GX390 & Clone Valve Spring Comparison

geoffw

I did this test to show the differences between the valve spring options available for the GX340, GX390, GX420 & GX440 Clone. The available options are:

1. Stock GX390 Springs (Not sufficient when governor is removed)
2. Stock GX390 Springs w/Stock GX200 Springs Inside (Supposed to be a cheap alternative to HD Springs)
3. Single HD Valve Springs
4. Anyone want me to add the Dual HD Springs?

This one is for you sho! :)















As you can see, adding the GX200 Springs inside of the GX390 springs is a complete waste. It doesnt give you any more pressure where you need it (the seat/closing ramp) and it only adds a few pounds at max lift (on stock cam) where you dont want it (the nose).

If you are having issues revving your GX engine, its likely because your springs are too weak and your valves are floating.

-Geoff

BGW

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sho305

Thanks for posting! My 200 springs are on top of that locator thing, but its not tall enough to really change your test anyway. They don't even hit installed with out it you are right, and barely hit with it.

One of these days I will install your springs and 1.3 rockers! They are collecting dust here waiting for me to pull the engine and put gasket in it. Things keep coming up, normal for a busy spring here but agitating none the less.

geoffw

I tested it with the shim and it gave me 21lbs on the seat and 56lbs open.

It will definately help but Im thinking that might be limiting your rpms.

TxPowderCoater

shim is ok as long as there is no coil bind. make sure to not get to much pressure so you dont wipe a cam.
your data is awesome, great job!

sho305

I was told on another site to use gx200 springs inside the stock 390 springs, builders toss them in stock 390s (or 420s) because they throw them out everyone puts better stuff on gx200s for karts. It is smaller and shorter, it does hit when on top of the spring seat. Its a washer with a raised center over the stud nut. Mine only had one had to fabricate the other valve. But it only lifts it what, 20-30 thou or something. I can pull to 4500 hard and it lets off gradually to 5K then it does not pull much. It has 4 degree key and .050 cut head, high flow airbox/exhaust header.

sho305

This weekend I put the HD (Geoff and Vegas Carts!) springs and 1.3 rockers in, along with a new secondary clutch. Oh yeah, it was set free. I'd say a little more power in the normal ranges but on top it certainly revs out, it goes much faster. I was afraid to go too fast as I didn't have my tach on it, but sure it is easily over 35mph and it was around 31 before. It just keeps pulling now, I never did wind it out till it stopped. Took a friend for a ride and he says I should get a cage on it lol, he has a razor 800. I don't know if any of this stuff would cause problems down the road, not heard of any, but can tell you it is an effective rev kit for sure.

Thumbs up for Geoff these parts work great. Also get the feeling the way it runs I need to adjust the primary clutch up a little higher shift rpm, it pulls harder once it starts to rev out. Part of this could be the new secondary changed the rpm, not sure. Also put a umax spring in and it keeps the rpm up some at low speeds, works great for my use. Will post on my build thread. I'm a happy lad for sure

natew121

im acutally about to place a order today for( 4deg key,springs,rockers) do I need to order the automotive style retainers & locks as well when doing the 1.3:1 rockers?

geoffw

No need for the automotive style retainers unless you order SS valves. SS valves arent required with the stock cam.

Why only 4 degrees and not 8?

natew121

i really thought it was a safer option . but if its fine to advance the timing that much then by all means ill order a 8 if you think it i will run better. I do have 93octane gas at all pumps in texas just about

geoffw

If you want it to RPM in the 5500-6000 range you are going to need 8 degrees of advance. the 4 degrees will limit the rpms to low 5000 range.

You can safely get away with 10.5:1 compression and 8 degrees advance with premium fuel. If you want to run regular..stick with 4 degrees and 9.5:1.