BAP News 12-09-10  Valve problems

While doing a runup it was noticed that when the throttle was retarded the manifold psi. would rise and the engine would falter. This is common of exhaust valves hanging open. The engine would run well at power settings and not much else was noted. Retuning to the hangar to investigate this problem it was decided to do a blow down compression test. All cylinders except the center two Number 3 and number 4 exhibited compression results of 78-79 over 80. Numbers 3 and 4 showed 0-5 over 80. Air was roaring out the exhaust pipes, something was definitely wrong. The heads were pulled and the valves of these two cylinders showed signs of erosion and or beating. At first we thought of all the things that could have gone through these cylinders but nothing showed in the head area and all of the pistons had no marks of any kind on them. The Valves were taken to a local Dyno shop for examination. The owner stated "Wow your using a lot of lead in your fuel" Yes, we are using 100LL with TCP added to scavenge the lead. "Well, your not using enough then are you?" was the quote. It seems as this engine is a 9:1 compression engine, 100LL would work, but it doesn't. The lead built up under the valves and pounded them. We are using hard seats in the head but they were fine. The valves took the brunt of the damage. We have since thought of building new tanks and using Auto fuel, but with the comments on the VAF site it appears that the new fuel with ethanol will attack even the new Proseal used in the sealing process. With this in mind we will be using 2 oz. of TCP per 10 gallons instead of the 1.oz. The exhaust stacks on the bottom of the plane exhibit a plume of grey, this turns out to be scavenged lead not oil. Our oil consumption for 40 hours is 0 but there are heavy grey streaks from the stacks, We use rubber gloves and cleaner to remove the residue. After 561 hours this is what we have to report Why the center two cylinders? It seems that they are the ones centered on the manifold directly under the carburetor and this would seem logical for the heaviest concentration in the engine. With this in mind we still recommend the use of TCP lead scavenger but in twice the amounts as stated. The engine runs very strong over a 10 year period. Compressions are high and zero oil consumption. It starts in 2-3 blades, runs like a rocket and is very inexpensive to fly.

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