Here is a tough one all try on you fellas.

I have an 04 Honda Foreman 450 es. It does not start after sitting for a few days, without pressurizing the gas tank by blowing in the gas cap vent. It also constantly stops at random speeds when trail riding. To restart it I have to blow in the gas cap vent line pressurizing the tank. So far trying to fix this I have:

I have run this by mechanics and ATV enthusiasts
Replaced the gas cap with a new one and vent line.
Cleaned out the tank several times
Removed the internal fuel filter
Cleaned and tuned the carb personally and professionally
Replaced all the jets
Replaced the drain hose on the bottom of the carburetor
Gone crazy

ANY IDEA'S

 

You seem to have covered nearly all the basics. It doesn't seem to me that your problem is ventilation regarding the fuel tank at all, but from tank to carb.

You mentioned you replaced the drain line on the carb. What about the <u>vent</u> line from the carb? That will definately cause your symptom.

Another is the fuelline from the tank to the carb. If it's been replaced by a line longer than what came stock and has "special" turns and bends unlike the stock one, it will sometimes impede flow on gravity feed systems. Also when this is often done, the replacement line is cheap quality and doesnt insulate the fuel as well as the factory line, causing it to over heat and cause a form of vapor lock.
Seen this many times on the sport models like the 400EX in particular.

 

does it do it with a full tank of gas? If you cant figure it out consider getting one of those diaphragm pumps like on the polars. good luck.

 

Past experience I have found a few fuel petcocks that will not allow enough fuel flow.
Something else that you can check anyway.

When it dies, pull the fuel line from the carb and check for flow from the tank
Do not get fuel on the hot engine

----- Gimp -----

 

Does it have an inline fuel filter....made of paper? Ive found them to be troublesome in gravity fed systems. The ones that have the fine brass mesh , dont seem to do it, but Ive experienced this myself...and it was the paper element fuel filter.