GX390 Questions on a build.

I’ll take any info you guys can give me.
Building a mini bike with a big block for 1/8 mile racing we do here. Not looking to be the fastest right now, just be competitive.
I have a GX390 and I am going to do a stg 1 setup 1st. Exhaust, air filter and jet, springs, rockers, push rods, been looking at NR site.
What rpm is the flywheel good up to? I read 6000, is that right?
I may upgrade the rod Incase also.

Then if I don’t do this all at once, I’ll do what’s below later
Light porting on the head
What pound springs and cam choices are good up to 6000?
What about the stock or better mm carb also?

Not looking to break the bank on a build just yet. I may go vc460 for that.
 
Last edited:

flattop1

Dawg 89
You may find at 6000 rpm your running out of steam .
No way too guarantee the flywheel , I'd say your safe there .
 

CarlsonMotorsports

Site Supporter
Personally, I'd do the rod and flywheel from the get-go.
Both are safety measures and both will ensure that the engine doesn't come apart and wreck everything you've worked for when you're tempted to twist up the rpm just a little bit more. Keep in mind that these stock parts were never intended to run above the rev limiter / governor of 3600 rpm.
ARC builds some of the best parts for our industry. We stock and sell them through our shop, or you can order them direct (same price.)

If you need anything (parts or advice) for your build, please feel free to reach out to us.



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Carlson, we’ll i may just bite the bullet and get the VC 460.
I’ll need some cam choices and info
I also got a good deal local on a 420 with cam, springs carb exhaust etc...
I’ll keep the 390 for a,Ayer build.
 
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