Tuesday, May 25, 2010

Fuel Starvation

I am in the process of trying to debug a problem on my R100RT. On my last ride, I was heading up a moderately steep grade leaving town towards Nenana when I felt a vibration a couple of miles up the grade. It felt like I was running on only one cylinder but it didn't feel like there was a significant loss of power. The vibration was something to do with the engine since I could pull in the clutch and the frequency would drop with engine RPM. After reaching the summit, the vibration continued for a couple of miles then went away. The only time this problem has shown up was when climbing this grade. Last summer, going up the same grade, the problem showed up but it was raining really hard. At that time I assumed the problem was ignition related due to the rain. I carefully checked the coils for hairline cracks but didn't find any. A couple of days ago, I posted a message on the BMW forum and immediately got back a bunch of responses pointing to fuel starvation, something I hadn't even considered. I checked fuel flow to the carbs and got about one cup in about ten seconds which seems to be more than adequate to me. I went ahead and replaced all of the fuel lines since I had ordered the "correct" size fuel line in my last parts order as some of the lines had some small cracks in them. Not enough to leak but possibly getting close. Several of the replies mentioned that carb work was necessary every couple of years on these old bikes especially with the ethanol in the fuel these days. I have not touched the carbs in the three years I've owned the bike so I went ahead and ordered rebuild kits. Something else to work on since I'm still not supposed to be riding for at least a couple more weeks. One of the other suggestions pointed to the tank vent. This allows air into the tank to compensate for fuel used. The suggestion was that the vent tends to clog up and if you start to pull gas out faster than air can bleed in, a vacuum will develop in the tank. The forum describes how to check this out and how to resolve the problem. Something else to check out.

I went into the office for half a day today and my new laptop had arrived. A 15" Macbook Pro with 8GB RAM, I7 processor, 256GB SSD drive, 1680x1050 display and high performance GPU. It took quite a while to restore all of my documents and information from my Time Machine backup to the new machine. It works pretty good. No complaints at all. Battery life seems about the same (contrary to the Apple site) no where near the claimed 8 hours but then again, I haven't calibrated the battery yet.


  1. Richard:

    I'm no mechanic so don't listen to me . . .

    I was thinking that you should ride up that mountain again and try to replicate the vibrations for a mile or so. then pull over and pull the plugs to see which one is WHITE. perhaps one side would be WET and be the cause of misfiring due to too much fuel.

    do you have fresh gas ? how about the fuel filter ? if there was a vacuum you could just open the gas cap to relieve the pressure.

    dont' listen to me, for I know nothing

    Wet Coast Scootin

  2. Thank you for the suggestions, all things I'm planning to try as soon as I can ride the bike again. It was running fresh gas and the fuel filters have been replaced. The flow from the tank to the carbs is 1 cup in a bit over 10 seconds which should be plenty to avoid fuel starvation.

    The gas cap is a real possibility especially since I have use a tank bag and the vent built into the cap.

  3. The gas cap should have a hose coming from the vent. Not sure about BMW, but my ST1100 did. Had the same problem until I cleaned out that tube. The carbs also vent and the bottom of those hoses can get blocked, too.

    As to the laptop, wow! That's almost supercharged enough to run on a track!!!!

  4. There is vent tube running towards the ground. Thank you for the suggestion, I'll check it out this afternoon.