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.
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.
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.
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.
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.
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.
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 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.
“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.
Ever consider that the alcohol in the fuel caused the whole problem and still might.
I never really thought about the fuel quality, if that’s what you mean? It runs OK now, so I’m more inclined to think it was an issue with either timing or the carbs or the petcock…or maybe even a vacuum leak around the intake boots. Plenty of folks are zipping around on these CBs, so I’m hesitant to question fuel quality.
This brought back lots of memory’s as I had one exactly like it! Never had idle issue’s LOL
At 18 I bought my first bike, a 71 CB350. I rebuilt/modded the entire bike, and upon finishing it, I had the same exact problem and noticed it in my clear fuel lines! So luckily it didn’t take me long. I rebuilt the petcock (one of the few items I didn’t tear down and rebuild… I did basically everything else). Oh, I also accidentally had the spark plug wires reversed at first which took me a whole day to figure out. Won’t make that mistake again!
Hello. Good article. I would think that if fuel supply was inconsistent, it would lower float level and the problem should be when on power( higher fuel demand ) than at an idle. Possibly a lean misfire at first.
Just my 2 cents.
I once had an issue where my tank vent was clogged, resulting in stalling at idle. When the throttle was open, there was enough suction to pull fuel through, but at idle it’d just stall out. I had to crank the idle way up to make it home.
The comment about fuel with ethanol is spot on. These old bikes were never intended to see alcohol in fuel and gaskets, rubber lines and such can deteriorate quickly. Find a source of non ethanol fuel for long term reliability. I have a ’76 CB 550F and have lived this nightmare myself.
That’s really interesting. I’ve got an ethanol-free gas station right around the block too–once the weather gets above freezing again, I’ll fill ‘er up. Appreciate the tip!
Having played with Hondas for 40 plus years, I have seen my share of these types of problems. As I started reading one of my first guesses was the petcock! Have had several bikes over the years get gummed up in this area.
I had the exact same problem! After a revision of my bike (CB250G) it stalled after a few moments, suddely only running on the left cilinder or sometimes dropping dead during idle!
After rebuilding and re-rebuilding the carbs…discovered it was the petcock! A tiny bit of tank rust had come in the way!
Oh dear, start at the beginning, always, even with a more modern pressure fed bike, the tank is the starting point. Glad it’s all sorted though!
Had the same problem with my old 6 cylinder trucks. Turns out the methonal in modern fuels boil at under hood temps causing vapor lock and forcing fuel past the needle and seat while running. There’s a simple inline fuel filter that has a return back to the tank to relieve the extra pressure. The engine run smoothly and always start right up now.
That’s funny you should mention that Frank–we just published another story a few days ago on those dual-outlet fuel filters (I have one on my Jeep CJ).
[…] other side of the caliper to measure length/height. Here we’re adjusting the float level on a Honda CB350 carburetor using a digital caliper to ensure it’s in spec. (Image/OnAllCylinders) Digital […]
Do you live in a hot or cold climate? I am in San Francisco where we have cold mornings and warm days. Ive been struggling with Idle issues and I ran through the same steps. I just cleaned out the petcock – I solved some fuel delivery issues, but the left carb is still getting less fuel and I struggle with idle. I have to keep the gas going while braking (which has inhibits safe stopping). Any ideas how I can fix this? Not sure if its just cold weather and maybe there is a way to improve cold weather running.
Hey Brian, I’m in Cincinnati, Ohio and we get a mix of hot and cold–truth is, I got it running properly around January with below-freezing temps outside and even now with 90+ degrees, it’s still running great. Ambient temps shouldn’t be much of a factor once the engine is warmed up.
That said, have you checked your gas cap? Believe it or not, the little vent passages in the cap can get clogged, creating a vacuum that won’t allow fuel to gravity-drain down the lines. I cleaned mine out with a guitar string and pulled out plenty of gunk.
What I’ve learned, casually glancing at the clear lines from time to time, now that the bike is running great, is that air bubbles aren’t that big of a deal, provided the fuel in the lines stays above roughly where the carburetor floats would sit.
Speaking of carb floats, make sure yours are both set properly to get consistent fuel to the jets. I use this method with a digital caliper. You’ll find plenty of opinions on the good height, but for what it’s worth, I’ve got mine set around 25mm and it’s perfectly happy, even to the edge of redline.