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Discussion Starter #1
Im having this weird problem with my 848 trackbike.

It revs freely up to 9000rpm, but gets a stumble/soft limiter at exactly that point in every gear. Just around/below the first light on the limiter.

Ive changed fuel filter, fuel pump, TPS reset and new plugs.
None of the above made any difference.

In neutral or with the clutch held, i can rev it all the way to the limiter.

Ive read probably 20 posts on here about this, but they all point towards the filter/pump.

Any ideas folks?
 

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Discussion Starter #3
Dirty air filter.

Incorrect (iridium) spark plug gap.
Sparkplugs (MAR10A-J) both old and new made no difference.
And wouldnt the bike smoke or cough all through the register if it was the air filter?

But i havent tried the filter, so i can change it to one of the other 3 that followed the bike.
 

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I’d be shocked if an air filter was so dirty it caused a stumble at a certain RPM. I can say I’ve witnessed similar issues from clogged fuel socks or linked fuel hoses, but the issues are typically load based, not rev based. The only other mechanical common failure is sticking of the exhaust servo valve at a certain point in its sweep. Remove the cables and wire it open and Check it again.

If it’s readily consistent at 9k rpm regardless of load (half throttle 9k vs full throttle 9k for instance) I’d start ohming out crank position sensors.


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Discussion Starter #5
I’d be shocked if an air filter was so dirty it caused a stumble at a certain RPM. I can say I’ve witnessed similar issues from clogged fuel socks or linked fuel hoses, but the issues are typically load based, not rev based. The only other mechanical common failure is sticking of the exhaust servo valve at a certain point in its sweep. Remove the cables and wire it open and Check it again.

If it’s readily consistent at 9k rpm regardless of load (half throttle 9k vs full throttle 9k for instance) I’d start ohming out crank position sensors.
Yeah i dont think its the air filter.. but il try and change it just to make sure.

Exhaust valve has been removed, well the cables have. And from what ive understood, the valve is then stuck in the "open" position because of the spring on the valve. i will safetywire the valve just to make sure the exhaust pressure doesnt close it.

Fuel sock was changed with fuel filter. aswell as fuel hose between filter and pump.

would it be an idea to change the speed sensor in the rear with the crank sensor? (from what ive read these are the same type of sensors)
 

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They’re both Hall effect type sensors, but I’m not sure what (if any) relation the wheel speed sensor has with anything aside from showing road speed or ABS/TC function if so equipped.

My only experience with failed wheel speed sensors has resulted in ABS lights coming on or things like that, never rideability issues.


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Discussion Starter #7
Just got back from the track and some testing.
Replaced the air filter to rule that out. No difference.

I did put it in neutral and raised RPM slowly. The RPM raises nicely up to about 9k, it then drops 1-2k before rising again and reaching the limiter.

Any idea what it could be?
Ive sourced another Crank sensor to put on the bike tomorrow for some more testing, also gonna check and recheck all connections to see if that could be the culprit.
 

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Throttle position sensor perhaps? If it's doing it in neutral and every gear, at the same RPM regardless of load maybe the TPS has an anomaly. Check ohm reading as you rotate through throttle opening and see if there are any spikes or dips.
 

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Discussion Starter #10
Throttle position sensor perhaps? If it's doing it in neutral and every gear, at the same RPM regardless of load maybe the TPS has an anomaly. Check ohm reading as you rotate through throttle opening and see if there are any spikes or dips.

I reset the TPS, and did a quick turn of it with the workshops diagnosis tool hooked up, and it showed it going up gradualy. But i wasnt really looking for a fault either, just that i could see it move.

In neutral it does stumble at 9k, go down a bit and then keep on going up, all the way to limiter, but in gear, it just hits a softspot/softlimiter at about 9k and wont rev past it.

If it was the TPS i would think the engine would be having more trouble than just at 9k, seeing as im at 100% throttle from 3-4k and up to 9k out of the slow corners towards the straights on track.
 

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I don't think I can contribute much beyond what's already been posted, except an oddball theory... I've used TuneECU to change the rev limit on my 848 to 9500 RPM... no chance that a previous owner has done something similar?
 

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Discussion Starter #12
I don't think I can contribute much beyond what's already been posted, except an oddball theory... I've used TuneECU to change the rev limit on my 848 to 9500 RPM... no chance that a previous owner has done something similar?
Thanks for the tip. Ive ordered an OBD connector for the bike, so if theres no luck il go down that route. :smile2:
 

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I'll bet that this is crankshaft rotation sensor related. Air gap spec is 0.75 –0.95 mm but half that gap may be needed. A loose flywheel or a cracked engine casing will affect the gap, particularly for a hot engine (thermal expansion increases the gap) at high rpm. A clue is that the tachometer rpm drops during cutout. The rpm signal is sent by the ECU. When it looses the sensor signal it cuts ignition and stops the fuel pump — a post-crash safety feature. An incorrect gap will give an irregular signal causing an intermittent cut-out. An ECU problem, or sensor wiring loom/connector may also be involved.
 

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Discussion Starter #14
Well well well,

About a year after starting this process i have finally found the fault.

The problem was that some previous owner had removed the clutch switch from the clutch master (well its a trackbike, so why not) but they had then bridged the connector with a metal paperclip so they could start the bike in any gear.
This made the ECU think the clutch was always pulled in. And apparently the ECU on these bikes has the possibility to hold the revs at 8-9000rpm. If this is a poormans launch control or why it has this option i do now know, but both Termi ECUs i now own have this, aswell as the stock one.
So there you have it.. a Paperclip has held my 848 captive for over a year.
This is most likely something that can happen to all of these bikes (848/1098/1198) so now we know. Spread the word. Never bridge this connector, unless you WANT a 102hp 848.


Im just happy that i was able to keep up with 1098Rs on track with a limp 102hp.. wonder what 30more will do to my laptimes..:nerd:
 
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