WR400 Compression issues

Hello everyone,

I have asked similar questions before, but am now trying to get the bike running again.

I've got a 2000MY WR400. Over time it got harder to start, particularly when hot. So, I checked and adjusted the valve clearances. It had been put away after washing and lacked compression, so at the advice of people on here, I put a little oil down the bore, and it started straight up. Since then it's refused to start, and I think it's a low compression issue.

I don't have access to a leak down tester, but have compression tested it, and get around 70psi. With a little oil down the bore, I get around 90psi.

Can someone tell me what values I should be expecting, as I can't find anything in the manual?

I'm thinking maybe it's time for a piston, as some on here have already suggested. What are people's thoughts? I'd like to be sure it's that before tearing it apart.



I would think anything under 150 psi would require rings / rebuild. Normal range is 170 - 240 (I think...)

Only thing running on 70-90 psi is a lawn mower!

You have no auto decompression system, so you are absolutely low on compression, BUT, YOU ARE TESTING A COLD MOTOR which means just about nothing


A 14 year old motor; if you have not rebuilt it, if you have no proof that the prior owner has rebuilt it, it's time to rebuild it. The top end anyway...

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