My friend Zhuul recently picked up a vintage 1978 Kawasaki KZ650. The previous owners purchased it as a project bike where a father and a son could restore it. They had purchased it from a man who wanted to restore the bike. Zhuul got a good deal on it, however, it ran like crap, barely got 10 miles to the gallon, and was excessively loud. Zhuul is an experienced rider, however, he did a true motorcycle move, and laid the bike down. He asked me to restore the bike to good, original working order. Evidently it takes someone who doesn’t even own this bike to restore it.
I don’t have any pictures of the bike after it was laid down, but the handlebar, which was a cafe racer style bar, was bent hard on the right side. The master cylinder cap was sheared off, and the carburetor was leaking fuel. I opted to rewire the handlebars, replace the master cylinder, rebuild the carburetor, synch the carbs, and call it a day. As always, I found there was quite a bit more wrong with the bike than initially assessed.
Damaged throttle.
Rewired handlebar. Both controls had to be routed inside of the handlebars. This was a major pain.
After rewiring the handlebars, I rebuilt the carburetor. The biggest carb I had ever rebuilt was the 2 barrel carb on my Honda Shadow, many years ago. This carb was simpler in design, but more complex in practice. I took my time, and rebuilt each individual carb, making sure I didn’t accidentally intermingle parts. I think this was the first time ever that the carb had been disassembled, as the gaskets and o rings were all in really rough shape. The fuel lines especially had o rings that were hard and brittle, and the rebuild kit that Zhuul had purchased did not have any replacements. I used my best estimate as to what size they were, and replaced any o rings that weren’t with the kit with some generic hardware store o rings. Some of the gaskets were so old, that they had physically bonded with the metal, and I spent about an hour scrapping all gasket material off the carbs. Some of the components were damaged, as you will see below.
The carbs unmounted from their mounting plates. Notice the T pipe in between the middle carbs. I believe that pipe was the actual source of the leak. Nevertheless, the carbs definitely need rebuilding, so it was a good thing that Zhuul laid it down and brought it to me.
The picture is a little blurry, but this is a picture of a carb after I cleaned it with carb cleaner. The tall brass nut is the main jet, and the brass hole is where the float sits.
The float (the big round plastic pieces), the main jet (the tall brass nut), the pilot jet (the smaller brass nut next to it), and the air-fuel mixture screw (the screw on the right). I have not yet replaced any of the components.
This is the needle adjuster. You adjust this, along with the other ones, with the engine running, and a carburetor synching tool, to sychronize the carbs so they open the same amount, and let the same amount of air and fuel into the cylinders. This one has been damaged by a mechanic with a heavy hand. Zhuul had mentioned that the previous owner had never messed with the carb, as they did not want to synch the throttle-bodies. Either the previous owner, or the one previous to them, damaged this component.
The throttle body and the needle. Most carbs have some sort of diaphagm, but this one doesn’t. That big hole up top is where a rod is pushed through, which is connected to the throttle cables, which is connected to the throttle. When you pull on the throttle, you leverage the throttle body up, which allows more air and fuel into the engine, to make it run faster. It doesn’t get more mechanical than that. It’s for this reason I love carbureted bikes. It doesn’t get more raw than this.
Old gasket vs new gasket. The old one is completely soaked in gasoline, and was brittle, and fused to the surface of the carb.
The 4 carbs completely rebuilt, and ready to be mounted on the mounting plate, and mounted back on the motorcycle.
After remounting the carb, I went about resynching the carbs. This turned into a small nightmare, as the ignition key broke, and caused all sorts of issues. On older bikes, the key had 3 positions: Off, Run, and Brake Lights on but Running off. I’m not sure why, but if you twisted the key to the far clockwise position, it just left the brakes on. The key would not engage all the way back into the run position, and it was virtually impossible to set it to the off position. I figured that the ignition cylinder had a fair amount of corrosion. However, disassembly yielded no way to actually clean the cylinder itself. So I asked Zhuul to order a new ignition key and cylinder. After that arrived. I set about resynching the carbs.
The carbs prior to synching. The basics of throttle body synching, regardless of carbureted or fuel injected, is you adjust the highest/lowest carb first, then the next highest/lowest, and so on. With the KZ, I had to constantly adjust the idle speed as well, as adjusting the vacuum caused the engine to either run too fast, or too slow and stall.
Carbs synched between 18-22 cm-HG at 1050 RPM. Having them a tiny bit out of synch isn’t a big deal; The manual states as long as it’s within 2 cm-HG, the bike will run fine. This is the Carbtune Pro that I’ve mentioned before. Zhuul later mentioned that the bike ran basically like it was brand new, at least how the engine sounded and ran. I reset the air-fuel mixture screws, and I’ll bet that if the bike being laid down hadn’t seriously knocked the carbs out of synch, they hadn’t been synched in years. I reckon that the difference in air-fuel mixtures, and actual fuel pressures, were causing the engine cylinders to fire and burn different amounts of air and fuel, and caused the engine to become unbalanced. I honestly wouldn’t be surprised if longterm use in this condition could have resulted in a catastrophic engine failure.
The last bit was bleeding the brakes, road testing it, and putting things like the wiring harness and panels back in their place. There were some issues with the wiring harness, one of the last owners cut off the turn signals, and a lot of the wires led to nowhere. There was a random brown wire with a male bullet end connector that just ended at some random plastic end. During our trip out to Northwest Nomads, the headlight randomly shut off, and turning on the switch on the rightside controls on the handlebars caused the main fuse to blow out. Zhuul has some experience with electrical systems, so he opted to take a look at it himself.
The tank and crash bars only sustained some minor damage. A repainting will cover that up just fine.
The KZ650 completed, and running. It is one of the original superbikes from the days of the original Universal Japanese Motorcycle.