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Thought I would share this with everyone since just about everyone is
getting some of this heat wave and most of us aren't used to it.
Also, it sounds very similar to what Kevin is putting up with in his
RV-6A.
Interesting experience this last Saturday in my RV-4. Interesting
because it didn't happen at 800 feet where it would have probably made
the news but while I was just starting my takeoff roll.
Conditions:
90 deg F, about noon on Saturday
150 HP Lyc O-320 with carb, gascolator, mechanical and Facet electric
pumps.
Running last winter's last few gallons of auto-gas from my storage
tanks.
Engine was all warmed up from three laps around the pattern shooting
landings at Hillsboro. I had taxied back to the hanger to check for
anything out of the ordinary since I had recently been working on my
carb. Everything was okay so I headed back out.
While setting at an intersection for about 5 minutes waiting to be
cleared for takeoff my CHTs were heading for hotter than I like.
The wind was at my tail so the engine was not getting any cooling.
CHT was passing 335 on the Electronics International UBG-16 engine
monitor and I was about to call the tower and tell them I was heading
back. The controller beat me to it and cleared me for
takeoff. This airplane normally runs about 325 at power and I have
to have a hot day to get it to 350 but I never see it that hot prior to
takeoff.
Okay, I thought, a little air through the cowl and we'll be back to
normal. So I answered the controller "6RV cleared for
TO", lined up and pushed in the throttle. I got almost 2000
rpm and just enough of a push to make it to the next intersection and
coast off the runway. It was as if I had pulled the mixture.
The prop had stopped before I stopped rolling, I was looking at
everything thinking I had missed something like the fuel valve half
cocked or left the mixture mostly out (I taxi that way). Feeling
stupid I called the tower and said 6RV was NOT taking off.
After pushing it across the runway threshold I turned the electric pump
on and it was obviously empty from the noise it was making. I was
on the tank that was 95% full so I switched to the other tank and
viola. I recognized the sounds of the pump filling the apparently
empty fuel system and carb. Hopped in and after about 6 blades it
lit and ran fine. I called the tower and taxied back to the
hanger. Thankful this had not happened at about 800 feet leaving
the 5 o'clock news empty handed.
Pulling the cowling, the carb and fuel pump were so hot you couldn't hold
your hand on them. The gas had boiled enough that it pushed it back
through the mechanical pump, gascolator, Facet pump, and tank selector
valve. When I switched to the "cold" tank out on the
intersection the Facet could get hold of enough at that point to push
fuel back to the engine. From this tank the fuel was cold enough I
could get it started and taxi back to the hanger.
-->KEVIN:
I know you run 100LL but at the temps you are describing, upwards of 400
degrees, you can vapor lock 100LL, too. I think Dave's right, it
could be that you are getting some vapor generation on the ground when
you're trying to run at full power. You mentioned the problem only
happens when the engine is warmed up.
I'm going to try and duplicate last Saturday's scenario this weekend
(except the takeoff attempt) if the OAT is up enough.
Bear in mind that since I have never had any problems like this I never
saw any need for blast tubes on the fuel pump and carb like some people
have done. The cowling on the RV is pretty tight and I have four
exhaust pipes down there next to the carb, pump, and gascolator.
And since I have thousands of hours running auto gas in various planes,
while the extra volatility certainly contributed I don't believe it's bad
stuff and will continue to use it. I may keep one tank with mostly
100LL in it for ground ops on the very hot days for extra margin but
there were more things than fuel type that contributed to what
happened.
Obviously a fuel return line that would allow purging the hot fuel would
be another fix. We have another RV-4 on the field that has a vapor
bleed return line for just this reason.
Well, sorry for the epistle but I thought it was something for people to
keep in mind while your waiting in a long taxi line at a busy
airport. Especially those of us who rarely see 90 degree ramp
temps. It was a real eye opener (and I've only got one
eye..) P-)
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
At 20:50 2005-08-03, you wrote:
Kevin, I agree with Ken Powell
that it still sounds mostly like fuel starvation, probably in the
carb. So I won't beat that horse... However, if following
those suggestions doesn't fix it, then consider:
Since you have definitely identified temperature as a culprit, that
points at 2 possible causes: fuel vaporization and detonation.
Vaporized fuel going to the carb could cause a scenario where the bowl
cant fill. Also, detonation could behave like what you
describe. Maybe you cooling flow is no sufficient.
Doubtful, but a bad could maybe act that way. It's easy enough to
test that as a possibility.
--
Dave Leonard
Turbo Rotary RV-6 N4VY
http://members.aol.com/_ht_a/rotaryroster/index.html
http://members.aol.com/vp4skydoc/index.html
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