A question on vacuums in XJs

I realized yesterday that I don't understand how the vacuums work on motorcycles. What I'm referring to specifically is the tube that runs from our gas tanks to our carb boots. What's the purpose of this tube? Also, why does it only attach to one carb boots on my XJ550, whereas my girlfriend's Maxim 400 has the vacuum tube going to both carb boots?

I'm guessing that it's to create a vacuum in the tank so that it's not gravity fed through the petcock, but I'm unclear.

Thanks in advance for the education.

 

The petock has a valve in it that wont open without the vacuum.
or rather, the vacuum opens up the valve in the petcock
This will allow you to keep the petcock open at all times.

You can manually defeat this valve by turning the petcock to "Prime".
This will allow petrol to fill the carb bowls without vacuum.

dunno bout the Maxim 400.

 

Correct. The vacuum tube provides the vacuum "signal" to open the valve in the petcock and allow fuel flow.

As for the 400 Max, take a close look at the tubes. ONE of them operates the petcock for sure; I'm betting the other is an overflow or "vent" line. Unless someone before you "teed" the line to run it to both manifolds.

 

I get it now. Thanks for clearing that up in my head.

 

So,

If your bike runs great on "prime", but chokes and is super-lean on "on" it probably means that the intake mainfold "boot" between the carb and the head is not sealed (cracks, or bad gasket)? Gas tank has been sanded, and petcock taken appart and everything looks good.

-Rob

 

If you had a bad seal between the carb and the head I doubt your bike would "run great" with the petcock in the prime position. Are you reasoning that there would be insufficient vacuum to operate the petcock if you had bad seals? If you had that kind of leak, the petcock would be the least of your issues.

 

yeah I was thinking that was a possible deduction. I am curious on troubles and operations that are associated with the vacuum operation of the petcock. Petcock vacuum hose maybe cracked somewhere I can't see?

I have done the propane torch test on the intake manis and I have not detected any leaks...but I also don't trust my own handiwork sometimes

IF you leave it in prime, is that bad? Will it keep filling the carbs FOREVER?

Thanks!

Rob

 

If you leave it in prime and your carb float needles leak (when you're not running) you'll eventually fill your carbs (and engine) with gas.

Also, if you're riding in prime and run out of gas, you'll be unable to switch to "reserve" and find a gas station. This is pretty handy if you don't have a fuel warning light.

Dave