2004 DVX 400 fuel issues

[HeWasRidinDirty]

I rebuilt the carb on said machine. When I hook up the auxiliary gas tank it runs fine and everything is cool. When I put the fuel tank on and hooked up the lines, it completely flooded the cylinder and even about an inch of fuel in the air box. I'm pretty sure that it comes from the little hose on the petcock and hooks to the top of the carb. WTH is that even for? I mean the thing runs fine on the auxiliary tank, so idk And on the petcock when it's in the "on" position, I get no fuel. The petcock from the bike I took it apart and checked out how it operates. The sharpie marks indicate how it really works. Now the other petcock w/o the sharpie marks is an old one that was leaking that I removed from a Polaris quad. It's exactly the same thing, except the small hose and nipple have been removed. I'm so confused, LOL

 

 

Edited April 12 by HeWasRidinDirty

[Gwbarm]

What you have is a vacuum operated petcock in the prime position the fuel flows freely it the on position engine needs to be running pulling a vacum for fuel to flow.As they age the vacuum parts get hard and dont work well. The last problem i had with one it wouldnt run on on but would run on prime ran it for a while that way and finally replaced it. As far as the fuel getting into the cylinder it sounds like the fuel is leaking past the needle and seat or float level not correct i would check that, now it could be getting in through the vacuum hose if the diaphragm in the petcock is cracked.

[HeWasRidinDirty]

Thank you for the explanation.
 

My gawd, I haven’t been on here is several years, the freaking ads are off the chart! 
Anyway, so pic 1 is the ON position, and both fuel inlets are covered. So that’s where the vacuum comes in? I don’t see how it gets fuel, but it works somehow, obviously. The 2 pic is the PRI position and it opens up the reserve port. And pic 3 is the RES position and it opens up both ports but no fuel would into the carb supply? Couldn’t that vacuum be bypassed and would just have to treat the PRI position as ON and it would be drawing fuel, via gravity, but in effect you would have no reserve? 
 

I just looked up a $10 petcock and it only had ON/OFF/RES, so it looks like it deletes that vacuum thing. 
 

[mikeexplorer]

I have had models of Arctic Cats that used that type of fuel valve. They were gravity fed fuel systems. I have had this valve go bad and had to replace it. 

Flooding the carb and intake is usually the float in the carb not working properly.

 

[HeWasRidinDirty]

42 minutes ago, mikeexplorer said:

I have had models of Arctic Cats that used that type of fuel valve. They were gravity fed fuel systems. I have had this valve go bad and had to replace it. 

Flooding the carb and intake is usually the float in the carb not working properly.

 

Well as I explained in the OP, with the auxiliary tank hooked up to the carb the bike runs great, zero issues. 

[davefrombc]

38 minutes ago, mikeexplorer said:

I have had models of Arctic Cats that used that type of fuel valve. They were gravity fed fuel systems. I have had this valve go bad and had to replace it. 

Flooding the carb and intake is usually the float in the carb not working properly.

 

Likely his was leaking through the vacuum line to the carb.. The "This Fits" is the most common  valve for gravity fed systems.. As it is labeled the middle  is off .  Top  is on main and bottom  is the reserve. The faulty valve only opens the main under vacuum.. The reserve opens whether there is vacuum  or not..  Yes the no vacuum  valve will work  and give you both the main and reserve as dsigned..  TRhe vacuum operated valve depends on the fuel  in the carb bowl  to  start and open  the line..  If there is no vacuum the motor would die as soon  as the carb bowl emptied.

 

[Gwbarm]

I ran one for a while in the prime position when no fuel would go through the on or reserve position, and eventually replaced it with a manual one,  off reserve on and plugged the vacuum line, i did this because i like to run all the gas out of the carb before i store it. 

[HeWasRidinDirty]

That’s what I’m thinking of doing. I don’t see the reason behind the vacuum operating the petcock like that. Gravity always works. Anyway I got the thing back together plugged the vacuum line and capped the port on the carb. Went for a little ride, 2-3 minutes, and it was crapping out at WOT in 3rd gear. Then it just died. Couldn’t get it to start again. Checked the plug, it was wet. Spark is good, compression is good. I pulled the filter to look inside the intake and I see a bunch of oil in the airbox and in the carb. Is it possible that when the engine got flooded with gas that some seeped past the rings and into the crankcase? It happened 2 times, but I pulled the plug out and pumped the fuel out both times within about 15 minutes or so. The dipstick shows over full and it smells like gas. I’m assuming that nothing was damaged with the short ride time?

[Gwbarm]

It should be minimal seepage past the rings unless it is completely just pouring in there. You know its not getting in through the vacuum line. So i would take another look at the carb replace the needle and seat and make sure float height is correct. 

[mikeexplorer]

On 4/12/2026 at 6:43 PM, davefrombc said:

Likely his was leaking through the vacuum line to the carb.. The "This Fits" is the most common  valve for gravity fed systems.. As it is labeled the middle  is off .  Top  is on main and bottom  is the reserve. The faulty valve only opens the main under vacuum.. The reserve opens whether there is vacuum  or not..  Yes the no vacuum  valve will work  and give you both the main and reserve as dsigned..  TRhe vacuum operated valve depends on the fuel  in the carb bowl  to  start and open  the line..  If there is no vacuum the motor would die as soon  as the carb bowl emptied.

 

I did not see the "prime" position on the valve originally. I owned two Arctic cats that has a fuel valve, but did not have a "prime" position. They were strictly gravity fed systems. What your saying makes sense. If the valve is leaking by, it could very well suck fuel through the vacuum line and flood the carb. 

Another thing to consider since it is a 2004 model to check the fuel lines. Over time the lines tend to go bad and older models were not made to use E10 fuel. I had a 2007 machine that the vent for the tank collapsed because it got soft. I ended up replacing all the fuel lines and the vacuum line. (The machine had its gas tank in the back so it used a diaphragm pulse pump using the vacuum line) 

Mike

 

[HeWasRidinDirty]

Again it’s not the needle/seat/float. Needle and seat are brand new, OEM, and the float is set at 13mm. And like I said, when hooked up to the auxiliary tank, zero issues. Here’s what I found yesterday. Yikes! That’s mostly gas, just from the engine. The bike holds 2 quarts of oil and I think I drained probably 3.5 quarts total out of it. So apparently at some point with the fuel tank connected it was leaking into the vacuum port on the carb. Cuz that is a lot of gas in the oil. I must have left the petcock in the PRI or RES position overnight or all day long or something. As I posted earlier the thing died on me and after seeing all of the gas in the oil I was kinda freaked out thinking that I ruined the motor somehow. I put some fresh oil in it and it fired right up and everything seems cool. I do have a new petcock on the way that doesn’t use the vacuum port. 
 

Edited April 15 by HeWasRidinDirty

[Gwbarm]

On 4/13/2026 at 8:50 PM, HeWasRidinDirty said:

Anyway I got the thing back together plugged the vacuum line and capped the port on the carb.

Not sure how that happened with the vacuum port plugged, and new needle and seat, but im still thinking about it.

[HeWasRidinDirty]

1 hour ago, Gwbarm said:

Not sure how that happened with the vacuum port plugged, and new needle and seat, but im still thinking about it.

Come on man!!!! LOL I mean after the flood of gasoline I plugged the stuff up and all seems good. I appreciate the feedback and the help. My experience with these types have been if they leak replace it. I never looked into how they work. So now I understand a bit. Still don’t understand the thinking behind using these type of prtcocks. 

[Gwbarm]

Sorry! I read it wrong, thought you were still having problems after plugging it, hopefully the new petcock will solve the issue, i ended up replacing all of mine with the manual ones. I like running all the gas out of the carb when its not going to be used for a while. The only reason i can think for the vacuum one is if they are working properly, nothing to think about, just get on choke it and push the start button, same with automatics much simplier get on give it gas and go.