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1977 Suzuki GS750
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Discussion Starter · #21 ·
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.

:cool: 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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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.
 

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Discussion Starter · #23 ·
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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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
 

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Discussion Starter · #25 ·
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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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.
 
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Discussion Starter · #27 ·
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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Non vacuum petcocks are readily available on the ‘net. Measure the center to center distance of the mounting screws and look accordingly. I just purchased some 48mm petcocks for a project and they were about $75 a pair. This is a common change on XS 650 Yamahas. I think They use 48 and 50 mm depending on the year. MikesXS has them but they are more money. The petcocks are a bit nicer though. You can also purchase or fabricate adapter plates to use 1/4 bsp or npt petcocks. …British style. I have gone both ways on builds. Pay attention to where the petcock comes out, as the adapter/petcock combo sometimes creates clearance problems. I have never has much luck modifying the vacuum petcocks.
 

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Discussion Starter · #30 ·
I thought you said it runs like a champ now :unsure: 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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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.
 

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Discussion Starter · #33 ·
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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