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421 cub small bore shearer vs big bore


Cubshee421

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I have a 421 cub that had big bore shearer pipes on it. It ran pretty tall gearing with these pipes but if you had to lift off the throttle it would just bog out. Now I have small bores but it seems like it dont have the power to pull long gearing any more but its more forgiving if you let off throttle. So what are your guys thoughts on this, plugs are very clean with the small bores idk if I may be too lean on mains now.

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if its a high port cub.  its going to fall off the pipe no matter what.  loose weight on the bike or rotating mass on driveline.   the big bore's helped the pull down low and masked some of the lack of lower rpm power. ( 6500-7500)   the smalls dont have as much emphasis on the lower rpms but probably are helping up top  a tad bit more..

 

or your overall setup is a lil off..  domes , real porting ,may help? depends on the build of the motor. 

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Not sure where ur located, I'm in socal and that's lean for us I have same set up 39 pwk cub and sb shearer and I'm at 175,60 before dyno I imagine I'll drop 2 or more on main and maybe 1 on pilot and I'm only 6 degrees on timing and also running 110 with 19 cc domes and the super thin base gasket.

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Big bore pipes pulled 17-42 2nd gear launch with 160 main jets at 4 degree timing.

 

Now small bores with 15-42 2nd gear launch 158 mains at 7 degrees seems like it should be easy to pull.

 

It will pull hard first 2 passes then it does a weird bog when dumping the clutch

after that. Im going to check for a broken reed tomorrow

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