912 Question

[Guest DWB]

The last couple of times I've gone for a fly (last Sunday & today) my engine on take off at around 300' develops a bad vibration on full throttle, which I counter by throttling back to around 4000rpm. The Zephyr still climbs well at this rpm. At the same time as the vibration I note that the EGT on the front left cylinder drops approx. 250 F.deg. Runups on both ignitions do not display any thing unusual. Once I get to 3000' & level off if I open throttle up towards 5000rpm the vibe comes back, however, if I idle the engine for 20 secs or so then open it up it will rev cleanly to 5800rpm with EGT's being the same, then slowly the FLHC EGT starts to drop a little (about 100 deg) with little if any vibration felt. This process can be repeated with the same results over & over.

 

Last week I suspected stale fuel (6 weeks in tank since last flight) Topped up with 20 litres this morning over approx. 12 litres remaining. As it is the same cylinder though I am ruling out fuel issues generally. Spark plugs? Ironic if both are degraded on the same cylinder but could happen - they have 80 hrs on them & was going to change at 100 hrly.

 

Going to do a leak down test shortly to see if that identifies anything unusual.

 

Just thought I'd put it out here for discussion. I'll report back with my leak down results.

 

 

[Spin]

There's a puzzle Dexter! I'd be inclined to have a close look at the plugs on the suspect cylinder and compare them to the others - if anything it sounds almost as if that cylinder is going well lean of peak, hence the drop in EGT. That said I'm not sufficiently familiar with the Rotax to say how 1 cylinder of a pair fed from the same carby, could fluctuate like that. Interested to hear the outcome - good luck!

 

 

[Guest Michael Coates]

tyres out of balance, probably front tyre spinning and pressure low. happens on the front of the sting all the time especially abluve 140 knots when the front wheel starts to spin itself

 

 

[Scotty 1]

Dex

 

If it is the standard setup for Rotax there are only 2 EGT senders. One for right bank and one for left bank. If that is so it could still be in one carby as each bank is feed with a different carby. Have seen something similar on a 914 and it turned out to be a problem with a float chamber venting tube. I would also check the rubber carby sockets for cracking. Let us know how you go.

 

Cheers Scotty

 

 

[Guest avi8tr]

Carby and or balance?

 

Stuck valve

 

Electrical, what happens when Mags checked during vibration ?

 

Does Carby heat have any effect when it's vibrating?

 

 

[Guest DWB]

Thanks for replies guys. I'll try & answer each one.

 

Michael - No definitely not wheels balance, as yes I do suffer that but that vibration is minor compared to the one I'm getting which is definitely engine originated.

 

Scotty - No I have 4 EGT sensors & they are switched Fore & Aft for checking. IE. the gauge either shows left & right front or left & right rear. Carby sockets are good.

 

avi8tr - Carb balance is fine - I actually have real time monitoring of carb balance whilst ever the engine is going. If I don't get it solved, next time I'll try switching one ignition at a time off & see what happens (up high). Don't have carby heat.

 

OK, have done leak down test with excellent results on all 4 cylinders although the suspect cylinder #2 is slightly higher leakage than other 3 but still at the top of the green arc on the meter. Plugs from that cylinder were slightly darker than others when removed & had small deposits similar to lead fouling on them.

 

I'll get new set of plugs & try that next I think.

 

The vibration is very reminiscent of the one I experienced in the Gazelle which led to its demise which turned out to be carbon deposit under #4 cylinder exhaust valve, however leak test should have identified that if it was something similar.

 

 

[Wayne T Mathews]

Sounds like a weird one Dexter. Have you asked Wal at Bert Flood's if he's heard of it before? How many hours are on the engine? I haven't heard of it happening on a Rotax, but Continentals and Lycomings will occassionally get carbon on the valve stem which can cause the valve to intermittently stick. Can be a bitch because it will not show up on the differential pressure test if it's not stuck when you do the test. I'd give Wal a call...

 

 

[David Isaac]

Good point Wayne.

 

Just out of curiosity, will running an upper cylinder lubricant remove such carbon deposits or is a head job to fix (no smart ass comments please).

 

 

[Wayne T Mathews]

There are arguements/evidence that using an upper cylinder lubricant may prevent/reduce the build up of carbon if it's used from the get go... In my experience though, once it's there, it takes a head job to get rid of it... Having said that, before I put a spanner to a Rotax engine in this case, I'd talk to the Australian Rotax Distributer's engine man, IE: Wal at Bert Flood's... He knows and has seen a hell of a lot more about Rotax engines than I do/have...

 

 

[Guest DWB]

Spoke to Wal this morning - Going to be process of elimination. He agrees try new plugs first, then if that hasn't fixed it he wants me to remove the air vent tubes off the carbies & try that. The vent tube setup I have is odd according to Wal. Waiting for the plugs........ & good wx again.

 

EDIT: Wayne the engine just turned over 300 hrs

 

 

[mnewbery]

Any chance of posting images of the plugs?

 

 

[Wayne T Mathews]

Fair enough Dexter. I'd certainly listen to Wal... At 300 hrs, one would think it's a bit early for any plaque build up, eh?... Another area of infomation are the internet videos that are available. There's some good stuff on Rotaxes inbed in there. On one of the videos (sorry, I can't remember where I found it), I recall a couple of guys talking about problems from using Avgas with synthetic oil, causing lead build ups that restrict the already close tolerances in Rotaxes. When I get some time at home tonight, I'll have another go at finding it.

 

 

[Guest Michael Coates]

Any chance of posting images of the plugs?

Sorry just being smart...

 

 

 

[Guest DWB]

Now here's the ones I think you want to see in order 1,2,3,4 cylinder pairs

 

[ATTACH=full]906[/ATTACH]

 

[ATTACH=full]907[/ATTACH]

 

[ATTACH=full]908[/ATTACH]

 

[ATTACH=full]909[/ATTACH]

 

Now the surely you checked that bit, bit........... I was leaning towards the problem being on the left side seeing that side front EGT was dropping off. Wal did say that meant very little. So I've checked the float bowl on the RHS carb to find this, hmmm

 

[ATTACH=full]910[/ATTACH]

 

That little orange speck in the main jet well near the top is (or was) a piece of that lacquery scale petrol can create. It was sufficient in size to restrict the main jet. Maybe under full load as in take off the fuel uptake was sufficient as to pull it up over the jet. So the plan is now to test fly again with the old plugs & see what happens first. But I'm not jumping to conclusions yet. I do appreciate all the feedback btw.

 

 

 

[Guest avi8tr]

Color of the plugs isn't overly important in the 912 so Wal tells me.

 

I'd be looking at the Carby vent tubes, can you blow through them evenly, what happens when you take them off at idle etc? (don't fly without them unless you are sure it's running properly)

 

 

[David Isaac]

Plugs look fine in any case. Just check their internal resistance, cap to electrode and make sure they are all the same and that there are no cracks in the porcelain.

 

 

[Wayne T Mathews]

That little orange speck in the main jet well near the top is (or was) a piece of that lacquery scale petrol can create. It was sufficient in size to restrict the main jet. Maybe under full load as in take off the fuel uptake was sufficient as to pull it up over the jet. So the plan is now to test fly again with the old plugs & see what happens first. But I'm not jumping to conclusions yet. I do appreciate all the feedback btw.

In the picture of the carby bowl, Dexter: Is that water or something at the bottom, or just a trick of the light?

 

 

[Guest DWB]

In the picture of the carby bowl, Dexter: Is that water or something at the bottom, or just a trick of the light?

There's Premium unleaded in there Wayne about 6mm deep

 

 

[Guest DWB]

OK. All's good. Put the old plugs back in, tethered the Z to the hangar & gave it a good run at full throttle for a few minutes with no indication of any probs & had even EGT's. Put the cowls on & went for a fly (0.7) Motor ran sweet as.

 

I'm guessing that little speck in the RHS carby was the problem. Wal said it would be carb related.

 

Thanks all for you input. Happens again I know where I'll be looking first!

 

 

[David Isaac]

Good news Dex,

 

 

[dazza 38]

Excellent news mate.

 

 

[Guest DavidH10]

Great news Dexter, but it must be catching. My engine decided to cough and splutter on climbout, on Wednesday evening (second flight of the evening). A test circuit after changing the fuel filter and doing successful full power run-up resulted in the same, so will have to dig deeper this weekend. After reading this thread, one of my next checks will be to remove the float bowls and check them.

 

 

[Guest DWB]

Hope you get it solved easily David. Actually I think an "attention getter" every now & then is not a bad thing, as long as it doesn't turn out to be - a bad thing! You might like to let us know what your findings are as well thanks David.

 

 

[David Isaac]

Reminds me many years ago I had just commenced a climb out of Port MacQuarie in a C172 and at about 200 to 300 feet she coughed , misfired, spluttered and coughed again ....

 

I remember my first instinct was full carb heat followed by an expletive that sounded like "... you bitch ... comeon you bitch, don't you stop on me now ..."

 

I was white knuckled on throttle and tense for at least the next 15 minutes as we droned out of Port Macquaire ... everything was fine from there on.

 

As you said nothing like a cough or two to get your immediate attention especially when on climb out at 200' agl

 

 

[Wayne T Mathews]

Reminds me many years ago I had just commenced a climb out of Port MacQuarie in a C172 and at about 200 to 300 feet she coughed , misfired, spluttered and coughed again ....I remember my first instinct was full carb heat followed by an expletive that sounded like "... you bitch ... comeon you bitch, don't you stop on me now ..."

 

I was white knuckled on throttle and tense for at least the next 15 minutes as we droned out of Port Macquaire ... everything was fine from there on.

 

As you said nothing like a cough or two to get your immediate attention especially when on climb out at 200' agl

Oh nooo,,, David, David,,, You mustn't cuss at your aircraft... The little people might hear you (written with an Irish lilt), and they can be really mischievous, and sometimes downright nasty if they hear you speaking unkindly to your aircraft... ;)