I have an MTD mod. 604G, 42" deck with a Kohler 16HP engine (approx 7 yrs old). I recently changed the drive pulleys because it was slipping when going up hill. Deck belt works fine. Now when going up hill it seems to grab then slip - grab then slip repeatedly. It's worse at higher speed. I used OEM belts, cleaned and lightly lubricated the variable speed pulley shaft and adjusted the speed control pedal as outlined in the manual. I also doubled checked the belt routing and idler pulleys for free movement. Everything looked OK. Could it be the drive pulley on the engine is slipping - possibly a broken key? I had to lower it to install the variable speed belt but never took it completely off the shaft. Any ideas where to look next ?

O doubt that the key is broken, it would slip one big slip and never reengage. To test with out removing you can put a mark on the shaft and the pulley in a straight line. After the tractor slips check to see if the marks are still in a line. Check to be sure none of your grease got onto the pulley groove or belt. Is there any adjustment left in the tensioner pulley?

Thanks for the tip on the crankshaft pulley. I marked the pulley and shaft and it didn't move after the tractor slipped. No excess lube got on the pulleys or belts. The variable speed pulleys seem to move freely up and down. The tensioner on the smaller drive belt (from the trans to the variable speed pulley) is basically a idler pulley on a spring loaded arm with no adjustment mechanism. The spring seems strong. The tension on the long belt going from the crankshaft to the variable speed pulley is controlled by the a pair of idler pulleys operated by the foot pedal. That has an adjuster. I will try some adjustment there to see if it helps. Thanks for the suggestions!

I got the mower to work fairly well after an adjustment to the belt, but ultimately this turned out to be a transmission problem. It started to slip again and also started to make a metallic clicking sound. When I checked on the adjustment I noticed oil leaking from the trans. I ended up replacing the trans and the new one works great, a lot stronger and quieter than the old one ever was, even when it was new. I got the new trans off E-Bay and the replacement was very easy. I plan to do a post-mortem on the old one to see exactly what failed. Just information for anyone that may have a similar problem...