I have a 186D that had the same problem. I will discribe the fix on mine hoping yours will be similar. Sit on the seat and look straight down between your legs (not at yourself, the tractor)and under the cover is the power shift regulator case ( I have the column shift). At the rear of the case is a diamond shaped cover with two bolts. Remove the bolts and screw one into the end of the sleeve inside and carefully pull out the sleeve. Using needlenose or whatever means remove the relief valve and holder (there are shims inside, don't lose them). Behind all that are two springs, a smaller one inside a larger one. If you can get them out, you should see probably the larger spring is broken (compress it with your fingers, my had a hairline crack). Buy both springs (since they aren't expensive) from a Yanmar dealer. Don't buy one from the hardware that looks just like it (trust me). Replace everything the way it was, check the o-ring on the first sleeve you pulled out for damage and get one if needed. Hopefully you won't have to pull the whole case off. If you do, you need another gasket. This assembly regulates the pressure for the shift clutches and delays the shift so your neck doesn't snap while shifting. The broken spring prevents the proper pressure build up for shifting. I discovered this after the filter cleaning and oil change. I figured it out from my service manual. I hope this fixes it for you. It won't hurt to run it this way, it just won't shift well.
Skin
