opinion

by Clyde Poser in 912 / 914 Technical Questions 12 years ago

Here's one.
912ULS installed in aircraft in 2003,a fixed wing Sky Ranger. Engine has 542 hours. Prop was pitched to give 5200 at full throttle. The owner went for the $25 hamburger with a friend. After lunch during the take off for home the engine was only able to achieve 4800 RPM. They went ahead and flew home as there was enough power to climb out and get back to home field 60 miles away.
I just performed the following: Compression check - all four cylinders at 85/87. Checked all the spark plug caps at between 5k and 5.5k ohms. Checked all the secondary coils, all were within specified resistance and replaced all spark plugs with NGK DCPR8EIX gapped at .027. Balanced the carbs with a digital balancer right on at 4000RPM as well as at idle. The picture attached shows
the number 3 top plug on the right and the number 1 top plug on the left. Why would the #1 plug be so sooty? Why would the RPM suddenly drop 500RPM. I have been scratching my head on this one and after testing everything more than once can't come up with any reason the engine shouldn't be running perfect and pulling normal RPM.

2012-04-07_10-51-15_919.jpg (You do not have access to download this file.)

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Re: opinion

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Hi Clyde

It has almost certainly got to be a fuel problem and would suggest the easiest thing to do would be to remove the carbs and completely strip and rebuild them with new overhaul kits. When you balance the cabs you need to balance them at around 1800 rpm for tick over stop screws and at around 2500rpm for the cable adjusting screws. Don't adjust the cable adjusting screws so it is balanced at 4000 rpm as they will be way out of sync. I can explain why if you would like.

Good luck

,
Mark.

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Re: opinion

by Roger Lee » 12 years ago

Roger Lee

125 5.11k 4.52k

Hi Clyde,

While your doing this the WOT rpm is too low and can cause damage to the engine. Get the WOT rpm up to at least 5500-5600 rpm. It can even lead to a cracked case and other engine issues. Reduce the pitch about 1.75 degrees.
Back to your problem and this could be a couple of things. The black plugs may have come just from idling and can be normal. You would need to shut down the engine after a few minutes of higher rpm runs to see a better picture on the plugs. I would also like to know if the mag drop is fairly equal. You may want to do a quick test and just change the plugs. It never hurts and it's the first simple thing to try. I'm with Mark on one point that it could be fuel flow, but since they synced at 4000 then maybe not. If you just do a ground run can you get up to a full static rpm without the lost power? You could possibly have a small piece of debris in a carb. Since it runs fine and syncs up to 4000. The the place to look would be to inspect and blow out the main jet and remove the needle valve and make sure that passage is clean. For me when I get that far into a carb I just strip and clean the whole thing and then your done with it. Then the diaphragm can be checked too. It only takes a little longer and is easy to do. How are the temps while he was flying? A possible air leak might want to be considered. Check the carb sockets. It's a 2003 and he may not have done a 5 year rubber replacement and I have seen and heard of cracks in the carb flanges and they aren't noticeable unless you really put a little torque on them with your hand. The one place I disagree with Mark is setting the sync rpm at 2200. It's too much around the idle range and you don't fly there and the engine doesn't flow enough air and fuel to get a good high rpm sync. I think you are always better off syncing at 3500 or above. I find if you sync at the lower rpms that the sync is always off at the higher rpms where you spend most of your time. This is a subject we discuss in Rotax class. You will need standard dial gauge to see this and how far they can be off. The electronic sync devise is my second choice any more for a carb sync. The dial gauges tell me much more about each carb in relation to the other and the sync process is easier and faster.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: opinion

by Clyde Poser » 12 years ago

Clyde Poser

17 19 22

Thanks for the reply. Eric always said to sync at idle and at higher RPM to make sure the cable is in tension as that is basically what you are adjusting.
Roger, Prop RPM was 5200 which is minimum according to SL-912-016. Only recently did it drop to 4800. That just gave me an idea. Could be the tach is bad so I will definitely check it with an optical tach to make sure it is reading correctly. Also the plugs are a completely new set. In fact I changed the #1 plugs twice to make sure I didn't have any bad ones. Also, I did the higher RPM run at 3500 for 5 minutes and a quick shut down to check what the plugs looked like. Still the same as the picture. The mag drop is exactly equal on both sides and smooth. Mag check was done at 4000 RPM. I disassembled the 1-3 side carb and checked the diaphram and looked at all the passages. The diaphram didn't have any tears of damage and I didn't find any debris of note. However when I took the dome off the carb there was a little fuel up above the diaphram. I think it is because the owner uses a primer instead of the starting carb and the fuel from the primer gets sucked out of the aft part of the carb into the dome during starting. The carb flanges are new, they have the little spacer between the clamp ends. I am pretty sure the power loss is a fuel problem but I don't understand why the different appearance of th eplugs. The 2-4 side isn't like that. I will be rebuilding the carbs.
As to the balancer I've used my digital one since 2003 and understand what the readings say so I am comfortable with it. Just curious but if both carbs are sucking the same vacuum what would an analog gage tell you that a digital instrument wouldn't? Well, I just thought I would check to see if anyone had any ideas that I hadn't thought of. If you can think of anything else I might have missed please post. Thanks again.

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Re: opinion

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Hi Clyde, Roger,

The reason I suggest, as Rotax also suggest, you should sync your carbs at around 2500rpm is that it is more accurate.
Syncing the carbs at a low RPM gives a fairly consistent and accurate calibration between the two carbs due to the higher vacuum. If you sync the carbs at a high RPM of say 4000rpm, then there is less vacuum and so a lot of adjustment is needed on the cable adjusters to synchronise the vacuum difference of the carbs. This may mean that they are in sync at 4000rpm, but will be way out at all other power settings, both above and below 4000 rpm.
If they are synchronised as I and Rotax suggest, the very small difference in vacuum between the carbs at all power settings are balanced out by the balance pipe joining both inlet manifolds. That is what it is there for. I hope you understand what I mean as I am not very good at explaining myself.

Cheers Mark

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Re: opinion

by Clyde Poser » 12 years ago

Clyde Poser

17 19 22

Mark, The 500 RPM power loss and #1 plug fouling occured before I balanced the carbs so I don't think the fact I balanced the carbs at mid throttle caused the initial problem. I can easily go and do a balance at the RPM range you suggest. Do you think that will make the full throttle RPM go back up to 5200 and the #1 plug stop fouling?
After I balanced the carbs I did a runup and checked the balance from idle up to about 4300 and the difference throughout that range was only about .5 inches hg. I couldn't go any higher as the brakes wouldn't hold the aircraft. I don't know what you consider balanced but that seems to me to be pretty balanced. And the engine was very smooth with almost no vibration. Thanks for suggesting the lower RPM balance and I will try that. I just don't think that will fix the problem.

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