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Discussion Starter #1
Bike ran great this morning, and in the afternoon when I went to another office for a meeting. Right around the time the gas light came on, the bike stumbled a few times. I thought nothing of it, figuring it could be fuel starvation due to the low fuel level. After fueling the bike (3.4 gallons so it wasn't that emtpy) it was nearly impossible to ride home - severe stumbling at low rpms, with the occasional 'thump' audible from inside the airbox. Had absolutely no power from a stop, requiring quite a bit of clutch slipping to get it going.

The bike will idle, but it's a little low and it won't idle for long before it shuts off. I checked all of the sensors (coolant, pressure, air temp and timing) with the exception of the TPS, and they all appear to OK. My first thought was the no-name VR on the bike was flaking out, but the battery is charging at 13.5v at 3K. The AC output of the stator is a little low though. I have not checked the plugs or the plug wires yet, though I did verify that they were seated firmly on the coils.

I recently did the 12K service - belts, spark plugs, fuel filter and hoses and it's been running fine for the past 300 miles. It seems like an electrical issue and not a fueling issue...bad coil maybe?

Could it be a bad vent line for the gas tank? That's one thing I didn't do - check if it ran better with the gas filler open, though nothing seemed out of the ordinary when I filled it.


John
 

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Hmmm, definitely the easiest thing to check is the under side of the gas tank and the fuel lines. Make sure there's no leak or clogged tubes or etc. Then move on to the electrical bugs if that doesn't fix your problem.
 

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Hmmm, definitely the easiest thing to check is the under side of the gas tank and the fuel lines. Make sure there's no leak or clogged tubes or etc. Then move on to the electrical bugs if that doesn't fix your problem.
You should check the fuel lines inside of the tank as well... Open the filler cap, start the engine, and look / listen for swirling and excessive fuel movement. This would indicate that a fuel line is split or is loose. You should see a brief swirl in the fuel when you turn the key (as a result of the fuel pump priming). Any more than that indicates an issue with the lines....
 

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Maybe the fuel filter? You said it happened when you ran it low on gas, maybe some debris that was populating the bottom of the tank got sucked in and is choking the fuel supply? Just a guess.
 

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Discussion Starter #7
Thanks for the ideas, guys. I doubt the tank has much crap in it; when I did the fuel filter I replaced all of the hoses and clamps too.

The more I think about it, the more it seems like it could be bad gas, but it doesn't explain the stumble right before I pulled into the gas station.

Well, I'll be draining the tank anyway so I won't be putting that gas back in.

John
 

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Maybe the fuel filter? You said it happened when you ran it low on gas, maybe some debris that was populating the bottom of the tank got sucked in and is choking the fuel supply? Just a guess.
+1 I was thinking the exact same thing.
 

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Thanks for the ideas, guys. I doubt the tank has much crap in it; when I did the fuel filter I replaced all of the hoses and clamps too.

The more I think about it, the more it seems like it could be bad gas, but it doesn't explain the stumble right before I pulled into the gas station.

Well, I'll be draining the tank anyway so I won't be putting that gas back in.

John
Check your hose connections inside of the tank... it has been a common occurrence that the clamps have let loose after the hoses have been replaced....
 

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Definitely seems like a split fuel line inside the tank, dirty fuel filter or both. How long has it been since you changed out the hoses and fuel filter inside the tank? It's obviously starving for fuel or the fuel pressure is too low.
 

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+1 on the fuel. Also ensure you don't have a vapour lock going on (if the fuel cap has suction behind it or makes an audible psssh noise when you open it, a vent hose is blocked or kinked). Give the throttle bodies a glance as well to make sure you don't have a plug coming loose, or a loose screw (where you check cylinder balance).

Double check the torque on your spark plugs too. I recently forgot to torque a plug properly on my VF when I changed the plugs. Took me a while to figure out where that funny noise was coming from...
 

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Discussion Starter #12
Well, it wasn't bad gas, nor is it a split fuel line in the tank. Bike runs great when it's cold, but once it warms up the issues start. Disconnecting the ECU water temp sensor doesn't change the behavior, either - it still runs great when cold, and stumbles once the temp rises.
I tested the sensor with a MM, and it's giving borderline 'good' readings at low temp, but once the temp hits 120, it starts to fall off.

I've found the cross reference for a bosch part number, so for $14 I think I'll give it a try.

John
 

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Check your hose connections inside of the tank... it has been a common occurrence that the clamps have let loose after the hoses have been replaced....

Was thinking the same... is possible something wasnt tightened properly or has worked loose/fell off?? :)
 

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Well, it wasn't bad gas, nor is it a split fuel line in the tank. Bike runs great when it's cold, but once it warms up the issues start. Disconnecting the ECU water temp sensor doesn't change the behavior, either - it still runs great when cold, and stumbles once the temp rises.
I tested the sensor with a MM, and it's giving borderline 'good' readings at low temp, but once the temp hits 120, it starts to fall off.

I've found the cross reference for a bosch part number, so for $14 I think I'll give it a try.

John
The temp sensor is of a thermistor type and the readings should lower when the temperature
rises.or have i misread it and they go out of spec.

brian.
 

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Well, it wasn't bad gas, nor is it a split fuel line in the tank. Bike runs great when it's cold, but once it warms up the issues start. Disconnecting the ECU water temp sensor doesn't change the behavior, either - it still runs great when cold, and stumbles once the temp rises.
I tested the sensor with a MM, and it's giving borderline 'good' readings at low temp, but once the temp hits 120, it starts to fall off.

I've found the cross reference for a bosch part number, so for $14 I think I'll give it a try.

John
Aha... I had a similar problem with mine. What would happen is when the temp got into the normal range it would hit a wall - giving any more than 5% throttle would cause it to cut out. You could accelerate, very slowly, but give any more gas and it would feel like you were hitting the limiter. It would idle and cruise normally though. I replaced all the FI sensors (coolant temp, air pressure, air temp, the only one I left alone was the crank position sensor) and that cured it. I've heard that improper spacing of the CPS can cause a similar issue too. So you are probably on the right track.

The air sensors are located in the nose area - the density sensor is mounted on top of the coolant tank, the air temp sensor is attached below the headlamp bucket. Check those while you are at it.
 

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Discussion Starter #16
The temp sensor is of a thermistor type and the readings should lower when the temperature
rises.or have i misread it and they go out of spec.

brian.
You're correct, the readings lower as the temp rises. The one I replaced was somewhat linear until 120 or so, then it would just hang.

End result, a $15 temp sensor for '95 BMW 318 fixed the issue.

John
 
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