912 - low tank - higher engine hot start problem

[rans6andrew]

A friend with a 912 powered light aircraft is having a lot of problem with hot starting. If he warms the engine up, flies for a while and then calls into an airfield to pick up a passenger he struggles to restart the engine unless it is well cooled, ie the stop has been much much more than 10 - 15 minutes. The engine is in the nose and the fuel tanks are low down, in the fuselage, behind the seats.. The normal fuel return orifice and pipework is used and the aircraft has an electric fuel pump in series with the 912 mechanical pump. We think there is some sort of vapour lock situation occurring although he waits until the electric fuel pump brings the fuel pressure up to the normal figure before trying to start. I suspect that the electric pump would be better mounted closer to the tank rather than part way up the firewall ........

So, do any of you Europa / Eurostar / Zodiac / Pioneer / Sportcruiser / Dynamic flyers see the same issue?

Have any of you suffered in the past and found solution? Do tell.

We have compared his fuel plumbing with my high wing 912 powered aircraft plumbing and found little difference. Fuel return orifice at the highest point in the system, post pumps and just ahead of the carbs above the centre of the engine. Fuel pipes sleeved with insulation wherever they pass near to exhaust pipes etc. I don't have an electric pump as my high tanks keep the system primed. We are using super unleaded from the same outlet. We have then been and flown the same mission profiles, at the same time and my engine always fires up at the first press of the button.

Any insight would be welcomed.

Ta,

Rans6............

[mikehallam]

Hot restarts on my and a colleague's 912 powered a/c has been happennig fairly readily this hot season.
One 'solution' is to crank vigorously at WOT till it picks up then quickly throlle dttln to sensible rpm.
Usually works even if not quite orthodox.

mike hallam

[patowalker]

I have never had a hot restart problem with my Eurostar. This may be because my fuel lines are sleeved for fire (and heat) protection.

[Lascaille]

In general poor hot starting is associated with too much fuel, not too little.

The engine can run off the fuel in the carb bowls for a little while, and they should be full. If vapor lock was the issue I'd expect the engine to fire up, run for a while then stop.

Try shutting the engine down by cutting the fuel and letting it stop with the ignition still on. Then give it a minute or two, turn the fuel back on and go for an immediate start. If it starts up under those conditions I'd suspect that expanding fuel (due to heat) in the lines is being forced into the carb and overflowing the bowl and flooding the engine.

[Jan Olieslagers]

Starting a 912 warm - that is to say, if it has stood less than a couple of hours since thoroughly warm - is always a bit delicate. I was taught to open the throttle 5-10 mm (which obviously varies with the installation) and that has always worked for me. My own installation is quite similar to the S6's, tanks in the high wing, no electrical pump, only I do not have a return line. ISTR Rotax made them mandatory a year or two ago.

Lascaille brought me some new insights, I had never thought much about fuel expanding in the tubing. Perhaps not such a good idea to close the fuel valve right after shutdown?

[patowalker]

To add to my previous reply. I have built three of the aircraft mentioned, Zenair, SportCruiser and Eurostar. All have had vapour return lines and sleeved fuel lines. None have given me trouble with hot starts.

[Lascaille]

Originally Posted by Jan Olieslagers

Perhaps not such a good idea to close the fuel valve right after shutdown?

If you're getting easier starts when opening the throttle that means your starting mixture is too rich.

Opening the throttle lets in more air, but the airflow during cranking is 'jerky' so the carb usually under-delivers, leaning out the mixture.

Does it start cold without lots of choke? If it does, possibly the starting mixture is always too rich. Verify that the choke can close fully.

It's also possible for fuel to expand within the fuel lines and carb bowl, overflowing the bowl and flooding the carb. The exhaust during cranking will smell strongly of fuel then very sweet initially when running.

The quickest way to prove whether this is happening is to stop the engine by closing the fuel valve, then letting the engine die with the ignition still in the run position. This will empty the carb bowls and the fuel lines supplying them. When you are ready to restart, open the fuel valve and promptly start cranking the engine.

[Colibri49]

Hopefully your friend's aircraft is fitted with a separate cranking starter button and independent "mag" switches. Not just an ACS key switch with Off - R - L - Both - Start positions. The addition of a separate cranking starter button gives useful options.

When the engine is anything from slightly warm to hot, switch on the booster pump and leave it running. Crack the throttle 5 - 10 mm, as Jan suggests. Press the cranking button and let the prop get going for 1 to 2 seconds, then flick on both "mags" while still cranking.

Works every time for me!

[Lascaille]

Originally Posted by Colibri49

When the engine is anything from slightly warm to hot, switch on the booster pump and leave it running. Crack the throttle 5 - 10 mm, as Jan suggests. Press the cranking button and let the prop get going for 1 to 2 seconds, then flick on both "mags" while still cranking.

Works every time for me!

If you use this technique and have a muffler/silencer there is a small chance that it will explode.

But this may also occur after any period of time spent unsuccessfully cranking an engine with a healthy fuel supply.

💁

[patowalker]

The 3 low-wings I mentioned all have ACS switches. :-)

[Colibri49]

Originally Posted by Lascaille

If you use this technique and have a muffler/silencer there is a small chance that it will explode.

But this may also occur after any period of time spent unsuccessfully cranking an engine with a healthy fuel supply.

��

That would have to be an extremely small chance, because many others have used this technique for years without such a consequence. It's recommended by a recognised UK Rotax engine expert. Only 1 or 2 seconds of cranking before flicking the "mag" switches to ON isn't long enough to fill the muffler with fuel vapour, or by now there would have been reports of explosion.

This technique is especially helpful for starting high compression 912 ULS engines. I never had starting difficulties with the 912 UL in the same installation and cowls.

[ivorPhillips]

I know this doesn’t strictly apply but i have a 914 that will not start hot without a lot of hassle,
what i do now is before shutting down i allow the engine to idle for a minute then shut of both fuel pumps
down and allow engine to run until it falters then switch of the mags,
on a restart don’t touch the fuel pump switches, just start her, she will run a bit lumpily then flick the pump on then off then repeat as necessary until she runs smooth, this has worked with 40 C temps in spain, leaving fuel in the carb bowls above a hot exhaust is a recipe for localised vapour lock, but with empty bowls she will start fine,
obviously you cannot shut down the mechanical pump on the 912 so shut off at the fuel cock until she stops, my two cents worth,

[Colibri49]

Originally Posted by patowalker

The 3 low-wings I mentioned all have ACS switches. :-)

I retained my ACS switch, but simply added a push button for cranking and connected to the ACS by a couple of lengths of wire. Think hard about where to install the button on the panel (or elsewhere) because one hand is needed to push the button and another is needed to switch the ACS to BOTH within 2 seconds while still cranking the starter.

Retention of the ACS switch is a good idea, because a similar technique can be used for cold starts. Booster ON and remain ON. Throttle closed. Turn ACS key to START, then pull the choke within 2 seconds. If the engine fails to get going properly, don't use choke again but employ the hot start method, with booster ON and throttle cracked.

A separate cranking button is also good for simple (non-leak down) compression checks and pre-oiling before start if needed, or quickly circulating oil after an oil filter change.

[Gault]

My 912 has always been a bugger to start if left for about 10 minutes after shut down

Leave it a couple of minutes, fine it starts straight away....30 minutes +, fine starts great...leave it for 10 minutes and it is very reluctant, I have a vapour return

All the 912s at my field are the same.

[horizon flyer]

This is classic fuel expanding in the carb and flooding the intake. Happens on Injected IO360 Ltcomings as well also older Lotus Esprit/ and other Delorto/Weber carbed vehicles.
Mentioned in Weber carb manuals of the 1960s as well. Happens about 10 to 20 min after shut down. The Weber recommendation is
slowly open the throttle just in case there is an accelerator pump to enrich mixture on throttle opening. Then with throttle wide open
crank till it catches then close to idle.

So the best may be to empty the carb of fuel as mentioned when closing down, carb will need to filled be for start.

[muffin]

We used to have this problem very occasionally on our Eurostar. We always now leave the oil level inspection door open while parked in hot weather, and on occasion have removed the whole front cowl cover so the hot air does not collect in the engine compartment. This seems to prevent the problem.

[Flap40]

The Europa build manual for the 912 (available online) shows the restrictor as being in the return line AFTER the carbs.