Bonneville America Forums Forums Off Topic Discussion Forums Lounge gas mileage.

I have two problems to discuss with y'all:

1)I topped up the tank the other day and went out on the highway for an extended run on my SM. I got about 105 kms when the bike had to be switched to reserve. Does anyone else have this problem with poor gas mileage? This is only my second season on this bike and have yet to work out the average rate of consumption but my previous America would usually get me around 180 kms before I had to switch over.

2) Everytime I switch from fuel to reserve the bike vapour locks and I have to sit on the side of the road with my thumb up my you know what for 5 - 10 minutes before the bike will fire up again. A plugged vent maybe...Anyone else have this problem?

Yeah, I had this problem. I ended up running a piece of solid wire down the vent hole in the tank filler neck that finally took care of it after I had tried blowing out the rubber vent tube.

105 kms is only a little over 60 miles. That's some seriously bad mileage you're getting. Worst I've ever heard, and to get that kind of mileage you'd think gas was just running out of the tank onto the ground. Something isn't right.

maybe a leak that only happens when moving. Theres nothing that could be wrong enough to get such bad milage. Gotta be losing fuel somehow.

It's not vapor-locking, it's pulling a vacuum. As others have
mentioned, unscrew the gas cap to relieve the vacuum. The
tank must not be venting. There is some kind of a one-way
valve in the vent line that can cause this problem.

More informed members, please step in here.

As to gas milage, you must be leaving a trail!

Uncle Charlie

You're the only person I've heard who has worse mileage than I do. In mixed highway and city driving on my last tank, I turned over to my reserve at 112 miles and ran dry (again, but this time I was rolling into a station) at about 130.

Its been pretty consistant for me to hit reserve around 95 to 100 miles and then take about 3 gallons. I sit on the bike to fill it and hold it level to get to 3 gals. I am not a little guy (255 lbs.), so I'm sure that has something to do with it - I'd use a lot of fuel if I had to pull my fat a$$ around all day...

Running stock America, Nancy on back about 350 lbs.
STILL didn't get that bad of milage! Bout 140 never
hit reserve.

FJ,
I was afraid I was the only one with such a heavy right hand.
On the trip to the New Hampshire Rally, and back, I got
39-40 on the highway, and 36 in the pouring rain, stop & go.
I was running around 80 on the slab, with the occasional
blast to 100. (speeds as indicated, probably 10% less)

just got back from a trip to fresburg,ca.
with twisties, cross towns and the freeways up to 80. i still got 46 mpg. something is very wrong.need to check it out.

What's your set up, and how hard do you run?
I think I have 135/45, 1 shim, BUBs, AI gone,
snorkle gone, and a UNI filter, and about 1/4
of the time run it to the limiter. Man, these
mods feel great.

She pulls,
Uncle Charlie

I think these bikes get better mileage running gallons versus litres through them. Litres being smaller than a gallon, they run through the carbuerator plumbing and engine faster??

But seriously, there's so many variables affecting individual fuel economy, I'm never sure what to make of them. I do know that when completely stock I got ~48mpg and after epcos, rejetting w/160 mains and installing the K&N's I went to a steady ~43 mpg. And after adjusting the carbs to rid a persistent backfire, I apparently dropped another mpg or two. No hard records, it's all in my head. Trust me!!

If I could ever find some way to slow down and not run the gears at a high rpm I could probably find some of those missing mpg's. But where's the sport in that .

The mileage you describe sounds pretty bad however. Gotta be something going on as mentioned above.

JH

If I wanted good mileage, I'd get a moped.

My bike rips it up,
Uncle Charlie

I had the dealer rejet the carbs (since it had been so long since I done something like that) when they installed the 'off road silencers'. AI removed and I have the Leather Lyke bags which are pretty good size and drag. In suburban hilly country with some stop and go I get 46 to 48 MPG. The dealer thought the mileage would get worse but it has stayed the same or gotten a mile or so better. I've had the dealer tunes the carbs becuase of loud popping in 5th gear SOMETIMES. That drives me nuts-I can't tell when it's going to pop.

ANyway, I think everyone is right, you have something that isn't working correctly.

I usually get between 130 to 140 miles before hitting reserve. I think some of you guys have venting and petcock issues.

Somethin ain't right. When was the last time it ran right? Have you changed anything since then? Have you checked your mods against Pat's jet calculater to see if you are in the ball park?

Have you looked at your plugs? Are you sure you have spark on both sides? These bikes will run on one cylinder. You can get a spark tester for about $7 at the auto parts store. Another thing, if you have one side not sparking, you would be sending gas out the pipes AND into your oil. Not good. Could be something as simple as a loose plug wire. Maybe a broken wire or a bad coil.

If your gas tank vent was plugged, you would intermittantly run out of gas, but I don't think that would seriously affect your gas mileage. Not that badly anyway. I could be wrong.

If your plugs are sparking ok, I'd check the jets. Make sure one hasn't gotten loose or come out. Could be a sticky float. Luckily, I haven't had experience with that.

Good luck.


Cody

reaper, odometer is reasonably accurate? The vent issue is not unique, but switching to reserve was never what led to the fuel starvation/release cap to restore fuel flow. Fill tank and take a steady ride with minimal stop/go and no "racing" for 50-60 miles/kilometers and fill tank to as consistant a level as possible. Do the math and repeat process. After a couple of runs that you control most of the variables you will get a reasonable average fuel use to work from. It sounds as you may have a petcock that is clogged/installed at the wrong height and maybe a pinched/clogged vent. You haven't posted (or I missed info) the actual fuel amount added at the distance driven. This is really the data needed to diagnose the issue. Your bike would be mainlining gas and as others have posted, it just wouldn't be able to run that much fuel.

Good luck.
BobW

I am at 158 miles on my current tank and have not yet hit reserve and thats in town miles stop and go combined with highway speeds averaging only 50 miles an hour or less overall. The tank was well topped up to begin with.

Thanks to everyone for all the advice and tips, I'll let you know when I have found the problem/solution.

Reaper

I got 42.85 mpg today after a fill up today(can we call it 43? ). That's is my absloute best, in over 4 years. Mostly city. I think my problem is I hang on to the lower gears a little too long before shifting. I think a 19 tooth sprocket would solve everything for me.

Based on the odometer, I get anywhere between 33 and 44 mpg. 44 was the best - all highway. The lowest all city/2-up. I average probably 38-40. BUT, I've found out that my odometer reads fewer miles than I actually go. I don't have an exact amount yet (I will soon) but I estimate it's 12% off. SO, I would have to add 12% to all my calculations. If someone else's was off that much the other direction it could be 20-24% difference. Someone who says they get 50mpg might only be getting 44 equal to my 39 (corrected to 44).

There's really no clear way to compare unless everyone calculated exactly the percentage their odometer is off and compared "corrected" figures. You can either use a GPS on the highway OR note milage at a mile marker, ride 10 miles, and note milage. When I tried it the first time, I went 8.9 miles indicated for the 10 miles. I would have to add 12% to my indicated to get true milage. You can use an average of one tenth off = 1% off. But if you're less it will be a little more and if you're over it will be a little less percentage. Try it and see what you come up with. I would love to get a true mph comparison with others.

it sounds to me more likely that the vent isnt letting the tank breath and in reality you are not running out of gas that soon, it is just not letting the fuel flow. i would loosen the tank cap and see if that cures your problem, see if you dont get over over 100mi before switching to reserve, if that is the case, then you need to investigate the vent tube.

I can't believe you're not smelling gas fumes. For mileage that low there must be some drippage somewhere! I usually switch to reserve around 144 to 148 with a high of 156. I fill the bike on it's stand and have never experienced the vapor lock problem you've mentioned. How's the bikes general maintenance been? Is it time for any major scheduled pit stop?

Front sprocket size makes a little difference. I consistently get 40mpg with a 16 tooth sprocket and a heavy handed throttle habit.
--Tom

The first question to answer is: how much gas does the bike take when you go on reserve? If less than 3.5 gallons, you have a different problem, not gas consumption.
It sounds more like you have a vacuum lock/vent issue. This will aslo cause the bike to have to sit before it will start. Opening and closing the gas cap will prove this is the issue if it starts right back up, I bet it will.
You can screw around with the venting, or just get one of the breathing Kuryakin gas caps and be done with it.
One member here, BobA, had a bad venting issue the shop could not figure out. Same problem as you are describing too. I finally figured it out for him since the dealer was totally baffled after three trips to them to cure it.
I found the problem by attaching a hose to the little vent pipe on the very inside of the tank and tried blowing/sucking and no air would get through. The dealer replaced his gas tank and he has never had another problem. But with the new Kuryakin caps a lot of us have, this is a non-issue since the cap itself is vented.

If there is a venting problem here's another possibility. My dealer's mechanic said there is a ball check valve in the vent tubing just beneath the carbs. He said occasionally the ball can get stuck if you've ever overfilled the tank and some residual gas has dried and gummed up this valve. He recommended the fix was to just follow the vent tube down from the tank to the plastic piece between and under the carbs, pull it off and throw it away. You may have to reroute the vent tube so that it drains onto the ground not onto the back of the motor.
Haven't had to try this so I can't vouch for it. For what it's worth.
Regards,
Rich