Rough idling & fuel leak when tank is full

I've had my '95 Sprint (103k miles but new to me) on the road for a few weeks and have noticed that it runs rough and drips fuel out of the air cleaner box when the tank is near full. As the fuel level goes down, it smooths out and stops dripping. Any suggestions on what I should look at first?

Thanks.

[bigbird]

hi
maybe able to help with the leak from air box. My sprint did this and it was the floats in the carbs not shutting off and leaking back into airbox.To fix mine i had to take out carbs and remove float bowls and clean.If you want to try this its not too hard, a tip is to leave to throttle cable attached to the carbs and remove from handlebars.If i can be more help let me know.

[j98sprint]

New one on me. I have no idea how a full tank would do that. I'll venture a few guesses.

Make sure the tank is venting properly. I really can't see how a full tank would leak into the airbox. If a carb float was bad the level of the tank wouldn't make a difference, it would leak all the time. Now if it's running rough with a full tank, maybe it's not vented and the carbs are not filling fast enough. But again, if it's not vented when full, why would it be vented when half-full and runnig fine, as you say.

Makes no sense.

I suggest you go through the whole thing. Check all the gas lines, vent lines, check that the tank is venting and flowing fuel by checking the petcock. Check that the tiny in-line fuel filters are clean. Check the carb float height settings. The airbox and air filter are probably filthy. Clean box and replace filter. Check all your electrical connections for corrosion.......

Check, check, check, lots to check on a bike that has 103K miles on it, especially when they were put on by someone else.

And the answer to your original question, the first thing you should look at is a manual.

Keep us posted, I'd love to hear that your 10 year old workhorse (that's a compliment) with all those miles gets sorted and is running fine.

j98sprint

[ezride]

On 2005-10-02 06:48, j98sprint wrote:
But again, if it's not vented when full, why would it be vented when half-full and running fine, as you say.

I agree with j98sprint -- check and recheck. I do think a venting problem could be worse with a full tank though, so I would start there. With a full tank and no (or limited) venting, you create much more vacuum in the tank for a small change in fuel level. [With a nearly empty tank, burning 1 liter of fuel doesn't make that much difference in the air volume and thus the pressure in the tank.]

As for the fuel leakage, I think that must be a different issue.

Guys,
Thanks for your input. I've been tinkering with it today and have a 80 degree day off tomorrow to see if I've had any success! Happy riding.

[pendraig]

I had a similar problem this summer, (except that I didn't have the fuel coming out of the airbox thing). My bike started running "iffy" with a full tank, and then once shut down completely. When I checked things out alongside the highway, I heard the tank "whoose"ing when I popped it open. I took it home and cleaned out the vent-pipe that goes down through the tank along with its rubber drain hose, and then took the bike out for a road test. I found that it would run (very well) indefinitely with the cap open, but would show symptoms of starvation after about 7-10 minutes with the cap closed.
I went back into the garage and removed the cap once again, and pulled it apart. (There are 3 phillips-head screws that hold it together, and 5 springs that you have to keep track of). There is a pinhole right on the bottom-centre of the latch mechanism that leads to a channel that runs thorugh the plastic pieces, and through a chamber with a little 'figure-eight' shaped bit of plastic with two diaphragms, and then out a pinhole which leads to the circumference area of the cap, where the tank's vent-pipe is. My first attempt to ascertain that the passageway was clear ended in failure, with the road test ending with the same results as before. I took the cap apart a second time, got curious as to the function and condition of the little diaphragms on the 'figure-eight' piece, and ended up tearing one of them. I reassembled the cap without the piece, which I strongly suspect is the anti-spill valve for when the bike lays on its side. (Since removing it I have been very careful not to leave my bike on its side for any significant amount of time.) That seems to have solved the problem, and my engine has run 'just peachy' from that day forward.
I am aware that leaving parts out without knowing their function is quite inadviseable. I do not advise this course of action to others.
Also, theoretically, if the vent were clogged, a vacuum would result when the engine was running, which would explain the engine being starved. If the bike was not running, the tank would tend to pressurize on a warm day, pushing the gas out . Through the carbs?