Draining fuel tank on GS750

[Firewall]

Hello,

I'm trying to figure out how to drain the fuel tank on my GS750 (preferably with out removing it). I've done this on my other bikes, but this is the first bike I've had where the petcock does not have an "Off" position.

From reading around, it kinda sounds like with out a vacuum on the line fuel will not flow with the lever in the "On" position. So I should be able to pull the fuel line, then turn the petcock to Prime, right? I'd like to make sure of this before I end up with a big mess on my hand.

Thank you,
Matthew

[Adler]

That is correct.

[Firewall]

That is correct.

That's gotta be the fastest reply I've ever had on a forum (and I practically live on forums)

Thanks!

[Adler]

No problem.

Be careful unless you are positive that the petcock is working as is should be.

[Firewall]

Worked perfect, thanks again

[Firewall]

So follow up question, I see replacement of the petcock recommended a lot on here. As far as I know mine is a original 1977 part. However, it worked fine during this procedure (meaning it didn't let fuel flow except for on prime) and there's no fuel in the vacuum line. So I should be OK right?

[cowboyup3371]

Yea you should. Just keep in mind that if you start seeing fuel leak from the carbs you may be looking a petcock issue since the floats aren't designed to stop fuel.

[Texas Don]

So follow up question, I see replacement of the petcock recommended a lot on here. As far as I know mine is a original 1977 part. However, it worked fine during this procedure (meaning it didn't let fuel flow except for on prime) and there's no fuel in the vacuum line. So I should be OK right?

Replacement is recommended if you are having problems with it. Unless you know the complete history of the bike in detail, that part may have already been replaced or repaired prior to your ownership without your being aware of it. Unless you can condemn a part without question, using proper troubleshooting methods, why replace it? You gain nothing and only lose money on the deal.

[chuck hahn]

Cowboy..Just an FYI here`. If the needle valve seats and needle valve themselves are in good shape and the float height is properly set, then yes in fact the fuel should be shut off by the needle valves.. thats a basic function in any carb with a float. Thats what its designed to do. It sounds as if the petcock is operating properly. One test i do is to have about a gallon of fuel in the tank. Be sure the petcok is completely wiped dry. Put the petcock in the ON position and set it on a clean piece of cardboard. Let it set for about an hr. and then check for any wetness under the petcock.If its dry then the diaphram is sealing the flow properly..if its wet then you can purchase a rebuild kit from several sources. The thing is that you want to be sure the petcock and needle valves are doing their jobs in concert to prevent slow leakage of raw fuel into the crankcase and contaminating the oil.

[mindfulloffire]

hmm. I kind of wish I'd seen this a few weeks ago. I had to drain my tank the hard way by turning it over and spilling it into a funnel. I tried putting it on prime to drain but I didn't get anything.

[chuck hahn]

If you put it on prime and nothing came out then there was something wrong with the petcock.

[Adler]

If you put it on prime and nothing came out then there was something wrong with the petcock.

Ive heard that some of the petcocks require a tiny pit of vacuum on prime to get it started.

[cowboyup3371]

Cowboy..Just an FYI here`. If the needle valve seats and needle valve themselves are in good shape and the float height is properly set, then yes in fact the fuel should be shut off by the needle valves.. thats a basic function in any carb with a float. Thats what its designed to do. It sounds as if the petcock is operating properly. One test i do is to have about a gallon of fuel in the tank. Be sure the petcok is completely wiped dry. Put the petcock in the ON position and set it on a clean piece of cardboard. Let it set for about an hr. and then check for any wetness under the petcock.If its dry then the diaphram is sealing the flow properly..if its wet then you can purchase a rebuild kit from several sources. The thing is that you want to be sure the petcock and needle valves are doing their jobs in concert to prevent slow leakage of raw fuel into the crankcase and contaminating the oil.

Thanks Chuck.

[chuck hahn]

How does that make sense..the purpose of the prime feature is if the bike is totally out of gas or the carbs are being put back on and the bowls are dry..you need a way to PRIME them without vaccuum..If theres no fuel in the tank or carbs then you in cant start the bike like say on the side of a road and your out and someone stops and gives you some gas..At that point just how do you suspect you can apply a vaccum to get it started??? It should flow freely on prime without any assist from anything..thats how it works. Prime= free flow of fuel WITHOUT any vaccuum required..ON = fuel flow ONLY while engine running to supply vaccuum..Resereve = sama as on.

[Adler]

How does that make sense..the purpose of the prime feature is if the bike is totally out of gas or the carbs are being put back on and the bowls are dry..you need a way to PRIME them without vaccuum..If theres no fuel in the tank or carbs then you in cant start the bike like say on the side of a road and your out and someone stops and gives you some gas..At that point just how do you suspect you can apply a vaccum to get it started??? It should flow freely on prime without any assist from anything..thats how it works. Prime= free flow of fuel WITHOUT any vaccuum required..ON = fuel flow ONLY while engine running to supply vaccuum..Resereve = sama as on.

I suspect it was a design flaw? Anyways its just what I heard.

Also in normal circumstances the starter will produce enough of this allegedly required vacuum.