Air in Fuel Line

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

Bubbles are not good. Are you seeing the bubbles in the clear line prior to the fuel exiting the line?
My wild guess would be the pump is working harder when the bucket is high, and sucking air from the harder work, not working hard when going down hill, not sucking air.

Yeah, Don, you can see bubbles right where clear line is attached to gascolator. Sometimes they are tiny and only show up at certain points on the line. I'm hoping someone can explain this science...

Let fuel circulate for awhile

Jim,
How long has the new pump been circulating fuel? After having the system apart, it will take a while to get all of the air out.
It seems that you are making progress and there is a lot less air with the new pump. A trick used to find suction leaks is to apply a heavy oil or grease to the outside of the fittings on the low pressure side of the pump. Do one fitting at a time while watching for air bubbles in the fuel.
Joe

Joe, Done that. Fuel lube on all fittings, at collars too. Runs almost 100% clear at carb height. Just want to understand what's going on...

Jim,

Are you thinking that you have a vapor lock situation? Those are usually the result of fuel vaporizing because of heat reducing the pressure that feeds the carbs from the pump. It is notorious with auto gas and NASCAR is the usual victim but not no much with AVGas. You can boil autogas at lower temps if the pressure is lower as well. Just doesn't sound like vapor lock because it hasn't been that hot yet.
I am assuming that you are still concerned about your engine out situation in the air. The other possibility of course is that the carbs are getting flooded with too much fuel. Having just taken my carbs apart at 200 hours there are many jets/clips etc that manage the fuel/air mixture in your carbs. There are basically four different settings avail...two for average altitudes, one extremely high, the other extremely low. An incorrect mixture setting would most certainly risk a possible flooding situation.

Pete

Overrich was something we looked into. I think the very low fuel pressure/high fuel flow data (thank you Dynon) associated with the power loss got us thinking vapor lock. That and the fact that aberrations were only on hot engine, albeit at full power--which to my experience isn't the vapor lock scenario with which I am familiar in cars. The ramp temperature when this last happened was near 100F.

Jim