1981 Yamaha SR500 Project
I have always liked "Thumpers".  Too poor to buy a BSA Goldstar, instead I've had 441 Victors, an AJS 500, Honda GB500s, and most recently, this 1981 Yamaha SR500.  After running out of Trials bike projects this winter, an opportunity to get an SR500 set the wheels in motion for me to see if I could turn one into a nice cafe racer.  The SR was a running bike with only 7700 miles on the odometer.  It was fairly stock and working well, but cosmetically it needed help.  I dove into this project, forgetting to take photos as I went, but taking some as it neared completion.  Below is the story of this transformation.
Here's the finished project before testing and dialing in the 36mm OKO.
Not needing the stock tank or seat, they were given back to the seller for a discount.  My goal was to first eliminate everything on the bike that was not absolutely needed, and the next job was to go through the wiring.  What a rat's nest!  I got it down to the basics and just added wiring for the headlight and tail light.  A capacitor replaced the battery keeping the new wiring very simple.  The SR has an electronic ignition which provides a nice, hot spark.
Here's where I started taking photos.  All extra frame brackets that were not needed were cut off, as was much of the tail section.  Wheels were painted, as was the frame and front and rear fenders.  Headlight and taillight were purchased from the seller, shocks taken from another build that were never used, and the rear fender was found on Ebay.  The seat was another Ebay purchase.
After looking at many gas tanks, I decided on a chromed one made in India.  I was going to get one to paint, but Ebay feedback was best from this seller and their tanks were reasonably priced, so I went with what they had, which was chromed.
Fit was near perfect and it mated with the seat nicely.  Seating and bar position is very comfortable.  Electrics were arranged on a plate under the seat to have them neat and accessible.  The tube with the Yamaha logo is PVC pipe made into a tool container.  I kept the bars it had rather than go to "clip-ons" since old age likes me to sit up more.  The tachometer was eliminated and the speedometer was repositioned.  New open ended grips and bar end mirrors were added.
Engine work was the last to be done.  First, a Wiseco 10:1 piston replaced the original on a fresh bore.  Next was a call to Megacycle Cams to see what cams were available.  I decided on a cam that did not need to have piston or springs modified.  It gives all-around better performance, but it is particularly noticeable in low to mid-ranges.  The first time I grabbed a handful of throttle in first gear, the front end came up easily.
When I purchased the SR the previous owner had already gone over the engine.  With only 7700 miles on it, it was in very good condition.  With the new cam, valve springs and piston installed, I then added a new oil line that fed both intake and exhaust rockers.  The original feed only went to the intake rocker.  Of course, a change in carburetor was essential.  The original was a Mikuni VM34mm, so I started with a 34mm OKO.  With all dialed in I was experiencing a rich spot just off of idle.  When hitting the throttle, there was a little hesitation along with a puff of black smoke.  I tell my customers this is an indication the carburetor is too small, so I took my own advice and changed up to a 36mm OKO with similar jetting except a little larger main jet.  That was the cure.  The 500 single now idled smoothly, had good throttle response and good power.  Final main jet will require some ride time, but it is at least very close.  I used the stock header pipe, but put a "mini Goldstar" silencer on it.  I originally bought this for the GB500, but stuck with a Supertrapp for it.  This silencer is "straight through" with stainless steel mesh for absorbing the exhaust noise.  Exhaust note is nice, not too loud, and not too quiet.  Final sprocket gearing was achieved with a 15 tooth countershaft sprocket and a 42 tooth rear sprocket.  I also have a good 44 tooth rear in case I want to gear it down a little.  New tires have that period look and seem to hook up well.
Photo above shows the 36mm OKO K-104 carburetor.  The manifold is for 40mm carburetor spigots and the 36mm has a 42mm spigot, so I used the original manifold adapter over the new one I bought since it stretched a little better.  The carb. is coupled to a UNI sock type filter.  Engine has to be pretty cool to use the choke to start, and is closed rather quickly.  After sitting for a few hours I did not need to use choke at all.
Left side of the bike shows the front disc brake which I would like to eventually put some holes in.   Sprockets use a 530 chain.  A conversion kit is now available to change to a 520 chain, but I may do that at a later time.  The 530 should stretch less.  I found a chrome chain guard on Ebay and decided to paint it.  All paint was initially black, but the fenders and chain guard  were misted with a dark metallic green.  Looks like the chain guard got the most of the green.  I've put only about four miles on it so far, but initial testing results are very good.  The SR's engine is kick start only and requires you to "kick it like you mean it" to start it.  It typically takes three or four kicks to start, but then runs silky smooth.  I eliminated the exhaust valve lifter cable, operated from the handlebars and replaced it with an engine mounted lever.  That slows starting down a little.  You need to bring the engine onto compression and then briefly open the exhaust valve.  Don't do that and you'll just jump on the kickstarter and hurt yourself since it won't move.  After releasing compression, a determined kick (or two or three) will bring the engine to life.

I'm pleased enough with the SR500 to perhaps make another cafe racer out of one.  This bike is super light and handles very well.  I believe it will suit my riding style of mountain and back roads.  The engine has plenty of power, and in spite of not having a crank balancer, vibration is minimal, but maintains that "thumper" feel.