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Discussion Starter #1 (Edited)
Hello,

My my motorcycle is Yamaha MidnightStar 950 (2009). The issue is: I cannot get speed higher than 140 kmph (85 mph) with a full throttle on at the 5th gear. I've already changed spark plugs to a new ones, cleaned fuel filter, fuel pump and air filter, checked injectors (was clear as I think), tried to use 92 and 95 octane fuel - but no success. I cannot gain speed. Besides, sometimes when I pull throttle till the end, it feels like loosing power (rpm) on a peak at any gear. What could be the issue? :|
 

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Discussion Starter #3 (Edited)
I've purchased this bike at 15.000 km, current run is 18500. I didn't test it's max speed when I've purchased it, but main differences from stock is:
1. I've made a drill mod on stock exhaust (had same issue before the drill mod too)
2. The air filter is K&N

Here is the video where I'm riding on the full throttle at the 5th gear. The speed won't go up above 140 kmph :(

https://youtu.be/h226dFwa0Kw
 

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when i removed a baffle and opened the stock breather slightly on my 650 i lost some mid range torque and top end speed, i had to get more fuel to the engine to get it running perfect again, i don't know if the 950 ecm can be re mapped or re programed, if not a power commander might be the only way get it right, with a power commander and the bike on a dyno it can be dialed in perfect, it's lot easier and faster. it's expensive but it's a one shot deal. you can't really test an injector for a proper spray pattern, they can test fine electrically and be bad mechanically, what i'm saying is clogged injector will test good
 

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Agree with pauli. Altho i have no precise knowledge of the 950 if ur basics are in order then do some research to see if you need a programmer or reflash of ecm for running those mods. An injector being fired at a stock pulse width will also test good but the ecm would be firing it based on stock parameters. How is power and performance throughout the rest of your range? Is it only top end that seems lacking?
 

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Discussion Starter #6
As far as I feel, the isssue is only at the top end. I'm thinking that there is an issue with injectors or fuel pump. I hadd top speed issues before the drilling the exhaust, so I don't think drilling may cause such problem.
 

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Changing air filter or drilling a few holes in the exhaust should most likely not cause an issue like that. That stuff normally doesn't change AFR a whole lot.

FI is super nice, until something doesn't work right. Without the proper diagnostic tools, you're just guessing.....and that never seems to work when you have a computer working against you trying to cover up the symptoms.

Please let us know what you find...
 

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Discussion Starter #8
Changing air filter or drilling a few holes in the exhaust should most likely not cause an issue like that. That stuff normally doesn't change AFR a whole lot.

FI is super nice, until something doesn't work right. Without the proper diagnostic tools, you're just guessing.....and that never seems to work when you have a computer working against you trying to cover up the symptoms.

Please let us know what you find...
Yes, without computer diagnostics it'll be just a guess and nothing else. Unfortunately our official Yamaha dealer's support sucks. Does anyone could post a link where can I buy Yamaha Diagnostics Cable that will fit xvs950a? I would like to do diagnostics manually.
 

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Is there a possibility that the Rev Limiter (an ECU function) is causing the problem? Could it be programmed wrong or malfunctioning? Just a thought. Still need the diagnostics.
 

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Or a magic 8 ball...
 

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can you max it out in fourth gear? is the top speed in fourth higher then the limited speed in 5th?
Speed isn't limited in any gear....only RPM's are limited....and that is the same point no matter what gear you are in.
 

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i understand the rpm is limited. but if it is a fuel or exhaust restriction, then top speed in fourth would also be effected. maybe not to the same degree, but still would be.
 

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Discussion Starter #14
Hello guys,

Sorry for the late reply. Was waiting for a part to be shipped and received. So, the issue was with the Fuel Pump (P.N.: 5S7-13907-00-00) located in tank. I've replaced the part and it works like a charm. Thanks a lot for help to everyone.
 

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Hello guys,

Sorry for the late reply. Was waiting for a part to be shipped and received. So, the issue was with the Fuel Pump (P.N.: 5S7-13907-00-00) located in tank. I've replaced the part and it works like a charm. Thanks a lot for help to everyone.
How did you diagnose it to the fuel pump? Did you do a pressure check?
 

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Discussion Starter #16
How did you diagnose it to the fuel pump? Did you do a pressure check?
Nope I didn't. I've just found a technician who has nice skills. He told me to change the filter and pump. Too complicated to explain why I've trusted him :grin: Just in case - the quality of petrol in our country sucks badly ))
 

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I'm having similar issue, probably my fuel pump as well, fuel filter is integrated and not replaceable, fuel pump assembly is like $500 so I want to be sure before replacing it. Same year as yours but it did sit for a while which may be a contributing factor. Did you replace just the pump or the whole assembly?
 

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I would test the pump for pressure and flow before coughing up $500.

Also how much have you run the bike after it was sitting for a while? Seafoam has magical powers.
 

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you tube shows most people using seafoam with a carb, is it possible to just dump it in the gas tank? I'll have to preorder the fuel pump gasket as no one locally stocks it before i mess with it.
 
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