Topic: VM29 float bowl measurement - drain petrol out via the plug and measure it ?

[PeWe]

I'm in the long process of tuning my carbs. Mikuni VM29. I have tested some combinations and I'm still not satisfied with the result. I cannot count out the floats as a problem yet.
I have seen that carb 4 contains much less petrol than the other, 3 a little bit less. Bike is not standing straight, its leaning when it has side-stand only.

My idea now is to support the bike so carbs are horisontal, let it run for a minute. Petcock off and drain each carb via the bottom plug that the VM29 has. Measure how much petrol each carb contained. Later fill the carb float bowl with same amount and see the level.

- This must be OK, right?

It is a convenient way to check this if it is a reliable method.

I measure checked the floats briefly when I had the carbs on the bench, all 4 to be equal and appr 23mm. Might be different in real life. They worked 30 years ago, I have not touched the floats then or after.

[Sprocket75]

I used empty glass baby food jars to measure each of mine. You mentioned that your bike has only a kickstand, which makes it lean to the left, and therefore placing #4 and #3 higher than #1 and #2. (with #4 being slightly higher than #3 as well) So, it makes sense what you are saying. Also, be sure to check the fuel line going to #3 and #4 as they are fed from the same line, but separately from #1 and #2. At least they are on my 550. (might be different on yours) Make sure there are no clogs and nothing blocking the flow. (kinks, bends etc.)

[flybox1]

since you have already set your floats, the 'clear tube method' will tell you exactly what the fuel level is....
see the first picture on this thread.
http://forums.sohc4.net/index.php?topic=110188.0

Have you checked the actual fuel level in the carb bowls rather the just the mechanical float height?

1+ Check the fuel level in the bowls while they are mounted by using the "clear tube" method: unscrew the brass drain-plugs at the bottom of the bowls, jam/screw in pieces of clear vinyl tubing, and hold the tubes up along-side the carb body. The level in the tube should be 2-3 mm below where the top of the bowl seats against the carb body.

[lucky]

The floats are set by measuring the float height when the carb is upside down or on its side.
Not by how much liquid is in them.
No one ever did all of this liquid measuring stuff that you hear about on this forum.

But the amount of gas in the float bowls has to be below the over flow tube.
It has to be BELOW the over flow tube so that the fuel is not constantly overflowing.

So the float bowls usually end up being a little over half full.

[flybox1]

Lucky...that's all fine and all, but once a member here gets float heights properly set, YOU are making the assumption their floats are actually doing the job they are supposed to do 
When float height is set, by the book, and you don't get the result you were looking for, unless you have clear carb bowls or x-ray vision, this IS the next logical step to seeing what is going on inside.


Here's a prime example  -  Clymer says 21mm float height for 72-74 CB350F's. 
YOU would tell me to set it by the book and forget it, right? 
Well, 21mm is wrong.  Its actually closer to 24mm.  21mm puts the fuel height AT the top of the overflow tube. (stock carbs and floats)  My carbs were always dripping, even with new float valves and seats. 
24mm puts fuel level AT 4mm below the bowl gasket seam.  Perfect!  No more dripping carbs. 
The clear tube method was the ONLY way for me to see this.
Don't assume THE BOOK is always right, and correct for every situation.

[PeWe]

Yes, I did not find any liquid measuring method.
That's why I started this thread to hopefully get answer by someone that has tried it. And this is a forum where facts, ideas, questions, problems and our passion for CB fours are ventilated
The CB750 Kehin carbs do not have the bottom plug as Mikuni's have which might be one reason to not find it here.
I had the carb rack on it side (1 -4 vertical) and verified when the brass tongues touched the float needles.

Next step practical which is easy for me when I open those plugs rather often since 2 weeks, draining the bowls when switching main jets or before removing the entire float bowl when doing the pilot jets.  I hope to find same amount of petrol in each bowl. Then the supply must be good and float levels OK when I doubt that all are wrong at same wrong level.

[flybox1]

the clear tube method is used to see internal fuel level.
its very simple. 
connect clear tubing to your bowl overflow pipes.
secure the tubing next to the bowl it is connected to, and turn on the fuel supply.
the fuel will level itself and match the fuel level inside the bowl.
you'll be able to SEE if it is 3-4mm below the top of the bowl at the gasket seam
3-4mm below this seam is the goal

see this ..
http://forums.sohc4.net/index.php?topic=63342.msg690270#msg690270

[tlbranth]

Flybox is correct. Fuel level is what's important. If your floats are uneven weights or if there are dents in them, you won't get the right height using the fuel volume method. The clear tube method is the best and it's most easily done with the carbs on the bench. Actually attaching the tubes ain't so easy.

[bwaller]

PeWe, if you have an extra float bowl drain plug for your smoothbores you might modify it to use a clear tube to check fuel heights. The ideal fuel height for the VM29's is 2.5-4.5mm below the edge of the carb body. This is the most accurate. Regardless of what Mr. Lucky says I have measured volumes before, but you need to have at least one float adjusted correctly to use as a starting point.

The float height I have for these Mic 29mm carbs is indeed 23mm, but I always use the clear tube.

[lucky]

Lucky...that's all fine and all, but once a member here gets float heights properly set, YOU are making the assumption their floats are actually doing the job they are supposed to do 
When float height is set, by the book, and you don't get the result you were looking for, unless you have clear carb bowls or x-ray vision, this IS the next logical step to seeing what is going on inside.


Here's a prime example  -  Clymer says 21mm float height for 72-74 CB350F's. 
YOU would tell me to set it by the book and forget it, right? 
Well, 21mm is wrong.  Its actually closer to 24mm.  21mm puts the fuel height AT the top of the overflow tube. (stock carbs and floats)  My carbs were always dripping, even with new float valves and seats. 
24mm puts fuel level AT 4mm below the bowl gasket seam.  Perfect!  No more dripping carbs. 
The clear tube method was the ONLY way for me to see this.
Don't assume THE BOOK is always right, and correct for every situation.

QUOTE:
"Might be different in real life. They worked 30 years ago, I have not touched the floats then or after." 

You need new float needles and seats.

4mm below the bowl gasket seem would not be right.
I do not know who gave you that information.

The fuel has to be BELOW the over flow tube height.
Not below the float bowl gasket.

The fuel is always sloshing around in the float bowl so that gasket would allow fuel to leak out if that gasket is not a good seal regardless of the correct float height.

If you look at the float bowl overflow tube you will notice that the top of it is rounded and is not cut off square. This is made this way so that any sloshing fuel has a hard time getting in to the float bowl overflow tube unless the fuel comes directly in over the top of that tube. 


Just make sure your float bowl gaskets are in good condition and set the float level by measuring and it will run right.

No one else has this problem unless they have the wrong needle and seat installed,something like that. Or the wrong floats installed.

Books do have mistakes.
But if you do a search to find other owners having this same problem you should have your answer.


You will never know the actual height of the liquid in the float bowl because even with the gas turned off, when you remove the float bowl, gas is still pouring into the float bowl the second the float drops. There is no way to remove the float bowl to get an accurate measurement of the fuel because you cannot remove the float bowl without more fuel coming into the bowl while you are trying to remove the float bowl. That is why Honda uses a measurement system.

Mikunis are very similar to the Keihins as far as float operation.

[bwaller]

These carbs don't use a traditional Keihin standpipe overflow Lucky, plus have center bowl drain plugs. I set fuel levels at +-2.5mm from the gasket surface, the gaskets don't leak. It's how they're engineered, I didn't make it up.