For years I flew my Lazair Series II with the Rotax 185 engines mounted burning a fuel to oil mixture of 50:1 . I am confident that is a good gas to oil mixture for “most” two stroke engines. However, after reading an older Lazair service bulletin it may be needed to use a different oil mixture. The service bulletin recommends 35:1 gas to oil mixture. A things should be considered when deciding on the fuel to oil mixture:
- The original manufacturer from the early 80’s recommends a 35:1 fuel to oil mixture.
- The vast majority of two stroke engines today run 50:1 mixtures and some are even as high as 100:1.
- Two stroke oil formula’s have changed considerably since the 80’s and are far improved.
- In the 1980’s most two stroke engines ran much richer than today to prevent seizure.
- Too much oil will foul spark plugs and cause a premature engine failure.
- Too little oil can cause various problems ranging from high temperatures, to lack of lubrication causing seizure, to eventual failure of crank seals of a long period of time.
- Though Rotax recommended 35:1 with the Rotax 185, it later recommended 50:1 for the slightly large Rotax 277. These engines were designed only a few years apart.
Ultimately, as an airplane owner you need to make this decision. This website won’t seek to sway you one way or another. Keep in mind that there are a few “hot heads” who claim to be Lazair guru’s that say the oil mixture should be much richer and never approach 50:1. They may be right, but I have learned a long time ago to not take advice from crazy people. So make decisions on your own.
LAZAIR SERVICE BULLETIN
There have been some instances where operators have been using 50:1 gas/oil ratio mixes for the Rotax 185cc engine. These engines are used on the Lazair Ultralight Aircraft. This is not advisable. In the original configuration the factory recommended a 25:1 mixture.
However in the Ultralight configuration a 30 or 35:1 is recommended . However, during break-in and up to the first 10 hours of operation use a 30:1 ratio. After 10 hours a 35:1 is fine. A high quality petroleum base two-cycle oil is recommended.
NOTE using the larger Rotax engine oil mix ratio of 50:1 can and usually does lead to early crankshaft seal failure. Leaks at these areas can lead to lean fuel mixtures causing engine damage, and leaks at the mag side can result in oil on the points thus causing an engine failure.