Engine Spluttering

[Gyronaut]

Thanks T-bird. I have a slight conundrum (if I understand you correctly). On the one hand you say AVGAS (100 octane?) can burn valves on a 914. On the other hand I have heard from others also that one should use the highest octane available. It is rather more expensive but is available at my cabbage patch. Our unleaded is 95 octane. Should I consider going 50/50? Will be a hassle but worth it if its going to keep my engine happy. If my engine is happy then I am happy

[Morph]

I believe AVGAS is 103 Octane and high in lead content. I know they recommend replaceing the engine oil every 50 hours instead of 100hours on the 912 if you run on AVGA. It also gunges the plugs up quicker

[Gyronaut]

... by the way T-Bird, 2 more questions... where on earth is Tedderfield and that magni in your avatar looks a lot like Johan Stemmet's. Is/was it?

[Gyronaut]

Ta Morph, sorry I missed you at Morningstar the other day, Clueless made us feel welcome though! Look forward to meeting you.

[Morph]

Ditto

Will be there Sat morning

[t-bird]

Hi JetRanger

Tedderfield is South of Johannesburg. Next to Solitude.
I have moved to Summit, 10 minutes from the house to the west of Grand central.
Yes, this was Johan Stemmets old Gyro, bought it with 150 hours on the clock, sitting at 331 hours at the moment.

Cheers Callie

[Gyronaut]

Good on you Callie, thats quite a bit of flying! Let the Gyro's be seen.

I still dont know if I should mix 95/106 octane or just use the 95. Engine is running very nicely overall and the temps are spot-on.

Summit? Send me a google earth placemark? Have used the Hydromulch field in that vicinity, years ago though.

[t-bird]

Hi Jetranger

Another interesting piece on the SAGPA website regarding AVGAS and 914’s



Written by Mr K Katzke (Kevin)
Friday, 19 August 2005
I had been running my Rotax motor on either a MOGAS /AVGAS mixture or only AVGAS a lot of the time. The motor did 650 hours with no hic-ups.

Laura and I had just returned from a trip to the South Coast for 10 days where we flew for 30 hours in total. I said to Laura at the time that Cizy was not happy - I could feel it on climb outs. As soon as we got back to Gauteng, Cizy went off to Dr. Gideon to see what was wrong with her.

Here is what he found:

1. The Exhaust valves have flat ends, so a lot of horsepower go straight out the exhaust port. - Replace

2. The exhaust outlet ports have excessive carbon deposits. - De-carbon

3. The pistons and cylinder heads also have a lot of carbon deposits. De-carbon

4. The rings have worn away one third of their life. - Replace anyway

5. The gearbox is perfect, probably due to an oil-change every 25 engine hours.

In order to get some answers on the state of my engine, Butch ended up knocking on Johan von Ludwig's door. Johan is of the opinion that neither AVGAS nor low octane (assuming 93 and higher octane on the Highveld) are really an issue, save to say; that using AVGAS will leave deposits that will damage valve stem seals and the like.

The carbon and lead build-up can be the result of a combination of many factors, including things such as the use of the choke and/or turbo for extended periods. The choke creates the carbon and the turbo & lead in the fuel fuses the deposits to the pistons and valves. The exhaust valve damage could also result from carbon sticking to the surfaces or due to high exhaust gas temps.

At high power settings a lot of fuel is being dumped exacerbating the carbon problem - even though the exhaust gas temperatures shoot up, indicating a leaner mixture.

Johan also uses AVGAS and Petrol as available and has checked his engine's combustion chambers with a scope. He says they are very clean.

We know that Rotax does not recommend the use of Avgas due to the damaging affects of the high lead content. I must just mention that I saw more pistons damaged by detonation (mainly due to low octane content in our fuels) in Gideon’s pile of damaged spares than I saw burnt valves. So I choose to continue to use a combination of MOGAS and AVGAS.