Topic: Fuel tank pressure question

[emlupi]

Good afternoon,
I have asked this question in other forums but have not received any real answers. Here’s the scenario;
I have an ‘03 Honda RC51 which I recently bought used. The bike had sat for awhile so I went through it and cleaned it up, repaired/replaced parts as needed. The tank was rusty so I hit it with some Metal Rescue.
I took the bike out for a short shakedown ride and it ran strong, no problems. However, after riding it I noticed a slight hissing coming from the fuel tank cap keyhole. I opened the fuel cap and there was a short rush of pressure release from the tank. I checked the breather hose connected to the bottom of the tank and it is clear.
I have heard some say that it’s normal for some pressure to build up in the tank due to the hot engine sitting under there. Am I making something out of nothing or should I be looking more closely at this? I know the CB has a vent hole in the gas cap but I don’t see anything like that on this bike’s filler cap. I’m not an expert on fuel tank breather design so any help on this would be appreciated, thanks.

[Alan F.]

Perhaps https://www.rc51forums.com/  ?

[emlupi]

First place I asked, so far no response.

[Alan F.]

Sorry man, all I learned from looking at Partzilla was it's a fuel injected bike, there is an overflow drain on the tank but I don't think it's a pressure vent. There's a fuel filter and there is a return fuel filter (from pressurized FI system I guess).

I always thought FI required an unvented gas cap.

[emlupi]

I have attached a photo which is very similar to my setup (this is a 600rr). There is a breather hose on the bottom of the tank that connects to the ring just inside the filler neck as shown in the photo. I am not familiar with this arrangement but apparently Honda used it in several different bike models. I don’t know if there’s an issue or if it’s normal for this to happen. Anyone who can shed some light on this I would certainly appreciate it.

[Alan F.]

Are you able to snake out the vent tube with a length of solid copper wire in a drill?

[emlupi]

In order to do that I am going to have to take the tank back off the bike. I want to make sure there’s no other solution before I have to do that again. Kind of a pain to get it off.

[scottly]

All of my fuel injected cars vent pressure whenever the gas cap is opened; it's part of the vapor recovery system to reduce emissions, and a loose cap can trigger a Check Engine light. I don't know if the same applies to your modern bike, but if there are no other symptoms or problems, don't fix it if it ain't broke. 

[emlupi]

That seems to be the consensus. I haven’t ridden the bike for any extended period of time, only a couple of short 10 minute rides around the neighborhood. It hasn’t displayed any issues in that time and was running strong. I haven’t experienced this with other bikes so I wasn’t sure what to make of it. I guess I will just keep an eye on it. Thanks.