Fuel Pump Issue MX 4-Stroke

Lol! You just posted in a now pretty old thread to not use a return line, after so many running Mx type engines are running return lines. Explain as to why you should NEVER run a return line. I'm very interested in why?
 
You should work on the fuel pressure, and flow, and get that right. A return line is another possible cause of problem.
Think about what a return line does, and eliminate the need for it. THAT is the way to address the issue. KISS is good.
 
It might be enough, but what if it were still too much fuel? What then?

Ps, I've seen them run and be too much for the bowl, but not be able to supply enough during high rpm run.
Perhaps it wasn't the pump and it was something else. But, out a return line on and problems cleared up.
 
I run the square pump on my cr250 and have to run a restricted return line, otherwise it will dump fuel out all the vent tubes, not a big fan of it because of the possible added issues, but at least it runs this way.
 
I am not sure what added issues a return line adds to the mix? I have had no issues since I put my return line and never had a single issue with it over filling. Just curious what type of solutions PD proposed. But , needle and seat could be a solution. I just know that a return solved any issues I had and never once touched the carb. Just looking for a better method, I posted a link to this thread in another and figured if it could help someone I'd explore the other options.
 
Lets look back at stock motors. Smaller carbs on clone and animal. Means smaller floats and needles. That would lead me to believe that there would be more pressure at the needle and seat and less leverage by the small float to seal it up. Not once, ever, have I seen or used a return line on a stock motor. I actually took the pump from my stock clone and put it directly on my crf250, then my 450, and now my 400. Never an issue. Maybe you have a problem somewhere else if you have to run a return.

P S- the only pump I have ever used is the old square walbro. But to each their own. Everybody is different. Use what works best for you
 
If it works for you, whoever "you" are and whatever you're doing..................keep on doin' it.
"Don't fix what ain't broke."
 
WE SOLVED THE PROBLEM!!! We ran the return line (not restricted) right off of the carb. It seems this allows the fuel to be right at the carb when needed and if not, dumps into the return. We also moved the pump (round mikuni) closer to the carb. Pics coming soon.... The only issue we are having now is the header is turning cherry red by the end of a race (15 laps). I'll be checking the color on the spark plug tonight but ive been told that the yz250f seems to have a "glow stick" header. Any thoughts?
 
My pipe right after the head will glow when idling on my KTM 450, but should go back to grey when you rev it.

It's dumping too much fuel basically, and burning in the header. If it's still doing it when running you might consider jetting. Mine won't glow when it moves off the line and starts to burn fuel better. At idle it's going to be a little fat, so you don't want to let it run idling long. But, might be different built vs stock or whatever yours is.

I'll post pics of how my fuel pump is run, perhaps it will help.
 
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