Project My Fizzer

Aw c'mon Phil, look at the fun you are missing out on!
Back to the task at hand.
Sounds like progress Glenn. Maybe we can safely assume that your motorcycle does not run correctly with its fuel supply on. So hopefully that only leaves float height setting and fuel needles, unless I am missing something. If the needles were faulty shouldn't they just flood? Or are you thinking partially faulty? Can you turn your vent tubes to face down and see if they are leaking fuel?

I think I have the variable float level issue worked out. It would be the angle of the carburettor. If one assumes that the correct float level is halfway along the emulsion tube, or some value close to that. Then as you rotate the carb more towards the vertical, the correct float level will move further towards the mouth of the carb (the 'Mikuni' line).
and of course as the carb becomes more vertical (more downdraft) it will move in the opposite direction (more sidedraft) towards the pilot screw end of the float bowl. Most carbs never have this issue to any degree because most of them are just vertical. This implies of course, that different models of Yamahas that share the same carb bodies and have their carbs tilted at slightly different angles, will have different values for fuel level measured from the same point.

cheers all
Blair

 

Here are some models of my BDST carbs from my FZR400 3TJ1.
The first one is vertical.
next is with the carb tilted at 15 degrees from vertical and the other is at 20 degrees.
The distance is measured vertically from a horizontal line through the centre of the needle jet to a horizontal line drawn through the "float chamber line" on the float bowl.
At 0 degrees (vertical) the centre of the needle jet is 11.0 mm above the line.
At 15 degrees the centre of the needle jet is 20.1mm above the line.
At 20 degrees the centre of the needle jet is 22.8mm above the line.
At 21mm below the "Float Chamber Line", which is what the manual says, my float bowls will be empty. You have to love service manuals.

 

Great work Mr Sylvaner, the float level is the important part. A few good running standard bikes gives us the information we need. I think everyone has all the right bits and pieces as far as jets go.
Mr Grey you should have spare float needles from your 3LN3 carbs that you could try when you are ready for Round 3, or is that Round 33.

 

My two cents.
I set pilot mixture screws by ear, it seems to be some sort of black art as a lot of other bike mechanics I have worked with can't seem to do it.
As for needle clip positions, not much of a fan of this idea. If you have changed the header length or thrown the EXUP in the bin then maybe you would need to change the rpm at which the needle taper is doing its thing. It is not so much about richer or leaner as it is sooner or later. Changing the needle clip position to try to rectify the problem you are having sounds like a patch not a cure. Have you ever seen a standard setting for a needle clip that is NOT in the middle position? That is because the factories change the needle not the clip position.
When you open the throttle butterfly there are some small holes that are exposed. They are free to release fuel and air and thereby increase rpm. Once this happens the vacuum increases the slide lifts and away you go. As I understand it.

 

Gearbox selection problems are often related to the pawl mechanism that rotates the selector drum or the clutch dragging. The selector pawl can be bent, worn, badly adjusted, missing some parts etc. Also, as you have already alluded to, if the bushes in the crankcase and shift levers are worn the you are losing degrees of rotation at the selector drum. Clutches drag anyway because they are immersed in oil and the drag becomes worse if they are not disengaging fully. Finding neutral (which is located by a detent mechanism) is a lot easier with the engine turned off because there is no drag. The higher your idle the more difficult it will be to find neutral. Gearbox problems, on the other hand, are usually more like jumping out of gear or failing to engage (missing a gear).
cheers
Blair

 

This sound like the correct explanation. Another piece of the puzzle is solved.

 

Last (we wish) but not least, if your idle mixture is too rich and you have wound up the throttle stop to compensate then it will be really sick at low rpm. It is definitely worth winding those screws in and seeing what happens.

 

When a pilot screw is not a pilot screw. On many old Japanese carbs the pilot screw is an air screw and the pilot fuel supply is fixed. Adjusting that screw does not make much difference. A lot of 'bike mechanics' grew up on these things and their understanding is "n turns out". On the Italian bikes with Dellorto carbs it is a fuel screw and it is ultra sensitive. 1/4 turn each way and it falls off a cliff. Our little problem child is a hybrid. It has air and fuel and so although not as sensitive as a fuel screw it is still very important that it is set to optimum.

This is important and I think that you should check this before doing anymore medical experiments.

With a warm motor. The process is to find the mixture screw position which yields the highest idle speed. Once you find that position you lower the idle speed to the correct setting and repeat the process. Smarty pants types like me can do one carb at a time and 'hear' the engine/carb/exhaust. Most people cannot do this and I strongly suggest that you do not try doing it at this stage. However, if you do all four carbs together then you will be close enough and you will be able to easily detect if the idle speed is increasing or decreasing.
As you make these changes it takes a little while for the engine to react, it is not instantaneous.

Wind them in to 2.5 turns out and see if the idle speed increases. If it does lower the idle back to 1600 (I believe that is the setting for the 250)
Then repeat the process again. Set them to 2 turns out. If the idle speed increases then drop the idle speed again. If not then go back to 2.5 turns out.
When you have found the highest speed and adjusted the idle at that point then you can try 1/4 turn each way.