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Starting Anomaly

todehnal

Well Known Member
I am now approaching 70 hours on my RV-12, and since I bought the firewall forward kit early in the build; fall of 2010, the battery is now 4 years old. I always pull a few blades though before starting, which is an old habit from my round engine Cessna 195 days. Hey, old habits are hard to break! Anyway, I am experiencing an engine rotation stall, as the first piston approaches TDC. I immediately release the starter, then hit it again. It always goes on the second try, but I was wondering if anyone else is experiencing this, or if I need to do a load test on the battery. I do have soft start installed, and use ignition "A" only until I get a start. When pulling the prop though by hand, I have detected an increase in compression as the engine breaks in, which is perhaps why I didn't experience this stall on start-up until recently. I even tried leaving a battery tender on it until I am ready to fly, which did not help. Everything else appears normal, and I continue to see 13.8 volts during flight. I hesitate to start a systematic troubleshooting routine, if others are experiencing a similar stall on start-ups. Any thought out there??

Tom
 
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When the kit is of 2000 the battery is 14 years, but guess you mean 2010
I am pretty sure it is the battery.
It cannot deliver high currents any more due to degradation, which is typical for long time unused batteries.
Measure the voltage when the starter is running.
 
Thanks Jack. Yeah, I corrected my post to state 2010. I will check the voltage during start up, and I may go ahead and get a load test done on the battery at annual time, which is in October. We have a local shop that does it free. I was just curious if anyone else experienced a similar stall on start-up. Also, we head for Florida in December for three months, and if I need to replace it, I will wait until spring to make the swap...........Tom
 
So one thing to keep in mind. The 912S does not like to have a weak battery. It's high compression engine and I suspect you are experiencing what the Rotax guys call "kick back" which if you let happen too much you get the joy of replacing your Sprague Clutch. Which is a costly experience. I know because I had to do it on my 912S in my Kitfox. I had the soft start which was supposed to solve that problem, but over time it got worse and started kicking back more and more even though my battery was new. I finally got the clutch replaced, had a new battery, and replaced both A&B ignitions to the new ones that have the start retard timing built in so no more soft start needed.

With all that, it starts great now!
 
I thought it was standard procedure for Rotax to pull the prop through until it "gurgles" and then check the oil level before starting if the engine has been off for 30 minutes or more. Pulling a few blades might be asking for extra oil in the crankcase that could lead to more resistance to turning over and maybe hydraulic lock?
 
You don't need to burp after an idle time of thirty minutes. It is done only to accurately check the oil, not for lock. Easier to burp after the last flight of the day and check oil level. If no puddle underneath the next time you fly, should be good.
 
So you don't check the oil during your pre-flight inspection? Hmmm....

You don't need to burp after an idle time of thirty minutes. It is done only to accurately check the oil, not for lock. Easier to burp after the last flight of the day and check oil level. If no puddle underneath the next time you fly, should be good.
 
So you don't check the oil during your pre-flight inspection? Hmmm....

No need if you checked it last flight. It is much easier and faster to get the oil back in the oil tank when it is hot rather than cold. Fairly common practice for Rotax owners. ;)
 
Post flight check

It's harder to pull the prop through when the engine is hot, but it takes only 6-10 blades to burp the engine instead of 20-30. As stated above, if there's no puddle on the floor, it has to be all in the engine next flight.

Wayne 120241/143WM
 
I used to pull that prop like winding a rubber band and pulled 30 blades. Then at the ROTAX service course the instructor showed us that if you slowly pull through the prop stopping at each compression to allow the cylinder leakage to push the oil back to the tank you will get a gurgle in 3 revolutions.
 
The 912 series engine does not use an oil scavenge pump much like almost every other dry sump design uses. Instead the 912 uses crankcase pressure to force the oil through the banjo fitting in the bottom of the case and back into the oil tank, for this reason the engine does not use a breather attached to the crank case, but instead has the breather coming from the oil tank.

Because there is no scavenge oil pump, turning the propeller quickly will not force the oil back to the tank any faster than a slow steady rotation.

Some 912 pusher installations mount the oil tank on top of the wing in front of the engine which puts the oil tank much higher than the crankcase drain. These installations require many more crankshaft revolutions to force all of the oil back to the tank and get an accurate oil level check.
 
Mike,

Absolutely correct! The point the instructor was making was that if you pull through quickly most of the compressed air goes out the exhaust valve but if you pause the air leaks past the rings to pressurize the crankcase and you get the gurgle in just a few revolutions.

Rich
 
I sure hope that your compressed air isn't going out your exhaust valve - the exhaust valve doesn't open until after the piston has traveled to BDC, thereby relieving all of the pressure built during the compression stroke. I do agree that if you pause near TDC on the compression stroke that you get the most crankcase pressure.
 
That's exactly the point. If you just pull the prop through without pausing like many do then not mulch air leaks past the rings into the crankcase on each piston stroke. It just goes out the exhaust valve. Pausing allows the air to leak past the rings and pressurize the crankcase. End result is you get the gurgle in about 3 revolutions versus 15.
 
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