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losing mpg?

3399 Views 16 Replies 7 Participants Last post by  sfair
So, i've noticed that my mpg has dropped from a decent 45+ to around 31mpg, anyone have any idea why i might have dropped 10+mpg? at first i thought it might be jacked spark plugs, so i changed them out when i did my valve job a week or two ago- but no increase. with the exception of a k&n air filter, bike is stock. I've also noticed that i can smell gas the last month, so perhaps it's running rich? if this is the case, how do i change it with the stock fuel controller? and why would it suddenly start running rich? i'm running mid grade fuel through it (89), and it's a 2008 classic. thanks!
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1. How many miles on it?
2. Did the problem happen over night or did the mileage gradually drop?
3. How long have you owned?
4/ Any work/mods done, no matter how unrelated?
Around 19k miles- happened gradually. I've owned it for a year and a half, and put 15k on it as my daily commuter. No modifications other than the K&N air filter. No major work.
1. Start ok, engine hot/cold?
2. Idle ok, engine hot/cold?
3. Runs down the road ok, engine hot/cold?
4. Any backfiring?
5. What do the old/new plugs look like? Picture even better.
6. Did this happen after valve adjustment?
7. The exhaust smells rich? Any smoke?
I did the 15k valve adjustment, but did it late at 19k two weeks ago, was hoping it would solve the problem, didn't.
Starts, idles, and runs fine- no issues. Air filter cover might be getting hotter than it used to, or I could just be paranoid and looking for problems.
No backfiring, other than the occasional pop on decel.
Plugs looked great- I was expecting them to be fouled and jacked. Replaced them anyway.
Started before the valve adjustment.
The exhaust smells rich, no smoke.
You said you smell gas. Have you checked the fuel line? The tank was off for the valve check. The end that connects to the tank fitting is resin and can be cracked if you're not careful.
There's no fuel or oil leaks- the smell of gas comes from the exhaust.
Do you get your fuel from the same place?
I tend to get less mpg as I go up in octane, 87 for me from the local Chevron store.Same for speed under 60 or over 70 mph tend to drop below 52mpg.
I get my fuel from various different places- and my riding style hasn't changed from prior to the mpg drop.
What do you mean by plugs looked great?
Not a fuel processor on there that you do not know about?
Gap was a little wide, but no signs of anything wrong- looked like they should for the amount of miles on them. Will take a picture of them tomorrow, they're still out in the garage somewhere.
While it would be awesome if there was an aftermarket fuel processor installed that I didn't know about, it's highly unlikely- bike was bone stock when I bought it, the previous owner bought it new and decided he liked his moped better so he just let it sit and rust in his driveway. If there was an aftermarket processor, where would one typically be installed/where should I look?
I can't dig too deep into the bike right now, riding up the coast to Monterey on Saturday for a few days.
I used to easily get 150 miles on 3 gallons. Lately 3.01 gallons gets me about 101 miles. I guess it's been about two months since the mileage started dropping. No loss in power.
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I have the same problem with my bike. Mileage started dropping after adjusting the valves last year. Went from 57-60mpg down to 44-47 mpg now. It is a carbureted model 500. Plugs are tan and gapped, filter is clean. Some popping on decal. Tires are at proper psi, chain stays lubed. One would expect better mpg out of a 500cc motor. Driving the same routes for work also.
It wouldn't really follow that your plugs look great if you smell unburnt fuel in your exhaust.

Up here in Canada we get "winter gas" that is a different mix of gasoline and ethanol - and mileage drops a tad on the winter gas for me.

You're experiencing a HUGE drop in gas mileage - and if you are calculating the numbers correctly you've lost a third of your mileage. That does not sound possible unless you are driving a lot differently (loads of WOT runs now, none before), you have a fuel leak that you haven't noticed, the air entering your motor is severely compromised (air filter so dirty you are losing tons of inflow air), or your injection system is buggered (ECU?), or your brakes are dragging in a big way.

Its a pretty simple system, so you have to rule out the obvious stuff. I am unsure of the effect of maladjusted valves on fuel mileage. You have to be positive you didn't mess up the valve adjustment too.

Not sure what else to say. I consistently get 45 MPG on my Custom. Mostly city driving with maybe 20% highway. I'm not too tough on the throttle either.
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Didn't get around to taking pics of the spark plugs, my apologies.
I only really smell gas at idle- especially strong during its high idle warm up, and faintly at stoplights.
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