I ride a Brute Force KVF650 D8F, 2008 Canadian model. I've put about 1000km / 80 hours through it since it was bought (new). Maintenance meticulously performed (all fluids, air filter cleaning regularly) more often than suggested. Always using fresh fuel and adding stabilizer, since it is often left unused several weeks/months. In winter, I didn't drained the carburetors but it was filled up with fresh gas with stabilizer.

Last week-end, I rode my BF650 for a 3 hours ride, very low speed (<15km/h) with a passenger, driving on high gear. After about 2.5 hours, the bike was kind of hot, I could feel a strong heat through the seat. No overheating though. Probably just normal heat, from a bike riding at low speed on the hills. Then, we started to smell gas while riding. If I kept the speed slow enough, the smell was really strong, and it seemed to came from the front of the bike.

At the end of the ride, after shutting down the engine, I heard some periodic hissing from the rear end. Seems to come from a check valve mounted on the fuel tank. I decided to open the tank cap and a serious overpressure was relieved from the tank. Then, I took a small ride with my bike and the fuel smell came again after 10-15 minutes, but it was probably leaking less this time. I unscrewed the tank cap and again, there was still a lot of pressure in there...

My thoughts:

- Engine & fuel tank (completely filled-up) were pretty cold before taking the ride (around 10C)
- After riding 2.5 hours, engine heat was probably normal, especially since I was riding at low speed and with a passenger. This heat was also transferred to the fuel tank, which caused the fuel to expand/build vapor pressure.
- Overpressure in the fuel tank might cause the carbs to overflow, but I don't really understand all the interactions in non gravity-based fuel systems. Fuel pump, vacuums, could they prevent carb overflow? Or with strong pressure, it WILL happen?
- Since the fuel smell seems to be related to the bike/fuel tank temperature/pressure, I don't suspect deposits in the carb float valve could prevent the float from properly closing.
- While looking at online part diagrams, I can't get a proper picture of how/where the carb breathers are going. It looks like it is vented on the bike front, just under the handlebar. Probably from where it was smelling gas.
- It seems to have a overpressure system composed of a breather tank + a check valve venting in the rear frame, from the fuel tank part diagrams. How can I test both components? At what pressure should the tank vent in this breather? It is at least 35$ US to replace both parts so I would prefer to confirm they are bad before changing them...

I don't have the 2008 service manual (some sections of the 2005 KVF750 I got are useful but it is mostly useless for the fuel tank part) so I've no idea on how to check proper operation of the fuel tank venting system.

Any advice/idea?

Thanks!


François

 

There is NOTHING wrong on your quad except you put too much fuel into the fuel tank. You need to stop fueling when fuel reaches the bottom of the white tube of the filler neck. If you fill higher than that (as most do), the fuel will touch the vent fitting at the top of the fuel tank and fuel will exit out of the fuel tank via the 4-psi one-way check valve. The check valve is working because you said you felt lots of pressure build up when you open the fuel cap. The fuel smelling problem is more likely when riding in hilly area because all the fuel would be moved to the rear where the vent fitting is when going uphill. You may ask why does it still smell even after riding an hour or more? That's because the quad frame is acting as a holding tank for the fuel that already came out of the fuel tank, and as the fuel tank internal pressure expands the liquid fuel in the frame is getting pushed out but eventually it goes dry and you won't smell anymore.

Above is true for all v-twin Kawi quads.

 

I have had the same prob. and thought the same as Hayashi.I noticed that when I fueled up past the white insert and trailered or rode it I would get the smell. I also put a actual shut off valve in line under the seat and shut it off when trailoring, or if the carbs happen to leak by.,if someone steals it they wont get far and wont figure out what it is.