Marvel & other solutions to the lead problem

For all practical purposes, 80 octane aviation gasoline is no longer available, forcing the Cub driver to choose between 100LL avgas and 87 octane automotive gasoline (assuming that he has the necessary STC and that "mogas" is sold at the airport). Each fuel has its own problem. Despite the low-lead designation, 100LL contains too much lead for the older, low-compression engines. Mogas, on the other hand, doesn't have enough: some lead is needed to lubricate the valves.

Robert Parker uses Marvel Mystery Oil to reduce the buildup of lead from 100LL. He explained why in a post on the Cub Builders mailing list, which I have adapted here with his permission. Bob's article is followed by an email I received from Terry Lutz, expanding on Marvel and the alternative TCP. -- Dan Ford

That Marvel-ous Mystery Oil

by Robert Parker

Back in the late 1940s and early 1950s, at military air bases there were 55-gallon barrels labeled MMO (Marvel Mystery Oil). I forget the MIL spec number.... We had charts that told us how many gallons of MMO to put in per tank of fuel.

72 octane is the highest grade of gasoline that can be manufactured without [additives].

80/87 has 0.50 ml TEL [tetra-ethyl lead] per gallon and is dyed RED

91/96 2.00 ml TEL per gallon BLUE

100LL 2.00 ml TEL per gallon BLUE

100/130 3.00 ml TEL per gallon GREEN

115/145 4.60 ml TEL per gallon PURPLE

The amount of TEL in the higher grade fuels has increased the lead build up and fouling of spark plugs, along with valve erosion incidents, reported on some lower compression engines.

You know when you pull a magneto check at the runup area after a considerable taxi to get there, and you are running straight 100LL. You have a RPM drop that is excessive, and run the engine up to a higher RPM for a few seconds and then recheck gives a normal drop.

Why does this happen? There is a bromide chemical in 100LL that is supposed to keep the TEL vaporized and the excess pushed out the exhaust, and this bromide has to be at a certain temperature to work correctly. This temperature can only be achieved in our small engines at 1100 to 1200 RPM. How many of us taxi at this RPM? (If you answered, I do, how often do you have to change your brake pads?)

The automotive gasoline option. There there are two STCs on the market for mo-gas, EAA and Petersen. (I use Petersen's. If you have a question and call, Petersen has an answer for you. EAA has an answer, after 5 different people and the fifth one says let me check and I will get back with you. Sometime they do, and other times that's the last you hear.) Petersen's STC allows for mixing of av-gas and mo-gas. You can put mo-gas in the tank 75% (TEL 0.002 ml) and 25% av-gas (100LL at 2.00 ml TEL) and you have the same 0.50 ml TEL that 80/87 had.

Another way is to run about three tanks of mo-gas and the fourth is a tank of av-gas (100LL) Either of these work OK. That fourth tank however, you will still have the spark plug fouling syndrome.

The TCP/Marvel Mystery Oil option: These are most likely about the same thing, with small changes in the recipe, so as not to infringe on another's copyright. Both are high in detergents to do the cleaning job. Neither should be used for the first time on a high time engine; about half TBO or less should be the cut-off time, for first use.


    Use one pint Marvel Mystery Oil about 5 hours before oil change. If screen type system, be sure to clean screen. Then, in the new oil, install one-half pint MMO. This cleans the carbon and sludge collected in the engine passages and crevices and hydraulic lifters, and stores it in the bottom of the oil kidney. Some will make it to the screen, but either place it will be removed at oil change.


    Use the directions on the back for amount to put in the fuel per gallon of fuel (4 to 6 oz. per 10 gallons of fuel). MMO goes through the carburetor as a droplet, broken up like the fuel. When it enters the combustion chamber and the gasoline ignites, it is vaporized and soaks into the carbon buildup on cumbustion chamber walls, valve guides, around the valve stem, and on the spark plug. It soaks into the carbon and eventually loosens it up and it goes out the exhaust system.

    Now you know how it works, I will neither recommend or not recommend the procedure. Like Fox News says,"I report and you decide". I do know it will help when lifters start to be lazy and not do their job. When valves are beginning to stick, it will free then up. Some people swear by it, some people swear at it, but results is what really tells the tale.

    Marvel Mystery Oil and TCP

    by Terry Lutz

    Since 80-octane fuel has disappeared from Michigan, there has been a lot of talk about what to do so that lead fouling can be avoided in the smaller and older engines. The leading theory is to use Marvel Mystery Oil. This is generally regarded as good stuff, and a lot of people are using it. Mystery Oil is a top cylinder lubricant, so when it vaporizes during combustion, oil droplets are spread around the valves, guides, and rings to keep things moving. It works, and has for a long time. The old timers would use it to free up stuck rings and valves by giving the engine a "Mystery Oil Enema". The trick on a 65 hp Continental is to take the primer line off the carburetor, hook up a tube, and run Mystery Oil through directly through the carburetor while the engine is running. This creates great clouds of smoke, and extends engine a life for a few more hours. You can also just pour the stuff into the cylinders and run the engine.

    Then there is ALCOR TCP. This is a fuel additive specifically designed to scavenge the lead from the fuel to keep lead from ever forming on vital engine parts. But how does it work? Inquiring minds want to know, so I called the 800 number on the side of the can. A nice fellow named Rick told me that TCP stands for Tri-Cresyl-Phosphate. It was created during WWII because the cooler cylinders on the multi-row radial engines would lead foul, creating lots of engine problems. Remember too, that lead is added to aviation fuel to boost octane rating, and with the high manifold pressures the wartime engines were operating at, Tetra Ethyl Lead (TEL) was a necessity.

    When piston engines passed by the wayside in the military, Shell Oil bought the formula for TCP and used it for years in car gas, which was also blended with TEL. Of course, when unleaded fuel came out, there was no further need for TCP. But aviation fuel continued to contain lead, so the formula was purchased by ALCOR. 100LL fuel still contains 4 times the lead that our friendly 80 octane had. So, "How do it woik?" At the instant of combustion, there is a chemical reaction between TCP and the lead in the fuel to form lead phosphate, which comes out of the exhaust stack as a grey powder. The lead does not remain in the engine, and can't build up on valves and guides to mess up your engine. It only takes a few ounces of TCP to treat 10 gallons of gas. However, the carrier is toluene, and there's some xylene in there, too. You don't spill this stuff on your paint job, and is difficult to carry it with you in the airplane. The container says not to do it. Be careful and informed out there!!