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  1. #1

    Default Trapped air at Racor 500 filter

    In my sailboat with Perkins 4-236, i have a problem with a trapped air bubble at the exit from my Racor 500 filter/water seperator. I have temporarily put clear tubing in line before and after the Racor looking for air bubbles. No bubbles are seen entering into filter. Exiting the filter i see a long(2 to 4 inches)bubble travel into the clear tubing. If this clear tubing is led up the bubble will travel to the top of this tube and still allow fuel to past. This bubble does not seem to travel any appreciable distance down. Once the bubble is at the top of the loop no new bubbles are added to the system. This bubble just moves back and forth.
    If the clear tubing is instead led down, the bubble will exit the Racor into the clear tube but stay within a couple of inches of the Racor. The bubble does not travel downhill. I have varied the engine rpm and the bubble still doesn't travel down hill. Fuel does flow around this bubble and the engine runs fine for tests at the dock.
    I have tried filling the center tube of Racor to get the bubble to "percolate "out the top before i put the top and handle on the Racor. Same results.
    Will this bubble ever travel thru the system and cause problems? Better yet, how do I get rid of the bubble so when I get rid of the temporary clear hoses I have confidence there are no bubbles in the fuel line?
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  2. #2
    Join Date
    Sep 2008
    Maryland - USA

    Default Re: Trapped air at Racor 500 filter

    as a rule, physics will tell you the bubble won't travel downhill until the pressure inside the tube has bee reduced sufficiently.

    What caused (or led to) the 'experiment' with the clear tube??

  3. #3

    Default Re: Trapped air at Racor 500 filter

    After the annual filter change have had problems with engine stalling. It was intermittent, sometimes run a couple minutes, sometimes over an hour. I found and tightened a couple of suspect spots in suction side of fuel line. Also realized the lift pump wasn't working correctly. It was orignal (old). When i opened it found it full of crap. Installed new lift pump. I now do not see any new air leaking into system as the engine runs. Just have this bubble which is trapped. I can move the bubble to the top of my temporary clear tube by lifting the temporary clear tube higher than the Racor. No air is introduced. Why I can't get this bubble to exit out the racor when the top of the Racor is off and filled with fuel is bewildering.

  4. #4
    Join Date
    Sep 2008
    Maryland - USA

    Default Re: Trapped air at Racor 500 filter

    I think you are making a big deal over something not worth the effort.....as long as the engine is running (under load) and the 'bubble' isn't growing, I think you will be fine.

    If you really want to eliminate the bubble, I'd suggest going to the fuel pump end of the suction tube, removing it from the pump's inlet, and then applying vacuum until the bubble is eliminated....pinch off the hose/tube and reinstall it on the pump....you owners/service manual should have the bleeding procedure for the lift pump if required....

  5. #5

    Default Re: Trapped air at Racor 500 filter

    Successfully ran the boat over the the weekend. I had first tested the engine with the clear tube under load at the dock for half hour. The bubble was still there but never traveled far downhill. Then i put in real fuel line ran under load at dock for 3/4 hour. I can't see see the bubble but assume it is still there. Went for weekend trip and everything is fine. Still not 100 percent confident but believe my original problem was solved and this bubble will not be a problem. Why i can't purge this bubble back up and out the top of the Racor is still very bewildering.

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