Top and bottom end rebuild low compression

Basket
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5/17/2024
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Troup, TX, USA
Basket

Rebuilt the bottom end and top end on a 2002 yz125 all seals bearings everything replaced. Only reaching about 130 psi on compression. But I am burning some transmission oil. Noticed after the heat cycles I had to add some. Seals seem to be fine from what I could tell. This the cause of the low compression? Bike tires right up cold no choke first kick and idles good. 

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btroxler
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btroxler 11/11/2024 9:29am
11/11/2024 9:29am

If you are burning trans oil, your RH crankcase seal is leaking. Not going to cause the low compression. Make sure you check surface finish on cylinder wall, ring end gap and piston to wall clearance.

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Nairb#70
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Nairb#70 11/11/2024 9:49am
11/11/2024 9:49am

Case 1/2's leaking.

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MxAddic
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MxAddic 11/12/2024 5:56am
11/12/2024 5:56am
btroxler wrote:
If you are burning trans oil, your RH crankcase seal is leaking. Not going to cause the low compression. Make sure you check surface finish on...

If you are burning trans oil, your RH crankcase seal is leaking. Not going to cause the low compression. Make sure you check surface finish on cylinder wall, ring end gap and piston to wall clearance.

If the bottom end is not sealing it can absolutely effect cranking pressure. The combination of 2T oil and tranny fluid could also allow blow-by on the rings.

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