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Thread: Brief loss of power / bogging around 2800rpm? Otherwise fine... XB9

  1. #1
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    Brief loss of power / bogging around 2800rpm? Otherwise fine... XB9

    Hello again my Buell dudes & dudettes!
    So generally speaking, the bike is running really good. I have noticed about the half the time (?) that the output from the engine seems kinda inconsistent. Like the title says there's a real dead spot of power about 2600-3100 RPMs. Is this just the nature of the beast? Or should I tweak my fuel maps? The bike is mostly stock - engine wise. I'm wondering if the air filter but no aftermarket exhaust isn't jivin' with the maps. Overall, the fuel maps upgrade has been wonderful.

    2007 XB9 w/ K&N air filter, race fuel maps, stock exhaust, crankcase breather mod thing. That's pretty much it. 24,xxx miles.
    Found this dyno curve on the internets - look at that big dip in torque from 2500 - 3800 RPM!! (the stock dots not with race kit) That's the biggest reason I ask if this is the nature of the beast. We all know the internet is full of truths...
    158547.jpg

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    Senior Member 34nineteen's Avatar
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    Is everything else up to par? Is the fuel pump putting out enough pressure/volume? Do you have leaky intake seals? Are your valves starting to leak? What are your spark plugs looking like? Lazy O2 sensor? Clogged/leaky injectors etc?

    If you are having mechanical issues such as above, all the fuel map tweaking is going to do at best is maybe cover up a problem. Being that you didnt mention anything similar to the above, I'm going to guess you just plugged in the computer. Hell, it could just be a tank of bad gas.
    Last edited by 34nineteen; 01-20-2021 at 04:30 PM.

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    Senior Member Cooter's Avatar
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    I would:
    1) Dyno the bike to see if your butt-dyno is even correct, or if you are chasing a ghost.
    2) Datalog your bike to diagnose any issue you find.
    3) FIX whatever problem you find that makes it run poorly.
    4) Leave the tune alone, UNLESS you can prove it is the problem BEFORE you change it.

    The term 'tune' is misleading and makes people think that it will repair a problem, it won't. It will only repair a problem with the other TUNE.

    The easiest way to prove the tune is the problem is to install the stock tune back on it. If the bike runs and rides perfectly, then, and ONLY then, you need to modify the modified tune you put on it.

  4. #4
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by Thunderhead View Post
    Found this dyno curve on the internets - look at that big dip in torque from 2500 - 3800 RPM!! (the stock dots not with race kit) That's the biggest reason I ask if this is the nature of the beast. We all know the internet is full of truths...
    158547.jpg
    Yes, this is normal for the stock XB9 muffler, or a 12 with the valve disabled. The "ASB exhaust shootout" dyno charts reflect the same.

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    Is is bogging or is it stumbling at low speed? Either way that is NOT the nature of the beast. Are you also getting excessive decel popping? Check to see what your AFV is. If you aren't up in the mountains it should be ~100.

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    I guess that's the thing - I wouldn't say it runs 'poorly' at all. Spark plugs are pretty fresh, air intake is properly sealed (unlike that fiasco from a year ago). Pretty much no decel popping. Last I checked, the AFV was spot on - I will go check it again soon. Never datalogged before. That's where you hook up EcmSpy, put the laptop in your backpack, and ride around??
    The only time I really notice it it when I'm getting on it pretty hard, say about throttle 75%-100%. It just seems very unusual for a torque curve of any vehicle I've ever driven.
    Not sure how to best explain this, from an aggressive start (0-20 mph) there's a good initial pull then the power kinda 'falls off' about 2700-3100 rpm, then once you get through that - it's proper linear, powerful torque pulling you towards the horizon.
    Looking at some more graphs n stuff (I know some of these are not exact models but close)
    Seriously, look at that torque drop off around 2500-3000! I get it's tuned to be more rev happy/higher rpm horsepower. I've just never felt such a weird dip in torque mid-power band (ish). Think of driving a Honda (car not moto): there's no guts down low but once you rev it out, there it is. It's not like there was an initial push of torque and then it goes away. It's pretty consistent (consistently not there! hey o)

    I'm thinking this might not be an issue and I was more like I am musing about the quirks of Buells out loud. Hopefully.
    Check it, Torque curves.
    torque.jpg
    torque2.jpg

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    Senior Member Cooter's Avatar
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    You shouldn't be WOT at 2k RPM anyway. Downshift, man!

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    Senior Member d_adams's Avatar
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    wtf? If he's in first gear, why the **** not? WOT in 5th will suffer a little, but not in 1st. Whoolies man, whoolies. Or is that wheelies?

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    Senior Member d_adams's Avatar
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    Also, an xb9 in stock trim will only make 75 hp and maybe 60-63 ft/bs at the wheel, maybe.

    I just dyno'd one back in November with an exhaust (mine) tuned to match and it made maybe 85 hp and about the same for torque.
    XB12 made about 95/95 for hp/tq. Way too easy to loft the front end with these bikes if they're tuned correctly. No flat spot on a 9, just linear power. Bike in question was night & day difference, previous exhaust was a jardine that wouldn't run for ****.

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    Exactly. This is my test run . Stop sign leads to highway on ramp. Starting in first, so I'm hitting 3K rpms just after the intersection = torque drop & WTF moment. Then the banshee starts screamin' after that. I'm not rolling around town lugging it. Occasionally I caught caught out in a unexpected slow down (no turn signal doofus) and a quick jump to get going = catches me out of gear. I pretty much cruise at 3 - 4k RPMs in general, so it's usually ready to go.
    torque drop.jpg



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