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hyper 21 8 port sputtering

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Old 06-26-2003, 02:34 AM
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lilcap232
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Default hyper 21 8 port sputtering

okay first off here's the deal...
old style tmaxx
XTM 21 conversion with pipe
RPM wide maxx arms
Omega 15% fuel
RRP hardened diffs
RRP forward only conversion
stock tmaxx tires and wheels
72 tooth RRP steel gear
stock clutch and bell


now I have flown planes for years and I am a mechanic/engineer by Profession and have sucessfully ported and polished anything from a bigblock chrysler440 race engine down to my most recent TRX 15 pro that is a screamer as well.....
I am accustomed to tuning glow engines and Think the high and low needles are right the mid was left untouched as instructed...

the 21 has only been up to 268 degrees one time and instantly shut off (verified by temp gun)
I broke it in quite well follwing as best I could since it came with two entirely different methods( one by OFNA and one by HYPER)
I chose the Idle method in which it causes the carb to sweat due to the rich condition and slowly leaned it out after several tanks.

now it Idles fine,starts fine and screams all the way through second gear,however at the upper 2/3rds of first gear I hear this high pitched chirping hiccup that refuses to go away until the rpms drop when it shifts to second gear.....

I have tried it all, anybody have any Ideas or similar problem......
I am beginning to think XTM's pipe outlet may be too small because the engine turns faster at the upper end of first then any part of second hence lack of exhaust flow???
please help.........



Ohh I forgot to mention that on rough gravel I cant get it past this chirping/sputtering stage to shift to second gear, it will only hit second on the pavement.......
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Old 06-26-2003, 01:58 PM
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Kuma
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Default hyper 21 8 port sputtering

OUCH, sounds like it's getting very lean in some spots and is possibly already damaging the motor, could be an air leak. I'd do a full checkup and apply oxygen sensor safe silicone to the carb neck gasket and back plate. Also ensure the middle speed needle (opposite of throttle arm side) is flushed and untouched, I once accidentally was turning it thinking I was doing the LSN! :stupid:

I am an ex-mechanic as well and I thought my experiences would pay off in these little nitros, but they're quite different- I got stumped a few times myself.
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Old 06-26-2003, 01:59 PM
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Legendary
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Default I have 2 XTM .21 Maxx's!

And that is a tuff one! Does it sound like metal to metal? If it does maybe it is the spur gear rubbing against the clutch bell? I had this problem with one of mine and that is what it was! It was too tight a fit and I had to cut out a bit of the chassis where the flywheel is so I could move the engine back just a tiny bit!
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Old 06-27-2003, 12:43 AM
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lilcap232
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Default hyper 21 8 port sputtering

I got it!!!!!!!!!!!!
It was the damned glow plug, I bought a few of the fox BLASTER medium plugs with the engine, I just for kicks installed an OS #8 tonight and now it's off to the LHS for a new forward primary gear and a set of wheelie bars...!!!!!!!!!!!
man this thing screams and also makes for a lower profile rear bumper......LOL wheelies are easily acomplished...

I guess I as usual over looked the obvious...
I do thank you guys for the help. JIMMY

PS. it's running nicely at 225 degrees, consistent Idle as well..
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Old 06-27-2003, 01:36 AM
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Kuma
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Default hyper 21 8 port sputtering

Yes sir, the Hyper 8P really does scream! I don't use it much in my MBX unless it's race day because it's so fast- too fast for practice and bashing. I use a more mellow GS 21 more often.

And that's good you worked out the problem, better yet no damage to your engine.
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