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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: skidmore on March 10, 2015, 07:46:21 PM
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I am puzzled. My MPG has plummeted from a typical low to mid 40s MPG to 26.6666666667 MPG. Here is the tale the best I can tell it:
Up until the time I parked it at the end of the season in mid November I never noticed anything awry. The filler neck is not present. Normally I top it off to the brim more or less but since I was preparing to have her set the entire Winter I left some room for expansion. Probably could have pumped another 0.3 gallons but I stopped pumping early, added my Sta-Bil and that was that.
The day after Xmas was gorgeous and 60 degrees so I rode about 110-120 miles. I noticed while tooling down the road the fuel light lit up at around 90ish miles. Hmm. Seemed a little soon, but considering I left some expansion cushion, 90ish miles isn't too out of the ordinary. I call the light my 2 gallon light. The couple years I've owned the bike the light consistently lights up around 115-125 miles if I'm running sort of hard 70-75mph. If I'm running easy around 60mph it would light up around 140 miles. So again, considering the tank wasn't fully topped off, seeing the light come on at 90ish didn't startle me. I filled up a mile from home, this time pretty much topping it off as I normally would, not giving a crap about expansion. I didn't bother to calculate MPG because I wouldn't have a real solid idea what it would be because there was no way I could stop filling up at the same exact level as when I started the ride.
Spring is springing for me and I took my first ride of 2015 today. At 83 miles, the fuel light began to glow. WTF? I didn't freak too much because I was on the shoulder of an entrance ramp which had a downhill grade to it. I expected the light to go out once I got on the superslab leveled off. It didn't go out. Hmm, strange. I started the ride with the tank pretty well topped off.
I hit town and decided to fill up just to see how much it required. 3.34 gallons after 88 miles which equates to 26.6666666667 MPG!
The lowest I have ever averaged was low low 30s and that was when I was charging hard 80-90 MPH for a solid 100 miles or so. Today I was cruising 60-65 MPH which typically nets me 44-46 MPG.
What. On. Earth. Could be the problemo? :dunno2:
Worth noting, it seemed to be running fine. No symptoms of ill performance or anything. The temp was 60 degrees. No head or cross winds to speak of. Hell, it would take a hurricane to make me plummet MPGs down into the mid 20s.
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I do believe you should start looking for gas leaks of the high pressure variety.
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No, I hadn't googled yet, but after I did, one of the results brought me right back here to motobrick heaven :clap: Plugs, plug wires, vacuum leak, air filter......looks like I have some places to start. It is still puzzling how the drastic decrease seems to have happen all the sudden rather than little by little over time.
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Wowza! Seems plausible. If it is of the high pressure kind and significant enough to cause mid 20s MPG, should I be able to smell gas?
No, I hadn't googled yet, but after I did, one of the results brought me right back here to motobrick heaven :clap: Plugs, plug wires, vacuum leak, air filter......looks like I have some places to start. It is still puzzling how the drastic decrease seems to have happen all the sudden rather than little by little over time.
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Depends on if it's hittin' the bike at all.
Leave it idling for a bit and check.
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greetings skidmore...
my experience is that old fuel has less energy than new fuel... especially if its gotts some kinda stabilizer in there... i almost ran outta fuel for this very reason... figger i gotts 25mpg on old fuel compared to 40+ mpg on new fuel...
now... when my motobricks comes outta hibernation my goals are... 1-ride them down to a gallon left... 2-filler them up with no corn ethyl and 1/2 20oz bottle of techron... 3-repeat... then im back to 40+mpg...
j o
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Thanks Brad. I suppose tomorrow I will remove some fairing and start hunting.
Johnny, I hope I can be so lucky. I don't recall my first ride of the season last year netting such shitty MPG. All gas is not equal though, I reckon. If I don't detect a fuel leak tomorrow, I will head out to the superslab, wick it up to 80 and see how fast I can burn off the 2 gallons old/3.x gallons new. Perhaps the following tank will treat me better.
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Perhaps mice made a nest in your air filter
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Perhaps mice made a nest in your air filter
Or chewed something else......
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Johnny, it appears you are on your way to being the prize winner. Your "old bad gas" theory seems to fit my problem.
Wednesday afternoon I rode the same route. My topped off gas tank consisted of 3.34 gallons of fresh gas on top of the 2ish gallons of old gas with Sta-Bil. The fuel light came on at 108 miles! Worth noting is a headwind of 12mph on the return leg of the route which amounted to 54 miles. (Yesterday wind was not a factor). Without the headwind I think I could have hit the high teens and maybe 120. Assuming I again burned 3.34 gallons, my MPGs has jumped to 32.333. Reckon I will put another 40-50 miles on the remaining ~2 gallons before filling up again. I need to pull the tank and have a peek at the air filter plus I will feel better knowing, even in its diluted state, the old gas will be nearly gone. I'm feeling pretty good about the "old gas" theory because I didn't find anything blatant during my gander at the fuel system. Read on...
I pulled the lower fairing to have a peek at the fuel line etc. There were no leaks or sprays that I could see. The line(s) that I was able to lay eyes on were dry including the fuel pressure regulator's connections. What wasn't dry was the rear throttle body. It was wettish with black grime. The middle one was clean and dry. The front was grimy but not as bad as the rear and it wasn't wet looking. See the pic below. What could be the cause? FWIW, all three of them got a bath while I was spraying them down with carb cleaner trying to detect a vacuum leak.
(http://i63.photobucket.com/albums/h143/grindking63/IMAG0332_zpsg4lx8hrd.jpg) (http://s63.photobucket.com/user/grindking63/media/IMAG0332_zpsg4lx8hrd.jpg.html)
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The engine vent hose is near the rest TB. Check it for cracks
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The crankcase breather hose? Yup, it is cracked. I can't seem to find it now on any of the online microfishies but do remember seeing it a while back.
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10....Part #11 15 1 460 480.
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There it is! Thanks Inge.
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Ran my tank up to 140 and then filled up. It took 3.9 gallons yielding 36 MPG. I did 121 miles today with no sign of a fuel light. Even if it takes 3 gallons right now to top off, I'd be at 40 MPG. I reckon ol' Johnny was spot-on. Now that i am about 4:1 good gas to old gas ration, the ol' girl is running smoother than 4 days ago when the debacle began.
Thanks for all your feedbacks guys :2thumbup: