2 foreman 400 1-1997 & 1 1999 both missing at higher rpm

I would look at the size of jets you are using. Check what the stock jets are and compare them to what elevation you are riding in. It sounds like 5 to 10 thousand feet is high for stock jets. Seems to me that stock jets are for 3 to 5 thousand feet.I am sure someone will confirm or correct me on this.

there has been a few post on different factory jets sizes for the 400's, down south the 400's run a #130 and some of the bikes up north run a #125 for the higher elevations.

Thanks for the assistance I will check the jetting on both bikes and see if I can find out which one is in them. Would the K&n power kit be something worth looking at? Also, Is there a spark plug that works best in these bikes?

I have nothing against the K&N filters other than the price. I use the UNI filters on our bikes. They flow just as good for half the price and have always used the NGK plugs.

OK I found that both bikes have 130 jets in them so now the big question is do I install 125 or 122.5 jets? the dealership really had no answer for me they just said 8-10% leaner is a good guess. So I purchased both jets so I may have to do the trial and error method starting with the 125 so I don't run it to lean with the 122.5. Any suggestions? I will have to price that uni filter Thank You!

Try the #125, I think the #122 will be to small. I paid around $25 for the uni filter

ok so I installed the new jets now I just need to go try it out and see how it goes. I will try and do it soon so I can let you guys know how it goes. Thanks for all of the help!

I swear by the Uni Filters, I have one in each of my machines. Had the K&N with the precharger and the precharger keeps the fine dust out of the engine but reduces the air flow. My 400 was flooding itself at high speed and this was the problem. I'd go with the Uni or the Twin Air.

Well my foremans seem to be alot better. They have alot better throttle response now and don't seem to miss as much. I also purchased 122.5 jets so that when I am deer hunting above 9000 ft I can switch out and try them.

Well I was finally able to really test the new jets that I put in they seem to have cured the missing under power although the 99 still seems to backfire alot when decellerating so much so that the baffle is blown out. But I do appreciate all the help that I recived for the missing issue!!!