In distinction to a collection of misadventures with different bikes, my 2000 CB 750 Nighthawk has been the epitome of motorbike reliability. Yup, save for some leaky gaskets and typical upkeep, the standard Honda has carried out amazingly properly for 20+ years and 80,000+ all-season miles.
Till a few month in the past, that’s.
The Set Up
One morning, as a substitute of firing up immediately and settling into a pleasant, comfortable idle, the 750cc powerplant cranked over for what appeared like an eternity earlier than begrudgingly burbling to life. There was a noticeable miss within the engine too, although I attributed that to a chilly, choke-less begin.
As soon as underway although, one thing was clearly awry. After sputtering across the block, I did a U-turn and pulled again into my driveway to research.
A stream of gas spilled out of the vent tube after the plug was eliminated. (Picture/OnAllCylinders)
Repeatedly twisting the throttle led to uneven, inconsistent races up and down the tachometer. Then I started to note the unmistakable stench of unburned gas and, once I uncorked the vent breather tube, a stream of gasoline spilled out onto the concrete.
Yup…this Honda’s engine wasn’t firing…
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…OnAllCylinders.
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Discovering the Suspect Cylinder
For the reason that CB 750 has a person carburetor for every of its 4 cylinders, I suspected (learn: hoped) that the issue was remoted to only one cylinder. To verify that, I popped off the gasoline tank and commenced eradicating the spark plugs to determine which cylinder wasn’t having fun with itself.
Plugs one, two, and three seemed positive—darn close to excellent, really.
Whereas all the opposite spark plugs had a standard whiteish-tan discoloration, quantity 4’s plug was a black, sooty mess. (Picture/OnAllCylinders)
However quantity 4? Whoa nelly, it was dramatically completely different than the opposite three, coated in a black, sooty residue. If you know the way to learn spark plugs, that instantly hints at an air/gas combine that’s approach too wealthy.
Engine Diagnostics 101
At this level, it seemed like there was one thing amiss with cylinder quantity 4. And if you happen to keep in mind your fundamentals, you basically want spark, compression, and gas for an engine to work.
I used a primary in-line ignition tester to make it possible for the cylinder was getting spark. And because the bike had been working completely up till this level, I assumed (learn: hoped, once more) that compression was good.
So, the one different apparent offender was gas.
However earlier than blaming the carburetor, I checked the air filter. A clogged, soiled air cleaner can actually mess along with your air/gas ratio they usually’re a heck of rather a lot simpler to interchange than a carburetor. Popping the airbox lid off gave me one other huge whiff of gasoline, and revealed a puddle of gas trapped within the backside of the airbox—which was promptly wiped up to make sure that all the pieces was spic-n-span.
The air filter itself? Nicely, it seemed fairly good, but I cleaned it too, simply to verify.
Years in the past, I changed the OE paper filter with a washable, reusable air filter from Ok&N. Although it seemed fairly clear, I washed and oiled it once more anyway. (Picture/OnAllCylinders)
After ready just a few hours for the air cleaner to dry, I went to reinstall it into the bike. After which I observed a brand new puddle of gasoline gathering once more on the underside of the airbox.
That little clue lead me virtually on to the issue: an open carburetor gas shutoff valve, maybe attributable to a caught float.
So let me clarify how I figured that out.
That is contained in the Nighthawk’s airbox. Although it’s robust to inform on this pic, the arrows are pointing to proof of a stream of gas flowing from the #4 carb consumption, all the way down to the underside of the airbox the place it puddles previous to draining out the underside vent tube. (Picture/OnAllCylinders)
How a Typical Motorbike Carburetor Works
Earlier than EFI confirmed up, many bikes used a Fixed Velocity carburetor (Or CV carb, if you happen to’re in a rush). We received’t go in-depth on how they work on this article, however you’ll be able to catch a really sensible clarification right here.
What’s necessary, nonetheless, is realizing {that a} CV carb like this usually retains a gas reservoir in a bowl on the backside of the carb physique. That bowl is gravity fed by the tank situated above the carburetor. Contained in the bowl is a float/pontoon system that, when the gas stage reaches a selected level, the float rises and prompts a shutoff valve to cease the gasoline supply from the gas tank, making certain a constant quantity of gas within the bowl whereas stopping the carb from overfilling.
Right here’s an illustration of what we’re speaking about, made utilizing our state-of-the-art pc rendering software program. (Picture/OnAllCylinders)
The Downside with a Caught Carburetor Gas Shutoff Float Valve
However what occurs if that little float or valve will get caught?
Nicely, because the carb is gravity fed, it means there’s a continuing push of gas down the road into the carburetor. When the shutoff valve is caught open, it signifies that gas will fill past its most level and push by means of the jets, leading to a super-rich air/gas combination—significantly at idle, when there’s not a whole lot of air transferring in to lean it out.
Worse but, since gravity doesn’t cease, it’ll preserve forcing that gasoline by means of the carburetor even when the bike’s not working. The gas will preserve flowing and inevitably again up into the airbox and pour into the cylinder(s) till the gasoline tank runs dry.
And even closing the principle gas petcock on the tank once you’re finished driving received’t assist right here, as a result of it’ll nonetheless drain a gas line’s value of gasoline into your cylinder everytime you park. Past the poor gas economic system, you could possibly be dumping a shot glass stuffed with gasoline into your oil every time you decrease the kickstand.
When gas mixes along with your motor oil, you’re going to have a foul time. The gasoline will considerably dilute the oil—thinning it out and decreasing its viscosity. In different phrases, it received’t lubricate as properly, which might trigger severe friction-related issues. And that’s doubly true in a bike, the place the engine and transmission typically share the identical oil. So if you happen to suspect your gasoline is mixing with oil, change your oil instantly.
In case you suspect a bum carburetor is dumping gas into your cylinders, verify (AND CHANGE) your bike oil. (Picture/OnAllCylinders)
Since I already noticed a brand new puddle of gas forming within the airbox after its preliminary cleansing, I instantly unscrewed the engine’s oil cap and, positive sufficient, it reeked of gasoline. Pulling the dipstick out, I might see that the oil stage was properly above the complete line, and the quick drips of diluted oil off the dipstick advised me that the oil was significantly thinner than regular.
Yup—with the caught shutoff valve, the gas stored flowing by means of the carb, pouring instantly into the cylinder, the place it was creeping previous the piston rings and dripping down into the crankcase. Fortunately, I had caught it earlier than placing a lot stress on the engine.
One other explanation for a wealthy combination/gas extra is a broken float pontoon. Particularly on older bikes, which regularly use a brazed brass float, the pontoon can develop pinhole leaks and progressively refill with gasoline. In these circumstances, the pontoon will completely sink to the underside of the bowl, so the shutoff valve won’t ever shut, leading to the identical over-fuel situation as a caught shutoff valve. (Newer carbureted bikes typically use a plastic pontoon, which gave me hope that I merely had a caught valve right here.)
The Repair
In fact, one of the best ways to repair a caught gas shutoff float valve is to take away the carburetor, disassemble the underside bowl and clear/examine/exchange the gas shutoff needle and verify the float for correct operation.
Downside is, eradicating the Nighthawk’s quartet of carburetors is laughably cumbersome. In actual fact, the manufacturing facility store guide says you need to take away the rear wheel, to take away the rear fender, to take away the battery tray, to take away the airbox, to take away the consumption plenum, earlier than eradicating the carbs.
Suffice it to say, I needed a better path.
I’ve had good luck with a product known as Seafoam Motor Remedy up to now. The parents at Seafoam say it might lubricate inner parts and take away deposits out of your engine and gas system. I figured it was value seeing if it might work some magic on a caught gas valve.
The repair begins by draining the gas out of the carburetor, finished by merely backing out the drain screw. Connect a brief part of gas line to the drain and run it to a bottle to gather the gas because it pours out, elsewise you’ll have a puddle of gasoline in your ground. (Picture/OnAllCylinders)
The excellent news is that the wonky valve was in Carb #4, which is the best one to get to, proper on the surface of the cylinder financial institution.
So I began by draining the carb’s gas bowl by merely opening up its drain valve, then I flushed out the bowl by spraying some carb cleaner up the drain valve for a second, earlier than letting it run again out of the carburetor.
Carburetor cleaner can injury paint and etch metallic, so make sure to dilute/wash away any extra drips/puddles with water!
Seafoam Spray comes with this super-long, versatile hose that actually makes it straightforward to get the stuff precisely the place you need it. (Picture/OnAllCylinders)
Subsequent, I reached for a can of Seafoam Spray, which is similar because the common stuff solely in an aerosol can. And Seafoam’s good sufficient to produce you with a super-long, versatile hose, virtually tailored for what we’re about to do.
With the carburetor’s backside drain valve nonetheless open, I threaded the lengthy tube up by means of the drain port, after which mashed the button on the can. The Seafoam started filling the carb bowl and once I noticed foam start to bubble out of the highest carb vent gap, I shortly pulled the Seafoam tube out and closed the carb’s drain screw, trapping the Seafoam within the decrease carb bowl.
The tube is fed proper up into the drain valve from the underside, and the stress from the aerosol can pushes the Seafoam up into the carburetor bowl. (Picture/OnAllCylinders)
Now, we wait.
After about 24 hours, I went again and drained the Seafoam out and closed the drain valve. Then I rapped the bowl gently with the deal with of a picket mallet just a few occasions to free-up any doubtlessly caught components.
Subsequent, I opened the petcock, and waited a second for some gas to refill the #4 bowl.
Lastly, I began the engine.
After just a few nerve-racking sputters, the Nighthawk’s acquainted, rock-solid idle returned. I threw a leg over the bike and rolled into the road. Admittedly there have been a handful of early stumbles at pace, because the gas bowl refilled completely and the remainder of the Seafoam labored its approach by means of, however quickly the bike was working just like the day it was made.
Subsequent jaunts by means of city over the following three weeks confirmed that no matter was inflicting the caught valve had been labored out.
(Picture/OnAllCylinders)
Epilogue
All advised, what might’ve been a time-consuming and dear restore was solved with a couple of minutes of labor and a day’s wait. Whereas this repair isn’t a silver bullet for each carb-related downside, hopefully this text gave you some choices to discover as a substitute for a full-blown carburetor rebuild.
Am I on borrowed time? Maybe. These carbs are 20+ years outdated and can inevitably want a rebuild sooner or later, but when a couple of bucks and somewhat little bit of effort can stave off that job for one more 12 months, I’ll fortunately oblige.
