Project Café: 1973 Honda CB500 - Part 1
(Page 2 of 3)
January/February 2009
By Landon Hall
Finding spark
Goal No. 1 was to get the bike running, so we hauled it to Backus’ “shop” and went to work. First we drained and rinsed the gas tank, then took the Honda’s four carburetors off and gave them a quick cleaning. Carbs back together and a new battery installed, we gave it a few shots of starting fluid and hit the go button, but it wouldn’t fire. Testing showed we had full voltage to the ignition points, but we weren’t getting anything at the plugs. Our handy voltmeter pointed to bad coils, but after installing a new pair of coils and still no spark, we discovered the coils were only getting 7 instead of 12 volts, not enough to fire them. Some quick work uncovered dirty and corroded connectors in the wiring harness, and once cleaned we finally had good spark at the plugs. OK, we thought, now we’re getting somewhere.
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After a little more work, we got the CB to start, as our video proves. Thankfully, we didn’t hear any weird noises coming from the bottom end. But even after some fiddling, it wouldn’t idle and we were getting more blue smoke out of the exhaust than we liked. We decided a compression check was in order, as we were beginning to fear we had some very worn piston rings.
While we were hoping for compression readings between 120psi and 140psi on each cylinder, our testing revealed a low of roughly 50psi on one cylinder and a high of 90psi on another, with the other two cylinders at around 70psi each. This was not good.
Adding a teaspoon or so of oil through the spark plug holes and retesting each cylinder gave a much higher compression reading all around. Adding oil on top of the pistons helps the rings seal, so if you instantly get much-improved compression after adding the oil, it points to worn rings. It quickly became apparent that our CB500 needed a top end job.
The teardown begins
We had already decided to completely disassemble our Honda to have the frame refinished. We removed all the body work, and with the old bike stripped bare we began to realize how rough nearly every piece of it was. We started stripping components, bagging and labeling parts, and taking reference photos as we went.
We removed the airbox, carburetors, then the engine and transmission, the handlebars, gauges and all the cables, both wheels, the rear swingarm, the sidestand, the forks and the centerstand. Unfortunately, the forks were a lot rougher than we expected. Though we planned to rebuild the forks with fresh seals and oil, the top fork legs are pretty rusty. While this won’t affect their performance, we’re still mulling over our options.
Engine woes
With the engine out, we began its disassembly. By now we knew it needed new rings, but would we find even bigger problems? Unfortunately, the answer was yes.
The piston in cylinder No. 3 had, at some point, experienced the effects of severe detonation. The top ring was broken into four pieces, and part of the top edge of the piston had literally burned away. The good news was that the cylinder walls looked good, and the head and all the valves appeared to be in fine shape. At least something was going right.