lousy gas mileage

B

brucemg51

New Member
right now on my 70cc I've got a one quart gas tank from a lawnmower. Figured that would be fine for the short hops I make. Went out for a haircut the other day, 16 miles round trip, and didn't make it! Ran out of gas two miles from home, and I had topped it off just before leaving. So that's what, not even 60mpg? What's up with that? And I do a lot of pulling the clutch to roll down hill, and even shutting the engine off on long down grades. :-||Bike runs fine. Checked the plug and it looks fine fine.
 
Bruce,
Can you offer us some information such as your weight, rear sprocket size and how much wide-open-throttle time you ride? These figures would help determine why you seem to be getting poor milage. The 150 to 160 MPG figures offered by the kit suppliers might be a bit overstated for every condition but your milage does seem a bit low from the norm.
Tom
 
I'm no lightweight, around 200 lbs. I never ride with the throttle wide open. 15-20 mph is the perfect cruising speed for me. I have an over sized rear sprocket (52 teeth, I think) because I live in some very hilly terrain and am willing to swap off some top end speed for extra torque. My bike is a Kulana beach cruiser, which I realize is not the lightest bike around either. Still, last year I was driving an 1100cc, four cylinder Yamaha and getting 43 mpg. Something is seriously not adding up here.
 
Retmachinist said:
The 52 tooth sprocket is certainly not helping your gas mileage. Thats alot of R.P.M.'s at low speed.

John
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I understand that's a factor, but that much? I drive a 95hp motorcycle at 75mph and get 43mpg, and I don't have to mix oil with the gas. Going from a 44 tooth sprocket to a 52 tooth sprocket I would think would cause a barely perceptible drop in gas mileage. But 55 mpg? Maybe it's part of the larger picture, which would include my weight, the weight of the bike, the terrain I ride on, my usual cruising speed. Most of these are factors which I can't change. I never took seriously the claimed 150mpg, but if this is the best I'm going to get, then I have to consider whether I want to bother with a motorized bicycle.
 
brucemg51 said:
I understand that's a factor, but that much? I drive a 95hp motorcycle at 75mph and get 43mpg, and I don't have to mix oil with the gas. Going from a 44 tooth sprocket to a 52 tooth sprocket I would think would cause a barely perceptible drop in gas mileage. But 55 mpg? Maybe it's part of the larger picture, which would include my weight, the weight of the bike, the terrain I ride on, my usual cruising speed. Most of these are factors which I can't change. I never took seriously the claimed 150mpg, but if this is the best I'm going to get, then I have to consider whether I want to bother with a motorized bicycle.
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Don't know what else to tell ya. The rest of us are getting well over 100 m.p.g.

John
 
I'm 300# and I figured the last gallon went thru at a little over 90MPG. And that was mixed 20:1. I'm running 24:1 now and it looks like maybe I've increased mileage to maybe 130.
 
Well, your plug seems fine so you don't want to lean it out any more....you do it by moving the C clip up for leaner, down for richer. You can burn the engine up real fast if you are too lean.

Does your carb leak gas anywhere when it's sitting?
 
Bikeguy Joe said:
Well, your plug seems fine so you don't want to lean it out any more....you do it by moving the C clip up for leaner, down for richer. You can burn the engine up real fast if you are too lean.

Does your carb leak gas anywhere when it's sitting?
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If it is leaking, it's such a small leak that it's undetectable. I haven't noticed any smell of gas around it when it's sitting either.
 
Well maybe if this was the first time you filled up with this new small tank, maybe a good portion went to filling a fuel filter and your float bowl. I'd also double check your volume of the lawn mower tank. My lawn mower tank is really small, like 1 pint which probally would only get me 15 miles down the road. If you were sucking fuel like you said then your plug would be black charred and you might see a lot of black smoke. Any fuel wetness under you carb? A leaky tickle button can let fuel leak out which if you drove to the store, parked for awhile without tuning the petcock off it could drain out.
 
bigdreamz2000 said:
Well maybe if this was the first time you filled up with this new small tank, maybe a good portion went to filling a fuel filter and your float bowl.

Hadn't considered that, but no, it wasn't the first time using the lawnmower tank.

I'd also double check your volume of the lawn mower tank. My lawn mower tank is really small, like 1 pint which probally would only get me 15 miles down the road.

Yes, I think I should check the volume of that tank again. Pretty sure I measured it to be 32 oz., but I may have been wrong.

If you were sucking fuel like you said then your plug would be black charred and you might see a lot of black smoke. Any fuel wetness under you carb? A leaky tickle button can let fuel leak out which if you drove to the store, parked for awhile without tuning the petcock off it could drain out.
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There's no indication that it's burning an excess amount of fuel. I've owned enough motorcycles with carb problems to be very familiar with the symptoms. If it was leaking, I'd see something on the floor of the workshop and I'd probably smell gas all around the bike, but neither is happening.
 
I've checked the volume of the fuel tank. It's 32 ounces, one quart. And that only got me 14 miles. I can't say that I'm completely surprised by this. Although I never bothered to check the mileage before, when I had the stock tank on the bike, I suspected that it wasn't very good, but I had no idea it was this bad. It's hard for me to grasp the idea that the oversize sprocket would cause the bike to get half the mileage that everyone else is getting, but I can think of nothing else.
 
Lean it out some and see if it still runs fine and check the mileage again. Sometimes the plug doesn't tell the whole tale.

I agree that the sprocket would not make that much difference. Plenty of people here run well over 20 mph with the 44 tooth and get better mileage. That would be the same rpm range and even more aerodynamic drag.
 
Just don't lean it out too much, or you may need another engine...

Running at higher rpm may well be the problem.
 
brucemg51 said:
I've checked the volume of the fuel tank. It's 32 ounces, one quart. And that only got me 14 miles. I can't say that I'm completely surprised by this. Although I never bothered to check the mileage before, when I had the stock tank on the bike, I suspected that it wasn't very good, but I had no idea it was this bad. It's hard for me to grasp the idea that the oversize sprocket would cause the bike to get half the mileage that everyone else is getting, but I can think of nothing else.
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Lets try some algerbra: 4.5dia/44tooth=x/53tooth x=5.42diameter of your sprocket. Is that right? Now some geometry: The circumfrance of the 53tooth is 17.02inches vs. the 44 tooth 14.13inches.

There are 63360inches in a mile. 44tooth turned 63360/14.3 = 4484 revolutions. 53 tooth turned 63360/17.02 = 3723 revolutions. Hmmm... On a normal bicycle your crank gear is bigger than your hub, but on these it is the other way around. The circumfrence of the 13 tooth is 4.17 inches so it turns 4.08times on the 53tooth sprocket per revolution. It turns 3.38 times on the 44tooth.

13 tooth spins 15189/mile with 53tooth and 15155/mile with 44tooth. A difference of 33 revolutions per mile or 495 more times in your trip. Well that told me nothing because 500 more revolutions isn't going to zap your fuel economy that much. Must have something more to do with engine torque and RPM's per mile. My math has never been very good, but I had fun :). Maybe someone could expand or get Geeky on that and enlighten us!
 
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