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Mikuni BSR / vacuum petcock question

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Hi,

I am putting a clarke tank on for next season. I found a replacement second-hand vacuum petcock. Once I had everything tightened up (Tight!) it seems to have only a small (1/2 cc per day) drip.

So here's the question - Is having a petcock somewhat redundant when there's a valve in the float tank?confused.gif

Specifically for me - Given that the carb has a float valve which should shut off excess fuel, do I need to worry about this slight fuel drip (I have read the horror stories of fuel drips from vacuum petcocks destroying engines etc)? Should I put an extra petcock in (as seen on this forum). I like the convienience of the vacuum petcock.......

Thanks for the help

Neil

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Hi,

I am putting a clarke tank on for next season. I found a replacement second-hand vacuum petcock. Once I had everything tightened up (Tight!) it seems to have only a small (1/2 cc per day) drip.

So here's the question - Is having a petcock somewhat redundant when there's a valve in the float tank?confused.gif

Specifically for me - Given that the carb has a float valve which should shut off excess fuel, do I need to worry about this slight fuel drip (I have read the horror stories of fuel drips from vacuum petcocks destroying engines etc)? Should I put an extra petcock in (as seen on this forum). I like the convienience of the vacuum petcock.......

Thanks for the help

Neil

Bad assumption. The float valve does not provide any kind of real seal, it's more of a resistance to flow, hence petcock requirement for all gravity feed carb motors.

Put a manual petcock on and don't forget to close it whenever you park the bike for more than 10 minutes. Don't worry about forgetting to open it. The bike will let you know when you do that. :smirk:

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Petcock leak - Yes you have to worry about it. Petcock needs to shut off absolutely.

Carb float valve - The primary purpose is to regulate the fuel level in the carb not shut of the fuel long term.

Many float valves will seal long term but it is unknown if yours is reliable or not, many are not or inconsistant. Carbs with a high fuel level drain, leaky float valves are not a problem because excess fuel ends up on the ground, Your carb does not have a high fuel level drain so any excess fuel ends up in the cylinder and eventually in the crankcase oil.

You need a good reliable petcock (manual or vacuum) and a good reliable float valve. Often the problem with a leaky float valve is the $.50 "O" ring.

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Thanks for the help. I went back to my petcock and cleaned out the vacuum mechanism - it now seals properly :smirk:

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