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jett 50 problems

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Old 09-20-2009, 03:01 PM
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zero-cool
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Default jett 50 problems

ok.long delay on needle setting.I start to lean 14k then 15k.then one more little nudge then im all the way lean.and im still 4 turns out at 18,300 with 8.5x7.5 with ss muffler.ive had other engines on this viper with no tank problems.but when I try to put on jetts the needle settings are all over the place the tank has as much foam around it as i can get.I did buy a jett bubbless that is made to fit.but its long and gets in the way of the throtal servo.I just dont want to get up in the air have this thing go way lean.then cook it
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Old 09-20-2009, 07:00 PM
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Default RE: jett 50 problems

Your caution is wise, and thank you for writing....

A question first - did you perform the break-in proceedure on the test stand first? If so, what prop was used, what peak rpm did you observe after the 20 minute run time?

That jump you see from 15K to 18300K is the pipe staging. It is suppose to do that, and it will stage deeper onto the pipe another 1000 rpm once in flight with that prop. Find peak ground rpm (briefly), back out the needle so the engine drops about 800-1000 rpm to around 17,500, and that is where you fly it. If the needle is too sensitive to allow you to have that needle range, the prop is too big, or there is another issue to address.

Forget about "number of turns" they mean nothing. You have to go by the engine rpm and its response.

Once it gets over 18,000 ..... that is a whole new energy level. No other engine on the market comes close, that is why you have never seen other engines run into problems before. All sorts of problems can happen in the fuel system with that sort of power, and in fact the entire nose of the aircraft can react like a spring that will drive you and the engine nuts. The bubble tank will help with the fuel system. (stock tank installation is often way too tight). You also want to make sure the front end is solid.

Your next step - put the engine on a test stand. Run it there under controlled conditions. You can use the 8.5x7.5 prop that you have, and perhaps try an 8.5x7 as well. The needle should have some range after it stages up over 18000 you should be able to easily back it down 800-900 rpm without it falling off of the pipe. Find peak rpm (briefly), back off 800-1000 rpm and let it run. If the engine gets overheated and sags while finding peak rpm, back the needle out 1/2 a turn, shut it down, let it cool 15 minutes, and then run it again (no good data from a hot engine). Note that peak rpm and the flight-rich rpm. Note the appx needle position. Note the sound of the engine (should have an audible rich crackle to it. The break in is to help the engine seat in perfectly, and also so you can learn how the engine responds and what it sounds like when running properly This can only be performed and learned properly on a solid test stand.

If you see the engine perform and respond significantly better on the test stand, then you have isolated the problem to something in the aircraft.

The regular fuel tank can work, but no part of the tank may in any way touch the airframe. Sounds like it is padded ok, but double check. One point of contact, and it will buzz the fuel.

If there is a removable hatch on the Viper fuel compartment, it must be solidly glued in place to make a solid box structure on the nose of the plane. Screws or tape will not cut it. Also helps to add triangle stock inside of nose section of the plane too to make it more rigid.

If you can, post a few photos of the engine installation, and anything else you think might be helpful

Bob
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Old 09-21-2009, 06:49 AM
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Default RE: jett 50 problems

thankyou for taking the time to respond.im almost 100% sure its the fuel tank foaming.the only problem i have ever had with a jett engine.is always tank related.so i pulled the tank last night and made the bubbless fit.as long as i have a good solid stream of fuel.that will make setting the needle easy.also if im seeing 18,300 with a 8.5x7.5 do you think ill see better speed with a 8.5x7.0?thanks for the help.
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Old 09-21-2009, 12:14 PM
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Default RE: jett 50 problems

Hard to tell on the speed. Only way to know for sure is to try both. Speed comes down to a lot more than "Ground rpm" x "pitch".

The engine will unload differently in the air with each prop, and the plane will fly differently with each prop.

As long as the needle on the 8.5x7.5 will allow you to back it off cleanly at least 800 rpm before launch, then you should be good to go there.

But if the engine is new, you may want to run the lighter 8.5x7 prop anyway for a while. RPM will not hurt the engine... will only make it run better.
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Old 09-21-2009, 03:31 PM
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Default RE: jett 50 problems

thanks bob,I was also seeing 17,700 on a 9x8.and that seemed pretty darn good for a 9x8.would that work with the ss muffler?
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Old 09-21-2009, 05:37 PM
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Default RE: jett 50 problems

bob,I just got done testing the bubbless tank.and it works so much better.but still as i begin to lean it stages on the pipe at 16k then jumps to 18,400 and i try to back off and nothing happends.I keep richening slowly.then it just falls off again back to 16k I cant get it set at 17,500 it likes 16k or 18,400 what am i doing wrong? also im using a 8.5x7.25
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Old 09-22-2009, 09:27 PM
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Default RE: jett 50 problems

its working fine, you need time on the engine.

Set the needle just at the point where it drops off the pipe, and fly it there. Once in flight and the prop unloads, it will jump back up. Just be patient with it.

Error on the rich side, then sneak up on it as you get use to the engine and it gets some run time.

Also, did you reinforce the front end and glue the hatch on? That will cause some of that goofyness too. Not tank related, engine/airframe interaction causes it.
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