1998 Honda ST1100 gas leak

Question

I had 2 gas leaks on my 1998 Honda ST1100 in last 12 moths. Only when the engine is on. When turn off it stops. Both times happened after sitting in my garage for couple of weeks. And both times, while waiting for friend to help me, after couple of weeks, it "fixes" itself. I tried to find what's causing the leak but after gas evaporates it's hard to follow the trail. I'm assuming one (or more) of floats gets stuck while sitting in the garage. And I'm assuming I have to take off the carbs and clean them. Since taking the carbs off is PITA, I wonder what else can cause the problem?

Thanks for any help, Afan

Answer 1

If it's only happening when the engine is on, I would check the fuel line. I just had a quick look at a parts diagram and it seems your bike, like most carb'ed bikes, has a vacuum operated petcock.

When the engine is on, a vacuum source (most likely a hose connected to the airbox) provides suction to the petcock. This suction pulls on a rubber diaphragm and opens a valve, which allows fuel to flow from the tank through the line to the carbs and fill the float bowls. When you turn the bike off, the source of suction disappears and the value shuts. The idea is this helps prevent your entire tank of gas from spilling out onto the ground if your leave your bike sitting somewhere with the float bowls stuck open.

I would check the rubber fuel hose between the tank and the carbs. Rubber hoses will become dry and brittle with age, and it's quite possible after 19 years it's developed a small crack somewhere.

Answer 2

A fuel leak is a very serious problem, especially on a motorcycle where the gas tank is located immediately next to the engine/leak area, unlike a car where it is completely on the opposite end of the vehicle. If you don't feel you can safely perform this maintenance, don't, and take it to a mechanic.

In my experience on other bikes, the only way you are going to find the source is to remove everything from the bike so you have at least flashlight-visible access to the entire fuel delivery system. I prefer to fill every gap I can with balled up paper towels to prevent the fuel from pooling, and I quickly replace them from the engine so they don't catch fire themselves.

Then, start the bike cold. Depending on where the problem lies, it may leak immediately, it may leak during warm-up, it may leak after getting completely warmed up, or more than one of the previous two. Don't assume that because you found one leak you have found all of the leaks

If you can't get the leak to start with the bike running in your driveway, then go for short rides with the fuel delivery system still fully visible - you may need to zip-tie some dangling components - and stop regularly to see if the leak has started up again.

Also, don't let the bike get hot if you feel it is going to start on fire - like if it's dripping gas on an exhaust manifold or other very hot component.