Frans,
My own experience with vapor lock was nerve wracking. After fighting it
for months, I finally figured it out. The carb bowls were getting hot.
Check your exhaust for a good seal and build a baffle to fit behind and
inside on the rear exhaust down tubes. These will deflect the hot air
away from the carbs. I also fire sleeved all exposed fuel lines, under
the cowl. Worked for me. Zero problems since.
Jeff - Baby Blue
On 7/30/2012 5:58 AM, Frans Veldman wrote:
>
> Hi everyone,
>
> Just returned from a trip to the French Alps.
>
> I have had some worrying experience, so I hope to gain some wisdom from
> the group.
>
> What happened is that during the take off roll from a 4000ft altitude
> field on a hot day, the engine hesistated and didn't develop full power.
> As it was a rather short runway in a valley with absolutely no suitable
> fields in the proximity for an emergency landing I aborted the take off
> (and was happy it happened early in the take off roll).
>
> Associated with the drop of power I saw the fuel pressure drop (note a
> recent discussion where I discussed the importance to have a
> conventional fuel pressure gauge at the top of the panel, as an unusual
> swing of a needle surely catches your attention). A quick check of the
> fuel system didn't indicate any abnormalities. Idle and medium power was
> ok, but at full power the fuel pressure dropped everytime. My conclusion
> was that it had to be vapour lock in some way, so I decided to try
> another day in the morning. Indeed the problem appeared to have solved
> itself and ground runs gave enough confidence to attempt a take off. The
> engine developed full power as it usually does and the 4.5 hour flight
> home was just like normal.
>
> I don't fully understand how this could happen however. I have taken off
> in hotter weather.
>
> My assumption so far is that it happened because of the combination of
> altitude and temperature, and the fact that I refueled the airplane in
> the morning while the take off was in the afternoon and the fuel had
> plenty of time to heat up while the airplane was parked in the sun.
> Another difference was that the floor of the bagage bay was now covered
> with a black piece of carpet instead of the white carpet I normally have.
>
> A few questions here:
> 1) Is there any difference it the type of fuel (AVGAS, MOGAS) regarding
> to the sensitivity to vapour lock?
> 2) Is there anything that can be done about it? (I assume that it must
> have occured at the fuel pump inlet, under the bagage bay).
> 3) My habit is to switch on the second fuel pump while lining up. Maybe
> it is better to switch on the second fuel pump earlier? (This second
> fuel pump is also used for starting the engine, so it had already run.
> After startup I switch fuel pumps, so before lining up both pumps have
> already ran and found to be ok, and should have been "cooled" by the fuel).
> 4) Will a cockpit cover help? (Maybe the answer is obvious, but I'm
> hoping to hear from someone that the very same problem was cured by
> using a cockpit cover).
> 5) Is my conclusion and assumption about the vapour lock correct, or do
> I need to investigate further?
>
> Thanks,
> Frans
>
>
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>
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