The float bowl
All of the carburetor's circuits are fed from a reservior of fuel called the float chamber or bowl. The bowl is vented just like your bike's gas tank is, and it uses a shutoff valve as does the fuel tank. This is called the float valve. That valve is automatically closed by a float that rises with the fuel level. The bowl will have a drain for maintenance needs, and most also employ an overflow control system. How many things can you see in this description that can go wrong? The float bowl's gasket can fail. The float bowl vent can become obstructed. The float valve can be dirty or worn. The float can become damaged. The bowl drain can malfunction. And the overflow system can malfunction. Even the fuel itself can go bad. That's seven possibles! And the float bowl is just one area of the carburetor needing regular maintenance.

As an illustration of how important these float bowl characteristics are, consider how much just one of them contributes to normal carburetor function, float bowl venting. For gas to flow from the fuel tank into the float bowl, the bowl must allow air out at the same time. When venting is compromised by incorrect assembly, dirt, and other problems, fuel flowing into the bowl is pushed back out by the air in the bowl becoming pressurized. The result is overflow into the carburetor bore, i.e. "flooding". This is in fact a common problem on Honda carbs due either to neglect or modifications.

Carburetor fuel level
No matter the fluid, as an object is submerged deeper, the pressure on that object increases. This is just one of the principles at work in the float bowl and why the manufacturer specifies a particular level. It sets the baseline level for all of the carb circuits that feed from the float bowl. Misadjusted fuel level therefore richens or leans the whole carburetor. Many believe that the specified float level on a Honda carburetor is what it is for the purpose of liquid tightness. They therefore proceed to alter that level when experiencing float bowl overflow in hopes of stopping it. There are literally dozens of possible causes of fuel leaking from the carburetor, enough to put them into two large and different categories even. But fuel (or float) level is way down on the list, for one important reason: high fuel levels result in richness throughout the carburetor’s entire operating range. If float level was to blame for your carburetor overflow, long before complaining of that you would have got off the bike in disgust due to its running so badly. Think about it. Don’t try to solve carburetor overflow by changing the float level adjustment. Fix the actual problem, which the most likely on old Honda carbs--after bad float valves--is cracked overflow standpipes.

Adjusting float level
My video on adjusting Honda carburetor float level is the most-watched of all my videos. More than 500,000 views. The first thing this brief video points out is that the float valve's spring-loaded pin is not included in the float height adjustment. The pin is not supposed to be compressed during adjustment. It is there only as a shock absorber that lengthens the life of the float valve seat. Another thing shown in the video is doing the job right-side-up, the way Honda's early literature shows it. While the sideways method may work alright, I consistently find float levels 1-2mm off on the Honda carburetors I rebuild. It also keeps things intuitive. Late closing (too close to the carb body) results in richness, while early closing (too far from the carb body) results in leanness. This is easily visualized and understood. Carefully bending the float's adjustment tang corrects misadjustment. Bending it upward makes shutoff sooner, and bending it downward makes shutoff later.

The clear tube method
The clear tube method of determining fuel level often promoted on the Internet for use on vintage Hondas is one of a myriad of things online communities take out of context. Early examples of non-Honda (Mikuni) carburetors had inherently very leaky float valves, thus the OEMs that used Mikunis--Kawasaki for example—had to use a fuel level measuring method that accommodated their carbs’ “dynamic” fuel level (as opposed to its static level). Yamaha also had this problem. Look around for Yamaha triples or XS1100 fours. Not many to be found, are there? So bad were their float valves that their roller bearing crankshafts wore out ages ago, due to fuel dilution of their crankcase oil. Though advocated on many Honda forums, the clear hose method is irrelevant to Hondas. This is proven by the fact that the procedure has no reference point in connection with Hondas, no standard. That’s because Honda never published and never adopted this method. They didn't need to. Honda uses the simple float height method because their float valves don’t leak. Do your Honda float measurement the Honda way, using the Honda tool, and the Honda method (with the carbs right-side-up). You can't go wrong. The clear hose method is not one that is universal, is not a technique for solving float bowl overflow, and is not superior to Honda’s float height method. The forums are wrong in characterizng it as a superior way to do the job. It isn't. Unless your bike is a 70s Yamaha.

Float valve needles
When I began doing carburetors full-time I tried using aftermarket float valves. They were uniformly bad. I began testing brand-new ones right out of the package with a Mityvac before installing them. The result was I had to throw half of them away. I had to buy eight to get four that would seal. Sometimes ten. And then, more often than not, even the ones that sealed initially quit sealing after a short while. And if that didn't happen then the plating that is put on these crappy float valves would begin to peel, resulting in fouling of the valve and carburetor overflow. One hundred percent of all new factory Keihin (Honda) float valves pass the vacuum test. Every one, steel or Viton. The first thing a carburetor customer looks for is fuel tightness. I'm going to send him carbs I know are going to leak? Hardly. I can't understand why anyone would use these parts, especially people who rebuild carburetors for a living.

Floats
Really early Honda floats are made of thin brass sheet soldered into hollow pontoons. These are very delicate and have historically been subject to leakage of fuel into them, which makes them heavy and shutoff late, richening carburetion. Mechanics shake test, and also submerge into a container of water and look for air bubbles. These floats were replaced in later years by solid plastic floats that do not take on or absorb fuel. The last Honda carbs had hollow plastic floats that can take on fuel, but aren't as known for it as are the brass ones. They fail in another way, by becoming distorted and delaying valve shutoff. Speaking of float distortion, all floats can be manhandled and malfunction as a result.

Float pivots
The pins that the floats pivot on should never be removed using pliers. Use of an appropriately-sized pin punch is best practice. Leaving pliers marks on the pins makes them harder to remove later. Carburetors whose pins are seized due to corrosion and varnish should be run in the ultrasonic to free up the pins before attempting removal and carburetor disassembly. I replace dozens of float pivot pins every season.

Float valve seats
All over the Internet is advice to burnish float valve seats, that is, to rub abrasive on them to clean them. You must not do that. It will change the delicate shape of the seat's internal dimensions. Clean them the same way you clean the rest of the carburetor.

First-gen V4 bowl venting
The 1982 through 1985 Sabres, Magnas and Interceptors are unique in respect to float bowl venting. Their bowls vent into the airbox. Honda discovered early on that the oscillating pressures inside the airbox were causing unstable fuel levels in the bowls. In 1982 there was a bulletin and service program to fit plastic restrictors into the vents to damp the oscillations. This helped considerably and was an important part of the first-year after-sale corrections. The V65 models were included in the update later. Later still, Honda changed their mind about the restrictors and had dealers remove them, and also directed on certain models to disassemble the venting system so that it was isolated from the airbox. Once this was done, the carbs' jetting had to be changed as well. Think about this paragraph for a while. It will educate you.

Overflow hoses
Give some thought to Honda carburetor overflow hoses also. There is a history of their contribution to carburetion. The first-year CBX1000 had hoses that actually caused bent connecting rods. The overflow hoses on the 1977-1978 CB750 caused V4-like fuel level instability because they hung too close to the roadway. And on all Hondas, the presence of these hoses makes it more difficult to maintain your carburetors. When overflow happens, you can more readily identify which carburetor if the hoses are absent.

Part 20