When Bench Testing Carb Float Levels, Should Fuel Be Added Under Pressure?

[Bob K.]

I've consumed a dozen or so threads about bench testing the carb floats by adding fuel. A few just add fuel without any pressure and a few seem to imply that fuel should be introduced under pressure in the same way that the fuel pump introduces fuel under pressure.

 

When the carbs are connected to the bike, the fuel pump applies some amount of fuel pressure while filling the bowls. I suspect this would affect the amount of fuel the float and float needle allow into the bowl. In other words, some amount of pressure can overcome the float buoyancy and needle pressure, which would force a bit more fuel into the bowl before the needle closed. That's in contrast to introducing fuel with no pressure during a bench test where the float and float needle might block the needle port faster because it's not fighting against any fuel pressure.

 

In order to accurately bench test float levels, should I introduce fuel with a little bit of pressure?

[Marcarl]

I really don't think it would make a whole hill of beans difference, the fuel pump only puts out 3-4 lbs pressure, so about as much as a gravity system.

The way I did my floats, was to hold the carb upside down with the floats installed. As you do that you will notice a casting mark on the jet body and that should line up with the contour of the float, worked every time, never an issue that I noticed, but then maybe I'm not as fussy as some others.

[Patch]

I've consumed a dozen or so threads about bench testing the carb floats by adding fuel. A few just add fuel without any pressure and a few seem to imply that fuel should be introduced under pressure in the same way that the fuel pump introduces fuel under pressure.

 

When the carbs are connected to the bike, the fuel pump applies some amount of fuel @ -4 lbs pressure while filling the bowls. I suspect this would affect the amount of fuel the float and float needle allow into the bowl. In other words, some amount of pressure can overcome the float buoyancy No or very unlikely, has to do with sq. area of pressure and needle pressure, which would force a bit more fuel into the bowl before the needle closed. ​The shape and buoyancy of the float produce the counter force levered which closes the gate. Again the sq. area levered should overcome the pump pressure.

That's in contrast to introducing fuel with no pressure during a bench test where the float and float needle might block the needle port faster because it's not fighting against any fuel pressure. This is reversed thinking, the pump pressure is for 1 timed, and 2, working to counter the float pressure @ the needle head only (sq area). When the Volume drops the float drops and the pump pressure displaces the air out the vent refilling or maintaining a level which is merely replacing spent fuel. (while throttle is open).

 

In order to accurately bench test float levels, should I introduce fuel with a little bit of pressure?

Of course you can - if you suspect an improper float needle valve function /then run it under load, just have a safe length of tubing above the bowl.

 

Good question;)

[Prairiehammer]

I really don't think it would make a whole hill of beans difference, the fuel pump only puts out 3-4 lbs pressure, so about as much as a gravity system.

The way I did my floats, was to hold the carb upside down with the floats installed. As you do that you will notice a casting mark on the jet body and that should line up with the contour of the float, worked every time, never an issue that I noticed, but then maybe I'm not as fussy as some others.

 

Marcarl's method is valid; consider how the factory (either Mikuni or Yamaha) would do the float adjustment. (The factory is certainly not going to do a wet adjustment for every carb.)

This pic shows the casting mark (a circle) on the jet block and the correct location of the float in relation to that casting mark.

 

[Bob K.]

Thanks, Kevin. Great pictures! I followed that method to set the floats while dry. I'd now like to do a wet test while I have the carbs off the bike. The pressurized fuel concept is what's currently intriguing me.

[Patch]

Thanks, Kevin. Great pictures! I followed that method to set the floats while dry. I'd now like to do a wet test while I have the carbs off the bike. The pressurized fuel concept is what's currently intriguing me.

 

I get what you are wanting to double check. If you want to be sure what ever the motivation you can check while under 4 lbs.

Just keep in mind the footprint under such a test is very small and pointed.

 

God help those carburetors if they start leaking after install! Right?

[Venturous Randy]

Are you sure that circle has anything to do with setting the float level? I ask this because to me that is just the indication from an ejector pin the pushes the casting out of the die after casting. I would be really surprised that the casting die maker considered the location of the pin for anything other than ejecting the casting without it binding in the die. But, that doesn't mean that it could not be a good reference point if it has historically worked.

Randy

[Marcarl]

Are you sure that circle has anything to do with setting the float level? I ask this because to me that is just the indication from an ejector pin the pushes the casting out of the die after casting. I would be really surprised that the casting die maker considered the location of the pin for anything other than ejecting the casting without it binding in the die. But, that doesn't mean that it could not be a good reference point if it has historically worked.

Randy

 

Nope, not sure at all, fact is I think it would be rather strange, but it works, seeing as all the castings are made from the same mold, and hey''''' if it works why not use it.

[Bob K.]

@Venturous Randy--I think it's completely coincidental. The round circle--a mold mark--just happens to be in the same neighborhood of the dry float level measurement that happens to yield a pretty close wet measurement fuel level for a lot of folks.

[Bob K.]

I ended up not adding any fuel pressure when I wet tested the carbs. As I discovered in my other thread (https://www.venturerider.org/forum/showthread.php?138154-Carb-Float-Adjustment-Problems-Standard-Dry-Adjustment-Results-in-Low-Fuel-Level), I think adding vibration to the carb in order to get an accurate float level measurement is more valuable than adding fuel pressure.

[saddlebum]

Here is the setup I used to wet set my carbs sitting level on the bench and I am very pleased with the results.

 

https://www.venturerider.org/forum/showthread.php?86821-home-made-set-up-for-wet-setting-floats-on-the-bench