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Dorman10

400ex possible fuel issue

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450 tank have its own cap? Or did you use your 400ex cap? Did you try running no cap? It has to be lack of fuel flow. Be prepared if you try a fuel pump you will need to put a return line back into the tank or regulater to slow the flow. Otherwise you are 100% going to push passed the float needle and have fuel running out the over flow tube.

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Not to mention if the fuel flow isnt able to fill the carb bowl fast enough it wont be able to feed the fuel pump fast enough to keep primed so its not gunna really work anyway. It has to be a tank vent issue. We can rule out petcock. Carb.(Unless you got 2 with the exact same problem? Not likely) Time to run it on a bottle with a vent hole in the top and see if it runs perfectly fine.

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450 had its own cap. I ran it with the cap on and off. Ran it with a bottle too and it still has the issue. Had a vent cut in the bottle and everything. Could it be electrical, like in the stator and firing?

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Honestly at this point it could be anything. But we confirmed its running the bowl out of fuel. It doesnt make any sense. Stator, coil, cdi. None if those are gunna have any control over filling that float bowl. Gravity fills it. Does it do it when engine is cold and hot? Could it be getting so hot it boils the gas in the fuel line? Im grasping at straws here my man. Nothing about this makes any sense. Unless you have 2 bad carbs in the exact same way and one is brandnew right out of the box bad? Wow thats a stretch! The main jet didnt maybe fall out or something? That would pull so much fuel so fast it maybe could out run the gravity feed?

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What would be the ideal main jet for me to use with the set up i have:

440big bore kit

stage 3 hotcam

ported heads.  
slip on HMF pipe

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Stock carb i would be guessing somewhere in the 180-190s. Pilot jet will be 45 or 48 i would guess also.

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Ok so this is going to sound weird, but blowing in the hose to the gas cap seems to keep it running on top end, like it needs pressurizing. 

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your fuel cap vent is clogged up , do a video search of fuel tank  vent blockage , how to check and results   

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Taking apart the carburetor now to see if something from the tank went down in it. The floats in the carb aren’t doing their job anymore because it leaking gas out to the run off hose

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Leaking gas out the "run off tube "is a sticking needle valve , you'll got ethanal gas around where you at 

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It’s like it’s not letting enough fuel flow down to the carburetor, but nothing it’s stopped up from all we’ve tested

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Yeah man im stumped. As long as gravity hasent changed it has to work. The fuel flows out of the tank down the fuel line into the carb. There is a direct connection from the fuel line nipple on the side of the carb and the float bowl. The only thing left in this process is the float needle. I wonder if something was in the fuel line and got into the original carb. Then when you put the new carb on the same stuff was still in the line and went in the new carb and did the exact same thing? Maybe sand or dirt or something? That would stop the float needle from moving freely. Which would then slow the fuel flow into the bowl. This new symptom of leaking out the overflow is telling us the float is sticking for what ever reason.

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just a shot in the dark, but...what size is the fuel line ?, and do you have an inline fuel filter ?, if both are too small ?, it will cut down on the fuel flow to carb !.

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I'd look over your carb and tank again, something still isn't quite right, did ya get it all cleaned? Probably unrelated but I run Sunoco Non Ethanol Fuel it's the only stuff I run, if you can find it in your area I highly encourage you to get it.

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Hey Dorman10 did you ever figure out your problem? I have same exact problem and done EVERYTHING you have done and nothing!

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