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