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Discussion Starter · #1 ·
Had to do the top end on an 04 quadmaster. Local engine machine shop isn't able to go small enough to bore out the oem cylinder, so I had to resort to one of the aftermarket ones (Niche). The original has a vacuum port that runs to the carb, pic below
Gas Toy Wood Machine Auto part

. Once started, supposed to switch the petcock from prime to run, and this vacuum port apparently controls the flow of fuel with the help of that vacuum, if I understand that right. None of the aftermarket cylinders have this port. So I'm guessing the correct procedure would be just to run it with the petcock in the prime position? Or am I missing something

Also, any ideas on a shop that can bore out these little guys? I'm in MI, but I can ship anywhere. Be nice to have that as a back-up, or just switch it when it came back.
 

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You say the port runs to the carb. Where does the petcock get its vacuum from?
If operating correctly, you would never use the prime function on the petcock. It will pick up fuel on its own.
Leaving it on prime will by-pass the need for the vacuum port, but remember to put it back to run when you shut if off, to prevent flooding if you have a leaking needle and seat..
Give Millenium Plating a call, they say they will do cast iron as well as plated cylinders.
 

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Discussion Starter · #3 ·
The port circled in the pic runs to one side of the petcock. If I'm understanding the system right, the petcock has 3 positions. Run, Prime, and Off. To start you put it in the prime position, which allows a straight flow of gas from the tank to the carb. After starting, the manual says to switch to run - and the vacuum from the cylinder is supposed to control the flow. So the petcock is getting its vacuum from the port that is circled. So absent that port and vacuum, I'm thinking my option is to just leave it in the prime position and let the carb float do it's thing.

And thanks for the Millenium suggestion - I'll give them a call
 

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On the 400, the 3 positions are Run, Prime, and Reserve. Off is taken care of by the lack of vacuum in the run position. A simple test would be to turn yours to off and see if fuel runs out of the petcock. The carb bowl and gas hose are full of fuel. The only time you would need prime in a normal situation is if everything were drained.
 

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This sounds like it could be solved a handful of ways. You can drill and tap the new cylinder and stick your own fitting in it. Or, is there a manual petcock available that will fit your tank? The auto shut off was a quirky unreliable thing anyway in my opinion. Or, you can run it on prime. Outside of not having reserve, the only way running on prime will cause issues, is if the carb has something wrong with it.
 
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