Topic: CB750 Float Bowl Level

[gwallis]

OK... so I removed the carbs off my 1973 CB750, for the third time, and set the carb floats to 26MM.  I had mistakenly set them at 22MM for the first two rounds.

However, when I check the float height using the clear tube method (I purchased this little device) it shows that the fuel height is only 3/4 the way up the float bowl.  I must be doing something wrong here... but what?

As a side note... the bike will start but will not stay running.  I start it with the choke on and if I give it throttle the engine dies.  Like it is sucking only air.  Is this because the float bowl height is so low?  I should say all the bowls are at the same height using the clear tube.

Any help would be greatly appreciated!

[Davez134]

How are you measuring float height? With the carbs on the bike, or off, tilted at an angle?

[gwallis]

The measurement takes place with the carbs off the bike... tilted slightly so that the float 'tab' just touches the float valve.

I have seen this done and have done it this way before.  That is why I can not believe that the fuel height is only going up 3/4 of the way to the top of the bowl.  Isn't it supposed to be all the way to the top of the float bowl?

[Davez134]

Not quite to the top. I am having trouble with my pictures right now for some reason, so I'll just post the links. It should be right in between here:
http://forums.sohc4.net/index.php/topic,156215.msg1793030.html#msg1793030
and here:
http://forums.sohc4.net/index.php/topic,156215.msg1820926.html#msg1820926

The first pic I was on the lean side, last pic I was rich. I am about 1mm lower than 2nd picture right now. Yes, it does make that much difference.

[gwallis]

Well ... aren't I the dumb one!

I mentioned that I had rebuilt the carbs.. new floats, jets, Air screws and springs.  I had always heard that you should not over tighten the air screws for fear of damaging them.  Installing the air screws with the new springs were tight and I had thought that they had bottomed out.  In watching a few CB750 Carb video's I noticed that the air screws were turned in way past the lip of the A/R port.  Mine were just about flush with the opening.

I went out to the garage and turned the air screws all the way in until I knew that were seated...about 1/8inch in from the lip.  The bike started right up and idled much better... even with the choke off.

I got some more work to do but it looks like I found my problem.

"Thanks' to all that contributed to this thread!

[flybox1]

Some idle screws, especially aftermarket ones, are taller than OEM, so setting them by how they LOOK is not correct.
Your IMS should be set to get the correct air/fuel mixture at idle, and if you dont have a sniffer on a dyno to tell you how to set them correctly, you default to idle chop plug color.  Light tan plug insulator is the goal. 
As they are set now, who knows if they are right. . .



Floats should be set to achieve a fuel level of 3mm below the top of the bowl.