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Old 11-20-2012, 06:49 AM   #1
Grad OP
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What is causing this engine stumble? (video)

I have a 2007 Triumph Tiger with 15,500 miles on it. Under VERY light throttle above 4,000 rpms, the engine has a stumble to it. Under regular throttle it runs very strong and has no issues, but if I am coasting down a hill at 4500 rpms and barely have the throttle open, the bike will lurch like I have chopped the throttle off and back on. You can see evidence of it in this video:

http://youtu.be/uwrcT4Mn25A

I thought it was an ignition miss, but a mechanic I showed the video to thinks it is too "smooth" to be a miss, and more of a fueling issue. I believe the previous owner changed the plugs at 12,000 miles. I am just trying to narrow down the problem to see if I should order any parts before I tear into it (I live a ways from a dealer, so I am going to buy off Ebay and then resell the ones I don't need).

I would love to hear opinions from experienced mechanics.
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Old 11-20-2012, 03:24 PM   #2
LuciferMutt
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Partial throttle lurching is badly calibrated fuel injection. Not much you can do about without some electronics trickery.

When is that bike supposed to have the valves adjusted? Throttle bodies sync'd?
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Old 11-20-2012, 03:56 PM   #3
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the TBS are synced, and the PO said that it had a 12,000 mile service which includes checking the valves, but I'm not positive it has been done.

The strange thing is it only does it above 4,000 rpms, and ONLY does it when the throttle is open about 1%. Otherwise the bike starts strong and runs like a scalded ox.
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Old 11-20-2012, 04:46 PM   #4
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Quote:
Originally Posted by Grad View Post
The strange thing is it only does it above 4,000 rpms, and ONLY does it when the throttle is open about 1%. Otherwise the bike starts strong and runs like a scalded ox.

That's really not that strange. A ton of fuel-injected bikes from the 2000s don't have a good on/off throttle transition. Have you tried googling your issue? I'd bet dollars to donuts (wtf does that even mean) that you're not the only person complaining about this problem.

Hell -- with drive by wire in cars these days, I've driven plenty of cars that have this problem.

Sometimes throttle tamer "cams" can help with this?
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Old 11-20-2012, 04:48 PM   #5
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The EPA is to blame.
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Old 11-20-2012, 05:26 PM   #6
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Quote:
Originally Posted by LuciferMutt View Post
I'd bet dollars to donuts (wtf does that even mean)
http://en.wikipedia.org/wiki/Dollars_to_donuts

Dollars to donuts is a faux bet in which one person agrees to put up the same amount of dollars to another person's donuts in a bet (where a donut is considered to be worth much less than a dollar). Betting someone dollars to donuts is a rhetorical device that indicates that the person is confident in the outcome of an event, but it does not usually involve an actual bet with actual payoffs (either in dollars or in donuts).

Betting dollars to donuts may not be such a good bet today. When this phrase was coined, you could buy many donuts for a dollar. Today, when a donut often costs more than a dollar, the bettor who bets dollars to donuts is actually making a bad bet.

Other versions of this phrase have been used in the past: "The almost forgotten terms 'dollars-to-buttons' and 'dollars-to-dumplings' appeared in the 1880s, meaning 'almost certain' and usually used in 'I'll bet you dollars-to-buttons/dumplings.' They were replaced by 1890 with the more popular 'dollars-to-doughnuts' (a 1904 variation, 'dollars-to-cobwebs,' never became very common, perhaps because it didn't alliterate)." [1]
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Old 11-20-2012, 07:36 PM   #7
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Thanks Sheldon
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Old 11-20-2012, 11:05 PM   #8
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I'd also recommend testing your throttle position sensor.
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Old 11-21-2012, 02:08 AM   #9
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Flash your ecu: http://www.tuneecu.com/
These guys have various maps, besides stock. Maybe your bike is modded and needs different settings.
Also, check your charging system. Upgrade to a mosfet regulator/rectifier and check your stator coils.
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Old 11-21-2012, 06:02 AM   #10
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Thanks so far everyone.

I have Googled the crap out of this and turned up completely empty. I have threads running on both Tiger boards and nobody has seen this before. They all agree that this is not normal behavior at all. I have loaded several different fuel injection maps and it hasn't changed the problem. They are giving me some ideas to check out, but nobody is certain what the issue is. My TPS is a little out of spec and I do have a high idle (1600 instead of 1150 rpms), but so far most feel that the TPS wouldn't cause this particular problem. I will try to fix the TPS, but I am trying to gather more information before I tear the bike apart (tough to get to the TPS) so I can check other things that could be the culprit while I have it torn down.

I was laying awake last night trying to figure out what the issue is, and I have come to the conclusion that this isn't some little engine miss or misfiring spark plug. It is way too abrupt and final. Essentially, it is like the bike is in decel mode for about a second. It isn't missing, it isn't stumbling, it is like I chopped the throttle completely closed, waited a second, then resumed exactly where I left off. Perhaps like something builds up, releases, then starts to build again. Any idea at all what could cause this?
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Old 11-21-2012, 09:26 AM   #11
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Non similar bike but i had a cannondale that would not run right when hot. Ran fine for short trips then the throttle would be goofy for lack of a better term.

I ran around replacing coils and such but replacing my tps fixed my issue. I think setting your idle to stock and syncing your tps would be a good start. You can check the resistance of your tps with an ohm meter and see if there is a spot where its value drops significantly.

Hope you figure it out.
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Old 11-21-2012, 10:58 AM   #12
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You don't sound certain that your plugs actually were changed. If that's the case, how hard is it to throw in another set and examine your plug wires and caps while you're at it?
I don't endorse throwing parts at a problem but this could be a simple precaution.
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Old 11-21-2012, 12:49 PM   #13
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It's the rev limiter, All FI vehicles have them. With some bikes, they been set around 4 to 6K when free revving to keep you from damaging the motor. Some bikes don't have them set at all, but most touring bikes do. If you rev it a little higher, the limiter will be more pronounced.
You don't feel it while you're riding, right ?
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Old 11-21-2012, 01:03 PM   #14
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Quote:
Originally Posted by Grad View Post
My TPS is a little out of spec and I do have a high idle (1600 instead of 1150 rpms), but so far most feel that the TPS wouldn't cause this particular problem.
Watching that video and having a 1600rpm idle? Put me with the least who suspect TPS.
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Old 11-21-2012, 05:08 PM   #15
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Tuneboy on my 2007 Tiger 1050 and it goes like a bastid with NO BAD MANNERS, anywhere in the fuel curve. Sweet running everywhere. I never rode it stock, bought with this in there.
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