The bike as it sat just prior to my most recent attempt to get it back on the road. (Image/OnAllCylinders)

I’ve had an on-again/off-again love affair with my father’s 1970 Honda CB350 motorcycle. With twin megaphone mufflers, chrome fenders, and a hipster-chic kickstart, the little street bike just oozes retro cool. It’s the bike my father learned to ride on—and the bike I learned to ride on decades later. Most importantly, it’s just super fun to throw a leg over and zip down a country road.

….Just don’t stop for too long.

For decades, my father and I have struggled with idle issues on this bike. It was the White Whale, we were Ahab. We’d ask it to idle. It would prefer not to. (Ermagerd! Double Melville reference!)

The routine was tedious: We’d rebuild the carbs, set and sync the idle, and then…it’d stall. So, we’d order another set of rebuild kits, check the plugs, fiddle with timing, and do it all over again. Sometimes we’d purposely set the idle high, only to have it settle for a bit, before revving to the stratosphere.

And then it’d stall.

Dad bought the Honda new in 1970 and it was ridden regularly until family life intervened. (Image/OnAllCylinders – Paul Sakalas)

This went on for several years before it was finally wheeled into a dark corner of the garage and forgotten. After two decades of sitting dormant however, I recently rolled the little bike out again—filled with a renewed sense of vigor, 20 years more wrenching experience, and perhaps a dab of hubris.

I rebuilt the carbs with proven, quality kits, using digital calipers to set the float levels. I adjusted the throttle blades and idle mixture perfectly. Timing was checked and re-set. Plugs were clean and gapped right. Compression specs were spot-on.

After kicking it over a few times, the bike roared to life, seemingly excited to see the 21st century.

Then it stalled.

This picture from the late 1990s shows our last attempt to really get it running right. It was garaged after this particular summer for a long dormant period. (Image/OnAllCylinders – Paul Sakalas)

Undaunted, I took apart the carbs again to double check my work, making only a few small tweaks. Then I went deeper, ditching the points ignition for an electronic one. I replaced the rubber intake boots to mitigate the chance of a vacuum leak. I even re-set the valve lash and cam chain tension.

I buttoned everything back up, flicked the starter, and the CB350 fired up almost instantaneously, before settling into a nice 1,000 rpm idle. It puttered happily in the garage for 10 minutes.

…And then stalled 30 seconds later.

The bike also benefits now from a modern electronic ignition, instead of the old mechanical points. Less moving parts means less things to go wrong here—we hope. (Image/OnAllCylinders)

Getting more than a tad frustrated, I got it started back up, jumped on the seat, and took it around the neighborhood. Up in the rpm range, it ran fine. But then it started to cut out—the unmistakable miss of a dead cylinder. And what’s weird, is that it was never the same cylinder. Sometimes the left side cut out, sometimes the right. Sometimes both.  

Oh, and then it stalled again.

After I limped the vintage Honda back home, a full scale investigation began…but as I pulled the carbs off for their four hundredth rebuild, I forgot to shut off the fuel petcock.

A Serendipitous Oversight

With the disconnected lines dangling from the petcock and fuel still flowing, I noticed that one of the fuel lines was putting a steady drip of gasoline on my garage floor. Curiously, the other one wasn’t.  

I pulled the gas cap, and there was plenty of fuel sloshing around inside. The good news is, a vintage bike’s fuel delivery is stupid simple—it’s just gravity fed with a shutoff petcock valve right below the tank.

So…seeing as it was the only thing in between the gas tank and the carbs, I removed the lines directly from the petcock and turned the fuel on. A pair of drips from the hose barb fittings suggested that the petcock was fine.

Having a few lengths of surplus clear fuel line helped diagnose a nagging fuel delivery issue. (Image/OnAllCylinders)

But driven by Ahab-like madness to finally solve the problem, I ran an experiment by replacing the original black rubber fuel hoses between the petcock and twin carburetors with clear fuel line.

As I suspected, the fuel coming down through the petcock was intermittent at best. Since the fuel lines are gravity fed, there should be a consistent amount of fuel on either side of the line’s main bend at the bottom. Sometimes the twin individual lines would be almost full, other times one or the other would develop a large bubble for several moments, right ahead of the carburetor—surely enough time to momentarily exhaust the fuel supply in the carburetor bowl and make the engine sputter.

And of course, it’d probably cause a stall while idling too.

The fuel in either line would occasionally run dry as it was drawn up into the carburetor. While it can be tough to photograph, you can see the fuel in the far line is being pulled up against gravity, while the line in the foreground has a nice steady reserve above the bend in the line. That hinted at an inconsistent fuel supply. (Image/OnAllCylinders)

Though it had been rebuilt already during an earlier resuscitation attempt in the 1990s, I enthusiastically disassembled the petcock again. And sure enough, despite being ridden sparingly since that time, there was enough crud built up inside the valve that it partially blocked fuel flow, particularly at the mesh fuel strainer screen. The disassembly also revealed some aging, decaying rubber gaskets and seals that likely contributed to bubbles in the fuel line too.

Inside the petcock valve, all of the rubber was deteriorated and deformed, no doubt restricting flow and causing small amounts of air to enter the fuel lines. (Image/OnAllCylinders)

I ordered a petcock rebuild kit, reassembled the valve, and put it back on the bike. I closed my eyes, pushed the starter button, and the bike began to idle.

And then it did not stall.

The petcock valve for a Honda CB350 has a built in fuel strainer. While I had emptied the strainer bowl several times during troubleshooting, it was difficult to fully assess the condition of the gunked-up strainer screen until I took the valve off the bike and inverted the petcock. (Image/OnAllCylinders)

The bike was running perfectly. Still skeptical, I let it idle in the garage for a pretty long time, then ran it around the neighborhood. Everything was perfect. No sputters, no cutouts, no stalling. I checked back over the next few days, running it around town and letting it idle in the garage. Satisfied that the problem was finally remedied, I swapped the regular rubber fuel lines back.

It also explained why we couldn’t replicate the problem during the handful of times we ran the bike with our small auxiliary fuel tank.

With the rebuilt petcock, both fuel lines filled up equally, time and time again. (Image/OnAllCylinders)

Epilogue

The drama’s done. Why then here does any one step forth?

Well, because looking back on it, I’m not entirely sure the petcock thing was the only problem. But it certainly was a problem.

The stalling was in all likelihood a confluence of issues related to things like improper carb float levels, imprecise timing, and ancient, hard rubber intake boots—so I don’t feel those earlier efforts were in vain.

But the important lesson learned here is Occam’s Razor. Start simple and, with regard to fueling, maybe start at the tank and move forward. I also learned the value of clear fuel hose for diagnosing fuel delivery issues, especially on a gravity-fed system like what you find on a motorcycle.

At any rate, I’m happy to report that the Little Honda is still going strong, and ready for a summer of neighborhood cruising.

The only bad news? I figured all this out in January…in Ohio…where there’s currently snow on the ground. So after some brisk “sea trial” rides around town, the little Honda hopped on its handy motorcycle dolly and got tucked in underneath the leaf blower and lawn mower for a few more months of hibernation. (Image/OnAllCylinders)
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Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or watching a 1972 Corvette overheat. An avid motorcyclist, he spends the rest of his time synchronizing carburetors and cleaning chain lube off his left pant leg.