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420 adapter

The gx390 adapter is what you need, you can blend it to match the port and the 420 carb. I'm pretty sure the 420 carb has a 23mm bore compared to 21mm bore of the 390 carb, thats what someone said the other day.
 
That's why I been looking at the 420 instead of the 390. Most adapters I have looked at I'm assuming need the insulator with it? Only the long one from NR says it does not require the insulator.
 
Not sure about the adapter part, the one I use is a long one and is made to use the insulator with it, I have seen guys use the ARC adapter with and without the insulator. I like the idea of having the insulator in place because it will keep the carb from getting hot like the head does, which I would assume helps with fuel atomization but I may be wrong.
 
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