Carb holder for wet test

I am going to build a holder for wet testing the fuel level on my xj750 seca carbs.
I have a few beat up engine side manifolds I can fasten to some plywood
to hold the carbs while testing. My question is, should the float bowl seam
be level front to back as well as side to side ? The carbs sit level on the bike
from side to side but they tilt uphill to the back when installed . should that uphill
angle relative to level be duplicated on the holder when wet testing ?

 

yes to the plywood. steel angle iron would be better just use 2 manifolds.
level left to right and front to back .

your bike will never be in a constant position, angle of carbs change with wieght of rider /passanger ,then add in up hill down hill and angle of bike in turns

 

Wet set with the carbs level in both planes.

http://www.xj4ever.com/setting fuel levels.pdf

 

Thank you. I will get to work on a carb holder asap.

 

And then, there are all the people that argue that one should orient the rack with the slight forward tilt to replicate the installed position. Go figure

 

But Yamaha (and Hitachi) said...

 

I screwed 4 long deck screws into a block of wood so they support the rack at 4 corners. Very easy to level, dead simple to make.

 

Yup...I'm just pointing out that there are other people out there who have a differing opinion (I'm not one of them)

 

I thought about this when I did my first wet set. I decided that the geometry (angles and distances), tolerances, and the directive to measure at the middle of the bowl would make any practical difference from the tilt of the bowl minimal (negligible, in my opinion).

 

when you make it, get a fuel valve of some kind right in close to the carbs and keep the carbs away from your plywood so you don't bang your knuckles

 

As I understand it, the fuel height that matters is right over the main jet , which is right in the middle of the carb bowl. In the end, it's the height of the fuel at this point which adds hydrostatic pressure to the negative pressure that the venturi produces to draw fuel up. The floats also surround this jet, and therefore opens and closes the fuel valve according the fuel level there (and not somewhere else in the carb bowl).

With a tilt installed on the bike, the fuel will we a little higher in the front, and lower in the back (wrt the plane of the bowl) but it will always be the same height at the center. Even as you tilt the bowl more, the level will still be the same in the middle.

On the bike, the measurement needs to be taken at the center of one of the outside carbs, right in the middle of the body. Carbs must be level side to side (and aligned) for this measurement to actually represent the level in one of the middle carbs.

If you build a rack holder, and level is front to back and side to side, you can easily measure the float height on the back of any particular carb body, because any measurement with respect to that plane will represent the height at the main jet.

 

Tilting the carburetor forward will reduce the fuel level as the buoyant force increases and closes the float fuel valve lowering the fuel level. Tilting the carburetor rearwards will have the opposite effect as the buoyant force is reduced and a higher level of fuel results.

The intention of the FSM to level the carburetors in a vertical direction is ambiguous and a debatable subject:

The instruction to level the carburetor vertically using jacks under the engine is impractical and would be very difficult to accomplish.
No verification method is instructed on how to verify vertical level such as with horizontal level the side to side fluid level is used.
The most common pictorial just happens to show the level checked at the center of the bowl, which would be proper to negate the effects of a tilted carburetor to determine fuel level.

So, the question of which method is proper is somewhat valid. Should the fuel level be 3mm +/-1 mm on the bike with the nominal tilt and after running momentarily as instructed to account for vibration forces acting on the float needle valve, or should the carbs be leveled as in the wet test procedure and set to 3mm +/- 1mm?

My inclination is to stick with the level setting as instructed in the wet test procedure as it is the most practical with reduced variables. Add to that the millimeter or two in fluid level height and the resulting head pressure of the fuel in the bowl begs the question of how much does this really matter? I guess it's just one of those things where if you are going to do it doing it correctly would be the preferred method - instead of just getting a result that is close enough.