figured out something but a queston as well.

[shadowfist]

ok, so i was having a problem with my bike running rich and fouling out plugs. and i am getting ready to re-do my carbs. anyway while i was reading up I found that my 'fuel mixture screws' that i was tightening up to lean out the mixture are not the fuel mixture screws at all they are the air mixture screws... so i was making my mixture more rich while trying lean it out... anyway here is my question..

I was having a problem with my petcock and it seems to run flakey lately. so it seems to run as long as i keep it on prime while i'm driving. now, if I inadvertently cranked those screws down thinking they were fuel adjusters and that vacuum line goes right to the petcock... that could have been causing my petcock problem couldnt it? and it semes to have only happened after i 'leaned out the mixture' bwahaha *smacks forehead*

well live and learn right? time to open those babies up now...

[Riding Again]

It could be a contributor, but I wouldn't blame the whole petcock problem on that.

The vacuum is generated by "fluid" movement through the carbs. Fluid is both air and fuel. The principle is similar to an airplane wing. The faster the fluid movement the lower the pressure. Air needs to move faster (higher velocity) on the upper section of a wing than on the bottom (it's curved more). This pressure difference, lower above the wing than under the wing, provide the lift.

Similarly in a carb, the faster the fluid flow, the lower pressure it has. This is the vacuum that opens the pulls more fluid into the carb. It self feeds itself.

Back to your original statement. By trying to run the bike too rich would reduce the amount of vacuum to pull fuel, but its because the engine wouldn't run well and subsequently the vacuum would be smaller, but I don't think it would have prevented the petcock from working.

[renobruce]

If your carbs are the stock VM26's, you DO have a pilot fuel screw as well as a pilot air screw. Which one were you playing with?

[tkent02]

The vacuum is generated by "fluid" movement through the carbs. Fluid is both air and fuel. The principle is similar to an airplane wing. The faster the fluid movement the lower the pressure. Air needs to move faster (higher velocity) on the upper section of a wing than on the bottom (it's curved more). This pressure difference, lower above the wing than under the wing, provide the lift.

Similarly in a carb, the faster the fluid flow, the lower pressure it has. This is the vacuum that opens the pulls more fluid into the carb. It self feeds itself.

Not quite the whole story, true at higher RPM but the intake trying to suck in air through the closed throttle creates much higher vacuum, at cranking RPM this is the only thing working to open the vacuum diaphragm in the petcock to turn on the fuel.

[Riding Again]

tkent you are correct. It's like begining a siphon.

[shadowfist]

If your carbs are the stock VM26's, you DO have a pilot fuel screw as well as a pilot air screw. Which one were you playing with?

the only thing that looks like an adjustment screw on my carb is the one diagonal flat head screw (leftmost in this pic, thanks cycleorings.com)


I have heard there is a fuel adjustment screw somewhere on the bottom but i haven't seen them yet. and what type of carb is this? it doesn't look like the pictures i could find of the VM26.

[tkent02]

tkent you are correct. It's like begining a siphon.

Not at all, it is vacuum opening a valve, then fuel flows down hill.
No syphon, fuel is flowing by gravity alone from one vented container (tank) to another (float bowl), flowing downhill all the way.
The vacuum line is separate, and only opens the valve, does not touch fuel at all.

[renobruce]

the only thing that looks like an adjustment screw on my carb is the one diagonal flat head screw (leftmost in this pic, thanks cycleorings.com)

I have heard there is a fuel adjustment screw somewhere on the bottom but i haven't seen them yet. and what type of carb is this? it doesn't look like the pictures i could find of the VM26.

Looks like a VM26 to me. The pilot fuel screws are the four screws you see along the top of the carb, centered right above the floats in this pic:



Yours could be covered by a metal cap if they haven't been tapmpered with yet.

Here's my VM26's... the bodies look the same as yours.

[silverhorse47]

It is engine vacuum that is opening the petcock diaphragm when it is on 'run', and also engine vacuum that is pulling fuel into the throat of the carb through the jets. The vacuum source for both is the same...when the pistons go down and the intake valves are open...they suck.
If your petcock supplies fuel when it is on 'prime' and has trouble when it it is on 'run' then either there is a leak somewhere in the vacuum line between the petcock and the engine, the vacuum line from the petcock to the engine is wrongly connected, or the petcock is needing attention. There may be a hole in the diaphragm, or the guts of the petcock may be gunked up. They are simple to dismantle and clean, if your search for a vacuum leak in the line is fruitless.
Your fuel screws will affect engine vacuum, but not so much that the petcock would not work on 'run'. One of the usual ways of tuning an engine idle is to set the fuel/air screws so that the highest even idle is achieved. However, if the engine is running at all there is sufficient vacuum to hold the diaphragm in a position to flow fuel to the carbs.
To clear up confusion in the future, as a general rule, the screws that add air to the mixture are on the airbox side of the carburetors, and the screws that add fuel to the mixture are on the engine side of the carburetors. This makes perfect sense as it is the air rushing past the open jet that sucks the fuel into the stream and into the combustion chamber. It wouldn't work if the fuel was introduced prior to the air flow...
This may not always be true, but I haven't ever seen anything to the contrary.
S.