Vintage Dirt Bike Q & A

there is a little oval opening with 2 bolts on either side on the under side of my exhaust. it looks like a plate and gasket bolt on there. does anyone know the purpose of this hole? i dont have the plate on it right now, could that be causing any problems other than putting oil all over my frame?

It's a clean-out cover, directly below the entrance to the spark arrestor. The idea is to poke and/or burn the spark arrestor clean, but it's not all that easy to get to the far-side openings.

The MT125 pipe is extremely restrictive even when clean. As an experiment, try removing the cover and see how the motor runs (it will be louder, of course).

I cut a large slot in the baffle that runs across the middle of the clean-out opening, drilled a few holes diagonally through the spark arrestor and rear baffle plate, and ran a giant drill into the tailpipe (from behind the bike) which dug into and removed a thin inner reducer sleeve; the tailpipe was unaffected. The pipe was louder, but the motor came alive - it revved quicker and had noticeably more top-end power, with a little less bottom end.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric