C0C1B9.jpeg

I'm fouling some plugs only when going slow tho an smoking alot I think the new topend made the right crank seal go bad or even bader 

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13 minutes ago, marjanboo said:

The simplest two-stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. ... As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase

The crank actually creates a vacuum. Either way, never a bad idea to replace crank bearings & seals on a 2-stroke. Then see how it runs, tune from there...

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3 hours ago, Diverdown said:

The crank actually creates a vacuum. Either way, never a bad idea to replace crank bearings & seals on a 2-stroke. Then see how it runs, tune from there...

Yea I just don't have the tools required to do clutch an flywheel I'm getting a guy to do it  will get all the tools I guess at some stage an split the engine but if i was to do the bearings aswell I may aswell do a full rebuild 

Edited by marjanboo
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Yep new crank bearings and seals always go good with new top end 🔧 see how worn your big end bearing is also , might still be ok.  But I always recommend new crank bearings and seals if older engine hasn't been rebuilt in while, run like new again with new top . No biggie if can do yourself

Edited by Motox367
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