September 12, 2011 10:19 AM PDT
First of all you can't just put new jugs and heads on it with out changin ur crankshaft for more stroke, that's a H2O cooled engine, you shouldn't fool with the bore or you might crack ur barrels from heat. A larger engine? You are planning to modify the frame to accomodate the engine size and alighn the chain drive I hope. From a salvage yard? Why do you think the engine is in the salvage yard as opposed to sitting in a used bike parts shop and in pieces(parts make more $$ than whole engines)? You're going to have to remanufacture the used engine too so why not just reman yours?
Sprocket change? Putting a 39 tooth sprocket on the rear as opposed to the stock 42 isn't going to change very much off the line performance unless you seriously need to go on a diet. The sproket change isn't going to increase torque, although, it may increase usable horsepower to the rear wheel, unfortunately, no where near what you may think. The engine determines the torque and horsepower all the sprocket change will noticeably do is lower your engine RPM at the top end so you put less wear on your engine if your a real biker who rides a min of 10K/year then your engine will last longer and the fuel economy will also improve, although, it'll take a while to notice it. Hey, the VT750 anything is an art form with me. Hopefully I'll have my custom work done by spring. It takes time to build a frame ya know.
How many miles are on it? I've 95K and I'm pulling real strong. Sounds to me like u may have used OEM spec'd engine lubricant or some other salesman's idea of motorcycle oil and you've worn ur engine out. How much do you weigh? Do you ride two up? There's a whole lot of reasons but it sounds to me like you'd benefit from a remanufactured (not rebuilt) engine and, yeah, if u dont know ur bikes then u better know your mechanic.
Good luck.