New 13 HP Clone Tear Down/Rebuild Suggestions

[rdenis]

So I just picked up a brand spanking new GX390 clone with electric start.

Where's the weakspot in these clones? I'm going to tear it down and rebuild it and since I am at it I might as well do it right.

I'm thinking new valves and springs, port and polish intake and exhaust, new rods, new cams and camshaft.

Not necessarily looking to increase power, just reliability for now.

I'll also measure the specs on the piston and cylinder based on stock GX390 specs to see how close they are - hopefully won't need to rebore.

Also, should I remachine the head - not sure how "true" they are out of the factory?

BTW - I'm going to try doing all this myself which I've never done before so any suggestions/tips/hints would be much appreciated!

 

[Kaptain Krunch]

If your not looking for power tearing the thing down for a rebuild would be a waste of time and money.

 

[Russ2251]

should I remachine the head - not sure how "true" they are out of the factory?

You will find that no engine mating surfaces are 100% "true" regardless of who the manufacturer may be. That's why gaskets were invented. Sometimes there are ways to by-pass gasketing, but is labor intensive and not very profitable (cost effective) for manufacturers to employ.

 

[jr dragster T]

If your looking for reliability and have no experience with this stuff when why tear down a brand spanking new motor? There incredibly reliable right from the box!

 

[bajagokart]

i'd say! any clone that ive seen runs perfect has had no problems starting and keep going! liek if you want power outta the fricken thing than i suggest go for it! see how much hp and torque you can get outta the thing, but to go and rip a 400 dollar clone apart just for the heck of it and do mods that you have never done by yourself before? good luck man

 

[Jdominique]

Only reason to tear it down would be to:

1) Remove gov
2) Install billet rod

 

[bajagokart]

and do valve springs so you dont float them

 

[rdenis]

I should clarify - I've never worked with these clones before - I've only worked sportbike engines - late 80's and up.

If I don't need to tear it down for reliabilty alone, I won't bother. I was just a bit suspicious of quality on a $280 motor - plus it was a good excuse to put some mileage on my new milling/lathe machine.

You guys have way more experience with these clones than I so thanks for the thoughts. Will definitely change out the valve springs.

But now my curiosity is peeked on the performance side - I'm guessing ungoverned this thing could run to around 6,500 rpm with heavy duty valve springs, new rods and a lighter flywheel.

It would be sick to get this thing to turn at say 9,000 or even 9,500 rpm - no way this block could handle the compression I'm betting - maybe bring the peak HP to around 40 at 6,500 RPM and peak torque at around 5,600 RPM - double the compression and change the detonation curve - who knows. Probably grenade but it would be really cool.

Anyways - someone should start a build comp to max performance on one of these things if it hasn't been done yet. I'd be in.

 

[Kaptain Krunch]

Haha right 40hp..not worth modding these things, if its power u want get a bike or sled engine.

Nowmatter what you do the engine wont spin more than 3600rpms with the governor, w/o
governor you could probably spimn it a little over 5k. More with lighter fly, stiffer springs, billet rod, cam, etc

But like i said if power is speed is what u want forget the clone.

 

[Rotore]

Should of gotten a v twin 9500rpm and can make moré then 40hp

 

[Kaptain Krunch]

Or a bike engine, 250cc two stroke rm, yz, cr, etc 40hp stock and easily could easily hit 10k.

Even my 1978 kawasaki 400 twin (which is a pretty low performance bike engine) makes 36 horse stock and redlines around 9k.

 

[anderkart]

You probably could get 40 hp out of that mill built to the max and running on Methanol. (meyby 50 by adding some nitro) If you wanna find lots of info on building these motors just enter the term GX390 in the search window at this forum: http://karting.4cycle.com/forumdisplay.php?f=81

You'll probably run across a few threads/posts discussing differences between clone and genuine GX motors too.

 

[bajagokart]

if you can get a clone to go 6500 rpm without blowing up, you will be my hero lol

 

[slideways]

You mean stock? They turn that much easily with alum flywheel and rod and decent springs. You just need the right cam and head combo to actually make more than noise at that point though.

 

[rdenis]

Yep - I'd love to see what the max is for one of these things - engineering things beyond their design limits appeals to me.

As soon as I finish my dyno, I've got myself another project and excuse to head back to Princess Auto.