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Jett .50

Old 07-03-2002, 03:34 AM
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dave long
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Default Jett .50

Reciently purchased a Jett .50 from a buddy of mine. He used 15% nitro and 18% oil, 9 X 8.75 prop with KandB1-L plug and had good results. Today I used Powermaster 20/20 and toasted several plugs. I maxed out the RPM (on the ground) at 17,500 RPM then backed off to 17,000 RPM before I flew. 1 minute into each flight I was burning a plug. Should I back down to 16,500 RPM (on the ground), switch to 15% nitro, add a shim, or change props? Any advice would be helpful.
Old 07-03-2002, 11:24 AM
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bob27s
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Default 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
Old 07-03-2002, 12:14 PM
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dave long
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Default Jett .50

Bob,

Very helpfull. You are right on about "lean" being the cause. The engine and plane were purchased together. The only changes I made was fuel and RPM (increase). My buddy was set at about 16,750. I tweeked it to 17,000 AND changed fuel from 15% to 20% nitro. Next trip to the field I will start by reducing the top end to 16,750 and see how it goes. The tank is wrapped very well in foam so if a lower RPM doesn't work I will try a different tank. Take Care.
Old 07-03-2002, 12:31 PM
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Default Jett .50

Hi Dave...

The extra nitro may have pushed the in-flight RPM to that critical vibration mode and caused the foaming. It is amazing what an extra 200 rpm can do Dropping the RPM back will most likely help.

In the past, the "easy" field fix for this is to put a larger prop on it, to slow it down just a bit. Try going to a 9 or 9.25 pitch using the 20/20 fuel. That should get you back down in the mid 16.5k rpm range.

Let me know how things work out.

Bob

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