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421 Serval plug Chop. Opinions.


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Both plugs look the same so I dont see how the carbs are bad. I doubt both carbs have the same exact problem. I have no idea what could be causing this. My only guess is weak spark. New coil (old OEM ran the same), new stator (OEM and tests good), second Flywheel (Bought it on Ebay and I think there may have been a VERY faint rattle doutb its causing this but who knows).Oem CDI with the plug cut off and differnt connectors installed (I guess it could be a problem but I doubt it). It runs pretty strong now. On the road stabing the throttle it spins the tires pretty good through 2nd. Other gears come up with a plus 4. Plugs look good. Any suggestions. Should I just run it? Doesnt look lean.

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Carbs are synced with fast tool. Floats are adjusted. Slides open all the way. With a size or two bigger jets sommetimes there is power there and sometimes there isn't. Just doesn't build power smooth. With the 152 it revs clean. It spins boadies on pavement no lack of power. One size bigger and it runs like crap

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Every motor behaves differently. Smaller jets don't always mean less power. You could simply have a stronger signal across your venturi which is pulling its correct fuel ratio with that orfice. Its good practice to ensure everything on your machine is in good health (as you were concerned with weak ignition) but in the mean time ride it like a banshee.

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Well I hope your right. Im not that concerned about the power out put. I think it runs great. Mostly concerned about hurting the motor. The plugs look great and it has no lean symptoms. I have a temp guage as well and its never got hot. I have a new flywheel comming. Ill try it and see if that changes anything.

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I kind of had the same problem with mine but its a 4mill cub with a aggressive port job by HJR. I am running smaller jets in mine, where a handful of guys with the same setup or close to it are running bigger mains. I started at a 160 main, needle in the middle and a 52 pilot jet. I went up and down with my jetting and finally put it on the dyno at Redline racing and ended up at a 152 main, needle in the 4th clip from the top and I went with a 48 on the pilot. It runs real nice. I am fighting a low end power problem right now. I am having a hard time starting/launching in 2nd. It bogs when I try to launch in 2nd when it didnt before. I used to be able the flip the bike on top of me and now I cant. I am going to mess with needle a little and check my reeds. Other then that the bike runs like a cub.

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If the plugs look good and it runs good I wouldnt worry about it. It could be a small difference in electronics , a 1 degree difference in timing can make a difference as well as different intakes or reeds. It sounds like you have one thing different on yours over the others running 160s. Intakes, timing, coil spark , many things can cause it.

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