Bored GX 390 carb on GX 270

#1
I installed an NR racing stage II kit in my Honda GX 270 . It included a cam /springs 4 degree key, intake manifold, air filter, a bored 390 Gx carb plus Robertson H loop exhaust system. The carb is 23.5 mm bore a .037 main and a .025 pilot with a stock E tube . The kit installed easily and the motor fired right up and idled smoothly . The problem is when the throttle is opened the engine starts stumbling and will not rev at all . I am hoping someone out there may have experience with this issue and point me in the right direction.
https://www.youtube.com/watch?v=9nNXWVA0Cuk&feature=youtu.be

 
#2
I run a #37 main in a stock GX200 with a filter and pipe. That seems awful small on the main jet? I haven't experimented with a GX390 yet, but I would think it needs much more fuel?
 
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#3
The idle jet needs to be opened up to at least .026... And the main I would at least start at a .038 and go up...
In our SA carbs we run .026-.028 idle jet and a .038-.041 main jet...
The 390 carb. I run on meth. And use a .035 idle jet and a .065 main jet.

But this is on a gx200...
 
#4
Thanks for the input . I re-installed the OEM carb and the engine ran fine, This allowed me to isolate my problems to the 390 carb . I disassembled it and cleaned it thoroughly. I re-installed it and started up with the main jet sizing . At .041 the motor responds well to the throttle and runs smoothly.. I am going to run it this weekend and try to get a plug reading. Thanks again for the help.
 
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