The best manual is and always will be the manufacturer's. I've seen OEM manuals on Ebay as low as $14 US.
Ps I'm in the middle of the same 521 Mods. Here are a few things you should know:
1) The 536 dosen't mind properly jetted MIC 40mm's (PTO 330 MAG 350 Pilot jet 40; Needle jet 224 AA5; Jet needle7DH2; Throttle valve 2.5(Brass) good for -20C @ sea level
2) The 1986 537 version of this mill had the highest spec HP @ 99
3) The '85-'88 537 had 6.5:1 compression ratio vs 536 with 6.1:1 accomplished with different Pistons, Jugs & Head.
4) Try to find an exhaust system from a 583 mill.
I have had pretty good results swapping out the carbs for 40s, Carbon Tech 141 rotary valve, pipe / can from a 93 Mach 617 & minor porting on the exhaust on a 1989 Plus Radared at 97 mph on a clear day at -14C
A forum community dedicated to snowmobile owners and enthusiasts. Come join the discussion about performance, troubleshooting, trails, maintenance, riding tips, modifications, classifieds, accessories, and more!