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new 421 cub cutting out on left side, need some ideas


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As the title says it cuts out on left side at about 4k rpm. So far I've switched out with parts off from a perfectly running shee, stator, flywheel,coil,cdi, wiring harness. swapped carbs side to side, redid a leak down test holds 6lbs for over 30 min with no drop. changed jetting up and down still no fix.Im baffled can't find the problem .

 

wiseco 4mil crank

cheetacub with cleanup porting with case porting, done by fast racing.

v4 reeds

chariot intake

39mm pwks with 8" pods

cpi oofs

prodesign coolhead with custom domes by fast racing

+4* timing

170lb compression

runing 110 race fuel with amsoil dominator oil 40-1

 

any ideas?

 

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Toytech, call us in the morning , we will run through some scenarios with you and try to help...

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Toytech, call us in the morning , we will run through some scenarios with you and try to help...

 

thanks Jeff just trying to eliminate all I could before I bothered you. I know your busy

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I've had a problem like that and the carbs were not in sync. They appeared to be,but the cable was in a bind. I fought it for a while. BUT carbs out of sync show up at lower RPMs. Reeds, water getting in cylinder, can show up at higher RPMs.

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I had that problem with the stupid screw on caps on the spark plug. If it was loose the bike would miss and sputter on that side

thats what it acts like but worse. I've had that before too.

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  • 4 weeks later...

well my wrist is mostly healed, so started to work on the cub again. we talked to Jeff at fast. he gave us some things to check.  we set the plug gap at 24 and the pickup at 15. and added a ground wire from the coil to the engine case. did another leak down, holds perfect. runs better but still not rite. left side plug wet, so just for shits we went up 4 jet sizes and it started to run much better. the left side looks lean still. we went up 2 more sizes and went to do a plug chop, and the clutch let go.

 

has anybody seen a motor so lean that it wouldn't fire and actualy look like it was rich?

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