912 starting

[MotReklaw]

I have a friend who has a 912 ULS powered Zodiac at our airport. He is having a problem getting the engine to start. The starter spins the engine, but no light off. There are no Rotax Mechs in our area. I had two 912s previously and tried to help him get the engine to crank and run. Here's what I did and maybe you've been there, done that and can offer a hint. The engine would sometimes start but run so rough that it sounded like it was coming off the firewall.

1. I check to see that there was fuel getting to the engine:
a. pulled the fuel line at the mech fuel pump and ran the facet fuel pump. No fuel came out the line.
b. Checked to see if the enricher valves were closing when the knob was pulled. They were, based on the movement of the leverage.

c. hooked up an external gravity fed fuel tank/line to the fuel pump and cranked the engine but still, no joy. Engine "almost" started one time but severe vibration.

2. removed a spark plug and turned the engine over using the starter. There was a spark at the plug.

3. The spark plug showed moisture, indicating that fuel was reaching the cylinder.

I initially thought the problem was the enricher circuit, but based on what I saw, it is not that.

The owner told me that on the occasions that he was able to get the engine to start that it ran extremely rough for several minutes, then as the engine warmed it smoothed out. He said that if the engine ran 10 minutes he could shut the engine down and it would crank up immediately and run smoothly.

I think that it is a carburetor/fuel issue, but I'm not sure.

Any suggestions?

Thanks,

[foxkit3]

I would take a look at the carb boots. They will split and cause a lean condition.

[DesertFox4]

Tommy, how many hours on the engine or is it a new engine/ new install? Was it running properly and this problem just developed or a gradual degradation over time?



Has a pneumatic synchronization of the carbs. been performed lately or at all ?

[MotReklaw]

Foxkit3, I checked the boots and there did not appear to be any cracks. They were replaced about 2-3 years ago during an annual inspection.

DesertFox4, The engine has almost 500 hours since new. It sat for a year down in Sebring, FL undergoing the Zenith wing update, and the owner told me he had to get help to get it started down there, getting a jet ski mechanic to work on the carbs. He said the mechanic synchronized the engines. If you know anything about the Zenith system, it is a rube goldberg type of cabling and rod system (my opinion). It is very easy to get the carbs out of sync just due to changes in the weather and the cables stretching. Anyway, he then flew the plane non-stop from Sebring to Anniston. He's had trouble starting it ever since. The nearest Rotax mechanic I'm aware of is in Lucedale, MS, Ronnie Smith. But the owner has to get the engine running to get it there.

I never had this problem with the two 912s I owned. They were great engines. and ran flawlessly.

I really think there is air getting into the system somewhere. The engine has the carb airbox from Rotax, so it is a little hard to get to the carburetors to check much other than the linkage.

I would think that on a non-running Rotax the only synchronization to the carbs would be a mechanical sync, right?

Any advice appreciated.

Thanks,

[DesertFox4]

Tommy, a mechanical sync is a starting point if no problems are apparent with the boots (a little difficult to discern without removing from the manifolds), or box and the enrich circuits are returning to completely off position. Also make sure the low speed screws on the bottom of the carbs are both set to somewhere between 1 1/4 to 1 1/2 turns from fully closed.
Again a base starting point to proceed from.


After sitting that long just consider the degradation of the fuel if it was auto gas. If it is 100LL Av-gas it should still be useable.

[kmach]

Hi,
I don't know if you have tried using a different battery. I would try boosting or a different fully charged battery .
If you don't get the proper rpms on hitting the starter, the ignition system won't provide the right spark. I think it needs to get to 250 rpm.

If it is slightly lower than the required , the prop turning can look like normal , but there will be starting problems. The battery is easy to try and is the first thing that came to mind when reading your problem.

You could try looking on the website www.rotax-owner.com under forums , lots of solutions can be found there. Good Luck,
Kevin

[kmach]

Hi ,
After re-reading the original post, I believe this aircraft has sat awhile, is this correct ?
If the good, fully charged battery( the easiest) doesn't work,
I would take the carb bowls off and inspect the gas in the bowls for debris, if any hint of debris or varnish,I would remove the carbs ,clean all the little holes and jets with carb cleaner and compressed air. with the carbs off, a good inspection of the rubber boots is also possible. Inspection of the air box for blockage as well ( I have air filters so don't really know if this is possible). Drain fuel tanks , and start over with fresh high octane premium unleaded gas.
carb sync out is possible, but unless the linkage has been worked on or has come loose somehow on one carb, the engine should run enough that moving the throttle arm of one carb ,on only one side, will smooth out the running or make it worse. Best of Luck; Kevin

[Dave S]

Tommy,

Going back to the original post - a couple things caught my eye.....

"1. I check to see that there was fuel getting to the engine:
a. pulled the fuel line at the mech fuel pump and ran the facet fuel pump. No fuel came out the line."

This doesn't make sense unless there is some other missing information (fuel valve shut off?) If the line is pulled off the mech pump and there is fuel in the wing tank - it should run out even without the electric pump running - is there a line obstruction?....If a person disconnects the fuel line at each carb, fuel should run out there also - should be enough head pressure from the wing tanks to overcome the poppet valves in the mech pump. After cranking - pull off the fuel bowls on the carbs - is there a correct level of fuel?

"2. removed a spark plug and turned the engine over using the starter. There was a spark at the plug". Good -we know we have spark - do we know the compression is adequate? - with those two ruled out - it's basically fuel/induction, etc

"3. The spark plug showed moisture, indicating that fuel was reaching the cylinder'. Is it certain the "moisture" is gas?...even so - is there the right amount of gas?

"I initially thought the problem was the enricher circuit, but based on what I saw, it is not that."
" The engine has the carb airbox from Rotax, so it is a little hard to get to the carburetors to check much other than the linkage."

Difficult or not - the only way to verify is to take it apart.

These things can be befuddling -

Good luck & let us know what the final analysis turns up

Sincerely,

Dave S
KF7 Trigear

912UILS Warp

[Monocock]

Have you checked the O ring seals that join the inlet manifold to the engine and the carbs to the inlet manifolds?

If it runs ok after 10 minutes that says that the heating of the engine expands and seals something (ie an air leak). Hence my suggestion to check the seals.

[szicree]

Start with the simple, common stuff: Pull and check ALL the plugs. Carefully inspect the wires too.