Av gas warning!

[SkySteve]

My hangar partner, John Oakley, has had trouble with his 80hp 912 for over a year. Just after take off it sputters and quits. Dead stick landings have become his specialty. We have tried everything we can think of to cure the problem. New carbs, carb balancing, float adjustments, checked all the wires, ignitions, fuel, fuel flow, spark plugs and wires, even had 2 A&Ps look at it. Nothing would cure the problem.

Finally, we removed the engine and opened it up. The tops of the pistons and valves are covered with ugly lead. When we pulled some of the valves they were also coated with lead along the seats. The result: The valves will not seat tight. It is more noticable when the engine heats up than when it is cold. He is sending the engine to a Rotax repaiman for an overhaul.

He has about 500 hours on the plane and has always used Avgas with no additive like Decalin.

Lesson learned: DO NOT use straight Avgas!!

[Slyfox]

were the rings stuck in the grooves. or was the main problem with the valve guides.

All the gas stations I have checked here has 5% alki fuel. I think this is just fine, I've run it for a good long time now. I think I'll go and scope my cylinders when I get a chance just to be safe. Rotax now says ok up to 10%.

I have a small bottle of decalin in the plane for out of town trips.

[DesertFox4]

has always used Avgas with no additive like Decalin.

Sorry to hear John has to spend that money and has lost so much good flying time.
He's been chasing this goblin for some time.
IF you want to or have to use 100 Low-Lead AV-gas , use Decalin fuel additive religiously or you risk potential problems like this. Much cheaper than an overhaul at 500 hours. It is safe to carry in the aircraft and it takes very little (maybe an ounce per tank per fill up.)
Also if you are using 100 Low-Lead Av-gas you should NOT be using a 100% synthetic motor oil.
If you use Av-gas you need to change your oil more frequently. 25 to 30 hours is a good rule plus change the filter each time. While I've never had a fouled plug while practicing these procedures and using Av-gas exclusively, I try to change the plugs about every 100 hours and at conditional inspection time. Rotax plugs are cheap compared to Lycoming or Continetal spark plugs.
Again our condolences to John. Glad he's found the gremlin and can get back in the air soon. I know what to get him for Christmas now. Decalin.

[SkySteve]

Slyfox,
The rings are not stuck but are very dirty. They don't seem to be lead coated. However, John is thinking he might as well change the rings while doing all the other work. It appears the main problem are the valve seats.

[Slyfox]

have the valves springs been released so the guides can be checked? Just currious because there should be a definate yes to the problem somewhere. If there are no burn holes in the seats than I would be looking at valve guide problems as far as the valve seizing when hot.