Troubleshooting Mikuni Float Valve Operation

[dcooke007]

I am working on one of my own bikes but it is not a Hodaka. It does however have three Mikuni carbs like those used on a Hodaka. I was researching info for my bike on another forum and came across this info. Maybe it is not new to you guys, but when I saw it I had an aha moment. So simple and I cant believe I have not seen this before.

On this particular bike there is no specified float level that can be checked with a scale or floats parallel to base. A test float bowel is used that has a clear hose attached at the bottom to gage fuel level and adjust accordingly. Of course no one has one of these test bowels unless you work for that particular dealership. One of the forum members used the pictured method to check fuel level. You do need a fuel supply attached to the carb and I used a Motion Pro Auxiliary Tank. This tank has an on/off valve and is necessary so you can shut off the fuel. Made nice with my wife and found this lone glass that the base of the carb could sit on and the floats can move freely. Ideally the depth of the glass would be just deeper than the extended float. This glass was much deeper and had to fill a good bit before the floats started to move. With this set up I can measure the fuel level and in this case also observe the float valve weeping fuel after seating.

Inspected the needle valve and it looked ok.

The seat was not so good. Pictured you can see the pitting that is causing the weeping fuel. I might be able to lap the bottom flat and salvage the valve, but for the sake of reliability I am going to replace the float valve assemblies.

I can see this method being useful for checking possible heavy floats and sometimes being able to identify old float valves that look ok but just wont seal properly or maintain correct fuel level.

Danny

[Dale]

Good stuff Danny. Thanks for posting.

[BrianZ]

I've always wondered how accurately the float maintains the fuel level while the bike is being hammered around a motocross track.

Brian

[Zyx]

I've always wondered how accurately the float maintains the fuel level while the bike is being hammered around a motocross track.

Brian

It doesn't, which is why float level need only be approximately correct for how you ride. It needs to be high enough to supply a relatively unfrothed supply of fuel at highest level of demand, but not high enough to dump fuel overboard when tipped, jiggled, bounced, jumped or whatever.

Not to mention that setting the float level while the carb is sitting on a level surface only sets the level if the carb spends its life sitting level to the ground. It's hard to find a motorcycle that has level carbs. If you tip the measuring cup method to approximate the tilt of the carb when in place on the bike, you will spill out fuel unless you are really careful or the carb is sitting as close as it will get to the downhill side of the glass cup.

Generally, for a carb that has been in service for a while, you can see the fuel level by looking at the floats, as there will be a scum line on the float where fuel level meets air. That line is not going to look horizontal, but it is when in place. So with the bowl off but the floats on, look at the side of the carb, tilt it to approximate the degree of tilt when in place, then lift the float until the fuel air line is horizontal. That's your fuel level, and it should be lower than if you set the level with the carb horizontal.

[RichardMott]

"I know you think you understand what you thought I said but I'm not sure you realize that what you heard is not what I meant”

Alan Greenspan

[Bullfrog]

Interesting post Danny! However, I'd suggest that the title mayhaps ought to be something like "Troubleshooting Mikuni Float Valve Operation".

. . . which is indeed what is shown. Do the floats float? Does the float valve actually shut off flow?

Good stuff!

Ed

[dcooke007]

Hey Ed,
Took your advise and re-titled as you suggested.

My bike is a 1974 Kawasaki H2B, 750 cc triple. The carbs are Mikuni VM30SC and are of the same type as most of the dirt bikes back in the day. I have attached a photo of the service manual so you can see the specified method of setting the float level. The glass method came about in order to find another way to accomplish the required task with out the special tool / float bowl.

Some of the old Mikuni carbs I have rebuilt did not turn out as expected.....fuel level to high even though float level setting was correct. Engine would usually start but minor fuel leak out of over flow. In this case float valve looked good as well as the float. Those were the times I wish I had a clear float bowel to see where the fuel level was and what was going on. Also this way I can check fuel level before installing the carb and save some time. Seemed like something worth while to share.

Danny

[viclioce]

Wow Danny! The photo of the needle In your post looks exactly like the needle in my Wombat carb when we were talking about whether or not to replace the needle & seat. I mean it looks EXACTLY the same. Mine had the identical scoring or crusting or what ever that is and was the reason my carb's were overflowing fuel into the Venturi and the cylinder! Funny how that works out, huh? And glad those gaskets worked for you! Who'd have thought Hodaka gaskets would fit a Kawasaki! But they were both Mikuni carb's! Enjoy the 750, and I want to hear first hand how she runs down the road when you get her licensed! ; D Victor

[dcooke007]

Hey Vic,
I am sure the pitting is caused by corrosion from moisture. The thing is the carbs were filled with two stroke oil when stored. I bought the bike a little over ten years ago and stored it. Now working on getting it road worthy.

Funny how you can find a needed part from other makes. I would guess SOME of the guys on the Kawasaki forum would not give a lot of respect to Hodaka's. The oil pump check valves that bolt to the H2 carbs are not available through any Kawasaki sources. However, the check valves are exactly the same as the 99 model Hodaka. So I have three new Hodaka check valves on my H2 that I purchased from Paul and I did not have to pay a small fortune for them. A lot of H2 parts are hard to find and very expensive.

Danny

[Bullfrog]

Danny,

I stand corrected. Since you have mfg. instructions for the proper float level using that method, you are indeed setting float level (and trouble shooting float valve operation at the same time ). But since none of us Hodie fans have specifications for setting the float level by measuring fuel level, the new title is better for us - thanks!

Ed
PS: Don't let the news get too far about those check valves. Owners of other makes could raid "our" parts suppliers!

[viclioce]

Ed. I'm sure the response from an H2 owner would be Hoda--- what? ; D Victor