Gas tank ventilation question

[GS750GUY]

I read in my Haynes manual that a fuel tank that has a plugged vent hole can be the cause of the engine to start and run beautifully and then slowly fade and then stop. This is exactly what my 78 GS750EC was doing just prior to deciding I needed to rebuild my carbs. On my 78 how do I know where the vent hole is located? Is it in the top front of the tank somewhere or is it in the gas cap itself? I have studied the cap and can see no hole anywhere in the cap. I have also studied the tank itself and can find no vent hole there either.
Anybody out there had any experience with this or have knowledge of the matter?
I value your help!
Thanks!
GS750GUY (Harold)

[GS850_Coug]

If you shake the cap and it rattles you're probably okay. If it doesn't rattle, you can open it up by removing the two screws on the bottom. Be careful, theres a couple springs, ball, and sliders in there to operate the lock. Clean it all thoroughly to remove any rust and "gunk". I used Naval Jelly to remove the rust-had to mix the parts up rather frequently to keep them in contact with active acid. Used WD-40 to lube it up upon reassembly. Works great now.\\/

[Pearson]

I read in my Haynes manual that a fuel tank that has a plugged vent hole can be the cause of the engine to start and run beautifully and then slowly fade and then stop. This is exactly what my 78 GS750EC was doing just prior to deciding I needed to rebuild my carbs. On my 78 how do I know where the vent hole is located? Is it in the top front of the tank somewhere or is it in the gas cap itself? I have studied the cap and can see no hole anywhere in the cap. I have also studied the tank itself and can find no vent hole there either.
Anybody out there had any experience with this or have knowledge of the matter?
I value your help!
Thanks!
GS750GUY (Harold)

Good question Harold. Was wondering the same thing myself.

Wait a minute...there are no screws under the cap. Are they under the rubber flap or do we have different caps? I have a plain jane '78 750.

[Guest]

Could be petcock, vent lines, fuel vent, kinked fuel line, clogged petcock screen or carbs.

[GS850_Coug]

Good question Harold. Was wondering the same thing myself.

Wait a minute...there are no screws under the cap. Are they under the rubber flap or do we have different caps? I have a plain jane '78 750.

Mine has a large plastic housing that protrudes into the tank, attached using two screws (see link). looking at the pic of yours, it's different. Sorry my post wasn't much help.

[tkent02]

How does it run with the cap loose???

On a 78 you have no lock on the cap, it's a vented cap. To troubleshoot open the cap while the bike is running poorly, if a bunch of air rushes in and it starts running better you are on the right path.

[GS750GUY]

Fuel tank cap.

I don't see any holes anywhere in the cap. It's hard to determine if it is the original cap or maybe some aftermarket cap. Would I hurt anything by drilling a very small hole in the cap to be sure it vented?

GS750GUY

[argonsagas]

I don't see any holes anywhere in the cap. It's hard to determine if it is the original cap or maybe some aftermarket cap. Would I hurt anything by drilling a very small hole in the cap to be sure it vented?

GS750GUY

This is not a recommended approach, although it would be effective if the built-up vacuum was your problem.

Should the bike fall gas could leak through the hole, and fumes would escape at all times, especially when hot. These are not necessarily a major concern, but they can be flammable.

[GS850_Coug]

Should the bike fall gas could leak through the hole, and fumes would escape at all times, especially when hot. These are not necessarily a major concern, but they can be flammable.

Imagine that....light a cigarette on a hot day and have a torch that close to your crotch:shock:

[GS750GUY]

Fuel Tank cap

Thanks guys for your wisdom. Should the problem persist after reinstalling my rebuilt carbs I will try the loose cap test and replace the cap if needed. I will forget about drilling a tiny hole. Sitting on top of a potential mulotoff coctail is not my idea of fun. One of the nice things about this wonderful GS site is that the wisdom of the experienced helps keep us greenhorns alive.
Thanks!!

GS750GUY.

[Road_Clam]

I don't see any holes anywhere in the cap. It's hard to determine if it is the original cap or maybe some aftermarket cap. Would I hurt anything by drilling a very small hole in the cap to be sure it vented?

GS750GUY

If you have rust in your tank, and on the fuel cap, there is most likely rust inside the check ball mechansim. I don't think the fuel caps are servicable. I certainly would rather spend 20 bucks on a good cap, rather than have no ball hair for the rest of my life

[GS750GUY]

Fuel Tank Cap

Thanks for the suggestions guys! I think before I pitch this one I will try soaking it overnight in my can of Berryman's carb cleaner (without the rubber seal of course) and see if that does the trick. I did notice some rust accumulating under the rubber seal and around the outer rim of the cap.
Thanks guys! You know what they say about more heads being better than one!
GS750Guy

[Guest]

You know what they say about more heads being better than one!
GS750Guy

That's what she said!:shock: