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Evolution 61NX and Jettstream


Old 12-21-2011, 03:24 PM
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Default Evolution 61NX and Jettstream


We have three Evo 61NX engines in our club and all were having a peculiar problem. The engines would not richen at full throttle with the high speed needle backed as far out as possible. All three engines were using Du-Bro exhaust diverters and 15% nitro fuel. I dropped the nitro in my engine to 5% and it made the needle effective enough to fly with the diverter still in place. We removed the exhaust diverter from the Jettstream on another engine still using 15% nitro fueland the engine immediately richened up and the high speed needlehad to be turned in 2-3 turns to operate correctly. I have run a Rossi 61 with the same setup on the same plane and have had no issues. I have installed a Mac aluminum diverter on my engine and being shorter and rigid seems to help. The props vary from APC 11x6 - 12x6. All exhibited the same problem.

Iam also using an EVO 40NX with a Jettstream JS-30, 15% nitro, and Du-Bro diverter with no problem.

Even though we have found a remedy, I am not sure what is causing the problem. I would have thought that the diverter would have increased back-pressure to the tank and had the opposite affect we are witnessing. Do you have any ideas or have you seen this before? Thanks.

Danny Jackson
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Old 12-22-2011, 09:57 AM
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Default RE: Evolution 61NX and Jettstream

Typically when you can not get the engine to go rich ...... there is one of a few problems

1) An air leak elsewhere in the engine crack in casting, missing screw, torn gasket, carb o-ring not seated. The EVOs have been good engines, so crack not likely. But a few years ago we saw a few of the Tower engines give people fits. After looking and asking, I found out each had a crack near the engine lug... thin casting and crappy mounting.

2) Fuel foam - usually, but not always, caused by vibration. Suction on needle valve systems (remote ones usually) causes microbubbles to come out of solution. So even with a good installation, if the engine is sucking hard - it will vapor out bubbles

3) Fuel line restriction some ARFs come with lousey clunks and fuel tubing. Verify on that.

4) Make sure the pressure line is clear too. That can get restricted.

And yes, a restriction on the exhuast causes backpressure and a rich situation.

Let me know how things worked out for you?
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