In the course of trouble-shooting a power loss problem, I was rechecking fuel flow per PAP and found the rate of flow had actually improved since initial testing a few months ago. Since I was using a clear plastic line to exit fuel from the gascolator outlet via the push of the boost pump, I noticed a good number of air bubbles in the fuel flow. Others suggested that air in the line might have precipitated vapor lock development of such magnitude as to overwhelm the capacity of the vent lines in the carburetors.
After draining the tank, I checked each of the components of the fuel line by various means, including soap bubbles and dye penetrant. It was determined that the Facet pump was producing some air bubbles in operation. Since an identical unit in an RV under construction did not, the pump was replaced with one which produced a clear stream.
Here's where my question to you comes... Reassembled, the fuel system (boost on) produces nearly no bubbles at all when the submerged drain line and gas can are held level with the carbs. When placed on the floor, substantially more bubbles can be seen (line still submerged). The line is carefully secured to the gascolator and I do not believe air can be sucked into the plastic line there. (Via venturi effect?)
It was suggested that putting the can on the floor would reduce the pressure in the line, thus allowing the formation of air bubbles. But if the line is secure to the tank, where does the air come from? Can bubbles be generated by the fast-moving fuel becoming turbulent going over the hump in the line just aft of the shut-off? Can the impeller in the flowscan churn up a froth? Does the presence of a few air bubbles in the line presage a potential vapor lock event? Moreover, can a "closed" fuel system generate air bubbles?
I would be grateful for any guidance on this issue, whether from scientific theory or operational experience.
Jim in South Texas
RV-12 #264
22 hrs
After draining the tank, I checked each of the components of the fuel line by various means, including soap bubbles and dye penetrant. It was determined that the Facet pump was producing some air bubbles in operation. Since an identical unit in an RV under construction did not, the pump was replaced with one which produced a clear stream.
Here's where my question to you comes... Reassembled, the fuel system (boost on) produces nearly no bubbles at all when the submerged drain line and gas can are held level with the carbs. When placed on the floor, substantially more bubbles can be seen (line still submerged). The line is carefully secured to the gascolator and I do not believe air can be sucked into the plastic line there. (Via venturi effect?)
It was suggested that putting the can on the floor would reduce the pressure in the line, thus allowing the formation of air bubbles. But if the line is secure to the tank, where does the air come from? Can bubbles be generated by the fast-moving fuel becoming turbulent going over the hump in the line just aft of the shut-off? Can the impeller in the flowscan churn up a froth? Does the presence of a few air bubbles in the line presage a potential vapor lock event? Moreover, can a "closed" fuel system generate air bubbles?
I would be grateful for any guidance on this issue, whether from scientific theory or operational experience.
Jim in South Texas
RV-12 #264
22 hrs