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Date: Sun, 21 Nov 2004 21:03:55 -0600
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Bubbles in fuel line from pump
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Transparent lines on ALL "suction" lines (upstream of firewall) would be 
edifying.  Watch the bubbles as they appear and if/when they increase.  
Might try a pump just downstream of the selector valve after you've 
studied the bubbles as outlined above.


Finn Lassen wrote:

> I'm near wit's end trying to find out why I get bubbles in the line 
> from my facet pump to my carburators.
>
> To recap my setup:
> Fuselage fuel tank -> fuel selector valve -> fluel flow sensor -> G3 
> automotive paper filter -> to drain point -> T-split  to each fuel 
> pump. Separate lines from each fuel pump to carbs. Three carbs. The 
> two outer carbs (sharing one pump) each feed a secondary port. The 
> center carb, with its own pump, feeds the two primary ports. One 
> throttle to the outer carbs, one throttle to the center carb. When 
> have climbed above pattern altitude I turn off the pump to the center 
> carb, ready to reduce center carb throttle when all fuel has been 
> burned out of the bowl, and use center carb throttle to lean mixture. 
> Output from pumps near fuselage bottom slopes up to carbs which are 
> appox level with center of fuselage tank.
>
> Bottom of fuselage tank about 16 inches above the facet fuel pumps.
> When removing the quick-drain valve, the system gravity feeds about 
> 17-19 GPH through the selector valve, flow sensor and filter.
>
> Various observations:
> Idle fuel pressure on ground about 3.8 - 4 psi.
> Max power fuel flow near sea level at full power only about 13.5 GPH. 
> 10 GPH at altitude.
> Fuel pressure to outer carbs is roughly inversely proportional with 
> observed amount of gasses (air?) in the line from the pump to the 
> outer carbs.
> Engine wants to quit when pressure drops to about .1 or .2 psi. (Only 
> happened one time after a continous full power climb to 10,000' to get 
> over clouds. Pressure dropped steadily to that point.)
> Removed flow sensor and filter and amount of bubbles appeared to be 
> significantly less.
> Put sensor back in, and bubbles reappeared, inconsistently.
> Have mostly observed fuel pressure drop at high power climb attitude. 
> Seems to get worse at higher altitudes.
> Have checked tank vent (used to be shared between the three tanks, but 
> as part of debugging now goes only to fuselage tank). Vent is pointed 
> forward, mounted below fuselage, center, just forward of main wing spar.
> On the last test flight today, I continued to see bubbles, even after 
> reducing power for several minutes and levelling off. Were at 7,500'. 
> Pressure about 1 psi. After a dive to get down below Class B 6,000' 
> floor, pressure came back up to above 3 psi.
> A few minutes after landing I noticed that the fuel line to outer 
> carbs appeared empty. Turning on the pump for a few seconds filled it 
> up. After turning off the pump I noticed that the line emptied out. 
> How can this be? By gravity feed alone, the fuel level should have 
> been a couple of feet up that line. I did not delect any gas leak.
> Maybe 10 minutes later, the line stayed full after turning on and off 
> the pump (or was now full - not sure).
>
> This brings me to questions about vapor lock. I actually don't know 
> what that is. Carb bowls are vented into airbox. So how could the fuel 
> in the bowls be under pressure, maybe boiling, and push gasses back 
> down the fuel line to the pump? Unlikely? I do have a SS heat shield 
> between carbs and exhaust headers. Does its ability to reflect heat 
> lessen if it is no longer shiny?
>
> I guess the next step is to replace all rubber lines going to the 
> pumps with transparent ones so I can see what's going on the the lines 
> to the pumps.
>
> But other ideas are certainly welcome. Up till the last observation, 
> I've been trying to locate air leaks in lines to pumps and was ready 
> to replace the brand new facet pump with an old one, in case it 
> somehow is sucking air.
> Now I'm mystified.
>
> Finn (please no jokes about wit).
>
>