Rotax 912 hard to start

[Guest veldthui]

Hi from New Zealand.

 

Our club has a Tecnam P92 echo which has a Rotax 912 (100HP) in it. Lately it has been extremely hard to start and is getting worse. We were told to pull the plugs out and reset the gaps to 0.50mm which was done and the first start after it was done was second turn but further cold starts took ages. We were told to take them back a bit further so took them to 0.478mm and again first start after doing this was perfect but alas next day back to usual. Re-did the gaps back to 0.504mm and again perfect start first time after the plugs went back in but next start two days later back to usual. Then told to jump another battery to it in case it was not turning fast enough but no change.

 

When it is hot or warm it starts fine but when it has not run for about 8 hours it is brutal. Sounds like it is trying to go and exhaust pipes are getting hot indicating at least some burn but all the shaking and sudden stops while it is trying just sloshes fuel everywhere in the carbs.

 

Anybody got any idea what else could possibly be the problem?

 

Really strange that it is a perfect start after taking the plugs out, checking them, and putting them back in and nothing else. Plugs look good and clean.

 

 

[Guest pelorus32]

I'm not sure whether you've been through a winter with this a/c before. If not: the 912ULS is very picky about starting when it is cold. Full choke, throttle must be absolutely closed and it should start first kick, then leave the choke on until the engine is running smoothly.

 

I have seen them start very badly - as though only 3 plugs instead of 8 are firing and then slowly others seem to chime in. I'm not sure what that is about. Others may be able to help. (Nev, any ideas?)

 

If the throttle is open at all or has been pumped then the engine will flood and not start at all - leave it for 20 minutes and try again.

 

Regards

 

Mike

 

 

[facthunter]

Nothing new.

 

Just what you have covered re choke. The throttle must be closed or it is ineffective. Are the chokes acting on both carbs? nev.

 

 

[jetboy]

While it may be choke/throttle settings the rough startup can also occur due to faulty slipper clutch, dog clutch preload, or starter sprag. There is a service bulltein specific to the starter sprag for the 912s. we found it applies to the 80hp too. ( low starter current = slippage in sprags. The dog slippage is checked by locking the gearbox with pin and scales on prop. Both problems on an engine with less than 200 hrs.

 

Ralph

 

 

[Guest veldthui]

Thanks all. Yep know about the choke/throttle issue and yes it has been through a couple of winters before. Chokes have been checked and are operating fully and the same on both carbs.

 

It appears the only way to get it going is to pull out the plugs, look at them and put them back in and it will start instantly or let it turn over with throttle closed for a bit and then a well timed push of the throttle and it may catch.

 

The way we start it is 1/4" throttle, fuel pump on till over 2.0bar, close throttle, mags on and full choke and start. This has always worked in the past and does so after the plugs have been out and back in. Does not work next cold start. Weird huh!

 

 

[facthunter]

technique.

 

Since these carbs are CV and the slide only opens when there is a vacuum, I cannot see what point there is in opening the throttle when pressurising the pump. This is a similar technique as used with the fuel injection (dribble) system, for priming the engine, and is not really appropriate. Two things happen when an engine is cold.

 

1. the engine turns over more slowly.This may be weakening the spark. Note comment about starter motor. Higher duty motors have been fitted. I don't have the details.

 

2.There is a need to richen the mixture to achieve a combustable fuel/air ratio and as the fuel does not readily vapourise you have to have more of it initially., This is all pretty basic. I really think its due to your engine cranking over too slowly. Nev

 

 

[Guest Maj Millard]

What type of oil are you using ?? sounds like you may have contaminated the starter Sprague clutch. When Castrol GPS oil was the one recommended, some thought they could do better and tried Castrol Magnatec instead. FAIL...it has teflon in it, and almost always will give you trouble eventually. It's not a recommended oil either by the way.

 

The latest Rotax SI-912-016 R1 lists all the newly recommended oils depending on wether you are running a lot of Avgas or not. We have all settled on Valvolene DURA BLEND 15w-50 and it is doing fine up this way.

 

If you have contaminated the Sprague clutch there is no way out, other than to put a new one in, and it is around a $2000.00 min job depending on Aircraft type, as it requires removal of Ignition stator, rotor, starter motor and rear case assy. It will not fix itself or get better unfortunatly...............................................................

 

 

[Tomo]

So If it's the Sprague clutch in the Starter, why does it work if you pull the plugs out and then put them back in again...?

 

 

[Guest Maj Millard]

They will always sort of want to start even when cold. Soon as they fire the clutch disengages prematurely and the motor will stop because it hasn't achieved the required RPMs to activate and sustain the ignition system. From memory this is around 400rpm and is a built in safety thing to guard against man handling of the propellor.

 

With a contaminated clutch they will start often when warm, but not cold.

 

If your battery is below nominal voltage it may not turn the engine fast enough to activate ignition sufficently as above.

 

Few years back I had a Storch owner who had contaminated his starter clutch by accidently topping up with...wait for it....diesel oil !!!! hey, it was handy he said.

 

I couldn't go to him, so he decided to bring it to me. Not able to start cold in the morning after overnighting on the way, he removed the prop and fired up the engine and warmed it up (NOT recommended proceedure !!) He then quickly refitted the prop, and restarted the warm engine and finished the trip.

 

The whole episode cost him several thousand, including a gearbox check after running it without the prop.

 

Moral of the story: use only the recommended oil lest thy wallet be thinner !!

 

 

[Guest veldthui]

Well we had everything checked out and it was indeed the spragg clutch. Being so hard to start it has been taking a pounding and getting worse and worse and finally nearly gave the ghost completely. During the overhaul we also gave it a new hi-tourqe starter motor and the change is amazing. Starts first time everytime.

 

Thanks for all the help.

 

 

[Guest Maj Millard]

You'r welcome, we are here to share experience...........................................