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Discussion Starter · #1 · (Edited)
I have gotten 24 MPG for the last 3 fills (only 60 miles for 2.5 gallons of gas!). Since I have gotten 40-43 MPG for the last 2 years, this is very alarming. The sudden decrease started randomly and doesn't correlate with any changes I made (the last thing I did was clean, balance and rejett the carbs... over a year ago). The performance, power, smoothness are all the same as well.

I checked my plugs after the second fill and they looked perfect (nice darkish red color). I cleaned them and checked the gaps for good measure)

I noticed my oil had a gas smell in it, so I changed it (synthetic)

I checked to see if my breaks were dragging, but with the bike lifted, both of my wheels spin freely.

There is no gas smell or visible gas leak.

After this little maintenance I filled it up and rode some more. After only 55 miles it took 2.3 gallons to fill up.

If it was gravity fed, I would think that the petcock had a bad diaphragm, but its not gravity fed. It has a fuel pump down by the battery that shoots the gas back up to the carbs. If it was the valve, then wouldn't gas just collect at the pump to get shot the carbs as soon as the bike starts? I'm at a loss here.... Where is my gas going??
 

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No obvious external leaks, bike runs OK, plugs look OK...but mileage is in the crapper.

What you got here, son, is a conundrum. Honestly, the set of circumstances you list pretty much rules out the usual suspects. The fact that the plugs are not fouled or oily REALLY presents a head-scratcher because 1) too little air going in; 2) too much gas going in; or 3) weak (or poorly timed) spark causing incomplete ignition would almost certainly show up on the plugs. And definitely in performance.

Eliminating the holy trinity of air, gas and spark as potential culprits doesn't leave much to go on. And you don't smell gas as you would if a fuel line or carb was leaking. I got nothin', not even a WAG. Sorry.

But if you figure it out tell us about it.
 

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^^^ Check your fresh oil again. Do you smell gas now?
 

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Is the 'power' down too? That would indicate bad rings which would explain the gas in the oil.
 

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1983 Honda vt750 Shadow
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Rats nest in the air intake box. Did you look at the air filter for debris?
 

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Discussion Starter · #7 ·
I don't think bad rings because the fuel economy loss happened so sudden, and power seems the same as always. Also, to lose about a gallon of gas every few days into the crank case would cause my oil level to increase wouldn't you think?

I wonder about the carbs, like a stuck or worn pencil valve, but it seems unlikely to be the problem because the carbs aren't gravity fed. Without the fuel pump running the carbs are dry (besides what's already in the reservoirs)

Looking at it again today, I noticed a slow drip coming from the tank vent hose at the bottom of the bike. About a drop every 4-6 seconds. I looked it up and found that most threads about such drips concluded that such is normal when these bikes sit on hot days (it was near 100 degrees today), as the tank somewhat pressurizes as it heats up. Let me know if you think that is normal. Could it be that the part of the vent tube inside the tank that is submerged may have sprung a leak?
 

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Try to loosen the gas cap and see if the drip stops. If it does, it must be just the fuel expansion. If it does not stop then the vent tube may be leaking in the tank.
 

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Oil smelled like fuel? I'd say you have a stuck float and/or worn float needle valve/s or seat/s in your carbs. Do this repair soon before you ride the bike again. Running gas in your oil is bad for the engine.
I thought about that but discounted it as a cause because he said his plugs looked OK and the bike ran normally. When the float valve seats on my GSX1100G began to leak the bike would not idle, ran poorly even at full throttle, and the plugs were actually wet with gas when I pulled them.

Dripping from the vent tube usually only happens when the tank is newly filled and then allowed to heat up by sitting in the sun or maybe from a hot engine below. Those of us who have CA bikes and remove the canister see this from time to time. But since the vent tube opening is near the very top of the tank (for obvious reasons) the loss should be very small and not effect mileage so dramatically. A leak somewhere further down the line is another matter, but again there should be some obvious gas smell as a result.
 

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I don't think bad rings because the fuel economy loss happened so sudden, and power seems the same as always. Also, to lose about a gallon of gas every few days into the crank case would cause my oil level to increase wouldn't you think?

I wonder about the carbs, like a stuck or worn pencil valve, but it seems unlikely to be the problem because the carbs aren't gravity fed. Without the fuel pump running the carbs are dry (besides what's already in the reservoirs)

Looking at it again today, I noticed a slow drip coming from the tank vent hose at the bottom of the bike. About a drop every 4-6 seconds. I looked it up and found that most threads about such drips concluded that such is normal when these bikes sit on hot days (it was near 100 degrees today), as the tank somewhat pressurizes as it heats up. Let me know if you think that is normal. Could it be that the part of the vent tube inside the tank that is submerged may have sprung a leak?
SOMETHING happened suddenly, you dont have this stuff happen normally. If you are leaking gas into the cylinders, it can leak past the rings and more so if you are driving it. Rings can go bad/break what ever you want to call it.
 

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I'm not really good at wrenching on my bike, but if it was mine I would not ride it till this was sorted out. Gas is sort of explosive and you don't know where its going, this could be a recipe for a disaster, you the bike or both could be injured.
 

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Discussion Starter · #14 ·
The amount drained was just about right, which is not a surprise because it looked right on the dip stick before... I checked this morning and it looks good now too, and if there is a fuel smell now, it is very faint, like normal.

I also watched for drippage from the vent hose, which was not happening this morning. I figure this must be because 1, it was only 60 degrees, so the tank wont have built pressure yet, or 2, the fuel has drained beyond the leak in the vent tube, if there is indeed a leak. Today, I am going to try filling the tank all the way, then I'll blow compressed air into the vent hose to see if it makes bubbles in the tank. If it does, I think that means there is a leak in the vent tube inside the tank. If so, I will plug the vent at the bottom of the tank and get a vented gas cap. Any other ideas to make sure I cover all my bases?
 

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Fill the tank, ride for ~50 miles. Fill the tank again. How much fuel did you use?

Fill the tank, park the bike for 3 days. Fill the tank again. How much fuel did you use?

This will tell you if the fuel is leaking or if it's being consumed.
 
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