Mid Range stumble/misfire - MVAgusta.net
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post #1 of 23 (permalink) Old 03-30-2013, 05:58 PM Thread Starter
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Mid Range stumble/misfire

My bike has developed a stumble coming on at about 5500 and clearing by 8000rpm. It runs fine at idle and revs out fine from 8k to redline. I tried pulling the fuse out of the power commander, but it didn't make a difference. My bike already has iridium plugs and the simplified fuel lines with a new filter in the tank about 4k miles ago. It ran fine until recently and the stumble seems to be the only problem. Before I start throwing parts at it is the anything that might cause a 100% repeatable predictable misfire at a specific rpm only? I am about due for a valve check and maybe a throttle balance but this seems like a fuel or ignition problem as the misfire returns on decel, and there is a lot of popping which it never did before.
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post #2 of 23 (permalink) Old 03-31-2013, 07:37 AM
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Hi Al! How many miles total on the bike? Have you had the valve adjusted before? And, finally, is is starting OK or is it getting a bit hard to start?

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post #3 of 23 (permalink) Old 03-31-2013, 08:04 AM Thread Starter
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Hey chuck! Just about 15k now. Valves were done at 7500, no adjustment needed. It's on my to-do list to check again shortly. The bike starts up perfectly and seems to run fine everywhere else, which almost makes me discount an air leak of some kind, though I'm not discounting anything at this point. If I had to guess, I'd say it feels like too much fuel, it feels like its loading up on fuel right at 5500 and on decel in this range it's popping a load of flames out the pipe
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post #4 of 23 (permalink) Old 03-31-2013, 08:08 AM
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Interesting. If it were getting hard to start, I'd say valve adjustment. Keep us posted and I'll think on it a bit.
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post #5 of 23 (permalink) Old 04-08-2013, 08:20 AM Thread Starter
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The valves checked out, throttles were a tiny bit out of balance 1-2 vs 3-4, and I reset the TPS very carefully, for shits I put new plugs in too. Its running better but there is some issue with the mapping.

I'm pretty sure the timing is jumping around to sections of the map where it shouldn't be. The last time I rode it, it was idling at almost 2000, but if i tapped the choke lever up and back down the idle would settle. The mechanicals and TPS show the throttle is fully closed when its idling high, so I assume its timing related. I might have a bad TPS or other sensor. Steady state throttle cruising is pretty much unrideable now.
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post #6 of 23 (permalink) Old 04-08-2013, 06:12 PM
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Did you check the coils.
My Brutale 750 has had similar symptoms when only one coil was partially defective.
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post #7 of 23 (permalink) Old 04-10-2013, 08:02 AM
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Hello to everyone!

I have the same problem with my Brutale 750s (MY'04). It stumbles from 10000 until 11500rpm. I have change recently the two muflers with the original MV Corse muflers. Before that i had change the air filter with a DNA one, i had also change the catalytic converter with an arrow pipe and the eprom chip with an MV Corse.
Has anyone any idea why is that happening?
Thanks in advance
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post #8 of 23 (permalink) Old 04-10-2013, 03:02 PM Thread Starter
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Quote:
Originally Posted by Miko View Post
Did you check the coils.
My Brutale 750 has had similar symptoms when only one coil was partially defective.

I am going to try the diagnostic software and see what it says. Thanks for the tip!
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post #9 of 23 (permalink) Old 04-10-2013, 05:47 PM
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Quote:
Originally Posted by Al- View Post
I am going to try the diagnostic software and see what it says. Thanks for the tip!
Actually diagnostic software won't be of any help, even testing a coils using multimeter as described in workshop manual (while they are cold) probably won't be of any use, at least it wasn't in my case...
I indentified the fault coil (4th cylinder) by obvious increasing pressure in vacuum line while miss firing, after that i swapped coils on 1st and 4th cylinder and of course increasing pressure now occure on vacuum line attached to intake on 1st cylinder while miss firing.
Before that my (authorised) mechanic spent two days working on bike, diagnostic software didn't show any problems, TPS was set correctly, vacuum balanced ???? after that they changed spark plugs, checked fuel pump, pressure regulator, fuel filter, hoses, even checked injectors (cleaned all and then instaled 2 new) ... ALL THAT FOR NOTHING!!
Btw, I list all above so maybe you'll find somethin useful in my experiance
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post #10 of 23 (permalink) Old 04-10-2013, 06:12 PM Thread Starter
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It will be helpful to eliminate all the other possibilities and sensor failures I hope. I'll try to locate some coils in the mean time as it seems like we're on the right track.
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