Hi guys. I'm about to pull my hair out over my 400 foreman. I'm having problems with it "loping" at WOT. I have had it serviced, valves adjusted, and carb cleaned and adjusted. When first fired up it does great, but after maybe 30 minutes to an hour of running my WOT acts like its in a bind, struggling. From 1/4 thottle to 3/4 throttle you couldn't ask for it to run better, but when I open it all the way up it's back and forth loping. It's sicking to me to see a 300 or especially another 400 pass me like I'm sitting still. I have done all I know to do with it so I'm open to any suggestions. I have made sure all my carb vent are clean and clear of any debis. Everything on it is factory, though I have thought about a jet kit for it, but I want to make sure this is remedied first. Please help if you can. Thanks.

 

Hey bud I had the same problem with my 400. Main jet is to lean. I got a 130 in it and got a #55 drip bit which is the next size up and should be close to a 140. I can tell a big difference, when I punch it just wants to rev up and go now. I got it close to 80kms before 70 is all it would do. It was still accelerating but it was revving way to high at that speed. Also you can check your spark plug just hold it wide open for 1/4 mile and hold your foot up on the shifter shut it off and use the brake to slow down. It a real pain to check the plug on those than the 450.

 

does it run any better if you turn the choke on? if so, it's probably too lean. If it gets worse, then it's too rich.