GX390 help

S

skidmarx

New member
Hi guys, My names David Im new to the forum. Im from England, UK. I was hoping you guys could give me some advice, GX390 parts and tuning over here is very hard to come by, Its mainly Rotax and 2 strokes.

Anyway, Ive bought a brand new genuine Honda GX390 engine for a bargain price ( around $400 in your money) but im not sure on how to get what I want from It.

Ive already bought the clutch, A Noram star 15 T with #35 chain.. Ive lathed down and shortened the crank so it doesnt hit the seat. Now, I want to make no more than 20hp as im lead to believe this clutch wont last very long with anything above that, secondly.. Our tracks over here seem to be more tight and twisty than yours over there, For e.g my local track I'll probably only hit about 45mph on the straight.. so really I need to be making power low down in the rev range. Obviously I will be gearing it low to suit this, 6000rpm 15T clutch 60T axle and 11" tyre will make about 50mph.

I was thinking, billet rod and Flywheel, head skim, standard carb overbored to 24mm and re-jetted, decent exhaust and a few degree's advance on the flywheel. Would this make 20hp.. and more importantly would this make power where I want it?

Please bare in mind, I want to keep costs fairly low (hence the Noram clutch) and reliability high.

Thanks from across the pond.

David.
 
Don't know about the hp but you will happy with it pulls in a basically stock form. I think you need better valve springs. Parsons racing supply; Vegas Carts; N R racing maybe some others can get you what you need. Take it easy on that clutch. It won't take much abuse.
 
Thanks for the advice.

Yea I think I screwed up with the clutch, Should have got an Amsbeck one.. still atleast it wasnt too expensive. If it burns out fast I'll just get something better.

I totally forgot about valve springs too, AGK do them at 32ib which is ideal for a fairly stock engine.
 
You need to add a cam and the valve train parts to make a solid 20+ motor, stock cam and springs will give up around 5200 rpm, with good stock springs.
A small cam like NR 280 will work perfect, You can use most of your stock parts but you will need to replace your springs, build this as my first GX 390 engine and it ran it ass off and never broke a part for 2 years, used same gearing you have to.
 
Ok I've ordered from NR Racing, 280 cam Torque grind, 50ib springs, Aluminium retainers, High groove keepers, Billet rod and a 10.9 bolt kit.

Alongside this I will be milling the head .100, 24mm carb de-choked, Billet flywheel and a decent exhaust.

Anyone know what compression ratio I'll get from standard piston and deck with .100 head?

Also recommendations for an exhaust and timing advance?
 
skidmarx said:
Ok I've ordered from NR Racing, 280 cam Torque grind, 50ib springs, Aluminium retainers, High groove keepers, Billet rod and a 10.9 bolt kit.

Alongside this I will be milling the head .100, 24mm carb de-choked, Billet flywheel and a decent exhaust.

Anyone know what compression ratio I'll get from standard piston and deck with .100 head?

Also recommendations for an exhaust and timing advance.

Here is a chart for head milling from a company call Vegas carts here is there link :http://www.vegascarts
Stock = 41cc (8.75:1 CR)
Milled 0.025" = 37cc (9.5:1 CR)
Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
Milled 0.100" = 27cc (11.5:1 CR)
Milled 0.125" = 24cc (12.3:1 CR)
Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended
This will get you close, it's not exact but very close, as for exhaust run what you want, I used a straight pipe it worked fine. Motor will have so much torque I don't think pipe matters much, just make sure it's big enough, <.900"
Timing is really one of the things you will have to test to find the best for your engine. start at 32°BTDC.
I think your carb is to small, you need to be around 34 mm with a 390cc engine, unless your rules will not permit it, 24 mm will work but you will feel the power start to fall off early in the band, around 5000 rpm, with 34 mm it will charge to the top of the cam band around 6500 rpm with NR280 cam.

Hope this helps you.
Click to expand...
 
level600a said:
skidmarx said:
Here is a chart for head milling from a company call Vegas carts here is there link :http://www.vegascarts
Stock = 41cc (8.75:1 CR)
Milled 0.025" = 37cc (9.5:1 CR)
Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
Milled 0.100" = 27cc (11.5:1 CR)
Milled 0.125" = 24cc (12.3:1 CR)
Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended
Click to expand...
what is the bore and stroke of that engine? There must be something wrong with my compression ratio spreadsheet. I don't get anything even close to those compression ratios. I must have the wrong bore and stroke entered in.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
Click to expand...
 
Al I think it from there 460cc engine because of the 41cc stock head number, my stock 390 head was 39cc, but the numbers are close ,I have a 390cc head milled .085 and it's camber was 29cc (measured), so chart is close enough for what he is looking for, like I said It's not exact but close.
 
A bone stock 390 that is nearly perfectly tuned with open air filter and adapter, governor removed and good open header will be just shy of 18 HP. Say 17.6 to 17.8. The one best increase is to increase compression. You did that with the .100 head shave. That should be about 10 to 1 compression or just shy of it. You can increase the flow with stainless valves, ratio rockers, cam and head porting but be careful, a stock head will flow just about as much as you will ever get that stock bored carb to flow. You will not see a huge increase in power, like Level600 said. from any of these flow mods until you go to a much bigger carb. Put the stuff in it you ordered from NR Racing. Don't get crazy with head porting. Just a little. Advance the timing to 32 to 34 with that fuel you mentioned. You will need stainless valves to use those keepers and locks you mentioned. I am a little worried about those 50lb springs. That cam should be fine with 26 lbs (or 32s) I would think at the rpms you will turn. Anything more will just cause way too much wear with little to no gain. Your Star clutch should be OK. Keep it locked up and it will last a long time. Good luck.
 
Was only going for the 24mm Carb as its cheap and easily available here, I will look at the Mikuni's instead.

Advice very much appreciated.
 
level600a said:
Al I think it from there 460cc engine because of the 41cc stock head number, my stock 390 head was 39cc, but the numbers are close ,I have a 390cc head milled .085 and it's camber was 29cc (measured), so chart is close enough for what he is looking for, like I said It's not exact but close.
Click to expand...
I'm just trying to prove out my spreadsheet. I put in a bore and stroke that came up to 390 cc and with a 29 cc combustion chamber the compression ratio was 14.44.

390+29 = 419 cc 419 / 29 = 14.44 compression ratio.

You say this chart is close enough, but I'm wondering.

A stock 390 cc engine with a 41 cc combustion chamber has a compression ratio of 10.51. Where am I going wrong? Maybe they are using what is called an "effective" compression ratio. Meaning they don't use the full sweep cc.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
alvin l nunley said:
I'm just trying to prove out my spreadsheet. I put in a bore and stroke that came up to 390 cc and with a 29 cc combustion chamber the compression ratio was 14.44.

390+29 = 419 cc 419 / 29 = 14.44 compression ratio.

You say this chart is close enough, but I'm wondering.

A stock 390 cc engine with a 41 cc combustion chamber has a compression ratio of 10.51. Where am I going wrong? Maybe they are using what is called an "effective" compression ratio. Meaning they don't use the full sweep cc.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
Click to expand...
You are missing, gasket bore and thickness, and how far in the hole the piston is also the dish in the piston....the way you are doing it is the theatrical, hope I spelled that right., maximum compression, not the realistic compression.
 
skidmarx said:
Was only going for the 24mm Carb as its cheap and easily available here, I will look at the Mikuni's instead.

Advice very much appreciated.
Click to expand...

If your budget will only let you use the 24mm then it can be used, you will get the power fall off, but if you are gear right it might not be that bad................I agree with jsstump the springs might be to stiff try the 32lbs AGK springs it should be enough.
 
Yea I've ordered the SS valves in standard size. NR don't do a spring with less ib than that but I think other companies do, Will NR valves and retainers work ok with other springs? AGK do them at 32ib for eg.

level600a, Just read your reply. Cool, AGK it is.
 
level600a said:
You are missing, gasket bore and thickness, and how far in the hole the piston is also the dish in the piston....the way you are doing it is the theatrical, hope I spelled that right., maximum compression, not the realistic compression.
Click to expand...
"theoretical". Close. And just so you know, the compression ratio I cited was an actual compression ratio, not a theoretical ratio. I have no idea what a theoretical ratio is.

You can't calculate a compression ratio with just the cc's in the head. Seeing as how the list of compression ratios didn't include a gasket thickness, or a distance in the hole for the piston, I had to assume the cc's listed were the actual volume of the combustion chambers in cc's.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
Amsbeck clutches last a looong time and handle 50+ hp easy, but make sure to get the fan driver cover for extra cooling of clutch!
 
Any tips for head porting? I was just going to smooth out the transition from the CNC'd part around the valve guide to the cast part of the port, Also clean off any rough lumps and leave it with an 80 grit finish. I've already cleaned up the chamber and polished it smooth.
 
Call Parsons racing he runs his on a tag and will hook you up with everything u need....... And go with a 32mm carb because they have way better throttle response
 
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