Can autogas cause ‘varnish’ in carburetors?

The GAfuels Blog is written by two private pilots: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.

Recently, the owner of a 1936 Boeing Stearman PT-13 with a 300-hp Lycoming radial engine contacted us regarding the availability of ethanol-free autogas in his state of Georgia. Despite the many autogas STCs that cover various Stearman models, this person’s mechanic had some reservations about its use. Following are details from this pilot and a response from Todd Petersen, president of Petersen Aviation, owner of these STCs.

Stearman owner: “My plane had an STC for MOGAS when I bought it but I have not used it yet. When I purchased the plane 10 years ago I had to send the carburetor out for a rebuild because the inside was all gummed and varnished up and the plane was running too rich. My A&P is telling me if I use mogas I will have to send the carburetor out every year to be overhauled because of the varnish that will build up. I fly about 75 hours a year using about 1,000 gallons of fuel. The last carburetor overhaul cost about $900 and I can’t pencil enough savings in buying mogas that would cover the cost of a carb overhaul. What are your thoughts on this and is there an additive that can be added with FAA approval that would prevent varnish? I have had a couple of friends who said they would mix 100LL and mogas but I do not see the savings in this practice either.“

Todd Petersen: “Incidences of varnish buildup on carburetor parts have been claimed off and on since the STCs came out. While I do not dispute that there may be some amount of varnish buildup in some airplanes I do not think it happens to all airplanes using mogas nor is it likely to happen to any of them so often as on a yearly basis. It may have to do with the way the engine is handled. Shut it off using the mixture or fuel shutoff valve so that the carb is devoid of fuel when you park it. Varnish tends to form in hot climates when gas is allowed to evaporate. There’s no way of knowing how long your airplane may have been sitting before you got it, or how it was handled by the previous owner. Also, don’t buy the cheapest automotive gas you can find but rather seek out name brand stations. Cheaper gasoline is made up of cheaper components which operate less satisfactorily. Mileage with this cheap fuel will generally not be as good, it might not last as long without going sour, and it might leave more deposits throughout the system. Pilots who are extra vigilant in this regard will burn premium instead of regular even if the engine can take 87 octane. You have a 300 Lycoming, hence you need to be burning 91 octane minimum anyway. Also since this is a radial engine it is recommended that you use a mixture of 25% 100LL and 75% unleaded autogas. Burning this mixture yields the same lead content as 80/87, 1/2 gram of lead per gallon. This was enough to protect valve seats but not enough to cause plug fouling. As far as the engine is concerned, using this mixture is just like burning 80/87 insofar as lead content is concerned. I’ve never had anyone tell me they had varnish problems when burning this mixture. It’s still a significant savings, especially in an engine such as this one. I hope this answers your questions.”