Honda clone GX390 help needed please

[Abtheman28]

Hey guys,

I have a reasonably new Honda clone GX390. I have a suspicion that it is down on power in the top end compared to what it used to be. Is there anything that typically can cause throttle issues with the governor? Definitely feels like the governor is doing its job but laggy...

Also for future reference when I rebuild the engine at a later date can anyone tell me what size connecting rod I would need as well as piston to fit to the stock crank? I am also intending on getting a race flywheel at the same time. Also what cylinder head work/parts would anyone recommend please? I want the best performance possible for top end speed. It's not for track racing and only personal leisure riding. Or if anyone can send a link with parts I would need to make the beast come alive please send it to me.

 

[bullseye670]

Hello Abtheman28. Sorry I can't help you on the performance issues. i was just going to comment on your parts issue. When your shopping for parts, they will tell you what it fits. Will you be removing the governor? I think your going to want to. Just make sure to get a billet connecting rod and a billet flywheel. You want a piston to match the cylinder bore, a billet connecting rod, billet flywheel, a new camshaft, a mikuni carburetor,and a header for your exhaust. That should get you going. I bought my last racing head for a gx390 from nrracing. Take a look there. They have a few options to choose from.

 

[Karttekk]

Hey guys,

I have a reasonably new Honda clone GX390. I have a suspicion that it is down on power in the top end compared to what it used to be. Is there anything that typically can cause throttle issues with the governor? Definitely feels like the governor is doing its job but laggy...

Also for future reference when I rebuild the engine at a later date can anyone tell me what size connecting rod I would need as well as piston to fit to the stock crank? I am also intending on getting a race flywheel at the same time. Also what cylinder head work/parts would anyone recommend please? I want the best performance possible for top end speed. It's not for track racing and only personal leisure riding. Or if anyone can send a link with parts I would need to make the beast come alive please send it to me.

I've never done any performance mods but the governor is controlled by spring tension. Do a wet/dry compression test to see if there's any cylinder wear and try new governor springs or move them into different holes on the governor arm to add tension to them. Make sure you don't have a sticking torque converter or clutch, check the drive belt for glazing and slipping. Check for any brake drag or tight wheel bearings.

 

[Abtheman28]

Hello Abtheman28. Sorry I can't help you on the performance issues. i was just going to comment on your parts issue. When your shopping for parts, they will tell you what it fits. Will you be removing the governor? I think your going to want to. Just make sure to get a billet connecting rod and a billet flywheel. You want a piston to match the cylinder bore, a billet connecting rod, billet flywheel, a new camshaft, a mikuni carburetor,and a header for your exhaust. That should get you going. I bought my last racing head for a gx390 from nrracing. Take a look there. They have a few options to choose from.

Thanks mate. Yeah I plan on removing the governor... I want this thing to Rev much harder than it currently does. Do you think I could get away with just the billet flywheel, billet rod and piston without any head work? Or will the stock spring tension not handle higher RPM?

I'm guessing I would need what you said which includes the carb and cam also? With the carb I've seen on NRRACING that the one I'm looking at does not have a chock on it for cold starts.... if I buy this one how do the guys typically start there engines from cold?

Any help would be appreciated

 

[Abtheman28]

I've never done any performance mods but the governor is controlled by spring tension. Do a wet/dry compression test to see if there's any cylinder wear and try new governor springs or move them into different holes on the governor arm to add tension to them. Make sure you don't have a sticking torque converter or clutch, check the drive belt for glazing and slipping. Check for any brake drag or tight wheel bearings.

Thanks mate. Yeah I'll check out what you mentioned today. I'm only running a centrifugal clutch on it so how would I tell if it's sticking? You mention changing governor spring location to add tightness, how tight is too tight? Sorry I'm new to this...

 

[redflash]

Also look up the various stages of horsepower for these motors.....stage 1 is remove governor, remove oil sensor, add free flow exhaust....
stage 2 is stronger valve springs, ratio rockers, moly push rods, larger carb. next comes billet rod and flywheel.......on and on

On youtube "red beards Garage" is dyno testing a 212 with each modification added, and the horsepower that comes with each individual addition.
So you can actually see where the hp comes from one at a time.....very interesting

Do your toilets actually swirl counterclockwise down there in aussie land?

Da reflash

 

[Abtheman28]

Sweet thanks I'll do some looking around. Haha well I'm actually based in New Zealand and yeah that's the way the water leaves the toilet counterclockwise

 

[panchothedog]

Billet rod and flywheel will keep the internals together at any elevated RPM. To get there you will need to get a cam with more duration then the stock one.
Also stronger valve springs are a MUST. A stock carburetor with a bigger main jet as well as a bigger or drilled out pilot jet will work quite well with a modified engine. It won't match a BIG Makuni for all out top end horse power, but none the less will work fine. This will allow you to keep the chock in place for cold starts, as well as the fuel shut off and no need to change throttle linkage.

 

[bullseye670]

I would think the carburetor would have a choke or fuel enrichener or whatever they call those things that richen the carb for cold starts. They usually just call it a "choke" but that's not the correct term.
It gets cold here in Michigan so I could never get a carb without some type of choke.

 

[panchothedog]

They have a " Fuel Enrichment Circuit " . I live in Southern California, so l really don't know much about cold weather starts.

 

[Denny]

If your just running a centrifugal clutch it may be going away that’s why it feels down on power.

 

[Karttekk]

Thanks mate. Yeah I'll check out what you mentioned today. I'm only running a centrifugal clutch on it so how would I tell if it's sticking? You mention changing governor spring location to add tightness, how tight is too tight? Sorry I'm new to this...

Clutches can "stick" when the brass bushing is seized or dragging on the clutch sheave or the clutch shoes are hanging up in the drum. As far as the governor springs, just move them to different holes in the arm to see if this is the issue or part of the issue. There's no way to measure how tight the springs need to be, they stretch over time and lose their ability to maintain engine speed under load which is their purpose.

 

[Karttekk]

This is off topic but I understand the torrential rain has finally let up. My wife visited her brother there over Christmas and it rained the whole time. Her brother had to move to higher ground since his road flooded.