jett 50 - blown plugs
Hi Dave...
Thanks for writing.
Your prop, plug and rpm selection are on target. Don't change a thing there. If the engine was running well before, there is no need to change shims.
The extra 5% of nitro is not blowing plugs. Setting 500 rpm down should be sufficient, although a couple of hundred more might not hurt.
Plugs blow for one reason... lean run. Those are cause by a few reasons.
You mentioned you purchased the engine from a friend. I will assume for a moment that you only bought the engine, and it is now in a different aircraft.
About 50% of lean runs and blown plugs are caused by setting the mixture too lean (you seem ok there) The other 50% of the time, blown plugs are caused by fuel system problems. Often leaks, kinks, tank problems are to blame. Start with the basics
Since you are able to get a decent needle setting, and back it off 500 rpm fairly well... and you said the plugs blow about 1 minute or so into the flight..I will suggest that the problem you are running into is..... fuel foaming .
On the ground, the engine probably behaves well, and you may not see air bubbles in the fuel line. After the plane is in the air, the engine will unload an additional 500-1000 rpm. With this extra RPM, the airframe/fuel system picks up some vibration mode, and causes the fuel to foam.
I have seen this, and experienced this several times myself. With racing airplanes, it use to be a common gremlin. After blowing a handful of plugs, it always ended up being the tank installation. It is a bugger to diagnose, but somewhat easy to fix.
Solution : Make sure your fuel tank is wrapped in foam rubber, and that no part of the tank actually touches any part of the airframe. Of course, make sure there are no kinks or leaks in the fuel lines. Make sure to balance the prop.
The way foaming problems are solved with racing aircraft these days, is the use of bubble-free fuel systems, such as Jett's Bubble-Jett tank or a Tetra tank. This system allows all of the air to be removed from a fuel "bladder", it is then filled with fuel. The muffler pressure pressurizes the outside of the bladder, so there is no way for air bubbles to create foam in the fuel.
I hope this is helpful. Feel free to drop me an email if you have any other questions.
Bob Brassell