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JETT 90

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Old 03-31-2011, 01:43 AM
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GAD77
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Default JETT 90

HI ALL

Can i have instruction for breaking in Jett 90 ? (The instruction in Jett website is too general)
Which prop to use ?
How much time to it?

Thanks.
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Old 04-07-2011, 09:36 AM
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bob27s
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Default RE: JETT 90

Thanks for writing ........

GRRRRRRRRRR sorry have to start over here.... What I wrote didnt post because of a "time out" thing. God I hate that.

Instructions on the website are general and basic for a reason. There is nothing complicated about it.

Lets keep this simple...

Put the engine on a solid test stand.

Fuel - what ever you will be flying with. Typically 10%-15% nitro, will run on anything.
Install a 11x6 or 11x7 prop (11x6 if running FAI fuel or 5%).

Start the engine with the needle 4-5 turns out - keep the glow driver on, lean the engine so it is running in a smooth 2-cycle at full throttle but obviously slightly rich.

If you have a tach, that is helpful, if not - not critical, you can do this by ear/eyes too.

Lean the engine slowly so it is running solidly but obviously still rich. Your goal here is to run the engine near flight rpm while still slightly rich. Your target in this case is about 14,500-15,000. For now, use 14,500 rpm as a target. With 11x6 or 11x7, NO problem reaching that.

Run the engine there for 5 minutes, let it cool 5 min, then run it another 5 min. Let it cool off 5 minutes. If it picks up rpm in the process, without touching the needle, that is fine.

Next run same prop full throttle, lean the engine slowly until you find peak rpm - use your ears. Listen... if it no longer gains rpm or sags, back off the needle rapidly, but note that RPM and needle position. Richen back up and run the engine here 500-600 rpm down from that peak rpm. Two tanks of fuel (5 min each) in this setup.

If at any time the engine goes lean, gets hot or sags - STOP shut the engine down, let it cool 5-10 minutes... open the needle slightly and start again.

Next, install your flight prop. Our baseline prop for the SJ-90 is a 12x6.

Open the needle slightly, start the engine, go to full throttle, let it run a minute to warm up. Again, find peak rpm with THAT prop, then back off 600-800 rpm. Note the peak RPM, the running rpm, and keep track of the needle position as a reference. Watch and listen to how the engine runs. Let it run like this for 4-5 minutes.

This is where you become familiar with how the engine operates.

You can also test throttle response, check idle. The low speed mixture is set by Dub before the engine is delivered. Resist the temptation to adjust it at this point.

Next, Install the setup in the aircraft. Run it as above with your flight prop. Find ground peak rpm (BRIEFLY) and back off 800-1000 rpm. You should see pretty much the same performance you saw on the test stand.

Fly it there at 800-1000 rpm off peak.

BE SAFE !! stay out of the prop arc - keep glow drive and tools away from the prop.

I trust this is all useful!

Bob
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