GX390 RPM's

2kdime

New member
Just had a tach hooked up and the engine wasnt revving as high as I thought it would

I need to double check my tach setting but I believe it should have been 6k

Whats holding me back?

GX390
Billet flywheel
Billet rod
.360 lift cam
1.3 rockers
55lb springs
4 degree key
40/35mm valve head job
Port and polish
Mikuni SBN 38mm
Running on gas
 
Be sure the Carb has the right air/fuel ratio. Lean or rich will/could greatly affect rpm. .468 total lift is a ton. How much you have head shaved and what piston? Just curious. Any idea what compression ratio you got? 38mm mikuni with 40mm intake and .468 lift will want rpms to make power thats for sure.
 

2kdime

New member
I can double check the key but it should be good.

I hooked my tach up but it said it was only getting to 3k

I believe I had it on the wrong setting and it was just under 6k

So only revving to 6, which I had thought would get to ~8
 

2kdime

New member
Stock piston with billet rod

Havent touched the deck or head as far as milling goes

Planned on getting it going first and then using some clay to check clearances to see how much I can take off. As you said, lots of lift so I want to be watchful of valve to piston gap.
 

joler1302

New member
Higher compression will be needed for high rpms, and that's a whole lot of lift. Make sure you're not running too big of a carb, or at least try and jet it down. I run a honda gx270 stock carb, nr .280 lift 220* due cam and 1.3 roller rockers so thats .364, it can be felt throughout the rpm range with the high lift, gives me an advantage, but i run a #90 jet, and i tried going down to a #95 and that was bad. Added 3-4 secs to lap time and made it fall asleep.
 
Higher compression will be needed for high rpms
No it won't. It will help make more power, but even an 8:1 engine can carry the revs up high.

Without knowing if the key in correctly or if the tach is set properly or if the carb is tuned relatively well or what the compression ratio is now or how the engine was being used, we're all just guessing wildly.
 
First, WHY ARE YOU USING A KEY ON THE FLYWHEEL? that is a real good way of ruining a crank and/or flywheel. Second to much compression on a single cylinder engine can slow down the rate of engine excelleration creating drag. This was real evident years ago with the old FH's.
 

Bob Drye

New member
What kind of valves are you using? If you got them from Parsons they are heavy. They float at not much over 7000.
 

2kdime

New member
They were Parsons yes

I got it running and up to around 7k

I had a big problem getting it to rev at all

Come to find out the billet flys don't cooperate with all the new UT2 coils. I got an old UT1 and all was well.

The head was sold and I have a TI valve head coming prepped for alky
 

cubcadet70

New member
Parson valves are heavy as an elephant! Lol I'm gonna try to face the head of the valve off to lighten it up. Especially the intake 40mm valve.
 

Bob Drye

New member
I see over 8000 in your future. Keep those valves adjusted a little tight and adjust them often initially.
Parson valves are heavy as an elephant! Lol I'm gonna try to face the head of the valve off to lighten it up. Especially the intake 40mm valve.
 

2kdime

New member
I'm just hoping for 80 in the 8th

I've since made some other mods too, including a 38mm lectron power jet carb

Noted on the valves, thanks for the tip.

Edit: just noticed you weren't talking to me.
 
Our 40mm stainless steel intake valve is 3 grams heavier than the stock 440cc intake valve. The 40mm valve is also a 4mm larger head and .250" longer valve to get the spring heights right and to accommodate high lift cams. We have tested engines over 7000 rpm in many applications successfully with our valves.
 

Bob Drye

New member
I'm interested in what karters are willing to pay for rpm. At Daytona we were aero limited in rpm. We had to gear for torque. When we installed titanium valves and dynoed them last weekend we did not increase our horsepower, just our powerband. This will allow us to turn the engines 9000 at the end of the 1/2 mile stright away. This will allow us to be right in our power band for the rest of the track. The addational cost of the lightweight valve train was about $300.00. My question is it worth it?
 

metalmagic

New member
I buy all my parts from parsons and I have no problems being well into the 8k rpm range, I'm a motor head though and these things don't always act like an olé small block Chevy lol




Our 40mm stainless steel intake valve is 3 grams heavier than the stock 440cc intake valve. The 40mm valve is also a 4mm larger head and .250" longer valve to get the spring heights right and to accommodate high lift cams. We have tested engines over 7000 rpm in many applications successfully with our valves.
 
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