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1st, try unhooking the shiter cable from the trannsmission shift lever, and manually push the lever into reverse, to see if that makes a difference (the cable may need adjusting, if that's the case)
If that doesn't help:
If it is a cone clutch design, like the km2a on my older Yanmar 2GM, you (or a mechanical friend) can take it apart and apply a bit of valve grinding compound on on the drive cone, and then lap this against the reverse (and forward if you like) drive gears to take off any glaze that may be making it slip in gear. I did this on mine, and it has worked great for 3 years now.
It does not take alot of force to lap the drive cones in this way, so just a half dozen - a dozen or so (1/4) turns while applying pressure between them as you turn them. You are only trying to take off the glaze, and do not want to actually wear the parts with the abrasive valve grinding compound. Wipe / rinse off the compound when finished and re-assemble.
Last edited by Northeaster; 01-20-2011 at 08:20 PM.