Bike Runs rough at 1/4 tank - reasons?

Hi Everyone,

whenever my bike gets down to 1/4 tank or so (maybe more than quarter), it will start to studder at low idle. Once you pass 2000 RPM, it will run smooth. If I fill the tank up, it runs fine at any throttle speed.

Any ideas?

 

How is it on RES? These need to go on RES way earlier than it looks from the fill hole.

 

Hmm, haven't tried RES. You're right on the visual. In past bikes, if I could see the fuel, I was OK for a bit. I'll try it next time it dips down.

What sort of millage do these get roughly. I got 90KM before it started to spudder.

 

I get at least 100 miles on a full tank (162 KM I think it is) before I have to switch.

 

Water in the tank?

Gasoline floats on water, so this might be symptomatic of water or some other not-gasoline in the tank.

When's the last time you completely drained the tank? Sounds to me like it's worth doing right now. Drain it, refill it, and see if it persists.

 

Hmm, i can't remember ever fully draining the tank. I'll have to try that and while it's drained, might as well take the Petcock out and investigate.

 

If you do drain the tank, I would pull the fuel valve and check the pickup screen for blockage!

I get about 190 km before needing to switch to reserve!

 

Actually, the fuel pickup is in the bottom of the tank. Water would be the first thing in as fuel would float on top of it.

I'd empty the tank though, and follow Nick's advice. If that screen isn't clogged most of the way to the top you need to find out where your fuel is going.

 

Drain the tank, put on a new fuel filter, and every couple tanks run a little fuel stabil in it when you fill the tank with gas.

 

On my 650 Maxim I get about 180 KM before switching to reserve.

 

Thanks for all the info. I may also consider presure or lack of to be an issue to. It's easier to presurize a full tank, rather than a 1/4 tank. I'll have to check my line from the carb to the valve too.

 

The tank does not get pressurized on these. The fuel is gravity fed to the carbs.

The line from the carb to the valve is a vacuum line used to pull open a valve to allow the fuel to flow in the "on" and "res" position when the engine is running. The fuel will always flow in the "prim" postion engine running or not.

 

You are probably getting the -- "It's time to switch to Reserve" stumble that happens when you get low on gas.

So, I'm not at all surprised it goes away when you gas-up!

 

In the city only, I have to swap to res around 80miles or so.

 

everyone is saying when they swap to reserve.. but here's the money question?

How far (km) do you get on reserver until your on the side of the road with no oar?

 

I'm not sure if it's a case of switching to reserve. 90kms is a far cry to 150 km. Plus, I can still see fuel in the tank, easily. My bet is compression.

 

I'd drain that tank and clean the screen as suggested. If the problem persists perhaps try the petcock in different positions, res and pri.

One unlikely possibility is that the valve is not opening fully under vacuum power and with a lower volume of fuel in the tank gravity is not working as hard to push the fuel through. As rpm increases so does the vacuum created and it may fully open the valve at that point. If that is the case then it would run normally on the prime setting.

 

The next time this happens open the gas cap while the bike is stumbling. If it immediately clears up then the vent for the gas tank is plugged. Is the vent in the gas cap? I haven't checked.

Removing the petcock and cleaning everything is a good idea. Also drain out the float bowls if you suspect you have water in your gas.

Phil

 

I wasn't aware there was a vent on these tanks, considering they need presure for the fuel to flow?

 

It really doesn't take much Vacuum to open the Petcock. It's not actually variable, either.

It's just an Open or Closed situation.

There might be a problem INSIDE the Petcock.
Use a piece of nylon Weed Whacker line and probe the Outlet going to the Carbs. Buildup in there is tough to get rid of. The nylon will make the corner and will break-up anything in there.