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Low compression on cyl. 4 only - Bike sat for about 20 years

Mars

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Hello all,

I'm in the process of getting a 1979 CB750L back up and running after something it sat for about 20 years in a shed.

I've got it running pretty well actually but when syncing the carbs notices #4 cylinder a little wonky. Have driven it just a couple miles.

Dry compression test showed very even 149-150 psi on cylinders 1-3 and 30 psi on cylinder #4.

Wet compression test on cylinder 4 showed 195 psi. I'm surprised its even possible to get that high, but that's what it read.

I'm assuming that it will need new piston rings, but one person suggested leaving Marvel Mystery oil in the chamber over the weekend and see if it loosens up the rings and drive it around a bit.

Am I being overly optimistic I can avoid a rebuild? Or will I risk doing more damage?

Thanks
 
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