Leaking gas tank and now this - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 6 (permalink) Old 06-23-2014, 10:03 PM Thread Starter
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Leaking gas tank and now this

Hi everyone. To start with I just want to say that I purchased my first SP2 back in May and I absolutely cant enough of it! The vtwin has amazing power and interestingly enough, it can keep up with my buddies 954. I geared down to 15/43 and he is geared down to 15/44 on his cbr. When we drag race we are either even or I pull him until the very top of his rev range and he will nudge ahead. But as soon as he shifts, his power is gone where as mine is still there. We've pulled from many different gears at many different speeds and its always the same outcome. I'm thoroughly impressed, never thought a twin could keep up with a Cbr954RR!!

Anyway, the bike came with some hidden issues that I have posted in another thread, but basically the bike had a leaking gas tank that had been brazed together (badly) and failed. It has BLOWN fork seals, and I mean blown! The rear shock is a leaker too. So I used Red-Kote on the tank and sealed it up great, no leaks so far. I prepped it well and used three coats. While I had the tank off and the red-kote was curing I decided to remove emissions equipment (California bike). The evap controls and purge solenoid, the charcoal canister and also the that white catch can tank in the engine valley. It really gave me some more engine space and removed clutter. i weighed it all and it came up to 8lbs savings too! This little piggy can use all the weight savings it can get. Currently I am having a tank venitlation problem. My tank is failing to vent correctly and is pulling large amounts of vacuum to the point that when I open my gas cap, the tank fills up with air and "pops" back into place. It seems that my vent has failed and my fuel pump is caving the tank in.

Question: The tank has two vents on the underside. Correct? If so how can both be plugged? One vent line is a bit thicker than the other. The thick one routes down below the fairings IIRC. And the other thinner one ties into both throttle bodies in maybe 4 places and then goes to (my now removed) charcoal canister. I plugged the line going to the charcoal canister and I see how I could have caused the venting process to fail because of that. But if there are two vents, why am I having this problem? Im tempted to just open the line to the charcoal canister back up but I fear that I will be introducing un-metered air into the engine and cause a lean condition if I do that.
Any thoughts?
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post #2 of 6 (permalink) Old 06-23-2014, 10:22 PM
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not sure about cali bikes, but my rc only has one vent. the thicker hose youre talking about is the gas overflow.
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post #3 of 6 (permalink) Old 06-23-2014, 10:26 PM
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I believe that the "vent" going down the side of the motor and ending at the bottom of the fairings is actually a over flow drain for the fuel tank.

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post #4 of 6 (permalink) Old 06-24-2014, 12:34 PM
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Thaddeus is right, there's only one vent and it's the smaller of the two.
I just have my vent line tucked out of the way, and ending at open air. Been riding like that for 20,000 miles now...

With the gas cap open, you should be able to blow air thru that little hose - that tells you the vent is clear. It's not easy, takes a tiny bit of pressure, but you can easily tell if it's clogged or not.
You should also try blowing thru that thicker hose too - just to verify that your overflow line is also clear. That one is very easy to blow thru - assuming gas cap is open.

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post #5 of 6 (permalink) Old 06-24-2014, 01:36 PM
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The smaller of the two lines is the vent. The larger one is the drain from the cavity around the gas cap.

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post #6 of 6 (permalink) Old 06-24-2014, 11:29 PM Thread Starter
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you guys are definitely correct about the vent lines. I looked in my repair manual at the vacuum diagram and the larger line is the fuel tank overflow drain. The smaller line (on a California bike) routes straight to the charcoal canister and can safely let hang and vent to open air. BUT coming back from the charcoal canister is another line that routes to the evap purge solenoid then to the throttle bodies. The throttle bodies are used to create vacuum in the evap system, it looks like.

What needs to happen when removing these items is: let tank vent line free-vent so the tank can breathe and not pressurize/ vacuum lock itself. Plug the two orifices on the throttle bodies to not draw un-metered air (causing a lean condition).

What my stupid self did: I tee'd the throttle body orifices into the fuel tank vent line and have created a constant vacuum on the tank. That combined with the suction of the fuel pump can really wreak havoc!
I should have this sucker taken care of this week. now I just need a new rear tire...

Last edited by mikeyd; 06-24-2014 at 11:31 PM.
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