Suzuki gs750 petcock

1402 Views 32 Replies 5 Participants Last post by Bobby05, 8 mo ago

Bobby05

Discussion Starter · 8 mo ago

Does anyone know if someone makes a non- vacuum petcock for a 1977 Suzuki gs750

I got new carbs and they don’t have vacuum ports on them. I now need to find a petcock that doesn’t have a vacuum diaphragm in it

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Bobby05

Discussion Starter · #4 · 8 mo ago

TrialsRider said:
There's a bush fix if you just want to defeat the vacuum part; you can replace the spring loaded diaphragm part with a chunk of rubber cut out of an old inner tube.

Would that make the on setting be on without any vacuum

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Bobby05

Discussion Starter · #6 · 8 mo ago

TrialsRider said:
Yes to that and it also blocks fuel from leaking out the vacuum ports.

So just take out that back thing with a “bolt” through it and remove the spring but replace both with a pice of inner tube should do the trick ?

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Bobby05

Discussion Starter · #8 · 8 mo ago

TrialsRider said:
There's actually 4 parts you won't need any more, the o-ring, diaphragm, spring and the spacer plate with the round hole will all become unneeded. Should be relatively obvious once you have it apart.
Pretty sure the off position will still work, but you might want to check that. would suck if you can't turn it off.

I would definitely suck if I can’t turn it off but I’ll give it a shot

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Bobby05

Discussion Starter · #11 · 8 mo ago

TrialsRider said:
It's tricky to drill nice clean holes through rubber for the screws to go through, tools that you use for leather or wood work best. Hole punch for leather would be perfect.

Sounds good

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Bobby05

Discussion Starter · #14 · 8 mo ago

TrialsRider said:
that would present a problem, unless you plug one of the ports.
although if there is no vacuum and the spring is strong enough and the diaphragm is still working that would be an off position. omg it just got complicated I hope he fixed it already

I have yet to try it because I didn’t have the time yesterday

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Bobby05

Discussion Starter · #16 · 8 mo ago

TrialsRider said:
I think you are going to have to find a way to plug the prime position opening so you have an off valve. The reserve fuel pipe probably still flows through what was the original vacuum controlled port opening.
confidence is high

I’ll give it a shot since I can now

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Bobby05

Discussion Starter · #17 · 8 mo ago

TrialsRider said:
I think you are going to have to find a way to plug the prime position opening so you have an off valve. The reserve fuel pipe probably still flows through what was the original vacuum controlled port opening.
confidence is high

So I tried putting a piece of inner tube in instead of the diaphragm and that didn’t work

I then put it back together but put the spring in front of the diaphragm hoping it would stay open butthe on position didn’t work, only the prime

I’ll have to find a way to block off the prime setting

any other ideas?

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Bobby05

Discussion Starter · #21 · 8 mo ago

TrialsRider said:
bummer, I don't like the idea of leaving the diaphragm as point of failure, if you can't see a way to modify it to do everything it needs to do, replacement might be the only option. Plugging the prime circuit sounds easy enough but keeping the reserve and lower drain both working might be a challenge, do you have a local scrap yard? I'd be taking the tank and finding something that has an off.

did you get nice carbs
Pingel is still a good recommendation if you don't mind the cost and order process

So I might try and find something that fits that has on and off valve, I just going to have to find the correct size petcock the can bolt back on the gas tank

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Bobby05

Discussion Starter · #23 · 8 mo ago

8ball said:
Right now you haveAn on/off petcock. It is just that the Prime position is “on”, and the On position is “off”. The only risks you have are:
1. There is no reserve position because the RES and PRI positions draw from the same level. And
2. If the diaphragm fails, it will leak fuel out of the vacuum barb, but that would be a tell tail that the diaphragm had failed. You could put a vacuum cap on the vacuum barb, but you’d lose the tell tale. But the chances of a failure on a diaphragm that isn’t being used is small. It won’t be in contact with the fuel that deteriorates it.

Yes but I don’t believe the prime setting gives enough fuel

also the res setting is off and the on setting is also off so the prime setting is the only way to let gas out of the tank

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Bobby05

Discussion Starter · #25 · 8 mo ago

8ball said:
The prime setting flows almost twice as much as the ON
setting. Test it. You’ll see. Put a hose on the vacuum port and suck on it. Let the fuel flow into a cup. Then compare it to the prime setting.

As I stated before, I have to run in the prime position on high rpm runs because the ON position can’t keep up with my fuel consumption

Interesting, my bike bogs down in high rpm’s

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Bobby05

Discussion Starter · #27 · 8 mo ago

8ball said:
Yeah. I’m not talking about a jetting issue. On short runs, the bike runs perfectly, but during a 9 mile pass that is above 8k rpm about 80% of the time, it will start to starve for fuel slightly at around mile 6 if the petcock is in the ON position. No issue in PRI position.
this is with a brand new OEM petcock. I’ve even went to larger ID fill tubes between the carbs. It’s just flow issue with the OEM petcock.

I have a Murray’s carb system on mine so it could be a timing issue or a fuel related issue

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Bobby05

Discussion Starter · #30 · 8 mo ago

TrialsRider said:
I thought you said it runs like a champ now Pods will do that to you, the fuel starvation issue 8Ball described is not something you would experience on the street unless you are trying to outrun a road rage situation.

Well I had it hooked to a gas I.v. bag thing and that just gave it all the fuel it ever wanted but I couldn’t ride it so I didn’t fully know if there were any problems

the things now is I put the gas tank back on after it was painted and these new carbs don’t have a vacuum port on the so I have to run it in prime mode. It could be running with to much gas or not enough. I don’t think it’s get enough but it does have tiny backfires every now and then.

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Bobby05

Discussion Starter · #33 · 8 mo ago

TrialsRider said:
Post up a picture of all the spark plugs right after a good hot run and you might get some helpful input on the rich/lean thing. One fuel line per carb would solve any fuel pressure concerns. Ignition timing needs to be spot on
& I think you are going to end up needing to stack the carbs to make them tune perfect.

What's the fuel consumption over distance like too? Fuel efficiency is a good indication of an engines total health.

Not sure what the fuel efficiency is because I haven’t payed attention to that yet

yeah I’m gonna take a look at the timing when I can
Also I’ll try the spark plug idea

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