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Hi everyone, first post here, need your knowledge on an issue I have.
I bought a second-hand 2008 DRZ400 SM with Keihin FCR mod installed on recently and everything was ok, until yesterday I had this issue while riding. I had fuel on and my petcock set to "ON" position, my engine failed. After waiting a bit, thinking what to do, I tried to start the bike again and it started for a few more meters. When I turned the petcock to the "PRIME" position, the bike was rolling all good. But now I am completely confused about what is going on. The petcock has 3 positions, the "ON", the "PRIME" and the "RES". For some strange reason, I cannot turn the petcock to the "RES" position at all. Then, when I use the "ON" position, after a few meters the engine will die, either when I downshift and rev-match, or while giving gas, with a lot of popping noise coming from the exhaust. On the "PRIME" position the engine runs just fine, but the manual suggests that it shouldn't be left to "PRIME" for long periods? I suppose the previous owner did some misconfiguration? Any info would be helpful.
 

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It's a vacuum controlled petcock , it sounds like the vacuum line isn't connected. You can use it like you have been in prime , but you'll have no reserve.
 

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It's a vacuum controlled petcock , it sounds like the vacuum line isn't connected. You can use it like you have been in prime , but you'll have no reserve.
Thanks, so in "PRIME" i only run the risk to run completely out of fuel correct? There is no risk of gasoline overflow with the "PRIME" on only when the engine is running? I have put it back to "ON" position after riding.
 

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Prime is just like on on a regular petcock.
When in Prime position, it by passes the vacuum fuel valve and allows fuel to flow freely. So, if you leave it on Prime, and your float sticks, it'll drain the tank onto to the floor. Sounds like you need to pull the valve off the tank, take the valve apart and clean it thoroughly. They usually come apart with a good small Phillips screw driver, and you’ll find a small screen inside that could be gummed up. Or even the pickup on the fuel valve pickup tube screen could be blocked. Just look at it closely as you take it apart, so you remember which way the diaphragm goes. Some also have a tiny spring in there. Watch its position. Or you could just by a new one, with a new base gasket, and just be done with it….
 

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The stock cv carb had a vac nipple on it for the petcock. When doing the fcr model there is a bolt at the head where a vac nipple needs to be added. Check to see if you have a vac line connected at all.
 

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Also, make sure your fuel vent isn't plugged. Some are in the cap, or the drain hose off the bottom of the tank if there is one, can be kinked. A way to check the vent is working, when it dies, open the cap and see if it restarts and keeps running. Oh, and make sure you have fuel. Ha....
 
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