Float level

Tips and Recommendations from Guru Mike Nixon

Moderator: Whiskerfish

Post Reply
User avatar
Early 'Wing Guru
Early 'Wing Guru
Posts: 997
Joined: Thu Dec 10, 2009 1:51 am
Location: Prescott, AZ

Float level


Post by mikenixon »

There is an astonishingly pervasive notion among motorcycle riders that the height of the carburetor's float is somehow linked to float bowl overflow. In other words, that a float's height is the carb's first defense against carburetor flooding. This is totally wacky. And worse, because it would then follow that an overflow condition could be corrected by float level adjustment. Not so. Yet this notion permeates the Internet. On my Youtube video showing how best to adjust floats, I get regular inquiries on how to adjust floats to stop overflow.

Overflow has many possible causes, however too high a float setting is at the very bottom of the list. The most common problem on many vintage bikes is cracked overflow standpipes, where they exist (they are absent in Wings). At least half of all the carbs I rebuild have to have their standpipes repaired. Next is poor bowl venting, whether due to blocked vent passages or improperly installed or routed vent hoses. And then of course there is the leaking float valve, whether due to debris, varnish, water, wear, modification, or simply poor quality valves. And that last is worthy of some thought. The Wing carburetor, in all its permutations, lacks an overflow indication system, meaning overflow is more serious than on other bikes because there is no visual that fuel is pouring into the cylinders. For that reason, if no other, using good float valves is critical, and there are no good valves except OEM.

Float level is equal to the volume knob on your radio. It regulates the height of fuel, which determines the amount of fuel accessible to all the carb's circuits. All of them. Controlling the richness or leanness of all. This is known by every career tech. So consider. Logic should first tell you that a float set to the manufacturer's spec can't possibly be the cause of overflow. Duh. Makes even less to change it anyway, as many do. Second, it is equally logical that if a carburetor were overflowing due to an excessively high float level, rich running would be a bigger problem than overflow. Your first concern would be to get off that slug and park it until you figured out why it runs like it's towing a motor home. Seriously. The bike would run so bad you would never even have *thought* of overflow.

Float level is all about calibration, not liquid tightness. Like the jets, slide dimensions and even the carb's bore size, the float level is one of the specifications predetermined by the carburetor maker to control air/fuel mixture. Period. Race tuners appreciate this, as they use float level in the same way they use jets, adjusting both to suit tuning needs, and even some of the bike manufacturers themselves have issued notices to alter float level to solve performance issues. Float level = mixture control. Nothing else. If you experience carburetor overflow, fix the problem. Don't try to strangle the carb dry by altering float level. Shutting off your house's water main isn't how you fix leaky plumbing.

But what if adjustment *is* warranted? Previous work has been inexpert, the adjustments have vibrated out of spec, whatever. It seems many have forgotten that before carburetors got so heavy, manufacturers used to consistently describe adjustment with the carburetor right-side-up, that is, in its normal position. It's still the preferred way in my view, no matter the carb type or vintage. Show me a float adjusted in any position other than right side up and I will show you one that is anywhere from one to three millimeters off. Also, many seem to think the exact point at which a float shuts off a mysterious, difficult to obtain goal. I'm not sure why this is. But I know done right-side-up much of the confusion goes away. Also, the spring-loaded pin on the float valve is merely a shock absorber that protects the valve and its seat from repeated impact. It has nothing to do with float level. Nor is it true that the float's bottom edge should be level with the carb casting. That's a an Internet myth, no matter how pervasive. There are many carbs, even most, whose correct setting is not parallel with the casting. Some above it, some below. Older Mikunis for example are almost all markedly below.

Then there is the unique issue on the GL1000: the slots. Unlike other vintage Honda carbs having slots in the float bowl flange (the CB450 comes to mind), the GL1000's slots aren't used during float adjustment. Instead, the height measuring tool must be kept out of the slots in order to obtain the correct adjustment. Watch out for that.

Float level is important. The main practical benefit aside from correct mixture is a wonderfully smooth idle. I see incorrect float levels every day. Most are minor, but many are so far off you have to wonder what went on the last time the carbs were worked on. A simple system, the carburetor float. But so widely misunderstood. Hopefully this exploration has put some light on the subject. Check out the aforementioned video at https://youtu.be/aiexehn33kg.
User avatar
Honored Life Member
Honored Life Member
Posts: 5318
Joined: Sat Jul 11, 2015 2:37 pm
Location: Nova Scotia Canada

Re: Float level


Post by 5speed »

Post Reply
  • Similar Topics
    Last post

Return to “Mike Nixon's Spot”