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421 cub jetting concern


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i'm just now completing my new build. its a 421 cub, ported along with the case. running 35 pwk's, 21cc custom domes, boss intake, t-5's, and v-force 3 and +4 timing. it used to be a 4mil stock bore dune port and i was running 152's. how much higher should i go to start out with. i was thinking 160. i'm about 3,000ft and temps will be 80 to 100.

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i ended up going with 168 mains and 55 pilots. i got her running and the right bank is spitting and sputtering at off idle. the right bank is what blew up the time before. no vacuum leaks and carbs are synced. if you shut the fuel off the right bank runs out of fuel faster. pulled plugs and found it fuel soaked. swapped wires and plugs with no change. is it possible the carb is the issue. compression is 160psi. any help would be appreciated.

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Got the same needles? Floats set the same?

yeah, needles are the same. other than replacing jets, i haven't touched the floats. it wasn't doing it before. it seems to run ok till it warms up. once its warm the right bank starts to cut out. i'm starting to think that 55 pilots are too rich. thats all the dealer had right now. have to hunt for some smaller ones.

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55 is what we run in methanol bikes at or near sea level...yeah, 55 is way fat for gas @ 3K feet.

 

Swap the carbs from left to right, if the problem follows it's the carb, if not, it's something else.

 

150ish on the main, CEL needles and 45 on the pilot is where I'd start at your elevation.

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thanks for all the help. i have some more pilots coming. played around with it and found if it was cold the thing seem to hit on both cylinders, then as it got warm the right bank started spitting. i would think that is a rich condition. i know why i went with a raptor 700 couple years ago. banghead

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I think u are going to have better luck with a 45 pilot,CEL needle leaned out/dropped 1 clip from the middle and about a 145,148,150 main since you are at 3000 ft elevation. Why the 100 degree temp I can't figure out. You never said what needles that's important as some pwks come with wierd needles made for 80-250cc dirtbikes. Radar

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I think u are going to have better luck with a 45 pilot,CEL needle leaned out/dropped 1 clip from the middle and about a 145,148,150 main since you are at 3000 ft elevation. Why the 100 degree temp I can't figure out. You never said what needles that's important as some pwks come with wierd needles made for 80-250cc dirtbikes. Radar

i just pulled the needles out. it has CEG needles. disregard the temp.

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  • 2 years later...

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