Yah, what sixto said.
Do a search. There's been alot of discussion here about that subject. I've not wanted to mess with mine as it runs great. But if your adventerous, there's a few things that can be done to perk up these engines I guess.

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)