GX390 upgrades on the cheap.

[jgern]

Hey guys I have a kart which had a gx160 and swapped it out for a gx390 since I weigh 100kgs. I am running a dry clutch with 73tooth on the axle and 15tooth on the clutch. It has lots of torque which is great but not a lot of top end. I was wanting to know what would be the cheapest route of getting some high rpm's from this engine. I have already removed the governor and am getting around 4400rpm roughly from using the speed calculator.
If I get the arc conrod, change exhaust to header and get heavier valve springs will I make around 6500rpm? Or will I need an ARC flywheel also. Do the cams contribute to higher rpm's or just hp?

Thanks very much
James

 

[OzFab]

Welcome to the forum. Australia is a big place, can you narrow it down any?

My first thought was a 4.8:1 gear ratio is a bit steep but, my kart runs 5:1 on 10" tyres with a GX160 so...

If I understand correctly, technically, you don't necessarily want more RPM, you just want more top speed, correct?

The upside is, with a GX390, you have buckets of torque &, with a few cheap mods, you can gain more so, changing the axle sprocket for a smaller one won't hurt the engine too much & will increase top speed; a 60t axle sprocket will give you a flat 4:1, which should give you a top speed of around 60km/h

 

[jgern]

Hey Fabroman, I live in Dysart, QLD. It's a coal mining town.
If I go to a smaller sprocket on the axle and are just driving around on dirt, grass and road stop starting all the time I think it would be too much on the clutch. I had a 75 tooth on the back but I did not have enough clearance so I went to a 65 tooth and it seemed too much on the clutch, it was getting really hot! The clutch started playing up. If I can keep the 73 tooth on the back and increase the rpm that will be good. Just wanting to know what parts are the best bang for buck in increasing rpm.
Thanks

 

[jgern]

Im not sure but I think I may have a 4500rpm rev limiting coil. It is a clone that I bought recently. Im not sure how to tell?

 

[KartFab]

You should re jet and get a high flow intake if you are going to get a header.

 

[jgern]

ok thanks kartfab. Does anyone know what rpm I would be getting with hd springs, arc flywheel, arc conrod and high flow intake with the "stock" cam?

 

[Joe-405]

After building my 212 I really don't think your gonna be even close to your 6500 rpm goal. You might get to 5000 if your lucky. The camshaft is what helps the rpm range more than anything.

 

[firemanjim]

After building my 212 I really don't think your gonna be even close to your 6500 rpm goal. You might get to 5000 if your lucky. The camshaft is what helps the rpm range more than anything.

you're not able to go over 5000? My gx160 is running up to 6400 before I get valve float, and it's a used/abused engine with stock springs. Engine originally came of a power washer, iirc.....
I got video with my engine running and tach (digatron ) showing 64++ rpms..... I've been tempted to swap in better springs just to see what happens..... LoL

 

[firemanjim]

OP, buy a tach, whether it's a cheap "tiny tach" or clone, or a mychron /digatron..... Speed calculator (on paper ) can't tell you real rpm.....
EDIT
You might have one of the rpm limiting ignitions.....

 

[jgern]

Yeah I need to get a tach for it firemanjim.

So I bit the bullet and bought some parts from NR Racing.


Items Ordered: Price: Status:
1 of : Camshaft, 306 Series - (GX240s to GX390s) [Camshaft Grind:306 RPM]
$119.00 Processing
1 of : Chain, RLV Xtreme, Gold on Gold, #35 - 106 link
$24.00 Processing
1 of : Clutch, Noram, Star, 1" [Clutch Options:13 Tooth]
$155.00 Processing
1 of : Exhaust, Flange, Header w/stub, GX390
$11.00 Processing
1 of : Flywheel, Billet, Raceseng, S2 without fins - GX390 & 420 Predators
$119.00 Processing
1 of : Gasket, Head GX390, Thin
$3.30 Processing
1 of : Jet, 102 (1.02 mm, .0402")
$6.90 Processing
1 of : Muffler, RLV, 1"
$35.00 Processing
1 of : Race Air Filter Kit, Velocity Stack Style (GX270, GX390s, 13hp & 420cc Clones)
$23.00 Processing
1 of : Rod, Connecting, Billet, GX390, 4.410 long (std Length)
$133.00 Processing
1 of : Springs, Valve, HD, Single, GX270/390, Pair
$8.00 Processing

Hopefully this shall give me around the 6500rpm range. Not so cheap though haha!

 

[firemanjim]

You're gonna be with no problems. Well, the only problem will be keeping the bugs out your mouth, because you can't stop laughin and gigglin......

 

[jgern]

 

[Joe-405]

you're not able to go over 5000? My gx160 is running up to 6400 before I get valve float, and it's a used/abused engine with stock springs. Engine originally came of a power washer, iirc.....
I got video with my engine running and tach (digatron ) showing 64++ rpms..... I've been tempted to swap in better springs just to see what happens..... LoL

Before ? Not even close. Last time I shut the engine off it read 9200 tho after the other mods. But it gets a little squirrelly above 6k at speeds so I've never tried it under load.

And that's a lot cheaper than what I have in mine for money wise !

 

[firemanjim]

Before ? Not even close. Last time I shut the engine off it read 9200 tho after the other mods. But it gets a little squirrelly above 6k at speeds so I've never tried it under load.

And that's a lot cheaper than what I have in mine for money wise !

LOL oh.... Ok cuz I was gonna say,
That parts list will definitely make the engine a brute to be reckoned with.....

 

[chancer]

Holy crap Mate! that is a nice kart and gonna have a reeeaaal nice power plant for:
" just driving around on dirt, grass and road stop starting all the time" Definitely bugs in your teeth grinning and laughin!

 

[firemanjim]

Looking at the pics again, swap the rear tires left to right. They're on the wrong side..... Look at the tread pattern.....

 

[jgern]

Yeah firemanjim I noticed that after I popped them on. I thought the logo would go to the outside but obviously you can only have the logo to the outside on one side. I dont think it will make too much of a difference for me since I won't be racing.

 

[jgern]

Hey Chancer I couldn't believe my luck but I actually won the kart at a local tool store that had a Sidchrome competition going. It was setup with a 160cc and wet clutch and didnt have much go about it. Ive done a few things to it now.

 

[jgern]

Joe405, 9200rpm! That thing must be screaming!!

 

[chancer]

Cool FREE always makes it twice as nice. BTW this thread title is totally wrong after post #10