Q-40 Racing Discuss AMA 422 and any other variants of Quarter 40 racing

Jett QM Questions

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Old 06-14-2010, 12:12 PM
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rbg
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Default Jett QM Questions

The Engine has lost 5K RPM WHY?

I ran it in on a bench with an FAQ prop. 3 cycles. to break in.

Put a 7.4 X 7.6 prop on and flew it 4 times real rich blowing smoke never on the pipe.

Put after run oil in it and took it home.

Following weekend (yesterday)
Used the same fuel and couldn't get peak over 18K.

Engine when cold still sqeaks at TDC.

Any suggestions on getting the 22K back?
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Old 06-14-2010, 12:41 PM
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Default RE: Jett QM Questions

I just had the same thing happen to me last night.. Changed the plug and all was good.. The plug looked good and seemed to glow fine but obviously it was short on heat..
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Old 06-14-2010, 01:00 PM
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Default RE: Jett QM Questions

Thanks Lee..
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Old 06-14-2010, 01:32 PM
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Mikey D
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Default RE: Jett QM Questions

I'm thinking one or your issues is your selected prop. We run a 7.4 x 7.5 Prop, 7.6 may be just a smidge to heavy?

Should peak 24 to 25 but most likely 25+ for a good motor. Launch around 22.5 to 23 with a 7.5 prop.

BTW, I am speaking in context of the QM2

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Old 06-15-2010, 07:02 AM
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Default RE: Jett QM Questions

The 7.6 should spin up to 22K no problem.. Still think it's the plug..
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Old 06-15-2010, 12:41 PM
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Default RE: Jett QM Questions

take the muffler off and see if the sleeve has turned in the case. check the headbolts and retorque them. try a new plug, double check the prop and make sure its not a 8 pitch.
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Old 06-16-2010, 12:14 AM
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bigtrev@xtra.co.nz
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Default RE: Jett QM Questions

When do you take the glo leads off the plug...........the JETT is a cold engine and needs, I believe the leads on the plug until after you've needled it to full rpm- then take them off the plug.what plug are you running?Makes a difference.....suspect you're plug is the problem though.
regards trevor hnz
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Old 06-16-2010, 03:15 AM
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Default RE: Jett QM Questions

Trevor what do you mean by the jett is a cold Engine?
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Old 06-16-2010, 02:26 PM
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Default RE: Jett QM Questions

Dave.....takes a little while to warm up. Thats why you cannot treat/tune like a nelson, big tnz
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Old 06-16-2010, 02:33 PM
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Default RE: Jett QM Questions

Trevor I thought that would apply to any engine ? Warm up before needling.
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Old 06-16-2010, 03:06 PM
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bigtrev@xtra.co.nz
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Default RE: Jett QM Questions

yes Dave, but the JETT is different, TH,
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Old 06-16-2010, 11:56 PM
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Default RE: Jett QM Questions

Trevor can you please explain what is the difference of the Jett engine starting proceedure, compared to the Profi & the Nelson Engines?
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Old 06-17-2010, 03:13 AM
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Default RE: Jett QM Questions

yes, I'll show you sometime, the three engines behave very differently at the warmup stage, TH
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