Vapor Lock?

[woodsideraff]

Had an "interesting" experience today. After a 20 minute flight to a neighboring airport, I stopped for about 1/2 hour. When I started up again, I noticed it was harder than usual to get the engine to run smooth. After a couple of attempts, it cooperated and I then taxied for about 5 minutes to the active runway.

Runup was normal. However, after applying full power fo takeoff, the engine sputtered, still running, but with almost no power. Electric fuel pump was on.

Having over 5,000 feet of runway left, I reduced power, swithced tanks, and did another run-up. Everything was normal. I flew the airplane back, climibing from 500 to 5500 ft.

Once back home, I tried another runup, both tanks. Everything looked OK. Full throttle application and smooth acceleration.

The plane is an RV8-A with a carburated O-360; about 100 hours total time and no trouble history. The tempreature on the ground was in the low 90's. I'm burning auto fuel which is available at my home base (KBNL).

I have read all the posts I found on the subject. Any comments and suggestions would be appreciated.

Regards,
RL

[Kyle Boatright]

I'd bet my last buck on vapor lock. You had at least 3 things working against you today.

-You were running mogas which has a higher vapor pressure than 100LL.

-It was a hot day outside. Your fuel was warm even before it entered the engine compartment.

-You did a quick turn around. Your fuel lines, gascolator (if you have one), fuel pump, and everything else under there were heat soaked, and warmed your fuel while the airplane was parked.

Things which can help are either faster (5 minute) or slower (1 hour) turnarounds. Also, leaving the oil filler door open may help a bit. Even better, if there's a breeze, make sure to point the airplane into it to get some cooling airflow. Finally, running 100LL will help.

[Rick_A]

I installed a purge line on my O360 just to avoid this issue. From the carb I ran a line to an electric valve (I used a primer solenoid) and ran the output back to one of my fuel tanks. This was relatively easy to do while the plane was being built - it would be tougher to on a flying plane.

I can definitely see the benefit on a hot re-start. On a hot re-start, the fuel flow is erratic until I open the purge valve (i.e. I don't get a notable slow down on the facet pump). After I purge the line, the pump slows down to 1 - 2 gph, which tells me I have good fuel pressure.

BTW. Credit for this idea goes to my brother Walt who did this on his plane after it had been flying a while.