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Old 02-23-2016, 04:40 PM   #1
my1423
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Default HONDA gx390 build parts recomendations vc.

I Picked up a good running real not clone honda gx390.
Never built an actual honda.
I want to upgrade the valves springs, keepers, maybe lifters, pushrods, cam, rod to allow 5500 rpm without detonating.
I am torn on changing the crank to the 69mm stroker. I can mill the piston.
I want dependability!!! Quiet! Like to build more power around 18-20 hp fine.

1. What length billet rod do i need for the stock crank 4.410? and or the 69mm crank stock piston? If i mill the piston can the same length rod be used. Does the stroker have the same rod journal size as the stock crank?
2. in gx390 stock piston what all needs clearanced for the 69mm crank? Cam, balance shaft, and? skirts and cylinder clear?
3. Does anyone sell a reasonably priced compete kit for the valve train? Keep costs down. 5500 rpm what lb springs? 55lb?
4. Valves what do you recommend size and type? Stainless ect?
5. I am almost about ready to just buy a Vegas cart engine that has all this in it. The parts for mine minus valves from vc is already at 350$. Makes upgrading my honda almost not worth it.
6. Anything else i am forgetting?
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Old 02-23-2016, 06:23 PM   #2
fstg21
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Default Re: HONDA gx390 build parts recomendations vc.

Quote:
Originally Posted by my1423 View Post
I Picked up a good running real not clone honda gx390.
Never built an actual honda.
I want to upgrade the valves springs, keepers, maybe lifters, pushrods, cam, rod to allow 5500 rpm without detonating.
I am torn on changing the crank to the 69mm stroker. I can mill the piston.
I want dependability!!! Quiet! Like to build more power around 18-20 hp fine.

1. What length billet rod do i need for the stock crank 4.410? Yes 4.410

and or the 69mm crank stock piston? 5mm longer than stock not likely.

If i mill the piston can the same length rod be used. Gotta take .200 off so rod yes, piston ..no.

Does the stroker have the same rod journal size as the stock crank? Yes.

2. in gx390 stock piston what all needs clearanced for the 69mm crank? Cam, balance shaft, and? skirts and cylinder clear? Have to replace it.

3. Does anyone sell a reasonably priced compete kit for the valve train? Keep costs down. 5500 rpm what lb springs? 55lb? The single springs from vegas will do 5500 all day on a stock valve train.

4. Valves what do you recommend size and type? Stainless ect? Run it stock first, then upgrade the springs, the cam and the rod, rejet the carb and tell me you still want more.


5. I am almost about ready to just buy a Vegas cart engine that has all this in it. The parts for mine minus valves from vc is already at 350$. Makes upgrading my honda almost not worth it. Almost always is better to buy from vegas.

6. Anything else i am forgetting?
Ask in the Clone Section next time.
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Old 02-23-2016, 08:07 PM   #3
raydhd
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Default Re: HONDA gx390 build parts recomendations vc.

Ditto
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Old 02-23-2016, 08:41 PM   #4
slonomo
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Default Re: HONDA gx390 build parts recomendations vc.

Moderators can we move?
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Old 02-23-2016, 10:27 PM   #5
my1423
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Default Re: HONDA gx390 build parts recomendations vc.

Sorry thought big block forum was proper since this is not a clone and everywhere else calls this a big block.

fstg21 thank you for the reply.


I have seen the longer rod listed with the stroker crank for clones just did not know if the stock legnth rod would work by notching the piston to clear the crank and milling the lip off the piston for stock honda.
Since the 5mm bigger crank and 5mm longer rod would be to big to mill down but 5 mm could be taken up with dish removal and a head gasket.
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Old 02-23-2016, 11:01 PM   #6
rib33024
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Default Re: HONDA gx390 build parts recomendations vc.

Quote:
Originally Posted by slonomo View Post
Moderators can we move?
MOVED
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Old 02-24-2016, 12:04 PM   #7
fstg21
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Default Re: HONDA gx390 build parts recomendations vc.

Quote:
Originally Posted by my1423 View Post
Sorry thought big block forum was proper since this is not a clone and everywhere else calls this a big block.

fstg21 thank you for the reply.


I have seen the longer rod listed with the stroker crank for clones just did not know if the stock legnth rod would work by notching the piston to clear the crank and milling the lip off the piston for stock honda.
Since the 5mm bigger crank and 5mm longer rod would be to big to mill down but 5 mm could be taken up with dish removal and a head gasket.
The longer Clone rod is available because some come with a longer rod, not all, also, when used with a stock gx390 it puts the piston nearly flush with the block, good for a stock motor, bad for a stroker as you'll have to find that extra .030" somewhere.
As far as I know, no one makes an 88mm piston for a 69mm stroke. your best bet is to find a flat top piston 88mm in diameter that has the wrist pin .200" higher. Worth the cost, not likely.

The thickest gasket I've seen for a gx390 is a .063". that's 1.5mm. but you don't want the piston sticking out of the block at all if you can help it.

You may have 1mm between the top of the piston and the top of the cylinder at TDC. so there is 2.5mm,

However, if you take 2.5mm off the top of the piston, you may severely weaken the top ring land, depending on the factory piston design. Many of the later GX390 engine have a very high top ring for emissions reasons.

Be careful, and don't buy until you measure everything.

I will bet you would be very happy with just a Rod, Cam and springs. My 390cc cammed clone makes more than enough power to scoot 4 guys in a long wheelbase yamaha and spin a set of 25's.

If you buy those things and aren't happy you won't spend any more buying the upgrade parts later.

30yrs of modifying engines and lots of money spent have taught me to move in smaller steps. Is the last 2hp worth it? For a Kart racer maybe. On the trail, nope.
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Old 02-24-2016, 07:09 PM   #8
sho305
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Default Re: HONDA gx390 build parts recomendations vc.

Right, and you could always buy a vegas and put a larger cam and springs in (it comes with a cheater cam a little larger than stock). Really there is not a lot of difference 390-460 in power from displacement alone. The crank is cheap, and it will help torque, but nothing like changing the cam which will change the powerband in a big way. I'm not sure I would bother to stroke it either, at least not at first. I can tell you the kart guys say a stock cam (390/420 clone cams appear to be the same) clone the 390 will rev about 400-500 rpm more than a 420 with the same setup. That seems about right to me considering the displacement the 420 is going to run out of air faster. Everyone says the heads are also the same.

Lot of people run high compression, some use the long rod to do just that. I cut .050 and run a 4 degree key and with the stock clutch it would knock at low rpm if loaded. If I let off and slowed and gassed it before the clutch shifted it would knock. With the new clutch it shifts back sooner and so rpm is higher it does not. Anyway, I know I am right on the edge of running regular fuel that way and I don't want to buy special fuel for it at this point. Many people say cut the head .100 and run premium.
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Old 02-24-2016, 07:19 PM   #9
my1423
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Default Re: HONDA gx390 build parts recomendations vc.

Thank You.
Ill do just as you recommend.
Reground cam vc 50$, Valve springs 20$, and a rod 160$.

So for a stock piston and crank what length billet rod do you recommend for a little more bang? Or just go stock length and possibly a little mill work to bump the compression?

Rather just use pump gas nothing special.
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Old 02-24-2016, 08:11 PM   #10
sho305
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Default Re: HONDA gx390 build parts recomendations vc.

I asked around and was told the max I can do is above setup with .050 cut and 4 maybe 6 degree key (I ended up with a 4 from NR). If the longer rod gives .030 you might run that with 6-8 degree key I don't know what is better maybe Fstg21 knows. I also have a stock metal gasket in my 420, it is very thin like .010 or something. I did all this before the first vegas was out the 440. You can also run the smaller 270 honda head I heard, Hookm ran one I think, I don't know what that gives you for compression he might have posted it. Most stuff I learned was at 4cycle dot com there is a lot of kart racers and minibike guys on there, lot of info, but most of it is for stock classes of the small 200 clone or open motors that are way too radical for typical golfcart use so you have to be careful what you take from there for a GC. NR and Vegas were also very helpful. NR is also a race place so lot of stuff does not apply to a reliable high torque motor you generally want in a cart, but nice place and good parts. Most Vegas stuff is tailor made for what we need if he has it in stock, he has more stuff as time goes on and is a sponsor here. But for example I don't want his long rod it would mess up the deal I have going, where a stock motor could use it fine. I had my head ported and cut by a guy local that builds lawn race engines, he had a couple kaw sitting there for a local dealer. Of course he also does 1,000+hp car/truck/boat motors too lol and sleds.
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