Blackdogdualsport.com Message Board

Here's something to challenge you all and let's see if any of you can figure it out!

Yesterday I was out exploring with my bud, Danno, and I was riding my old '87 Honda XL600R, when it suddenly started to idle rough and would die. It was hotter than heck out and Danno and I were idling around slowly trying to find a particular trail. I did the usual big-thumper drill of cleaning out the cylinder by kicking through 10 times with the throttle wide open and the compression release pulled in. After that I would kick through like normal and no throttle (which normally it starts right up and purrs) and it started, then faltered again. What the ???? I tried cleaning it out again with the usual drill and noticed that fuel was dripping fairly well out the carb overflow tubes.

I figured that I must've gotten some crud in the fuel lines down to the carb float bowl and it was blocking the float tang (even though I'm running an in-line fuel filter and I drain the float bowl after every ride), so I got out a straight blade and drained the carb bowl. After that, I kicked her over and she started up, but then again started to run rough and gas was flowing out the overflow tubes again, plus was even starting to drip out the air-filter box drain tube(!). I kept trying to start it and it would even backfire through the airbox.

Another hint is that it was NOT dirt in the carb or a bad float-bowl tang. It has the stock tank (which was 3/4 full and a one-way vent breather line in the gas cap) Before I post the answer, and for another hint, here is a pic of the bike and no more hints.

So can you figure it out with those details?
Danno: Don't give it away!
-Tom

Could it be a case of Vapor-Lock .......which Big Thumps are known for during hot weather.

Close, Scott, but here's the poop: It was the one-way, gas cap vent tube. Here's what was happening to those that may still be scratching their heads.

As I was idling around slowly searching for the Holy Trail, the heat from the air-cooled Honda mill was rising up to the metal gas tank, which heated the gas, therefore making it expand. The one-way vent tube would not allow any gas (or in this case, fumes) go the other way, so the least point of resistance was to push gas to the carb float bowl and overfill the carb, air box, etc.

I discovered this as I was wondering how much gas I had lost already and opened the gas cap and it had that "whoomp" sound of being pressurized. I removed the one-way valve, end of problem and the engine went back to purring again. Took me a few minutes to figure it out. So now it just has an open hose on it.

And the reason I had the one-way hose on there, was so I wouldn't lose any precious gas in case I took a digger 100 miles in the middle of nowhere, although I've never done that.
-Tom

I knew it had to be some sort of Vacuum prob related to the fuel!
I found that cracking the gas cap open on my XT periodically during my many trips across the pass last summer,
was an easy remedy to keep from losing any of the golden brew.