Gx390 build

Gx390 build advice needed

I am wanting to build my first 390, not nothing crazy, I was wondering about the counter balancer, what rpm is it good until it needs to come out. What will the stock piston take?
The rwyb around here aren't terribly fast and i think a good handling chassis with a mild motor will be a front runner. Just looking for some advice from some of the veteran 390 guys.
Was looking at
Billet flywheel
Dyno 308
Billet rod
Ss valves
Dual springs
Tilly
P&p
10.5:1 to 11:1

Thanks
 

mikelavite#72

Site Supporter
give the stock piston the BOOT save yourself the trouble . yes it is usable but they wont withstand lots of abuse . and toss the balance shaft out . DONT FORGET to get the bearings for balance shaft out of block and side cover . you dont want them falling out and finding their way into crank and rod durring a race
 
You can use the stock piston if its the Tier 3 UT2 short skirt piston. http://www.nrracing.com/product-p/1310x-z5t-800.htm You will need to mill the block about .080 (and piston) to make it a flat top. If you put in a 420 crank and mill the piston about .040 (leave the block untouched) that too will work and you will have about 408cc. Parsons has a fine piston too. Just get that compression up and dont go crazy with the porting of the head. Get a good 3 (or 4) angle valve job on the valves and head. Lots of the power to be found in the carb. I would leave the balancer in for now. If you take it out be prepared to use studs, or wire and locktite. No need to turn huge rpms for power. Good luck
 
Was going to stay stock crank
What's the pros/cons to leaving the counter balancer?
Also leaning hard towards a 32/34 mm Mikuni round slide ( free)
What are you calling huge rpm?

What kind of performance can I expect out of above combination. Also going to add 1.3 rockers and a good piston, the engine needs a bore job
 
Leave in Pros: Much smoother running. Much less vibration meaning less stresses on internals. Take out pros: Will wind up a little quicker (very little), Make a little more horsepower, needs to be balanced if removed to somewhere between 56 to 62% balance factor IMO. I like the Mikunis. My ignorance of the Tillys keeps me from using them. Ive tried with no luck, again Im sure because of my ignorance. Ive found very little reason to turn them above 7000 rpm. They will but they make a lot of power between 4800 to 6500. Rpm causes lots of wear and stress. Since you are boring anyway, look at Parsons 88.25mm (.010 over) pistons. Ive never tried thier .200 longer rod short compression height piston but would like to. Mike Clements has great stuff also. I would hate to guess the horsepower on your build but if done correctly with optimal tuning and a true 11 to 1 ratio is achieved, I would guess 28 to 32 hp on good gas. Alky 10% more. Thats lower than what most claim but I am a realist. Ive had great luck while using the stock (Honda only) flywheel. Adjustable billet allows for much easier timing changes for tuning. Keep that side cover tight. If it come even a little loose you will have a parts pile. Good luck
 
That's kinda what I was shooting for, under 7k. On smaller open motors we do away with side cover gasket, machine fit, yamabond and studs. Problems went away with coming loose. Clements has a good looking rod for sure. Gonna start machining after thanksgiving. So I will try to keep progress updated.

Also , y'all running a 1" clutch or turning the shaft, l don't like the Idea of turning shaft but if I have to I will..
 
I run a 1inch clutch. For a shoe get a noram enforcer or mini-cup clutch. Not the starr. For a disk get a jammer. Horstman redhawks are ok but becoming extinct.
 
Only seen 2. I think thier called Turbo's. Both dropped out from "clutch trouble". Never followed up. Could have been operator error. IDK. Jammer calls thier 1 inch the Eagle I think. Buller and Jammer both make fine stuff. Neither are cheap. Ive never turned down to 3/4 because i have plenty of 1 inch clutches. ive been told that turning done does work and others do it all the time. If you already have 3/4 clutches, then it makes sense to turn down. Not sure what it would cost to have one turned down and a keyway cut vs buying a new 1inch clutch to use over and over.
 

flattop1

Dawg 89
are you using the noram I been thinking of going big block and clutch is 1 issue .
I run a 1inch clutch. For a shoe get a noram enforcer or mini-cup clutch. Not the starr. For a disk get a jammer. Horstman redhawks are ok but becoming extinct.
 
I am lucky enought to have a lathe and a mill. I turned one down years ago for a flat head and I remember it being hard for .100 or so then a lot softer, never wanted to turn one down after that. Really don't want to drop out for breaking the end off the crank. Or it be soft and beat the keyway up
 
Flattop1, sent you a PM. Oldschool, I know a bunch of people who claim turning them works just fine. I shorten my pto ends some to reduce stress and the Honda ones I cut seem hard all the way through. Honda parts are far superior to clone 390 and 420 parts. Good luck
 

fredjones

New member
do what you want but...I used the stock piston, 360 lift cam same valves with good springs a 34mm mukuni carb on gas, .100 off the head. Leave the balancer in and I even used the stock flywheel. Ported the head, intake only and put 28 hp on the dyno at 6700 rpm. real reliable. I feel there is no sence in going to far so it will last for years. I also used 10-40 mobile 1 oil.
If you want more then 28 hp then you'll have to do alot more work but why if it works well for you. I know you'll be pleased...did I mention I used an ARC rod.....thats it, good luck
 
lol good thing i just got my honda gx390 off of a mini sotck car moded car cam gear lightened flywheel max torque clutch for racing heh 14 to 1 gear ration very fun
 

metalmagic

New member
I built a Honda block with 88mm (stock) Parsons piston and a Parsons stock length rod with a crank from a 440 (66mm) no balancer, 308 cam with 1.3:1 rockers stock head with no port work billet flywheel Anda 32mm mikuni round slide and it will turn 7500 all day without any problems and the power is incredible.... 36hp and 43 torque..... I'm a big boy and I don't have a problem getting the checker... And its easier to drive than any engine I've had
 

alvin l nunley

Site Supporter
36hp and 43 torque..... I'm a big boy and I don't have a problem getting the checker... And its easier to drive than any engine I've had
1.5 HP per cubic inch doesn't seem excessive. At what rpm are you quoting the torque and the horsepower?

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