Here's a chronology for the interested, and why the changes over the years. It explains how you can tailor your 750 to match your riding style, too!
K0-K1: These were the "performance years". The carbs were jetted 120 mains, the spark advancers were quick-advance units (all in by 2000 RPM) and the pipes were straight-through with fiberglass-wrapped tube inserts. Gearing was 16T/45T on K0, 17T/48T on K1. Gas mileage was about 32 MPG on premium (95 octane). These bikes developed about 67 HP, peaking at 8500 RPM, putting about 61 HP "on the ground". The K0 had 4 cables on the carbs, while the K1 had the bellcrank system most folks recognize.
Late K1/Early K2: The cam changed, getting advanced about 3 degrees, but the duration was the same (so the overlpa moved forward, too!). On the K2, the straight-thru pipes were changed to a 5-chamber baffled design. The spark advancer was delayed to full advance at 2500 RPM. The main jets on the K1 were 120, on the K2 110. The K2 jet needles were dropped 1 notch from the K1 settings (middle notch, K1 was 2nd from the bottom). The sprockets changed to 18T/48T. These bikes put about 58 HP on the rear wheel at 8000 RPM in 3rd gear.
Late K2: The cam moved to 5 degrees early (compared to K0) andoverlap was reduced about 4 degrees, while lift stayed the same. This was, I believe, an attempt to reduce the plug-fouling habits of the early K2 (1000 miles per set, typically). The spark advancer was given 2 degrees more "spread" with the same curve as the K2 early models. The static spark setting was reduced by that 2 degrees for better idle. The mainjets dropped to 100. The pipes were like the early K2. Sprockets stayed at 18T/48T. The airbox inlets were narrowed a little for quieter (but more restrictive) operation. This model put about 45 HP on the ground at 8000 RPM in 4th gear. They felt, and ran, like they were over-geared, and lowering the ratios helped both MPG and rideability.
K3: Most K3 pipes had 7 chambers. A few early ones still had the 5-chamber models, probably leftover from K2 production. Sales had fallen off dramatically at this point. The carbs received the "lifter collar" around the needle jet to make better atomization of the fuel and better MPG, about 40 MPG. (About this time, the U.S. entered the 55 MPH era, and gas shortages were causing Congress to cry out for better MPG on anything that moved.) The 5-chamber K3 had 105 mainjets, the 7-chamber ones had 100 or 95 (I saw both). The spark advancer was unchanged from late K2. These bikes had noticeably better midrange torque, from the "lifter collars", but less HP, about 59 at the crankshaft, and about 41 at the rear wheel at 8000 RPM in 3rd gear. These would also run on regular gas (octane 89). The K3 was the first 750 that would idle well, at about 1050 RPM.
K4: Pipes were like late K3. Carbs were similar, but the mainjet holders could no longer be removed after the first 4 months of production (to prevent "tampering" with "emission controls". Huh.) The earliest K4 had K3 carbs, the later ones were like the K5 and later, a slightly different design. Midrange torque, like at 55 MPH in high gear, was improved by having about .010" less exhaust cam lift and slower spark advance (full advance at 2800-3000 RPM), but 2 degrees less total advance. (We modified a lot of these advancers for more advance.) These bikes got real good MPG, even up to 50 MPG on premium, and would run on regular (87 octane). Power peak was at 7000 RPM (many were lower, like 6500 RPM), but only about 38 HP got to the ground unless you changed something. Idle was improved and very smooth, at about 1050.
K5: The only thing I really had time to "dig in to" on these was the mileage. They got real good MPG, frequently 45-50, running on regular 85 octane gas. They peaked at about 6500 RPM with noticeably less power than my K2. But, they were REAL quiet! And, they would idle endlessly at 1100 RPM.
I don't have specific notes after the K4, because I was by then only a part-time mechanic (weekends) and was getting away from the intense detail of it all. Sorry, k5-ers & later.
