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I'm thinking about changing fuel from C12 to U4.2. Does anybody have a rough idea on how much either way I should start out with my jetting? Any reasons I shouldn't change fuel? I'm just looking to try something different.

I have never ran the U4.2 {Isn't that for a four stroke} just the old U2. with the C12 I run a 172 main and the U2 took a 205 main

pstoffers wrote:I have never ran the U4.2 {Isn't that for a four stroke}

I thought the same thing, but I did some snooping around and I'm getting beat by people running it, so I might as well check it out.

it will make a big difference, but i wouldnt spend time in the u4.2, i'd go direct to mr9 or mr11, depending on which you buy cheaper in your area, mr11 here. Makes more power than the u4.2 by a ways, the mr11 is an overkill, wont cause heat or anything, but the price has been alot better here. Dont be scared by the crysal looking deposits in the exhaust port + pipe that'll make you consider your aircleaner is sucking dust, clean your silencer frequently, its harder to plug read. Drain your carb after you run it + fill the bowl with pump premium if its foing to sit
I think it was 4or 5 jet sizes on the ktooms

I'm not sure if I can get mr9 or 11 around here. I know I can get 4.2 in town. I'll fatten it up and see how it goes... if I hurt the motor, the cyl is getting bored this winter either way and the bottom end is getting overhauled with ceramic bearings.

I had a customer who raced Latemodel dirt cars, spent the big $ and went from aluminum hub w/standard bearings to titanium with ceramic. He never broke so many hub bearings. about 1/2 way thru the season and 1/2 a dozen hubs wrecked he changed the bearings back to standards but kept the ti-hub, workd fine and didnt break another set or hub for 2 years.

I've talked with an MX mechanic who ran ceramic bearings in testing for 250f engines, he's gone back to oem aswell

I'd say balance the crank + use new oem bearings

Thanks for that info, roostius.

Just picked up the 4.2. Can't wait til Friday to try it.

Roostius_Maximus wrote:I think it was 4or 5 jet sizes on the ktooms

4 or 5 jet sizes being like 180 to 185 or the 4th or 5th jet past 180? (just as examples only, I know you can't tell what to jet my motor) As long as I'm in the ball park, I'll be fine. I'd just prefer not to burn it up on the first run.

Ya 5 like 180 or 185, - forget what this carb has, but we run a different needle too

Thanks. I've only got about 2 needles, so I'll just have to go with what I've got. I don't spend much time on the needle anyways. Hammer down!

I would put a 205- 210 to start, But what do I know!!!!

Well, I'm going to be using my powerjet carb... so I'm going from 150 on the main (which was already fat) to 160 and from 92.5 (fat as well) to 105 on the powerjet. I may bump it up some more before I fire it too if I can find jets in the 160 range that are keihin brand.

Thanks for the help guys! Wound up with a 160 main and a 110 in the powerjet and it was just a touch rich. Next time I might try a 158 on the main and 107.5 in the powerjet and see what happens.

If you're looking for temp/altitude for your own reference, it's 50-55*, 90% humidity at about 1000ft.