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Offline 74cb750

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Air in gas lines
« on: May 25, 2010, 06:49:22 AM »
Hi All,
any idea why this is happening:
when I switch my 1976 CB550 over to reserve, the gas lines fill with air.
I have to pull over, let gas flow into the lines, then I can continue on.

Just started happening.
peace,
michel
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Offline BobbyR

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Re: Air in gas lines
« Reply #1 on: May 25, 2010, 08:14:49 AM »
The obvious is that you do not have a proper downward angle on the gas line between the petcok and the carb. If there is a dip in the line air can get caught in there. You could also have air entering the line at the petcock, which would also mean you would have a gas leak so tha is not likely. Sometime a inline filter can cause that if the gas kline angle is not steep enough. Or your tank is empty.  ;D ;D ;D
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Offline Bodi

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Re: Air in gas lines
« Reply #2 on: May 25, 2010, 08:49:21 AM »
If you switched to reserve because you ran out of gas, air is going to be in the lines... that's running our of gas.
Otherwise there's no reason, switching to reserve on a full tank just switches petcock inlet ports from high in the tank to low in the tank.
It takes a few seconds to fill the bowls after switching to reserve if you ran out in "ON". I have never had a situation where air bubbles obstructed the flow... fuel will either slip past the bubble or if there's enough flow just push the bubble on through.
The reserve port on the petcock is at the bottom of the tank where crud collects, the port can get filled with detritus and reserve flow gets very low. Plus if you're on reserve because you need to be... the fuel level is quite low and the effective fuel pressure at the carbs is lower than with a full tank, again reducing the flow.
What's happening exactly? When you're riding on reserve you have to repeatedly stop to let the bowls fill, or you have to stop for a while after running dry in "ON" and switching to reserve, but after that one wait all is well?
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Offline flybox1

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Re: Air in gas lines
« Reply #3 on: May 25, 2010, 12:22:40 PM »
remove the fuel line from your petcock, cut 1/2" off, and reinstall, and check for bubbles when it refills.
this will decrease any flat spots in your line which might hold bubbles.  if another appears, cut another 1/2" off and try again.

decrease in fuel level in your tank will decrease the downward pressure thru the line.  but if there is gas above the bubble, the gas will always find a way down(unless your pc is clogged)

ever tried burping your fuel line?  kinda like bleeding the bubbles out of your brake lines.
gets all the bubbles out, and as long as your tank has gas above reserve, you shouldnt see any more bubbles appear.
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Offline SOHC4ever

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Re: Air in gas lines
« Reply #4 on: May 25, 2010, 12:28:43 PM »
Quote from: Bodi on May 25, 2010, 08:49:21 AM
If you switched to reserve because you ran out of gas, air is going to be in the lines... that's running our of gas.
Otherwise there's no reason, switching to reserve on a full tank just switches petcock inlet ports from high in the tank to low in the tank.
It takes a few seconds to fill the bowls after switching to reserve if you ran out in "ON". I have never had a situation where air bubbles obstructed the flow... fuel will either slip past the bubble or if there's enough flow just push the bubble on through.
The reserve port on the petcock is at the bottom of the tank where crud collects, the port can get filled with detritus and reserve flow gets very low. Plus if you're on reserve because you need to be... the fuel level is quite low and the effective fuel pressure at the carbs is lower than with a full tank, again reducing the flow.
What's happening exactly? When you're riding on reserve you have to repeatedly stop to let the bowls fill, or you have to stop for a while after running dry in "ON" and switching to reserve, but after that one wait all is well?

Threadjack!

In my case, I feel the characteristic sluggishness from running out of gas, switch to reserve and have to stop and wait for up to a minute before it wants to run properly. (Naturally, this frequently happens on the highway or at an intersection.)
Does this have to do with fuel line routing?
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scrapvalue

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Re: Air in gas lines
« Reply #5 on: May 25, 2010, 02:35:54 PM »
Quote from: SOHC4ever on May 25, 2010, 12:28:43 PM
Quote from: Bodi on May 25, 2010, 08:49:21 AM



Does this have to do with fuel line routing?

Your best defense is to know how many miles you can go before you "have" to switch to reserve.
Then switch to reserve 20 to 25 miles before that and it will eliminate your problem. Both the pulling over and the inconvenient time it usually happens.

Oh, and don't forget to switch your petcock back to on after filling.
I left mine on reserve once and ran out, I mean really ran out! That sucked.
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