I recently replaced my fuel supply system and have a WALBRO pump mounted downstream of 75/900 racors. I set the system up with three-way valves so that I can manifold either around the pump or through it to the engine and also rigged it with a bypass loop out of the pump and into the return line for fuel polishing. I installed a short section of clear hose in the bypass return line just after the pump and when I am polishing there is a very fine but persistent stream of air bubbles in the fuel. I have methodically tested every section of the supply system to and through the pump with a vacuum pump and I am able to pump down the system to 10 or more inches of mercury which holds indefinitely, (both on the filter-mounted guages and on the vacuum pump itself) telling me that there are no air leaks anywhere--nevertheless there are fine bubbles with the pump running. Is it possible that the pumping action of the WALBRO is inducing these bubbles into the bypass return loop and that my supply system is airtight? The engine runs fine but I am concerned. TIA--Michael Oritt Durbeck 48--NAMASTE

Michael, There is only one thing that comes to mind that may be causing the bubbles in you fuel line. When you apply pressure to the system you expand the tubing, and make all the pressurized fittings looser, when you apply vacuum you tend to tighten all these fitting. This difference may be the source of your bubbles. Good hunting - John Harris

Michael, There was an extensive discussion about this very same thing over on the T&T forum a couple of weeks ago. Since I don't have the problem or if I do I don't know it (I have a Walbro pump but no clear tubing so I don't know if I have bubbles or not.) I didn't follow the thread. You may want to go over there and do a search for the subject. I think I did read one post where the possible explanation was cavitation within the Walbro. Good luck. Bill

Michael, >Is it possible that the pumping action of the WALBRO is inducing these >bubbles into the bypass return loop and that my supply system is >airtight? The >engine runs fine but I am concerned. I "temporarily" placed a short length of clear nylon-reinforced vinyl hose ahead of the Racor 1000 fuel polisher filter on the Celestial. I installed it so that it had a loop where bubbles would collect at the top. There was a major stream of bubbles in the newly installed fuel polishing system. Actually, I put hose barbs and hoses at each major fitting, ultimately leaving only the one just ahead of the filter. I found that all of the new "imported" ball valves I bought from Home Depot that I had installed to make up the fuel polisher and to replace some others on the fuel distribution manifold were sucking air in around the valve stems. At that time, I could find no ball valve specifications that showed their capabilities in a vacuum. I emailed Conbraco and asked about their Apollo valves, as a local plumbing store recommended them as the best. Conbraco replied that although their specifications did not (then) show their valve's capabilities in a vacuum, that they would, in fact, work for this application. I replaced all of the new valves with Apollo valves. All leaks disappeared. I left the "temporary" loop in the system as a tool to detect any future air leaks. In answer to your question about the pump being the culprit, many things are possible. You could have an air leak in the pump or a fitting. Also, if you have any water in the fuel, it could be vaporizing under high vacuum. At normal vacuums generated in fuel polishing (1-7in Hg), I don't think you have water vaporizing as a result of the vacuum. And, the vapor should return to water when the vacuum is removed. (When working on a refrigeration system, where it is necessary to remove all forms of water, the normal method is to "pump it down", i.e., place a high vacuum on the system on the system for a few hours and then replace the air/water vapor with refrigerant. Several cycles may be required to remove all of the water in a large system. When pumping down the second and subsequent times, refrigerant recovery systems are used. This "pumping down" is done with a vacuum pump which usually creates a much deeper vacuum than the Walbro pump pulls, unless, possibly when the fuel supply valves are turned off. Still, my Walbro pump can pull almost 30 in Hg. Thirty inches of mercury is as deep a vacuum as most refrigeration service manifolds register.) Although Walbro does not advertise it openly, the pump has a cleanable wire mesh screen in it. To clean it, the end of the pump is removed. Since this screen is on the intake of the pump, it is possible that the seal is leaking air. You did not say if, over time, the fuel level in the filter dropped as a result of air coming in from upstream of the filter (or a leaky filter seal). If so, I would suggest that you also place a clear loop ahead of the filter. If bubbles are present, then insert clear loops further upstream until you find the source of the bubbles. If there are no bubbles ahead of the filter, e.g., the fuel level in the filter does not drop or you don't detect bubbles in the loop ahead of the filter, place a loop between the filter and pump to check the filter's top seal and fittings. Don't despair....it only took me about 3 weeks worth of spare time to track down and correct the air leaks on the Celestial. The aggravating part was that they were all caused by the new valves I installed. Good luck. Take care and be safe. Wayne M/V Celestial Albin43 Sundeck