Project Café: Rebuilding a 1973 Honda CB500

Seventies Honda of some sort that one of Backus’ buddies e-mailed him a photo of his latest barn find, a 1973 Honda CB500 Four, which sounded like the

perfect project bike to us. The CB in question was complete (even if the front brake caliper was in a bag), but it didn’t run. It was for sale, and,

importantly, the price was right. It was obvious this bike had led a hard life and frankly, the closer we looked, the rougher it was. And while that didn’t

scare us, in hindsight, it probably should have.
So of course we bought it.

But we’re also optimists. We didn’t see a nasty, worn-out pile of junk before us. We saw a smooth, sleek café racer, complete with a nice 4-into-1 pipe,

low bars, a custom seat and/or tail fairing, maybe a different tank, new paint, new tires, custom wheels and more. And with BikeBandit.com sponsoring our project, we knew we had a great source for information and

parts. Bring it on, we said. 

The float bowl on the left already

got a quick cleaning. One down, three to go.

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.
After a little more work, we got the CB to start. 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.

Detonation does very bad things to

pistons.

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.
We dropped the head off with our buddy Doc at Automotive Machine &

Performance in Lawrence, Kan. (785-843-9333), for a quick clean and to have new valve seals installed. Doc’s already handed the cylinder barrel

back, cleaned, honed and looking good as new, and our new piston is on the way from BikeBandit.com, along with a complete set of new rings.

Once we have all the gaskets, seals and engine pieces we need, we should have the engine back together and ready to install by the next issue. There’s

that optimist thing again.

Muck, muck everywhere.

A picturesque frame

With the frame stripped of all its parts, we gave it — along with the swingarm and centerstand — a thorough cleaning before dropping everything off

at Custom Coatings and Metal in

Lawrence. Custom Coating’s Stuart Armstrong had our frame back in a week, and boy, what a job he did. We had it finished in gloss black to mimic the

original finish, and it’s almost hard to believe the frame we picked up is the same one we dropped off. It’s so beautiful, we’re scared to hang parts off

it.

Our CB came with extra electronics

for free.

So what do we know so far? Well, we’ve definitely got a lot of work ahead of us, that’s for sure. Aside from the obvious engine work we’ll have to rebuild

the swingarm, headset, brakes, and, oh, just about every other mechanical component on the bike. We’re still working to decide on a color choice, considering

several options for body parts, and trying to consolidate the many lists of parts our little project is going to require.

No, that’s  not a nail — it’s

a seat hinge pin. Really.