next mod gx390

natew121

Well as my build goes on I am hitting the half way mark and now it's time to adjust th3 motor to deal with the new weight added from the long travel kit ..Performance time !.
Here's what it has now
Gx390 : HD valve springs ,free flow long header exaust , and air filter , drilled jets,gov-removed,8deg timing key ..Did 31mph tops on 22 " tires

Next : nr racing flat top piston, .10 mm head gasket , and 1.23 billet rockers since I cannot find 1.3:1 (not in roller) ,heads will be polished to get rid of blemishes ..
From others : adjustable jet , ss vales and locks and retainers ,bored carb -vegas carts


Will post pictures of the entire build very soon anyone have any suggestions ?

BGW

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lake cart

big power in the head!!! we race those motors on go karts if you turn it over 6500 put a ARC rod in it. definitely a flat top piston , mill the block to a 0 deck height. and a good cam. the most power to be made is in the head!!!and a good carb. I have seen some flow numbers from the vegas carts heads there not bad for off the self heads

natew121

After the mods I listed above lastly would be a billet rod .040 longer from arc ,that with the flat top piston and .010 gasket supposedly pushes compression to 12.:5.1 and vegas cam done deal don't want to get into the heads more along the budget lines here BV heads are $$$$

lake cart

good port work goes along way!!! a billet flywheel will bring it too life!!!

BLACK DEATH

How much will you have in it money wise & HP wise?

TheGDFP

A couple thoughts on it:
Adding the VC 5mm stroker crank puts the piston 2.5mm farther up.
This allows cutting the stock piston's outer lip to make it a flat top.
And adds 30cc for not much more money than the piston. 420>390 :)
Of course, in that line, the 460cc block/cover/piston is $150. And so on... lol

The VC cam has a bunch more duration, low lift. Like 1960's muscle cars. Modern stuff opens the valves farther, faster, for area-under-the-curve. $135 Dyno Cams hardweld will make more torque, but, the 1.23's help make up some of the difference. I'd be inclined to leave the stock cam in place, and do a mild port job, especially with the tiny 21mm carb, even bored to 23.5 like NR and others do. 246 degrees at .050 is pretty aggressive and trades off torque for revs. Your rocker upgrade will add enough lift to make up for the mild duration.



IMO :)

natew121

The engine was free from helping a buddy's dad move gave it to me. Gov removed free,drilled jets free,air filter was 20-25$,timing key was 6$ or so, exhaust was 20 ,HD valve springs 10$.. the next set mods is going to run around 122$( flat top piston -30$,89 billet rockers and gasket is 3.30$. Just waiting on a response as to what that will put me at from nr )all from nrracing .. not going to get to crazy with this engine it's a hunting and beach cart just looking for the cheapest way to add more mph and power for now

Power mmm not 100% it should be between 20-25 with a big carb could see more .. some people have made as much as 35-40 I want to say on oldminibikes forum

Streetbu

Port the head. Very easy to do, only costs the price of a head gasket and some time. You won't notice a huge difference at first, but if you time your cart from stop to say 20mph before, then again afterwards, you'll see the improvement. Really helps upper rpm hp, not lower end torque.

BLACK DEATH

Is the 390 an 11 or 13 HP engine in stock form?

Streetbu

13hp