J

jtdehaan

Member
How does the venting for the fuel tank work? The cap is sealed as proved by the balloon test. The other day I pumped the tank dry to fix a leak and the tank started to collapse with the fuel cap on. Does the venting only work with the engine running? Inquiring minds would like to know.

Jake
 
jtdehaan said:
How does the venting for the fuel tank work? The cap is sealed as proved by the balloon test. The other day I pumped the tank dry to fix a leak and the tank started to collapse with the fuel cap on. Does the venting only work with the engine running? Inquiring minds would like to know.

Jake
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The fuel tank venting only works if you do the construction steps for adding the vent hole on DWG page 37-7 that come right after testing the fuel tank for leaks.
 
The tank vent is a 1/4 " line that goes from the outboard end of the tank to a connector on the inboard end of the tank. The line continues into the cockpit and ending as another fitting on the belly just aft of the firewall. It is free flowing and does not need the engine running to work. As pressure / vacuum in the tank changes, air goes in and out of the fuselage end and relieves the vacuum/pressure being produced in the tank. The first thing that I would check is if you have a plugged vent line. Bugs have been known to make a home in the end of the vent line under the fuselage
 
Drill holes in gas cap

Jake,
Like Scott said, you must have missed the step in the plans that say to drill holes in the gas cap. Jim Kinsey must be talking about the RV-7 which is different than the RV-12.
Joe
 
Joe,

You are right, I was talking about the RV7. I thought I saw in Jake's bio that he was building a RV9 not a RV12. My mistake!!!
 
Though there is no scientific evidence (from the SLSA) to verify this, myself and others actually put three vent holes in the cap (after certification) because we couldn't get adequate fuel pressure. Now all is good so either the Dynon had a bad cold or the three hole karma set things straight. Obviously I would try the single hole first.
 
On the 12 I don't see the 1/4 line as a vent, but rather a return line for excess fuel to the tank. The only vent is the hole(s) you put in the fuel cap unless I am missing something here.
Dick Seiders
 
pluged vent

What happens if one of the lower vents becomes totally pluged? I see drop in fuel pressure until the engine quits. Or the fuel tank is crushed from the vacuum.
 
burgundyja said:
What happens if one of the lower vents becomes totally pluged? I see drop in fuel pressure until the engine quits. Or the fuel tank is crushed from the vacuum.
Click to expand...

Reiterating.....The vent on the RV-12 is different than the other RV's. The gas cap has a hole(s) drilled into it. The hole(s) in the cap are the tank vent. The fuel return line is just that. It allows a small amount of fuel to continue to circulate from the tank to the engine fuel inlet, downstream of the gascolator and back to the tank. The fuel return line is NOT a vent line.

For one thing, the constant fuel return line flow keeps the fuel in the gascolator at the temperature in the fuel tank and not the temperature in the engine compartment. An RV-9A builder here has ram air directed at the gascolator to keep the fuel in the gascolator cool. There is no need for this in the RV-12.
 
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