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Running a larger Z dimenison for low end power. Lower PV valve then normal

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Has anyone experimented with running a larger "Z" dimension in a 250SX or any of the big bikes to get more low end power. That is running the PV flapper valve lower then specified.

I am replacing the top end on my 250SX with a reconditioned cylinder. Fresh repair and re-platting form Power Seal, USA sized to a Vertex #1 piston. The cylinder I am taking out was running great but compression was low, ring gap was large, and I have know the cylinder was out of tolerance since I put the last #2 piston in last summer. I had a used cylinder re-done to avoid down time and I am ready to put it in.

The cylinder I am taking out has a Z dimension of 51.5mm. The spec is for 48.5mm. So the valve is sitting lower at idle then spec. The rod lines up perfectly at this dimension. I am wondering if I should go back in that way. It always seemed to have a lot of low end for a 250 I thought. It has more top end then I can handle too. The hit is like they say a 2003 250SX should be, quite a bit.

I read somewhere about folks doing that to a 125 or 200. It might have been from the Langston setup.

Never seen any talk of this on a big bike KTM two stroke.

I know one issue might be flapper valve clearance to the piston. That looks OK but close after clean up. So was the last one. I don't know what the clearance should be? Maybe that is in the manual but I haven't seen it.

Any thoughts?

Should I raise it back up to the specified 48.5mm?

Will I loose low end if I do?


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