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Discussion Starter #1
I find that many times when I am getting gas, opening the gas cap, I get a pretty good amount of suction. So much so that I've learned to put my hand over the gas cap so the spray doesn't get on the tank. None of my other bikes do this but they are not MV's. :) Is this normal or should I be looking for a clogged vent? I will say that I don't have a stock gas cap. It's a CNC pictured below.
 

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There should be a breather tube inside the tank that vents to atmosphere. Usually nothing to do with the filler cap on any other bike I've seen or worked on. The rubber hose that this connects to on the underside of the tank maybe trapped, kinked or blocked. Has the tank been removed recently?

EDIT; I stand corrected...
There is a risk of damaging the tank as the fuel pump is pretty powerful.
 

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Discussion Starter #3
I've only had the 600 mile service done last January. I don't think they remove the tank for that. I'll have a poke around and see if there is anything that doesn't look right. It's under warranty so if I need to I can always take it to the dealer. Thanks for the help.
 

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Chas, stay current.
1. Do NOT quote eMoto or add links to them.
2. Research MV parts layout before advising on a very familiar topic.

Fingertight, you're in the maintenance section so do a little search, also check recent posts.
Blocked or modified fuel caps causing damage is a very real possibility.
 

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I've only had the 600 mile service done last January. I don't think they remove the tank for that. I'll have a poke around and see if there is anything that doesn't look right. It's under warranty so if I need to I can always take it to the dealer. Thanks for the help.
That is not a standard fuel cap ?
 

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Chas, stay current.
1. Do NOT quote eMoto or add links to them.
2. Research MV parts layout before advising on a very familiar topic.

Fingertight, you're in the maintenance section so do a little search, also check recent posts.
Blocked or modified fuel caps causing damage is a very real possibility.
Yeah, hands up, another lesson learnt. Bad call on my part.
 

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Discussion Starter #7
That is not a standard fuel cap ?
It's not the factory cap. I did do a search and found clogged vent tube posts but I didn't see any of them reference the CNC gas cap I have. One thing is that it's not starving for fuel at anytime which I think some of the other posts mentioned that. I think I need to have a close look around before there is any damage and head for the dealer if I can't see anything. I appreciate all the help. You guys are great.
 

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Discussion Starter #8
Quick semi update. I poked around and saw nothing pinched or bent so I took it to the dealer. Good news is it's not the CNC gas cap or the vent tube. They said they think they found the problem but haven't given me details yet. I should know tomorrow and hopefully have her back by the weekend.
 

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I have a "race" gas cap on my SPR....the hole for the cap vent didn't line up with the hole/tube in the tank. I had to drill another hole in the right place and fill the wrong one with JB Weld.
This cap was listed specifically "for" MVs. Just goes to show, check everything....leave nothing untouched.
 

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?

I find that many times when I am getting gas, opening the gas cap, I get a pretty good amount of suction. So much so that I've learned to put my hand over the gas cap so the spray doesn't get on the tank. ......
it's spitting gas out?

your tank is pressurizing from engine heat/the sun .....the vent isn't working

remove the cap and thread a wire through the holes that head towards the rear of the

tank recess where the vent line goes down through the tank either to the charcoal

canister or the fuel pump plate

the canister can fail and plug the lines...... remove it

after checking the holes in the cap the simplest fix is to remove the connection

between the cap and tank.....yes, on occasion you will smell fuel, but it can't screw up

caps that worked on my MVs

the 2 stockers

the CNC screw plug one ....sold

2 MV Corse ones (with the "S" handle) like your SSR one

a Tamburini Corse one on the F4

a Tamburini plug that fits the MV Corse cap

and a plug from Alex at GP Racing that uses an Allen wrench and goes on the MV

Corse cap too

if they don't line up with the tank hole.....i just remove the rubber connection.....

its all going the the atmosphere

:popcorn:
 

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I'm on my 3rd trip to the dealer for warranty repair for same tank issue. 1st time it swelled up so bad there was .500" gap between tank and cover on top. Then it sucked in so bad the fill cap was down about 1.5". So it went to dealer, they said it was repaired. Only for the tank to leak most its fuel out the bottom (it was full as i just filled up at gas station and it wasn't leaking and i never filled all way up because of what i have read on here about the evap issues) by fuel pump in a 20 mile ride! I didn't even notice until i pulled in my driveway. Luckily i did not crash or catch fire. But now the tank was rippled and deformed.

Went back to dealer and a new tank was installed only for it to swell up again. And once again fuel started leaking from bottom of tank. Not the vent lines as I pulled these out from tank and had them hanging out to see if venting/leaking properly. (not tucked down and pinched that can easily be done)

So basically they keep putting failed evap canister back in new tank. This is what I think is wrong. If it wasn't under warranty I would have done removed the evap canister by now and tossed it in the trash.
Its currently at the dealer now for the 3rd time to get this corrected. It's a 2015 F4 that's spent more time at dealer than with me on it. Has only 1300 miles on it.
 

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So we have gone from a suction (vacuum) problem to a pressurisation problem. As a long term mechanic fitter I would have thought that an easy test would be to leave the fuel cap open. If that all returns things to normal then the problem is a blocked vent. Fuel going out needs to be replaced with air. Fuel and air expanding in the sun needs to escape. No you will not die a horrible death if you ride with the cap ajar and half a tank of petrol.
 

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Discussion Starter #14
Latest update is they found an external vent tube slightly clogged and cleaned it out. They wanted to replace the tubes inside the tank but had to order them so I took her home without the tubes replaced and she seems to be fine. Went for a long ride on Sunday and got home with the fuel light on. Opened the tank and there was slight suction but seems to be normal from my experience with tanks that are near empty. Before it was difficult to open the tank due to the pressure...or lack thereof. No use fiddling with the inside if it's a non issue so we'll keep an eye on it. Probably be 3 months before the tubes come in anyway. :jsm:
 
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