919 Suddenly Runs Lean - Wrist Twisters
 
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post #1 of 8 Old 06-25-2018, 06:11 AM Thread Starter
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919 Suddenly Runs Lean

My 919 has always run rich, as they are known to do. Suddenly, however, it smells lean and seems a little weak in low revs. Once she spins up, though, she still pulls. I'm trying to figure out why the hell my mixtures just flipped to the other end of the spectrum though. 2007 CB900, 27k miles, stock as frig. Did a bafflectomy over the winter but it ran same as always without the baffles until recently. I'm planning on looking at the plugs, checking the air filter, maybe checking the throttle bodies. It is quite overdue for a valve adjustment, but if that was the cause, I would think the change would be way more subtle.
Any ideas?

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post #2 of 8 Old 06-25-2018, 07:56 AM
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I'd think it would be hard to tell if the issue is that the bike is running lean. Looking at the plugs and filter are good ideas but also look at the fuel pressure regulator (FPR).
The FPR is a common failure with our 919's and, if that's the problem, the bike will suddenly run poorly. Doing a search on this forum will show lot's of good references for diagnostics and repair. It's pretty simple - pull the vacuum line and check for gas. If there is gas or evidence of gas in the line then the FPR diaphragm has failed.
If none of those things solve the problem then I'd look at electrical connections and grounds.

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post #3 of 8 Old 06-25-2018, 08:11 AM
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If you think it's lean, especially in the lower part of the RPM range, then I'd be looking for an intake leak someplace. Won't be easy to find necessarily but certainly a good candidate.

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post #4 of 8 Old 06-25-2018, 09:07 AM Thread Starter
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Good suggestions, I remember on my old V65 the guys on the forums taught me a little trick to finding intake leaks: pass an unlit propane torch around the carb boots (throttle bodies in this case) and listen for changes in idle. I'm in the middle of a move right now so it'll be a bit before I can get to troubleshooting but I'll update once I get to looking. Thanks.

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post #5 of 8 Old 06-25-2018, 09:55 AM
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Flapper valve been disabled? If that line sucks in air it runs like shit.

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post #6 of 8 Old 06-25-2018, 10:38 AM Thread Starter
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Is that the doohickey that recycles exhaust gas to burn off sunburnt fuel? Either way, not yet. Think maybe its sucking atmospheric air in instead of exhaust gas and leaning things out? IIRC that should be a fairly straightforward connection to inspect.

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post #7 of 8 Old 06-27-2018, 07:07 AM
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Quote:
Originally Posted by Whelan View Post
Is that the doohickey that recycles exhaust gas to burn off sunburnt fuel? Either way, not yet. Think maybe its sucking atmospheric air in instead of exhaust gas and leaning things out? IIRC that should be a fairly straightforward connection to inspect.
I believe you're thinking of the pair valve, which injects clean air after the combustion chamber to burn unburnt fuel. The flapper valve is on the intake snorkel on the left side of the bike and is actuated by a vacuum line. Many people have disabled this valve by plugging the vacuum line.

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post #8 of 8 Old 06-27-2018, 08:08 AM
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Quote:
Originally Posted by Whelan View Post
Is that the doohickey that recycles exhaust gas to burn off sunburnt fuel? Either way, not yet. Think maybe its sucking atmospheric air in instead of exhaust gas and leaning things out? IIRC that should be a fairly straightforward connection to inspect.
No it isn't.
LDH is referring to a vacuum line that energizes the dashpot at the flapper valve resident in the inlet snorkel of the intake system.
If that line is not properly plugged, you get the result he described.
For that matter, split/cracked vacuum lines anywhere will cause grief.
Perhaps you have aged out vacuum lines elsewhere, if the line to the flapper valve dashpot is properly plugged and not leaking anywhere along its length or attachment points.

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