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It has Mikuni carbs,
The vacuum line for fuel tap should come from barb on number 3 intake (second in from right side)
I've seen a LOT of those fuel taps assembled wrong with the spring on wrong side of diaphragm holding fuel on all the time.
The o-ring seal can also deteriorate with E10 fuel. It gas a 'double diaphragm' with plastic spacer between sections, there is a tiny slot to allow atmospheric pressure, it can get blocked.
It's very unlikely a 'modern' shop will be able to fix anything as most techs don't work on carbs or only single carb MX bikes (it was pretty bad in 2011 when I was teaching at MMI, I doubt things have improved with age?)
 

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1977 Suzuki GS750
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Discussion Starter · #43 ·
It has Mikuni carbs,
The vacuum line for fuel tap should come from barb on number 3 intake (second in from right side)
I've seen a LOT of those fuel taps assembled wrong with the spring on wrong side of diaphragm holding fuel on all the time.
The o-ring seal can also deteriorate with E10 fuel. It gas a 'double diaphragm' with plastic spacer between sections, there is a tiny slot to allow atmospheric pressure, it can get blocked.
It's very unlikely a 'modern' shop will be able to fix anything as most techs don't work on carbs or only single carb MX bikes (it was pretty bad in 2011 when I was teaching at MMI, I doubt things have improved with age?)
What do you propose I do to fix this
 

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What do you propose I do to fix this
He's telling you to take it apart and learn how to fix it yourself, IF it's not presently passing fuel when a vacuum is applied to the fuel tap via a rubber hose connected to carburetor intake #3

If in doubt, test it first, not hard to do if it has a vacuum hose. Remove the fuel line and turn the fuel tap on, no fuel should flow. Suck on the vacuum hose a little (the small rubber tube) and fuel should start to flow out of your vacuum controlled fuel tap. Easy to test and not all that difficult to fix because there is nothing in there except a couple of o-rings and a plastic diaphragm that moves in response to a very small amount of vacuum pressure.
 

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1977 Suzuki GS750
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Discussion Starter · #45 ·
He's telling you to take it apart and learn how to fix it yourself, IF it's not presently passing fuel when a vacuum is applied to the fuel tap via a rubber hose connected to carburetor intake #3

If in doubt, test it first, not hard to do if it has a vacuum hose. Remove the fuel line and turn the fuel tap on, no fuel should flow. Suck on the vacuum hose a little (the small rubber tube) and fuel should start to flow out of your vacuum controlled fuel tap. Easy to test and not all that difficult to fix because there is nothing in there except a couple of o-rings and a plastic diaphragm that moves in response to a very small amount of vacuum pressure.
Alright thanks I’ll try it
 

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1977 Suzuki GS750
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Discussion Starter · #46 ·
He's telling you to take it apart and learn how to fix it yourself, IF it's not presently passing fuel when a vacuum is applied to the fuel tap via a rubber hose connected to carburetor intake #3

If in doubt, test it first, not hard to do if it has a vacuum hose. Remove the fuel line and turn the fuel tap on, no fuel should flow. Suck on the vacuum hose a little (the small rubber tube) and fuel should start to flow out of your vacuum controlled fuel tap. Easy to test and not all that difficult to fix because there is nothing in there except a couple of o-rings and a plastic diaphragm that moves in response to a very small amount of vacuum pressure.
So the petcock works properly but it put a fuel filter on the bike and it won’t even start now
 

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So the petcock works properly but it put a fuel filter on the bike and it won’t even start now
that's not good. Where are you at with floats working and the bike not leaking fuel onto the ground any more?
and can we assume it has air, compression and spark all happening at the proper time
 

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1977 Suzuki GS750
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Discussion Starter · #48 ·
that's not good. Where are you at with floats working and the bike not leaking fuel onto the ground any more?
and can we assume it has air, compression and spark all happening at the proper time
So I might readjust the floats a little bit but I have no idea if it still leaks
It does have park because I used a little carb spray and it tried to turn over
I’m gonna put a different fuel filter in today because one I used was an old one to test and it seemed to make things worse. It’s a piece of mesh between two clear cones things. It’s weird tho, it’s completely clean but when I turned the fuel on yo prime the carbs the fuel work go through the line and through the filter completely fine but the line doesn’t go down at the best angle so I think carbs can’t fill properly.
 

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If it was me I'd go straight to removing the float bowl off carb #1 then I would turn on the fuel tap to 'prime' which implies that it should now be bypassing the vacuum controlled valve and see if fuel flows out (taking obvious precautions to deal with the imminent fuel spill) then I would lift the float with my finger until the fuel flow stopped and if it didn't I know I found a problem. I might even repeat that process for each of the carburetors independently.
Now I know that everything works as it should and the float heights can be considered checked.

If I was looking for a fuel blockage, I would go straight to the main jet emulsion tube, because that is almost certainly where all the dirt is hiding. Take it apart and clean it under my stereo microscope using isopropyl alcohol and then put it back together and consider that ready to go too. If I have not cleaned and serviced my air filter recently I ... should do that right now come to think of it, I'll be back.
 

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1977 Suzuki GS750
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Discussion Starter · #51 ·
If it was me I'd go straight to removing the float bowl off carb #1 then I would turn on the fuel tap to 'prime' which implies that it should now be bypassing the vacuum controlled valve and see if fuel flows out (taking obvious precautions to deal with the imminent fuel spill) then I would lift the float with my finger until the fuel flow stopped and if it didn't I know I found a problem. I might even repeat that process for each of the carburetors independently.
Now I know that everything works as it should and the float heights can be considered checked.

If I was looking for a fuel blockage, I would go straight to the main jet emulsion tube, because that is almost certainly where all the dirt is hiding. Take it apart and clean it under my stereo microscope using isopropyl alcohol and then put it back together and consider that ready to go too. If I have not cleaned and serviced my air filter recently I ... should do that right now come to think of it, I'll be back.
I tried to clean them as much as I could the when I had them off last

I tried putting a different fuel on and it had the same problem as last time. The gas will flow through the line into the filter and kinda just sit there

I just tried using starting fluid and it obviously ignited but it seems like the carbs aren’t pulling any gas even when I have the petcock on prime mode

I put just a straight gas line on it and it does the same thing, it doesn’t leak now but it also doesn’t suck in fuel so I’m not sure what is wrong now and yes the vacuum line is connected
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Nice filter! Carbs never actually suck fuel in, they need fuel pushed in.
Fuel pressure on your bike is 100% dependent on gravity feed. The float, needle and seat has to shut that flow off, so fuel needs to pour right out of that inline filter when the tap is turned on prime or you still have a problem. When the tap is on the regular 'on' setting, fuel will only flow when you apply a vacuum to the smaller rubber hoses.

Rider here today had the same kind of problem needing to shut his fuel off every time he stopped riding or the carb floods. He's waiting on a parts order from a local KTM dealer, o_O because apparently they don't inventory something as simple as carburetor parts
 

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Discussion Starter · #53 ·
Nice filter! Carbs never actually suck fuel in, they need fuel pushed in.
Fuel pressure on your bike is 100% dependent on gravity feed. The float, needle and seat has to shut that flow off, so fuel needs to pour right out of that inline filter when the tap is turned on prime or you still have a problem. When the tap is on the regular 'on' setting, fuel will only flow when you apply a vacuum to the smaller rubber hoses.

Rider here today had the same kind of problem needing to shut his fuel off every time he stopped riding or the carb floods. He's waiting on a parts order from a local KTM dealer, o_O because apparently they don't inventory something as simple as carburetor parts
I later put a section of hose that barely bent but also fit better. But gas still did seem like it was filling the carbs

before this I have taken the carbs off multiple times and even when I put new floats in it but all the sudden I stick a fuel filter on it decides to act really weird
 

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Looks like I'm going to have to pull the float bowl off a carb and take photos of how to test it, so you understand the concept. The float bowl is that very lowest part on the carburetor that is held on with 4 star screws. If you remove that, the floats will drop and fuel will flow or you have a blockage. Then when you lift the floats with your finger the fuel flow should stop.
Riding again today but I'll see if I can get photos for you later.
 

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Discussion Starter · #55 ·
Looks like I'm going to have to pull the float bowl off a carb and take photos of how to test it, so you understand the concept. The float bowl is that very lowest part on the carburetor that is held on with 4 star screws. If you remove that, the floats will drop and fuel will flow or you have a blockage. Then when you lift the floats with your finger the fuel flow should stop.
Riding again today but I'll see if I can get photos for you later.
I believe I know what to do
I might take the carbs off and readjust the floats
 

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Discussion Starter · #56 ·
Looks like I'm going to have to pull the float bowl off a carb and take photos of how to test it, so you understand the concept. The float bowl is that very lowest part on the carburetor that is held on with 4 star screws. If you remove that, the floats will drop and fuel will flow or you have a blockage. Then when you lift the floats with your finger the fuel flow should stop.
Riding again today but I'll see if I can get photos for you later.
So I have made some progress the bike runs and doesn’t leak gas on the floor but it is sputtering when in gear.

I believe it just needs to be tuned properly now
 

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So you're going to first bench-balance the carbs (measure that the slides all open the same amount), then you want to check static timing with a light bulb, then balance the carbs, then get the timing light and check/adjust timing again.

That should get the bike running well, provided the plugs are good, the coils and wires work (check voltage at the coils with the Ignition on), and the points are serviced.
 

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So I have made some progress the bike runs and doesn’t leak gas on the floor but it is sputtering when in gear.

I believe it just needs to be tuned properly now
Are you sure it's running on all 4 cylinders :unsure: do all of the headers get hot the same,
have you tested compression on all the cylinders to see if they are the same,
if you pull all 4 spark plugs do they look the same?
 

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You're also going to want to service the valve shims
 
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