Fuel Pump Issue MX 4-Stroke

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iwinulose29

New member
I am currently building a kart with a 2001 yz250f engine (with tranny) and I am having major problems with the fuel pump system. I am running a round style minuki pump with an independent return line off of the pump (no tee's in line yet). I also have walboro round and square styles if they would be better. The issue is, i dont believe the carb is getting enough fuel. The second we get the engine turned over, you can visually see the return line from the pump full of fuel. We attempter to clamp the line (before installing a jet) and it seemed to run a lot better. Another issue is we have to run the engine with the choke half out. It will not run with the choke off. Can anyone send me some ideas on how to resolve this problem???

Here are a list of things to try next:
1. Get rid of the independent return line and install a tee off of the carb.
2. Jet the return line and keep it the way it is.

If anyone has any other suggestions please let me know. Thanks!
 
Did thoroughly check and clean carb, check for correct float height before install? If having to run with choke halfway, starving for fuel!! If all has been checked and suspect fuel pump issues, Dellorto makes a nice variable flow pump with a internal bypass. Float bowl fills up, closes off needle, excess fuel pressure bypasses to tank. Never any excess pressure to carb, with plenty of volume to keep bowl full when called for.
 
Yup, we took the carb apart yesterday and cleaned all the gunk out from the ethanol. That Dellorto pump sounds like it would fix all the problems. I'll have to try one of those real soon. Also going to run leaded race fuel once we get everything established so that should help a little bit with fuel consistency. Thanks!
 
Fuel tank to pump, out of pump to Honda Oddysey "fuel valve" (three way plastic fitting available from Honda or Ebay). Out of three way valve to carb, the other side of the valve (the restricted side) back to tank as a return. If pump will not self prime insert a priming bulb (available at any boat supply house), between tank and pump...UAS #7, CRF450/ Ultramax.
 
for future reference if you have to run any engine with the choke half out thats a tell tale sign the engine is not getting enough fuel.
 
No need for a bypass on any of the pulse type pumps. Ran my 250 all last year with walbro set up just like any other kart motor. Worked great. Put same setup on my 450 and was perfect. Doin the same with this years motor. Works awesome.
 
Jimbo925..that's great to hear your having good fortune with your walbro pump set up on your 250....I speak from experience that just because it worked for you that way is in no way an indicator that it works like that for ALL MX engine/carb applications. The MX engines normally work off gravity feed only and any of the pulse pumps we normally use can and will at times over power the stock needle and seat/float assembly and cause fuel overflow issues. There are obviously several methods that have been discussed to handle any possible fuel issues. Its all good information. I would be careful making claims that just because it worked for you that it is all that is needed. I assure you if we didn't need to run a return with our application I wouldn't have done so. I have karts with CRF250 and CRF450 and I run a return line on them both.
 
I am doing the same set-up for a 125 mx motor. I was going to mount small tank like a catch can just a above carb and let it gravity feed. The small tank would be feed by your normal fuel pump set-up. Any sugg. Thanks
 
tchase said:
Jimbo925..that's great to hear your having good fortune with your walbro pump set up on your 250....I speak from experience that just because it worked for you that way is in no way an indicator that it works like that for ALL MX engine/carb applications. The MX engines normally work off gravity feed only and any of the pulse pumps we normally use can and will at times over power the stock needle and seat/float assembly and cause fuel overflow issues. There are obviously several methods that have been discussed to handle any possible fuel issues. Its all good information. I would be careful making claims that just because it worked for you that it is all that is needed. I assure you if we didn't need to run a return with our application I wouldn't have done so. I have karts with CRF250 and CRF450 and I run a return line on them both.
Click to expand...

That's why I offered the Dellorto method. All still VERY useful and helpful info!! What is one mans gold could be another's headache!!
 
I am doing the same set-up for a 125 mx motor. I was going to mount small tank like a catch can just a above carb and let it gravity feed. The small tank would be feed by your normal fuel pump set-up. Any sugg. Thanks
Click to expand...

Tried that one once. Tank did not have a return, built pressure in tank forcing fuel past needle seat. Had to put a 3/8 return to tank 1/2 inch below fuel entrance. Fuel to carb exited out bottom. Volume in tank between exit to carb and return hole to tank was 2 1/2 times volume of carb bowl. Seemed to work Ok like that. Never overflowed the carb. Just didn't like the extra crap hanging near my head full of fuel!! Lol
 
I run a round pump and have both outlets running to the carb on my 450 with a T right at the carb. The return runs down and put a spark plug tip in the line at the tank. Never had another problem and it's built to the gills, so I didn't want any fuel starvation problems.

I read how to do this in another thread before the crash, so I don't remember who to give credit for the idea.
 
Thanks everyone! We also have a Honda CR250 we will be "fuel pump" experimenting with and I believe out of all the ideas submitted, one of them should work lol. We are a lot farther than before!
 
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