Bob,
The problem you describe is common this time of year for the Rotax 912UL/ULS burning only MOGAS, especially before local gas stations switch to summer-blend gasoline. And even then, hot summer days and heat-soaked fuel can still present a problem. The mechanical fuel pump absorbs heat from the gear-reduction drive and typically does not get much (if any) cooling while in flight. Heat soak after landing is a real problem.
Other responders have given you good advice, but I'd like to go into a little more detail and give you an expanded checklist that might help eliminate the problem.
1. Between April 1 and June 15 with ambient temperature >80 deg F, use at least a 50-50 mix of 100LL and MOGAS.
Here’s why. Gasoline volatility is measured by Reid Vapor Pressure (RVP). The higher the RVP, the more volatile the gasoline. In Texas, winter-blend gasoline has a typical RVP of 10.0 psi. Summer blend varies from 7.8 to 9.0 psi depending on county. In contrast, 100LL AVGAS has a constant RVP of 6.5 psi. Thus, a 50-50 mix of winter-blend gasoline (10.0 psi RVP) and AVGAS (6.5 RVP) yields a fuel with an RVP of 8.25. This is just under the average of summer-blends throughout Texas (8.4). But that still might not be low enough to stop vapor lock on a hot spring day with heat-soaked fuel.
To achieve fuel with 7.7 RVP requires a 75-25 100LL-MOGAS mix (3 gal AVGAS per 1 gal MOGAS). So depending on where you buy your MOGAS and the ambient temperature, you might be better off using all 100LL until the switch to summer blend takes place.
- With the extended use of AVGAS, remember to reduce the oil-change interval to 25 hours.
- This year (2019), refiners must start blending summer fuel no later than May 1, but retailers are not required to start selling it until June 15. https://www.eia.gov/todayinenergy/detail.php?id=11031
2. Before engine start, turn on the electric fuel pump and ensure positive and steady fuel pressure (usually 4.0 to 4.5 psi).
- If you don’t have a fuel pressure gauge and you’ve got the usual Facet Cube fuel pump, you will hear the pump sound change from a sharp clicking to a muted clicking when the pump fills with liquid. If you can hear the pump running but it won’t produce pressure, try switching fuel tanks. (Temp variation between tanks is the only explanation I can think of for why this works sometimes.)
- Ensuring positive fuel pressure before start is critical for all restarts after the first flight when the engine has not had a chance to cool down. On hot days and after sitting in the sun, it may not be possible to get winter-blend fuel cool enough to safely operate the engine.
- If you can’t get positive fuel pressure in the normal range before engine start, drain the MOGAS and add 100LL until you’ve got at least a 50-50 mix and try again.
- There will be a strong temptation to start the engine anyway and try to prime the electric pump with the mechanical pump. But if the fuel temp is so high that the electric pump will not prime by itself, if the mechanical pump fails the electric pump may be of no use.
3. With electric pump on and fuel pressure in the normal range, start the engine.
4. While idling between 2,000 and 2,500 rpm, turn off electric pump and observe fuel pressure. If the mechanical pump cannot maintain normal pressure (2.2 psi to 5.8 psi), shut down and increase the ratio of 100LL to MOGAS.
5. After the ignition check at 4,000 rpm and before takeoff, increase to max power and observe fuel pressure in the normal range for 30 seconds with a smooth-running engine.
- It takes about 30 sec to deplete both float bowls at max power. (assuming 6.9 gph fuel flow and 3.3 oz in each float bowl). (2 carbs×(3.3 oz)/carb×(1 gal)/(128 oz)×(1 hr)/(6.9 gal)×(3600 sec)/(1 hr)=27 sec)
6. During takeoff roll, include fuel pressure in your crosscheck. If fuel pressure drops below 2.2 psi, abort the takeoff. Increase the 100LL to MOGAS ratio or go to straight 100LL.
- Don’t assume fuel pressure will recover or that a power reduction will get it back. It’s just not worth the gamble.
If anyone sees anything I've missed or messed up, please jump in.
