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Unread 04-18-2013, 07:21 PM   #1
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Default gx390 perf/upgrade questions

i have a supposed to be freshned up gx390,,it runs great and has plenty of power to what my 2 stroke had..i just changed the oil after 1 hr use and it was real dark metallic"bad"looking..upon start up again i noticed a knock that sounds like a rod knock,,im no stranger to engines just not small ones lol.
i have not taken the engine over 3500rpm,s and only took it there a few times for a second..thats about the rpm,s the engine seems to want to really get up and go..it launches from the hole with my 25 yr old ezgo stock 2 stroke clutch very well,,amazes me..

what i want from my engine;; i want to be able to cruise at 3500rpm,s for 5 minutes or so at a time and sometimes would like to rev it out to say 4500-5500 rpm,s if poss w/out hurting the engine,and not having to rebuild it 2 times a yr or so..me and the wife drive around 3-4 hrs at times in the 2500 rpm range and at times i like to let it rip..so i want dependable before anything..if this is not the motor to have let me no now please

since the rod is knocking i hate the thought of a rod w/out a bearing so hear goes..
will i get longer lasting and better performance with a billet rod with a bearing? i read the arc rod gets better oiling iirc? either way i think i want a bearing in there myself.
can i use stock flywheel and cut off every other fin for better performance,or will this hurt the cooling capabilitys?
the engine is sound but a lil lean im gonna drill out jet to 102 using .040 drill bit,,sound correct? thats going up .010 from stock.
while i have the engine apart replacing rod i will cross hatch hone the cylinder.should i go ahead with new rings for the heck of it?
should i mill the piston down any for any gain as im wanting reliability also.
what other parts should i replace while im in the case?
thanks in advance for any info

also,will i benifit from a temp gauge since the engine is in a cart under the seat? it seems hot as heck but no diffrent than the 2 stroke felt..
and last..if i was a lil lean would this cause the knock i just started hearing?

im gonna get the carb right before tearing the motor down in a few weeks..unless i find a cheap used better one,but this stock carb is working fine it seems..i should just drill the jet like i read to do..

sorry for such a long post but had to get it all out before i forget
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Unread 04-19-2013, 07:54 PM   #2
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Default Re: gx390 perf/upgrade questions

main jet was a factory "92" i drilled it .040 which is close to a #60 drill but..which makes the jet now a factory "102"..it runs even better ,and i accidently reved it to 4000-4200 a couple times taking off..i must need a good clutch as it seems to slip at take off and over rev before the clutch catches up?any ideas? the rear driven clutch seems to have a very week spring as i can push the belt with one finger and open the clutch most of the way easy? suggestions?
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Unread 04-19-2013, 08:10 PM   #3
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Default Re: gx390 perf/upgrade questions

Quote:
Originally Posted by sassycat View Post
i have a supposed to be freshned up gx390,,it runs great and has plenty of power to what my 2 stroke had..i just changed the oil after 1 hr use and it was real dark metallic"bad"looking.
If assembly lube was used, that is quite common. I used just a tiny bit of black grease to hold my tappets in place during assembly, and the oil turned black immediately.
Is it still knocking?
You shouldn't be able to push the belt down into the secondary clutch. That's probably what's causing the issue on take-off.
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Unread 04-19-2013, 09:09 PM   #4
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Default Re: gx390 perf/upgrade questions

i got the engine on a trade from a older guy that had a dozen of them he rebuilt..in the end he lied about the rear end i got also so i dont trust anything he said about the motor now if ya no what i mean.
but yes,i ran it 20 min after the re jet a few hrs ago and i can only hear the knock when the seat is off and at low rpm,s..
im gonna try to drive it easy this weekend and keep my ear out incase it gets worse..
i cant get much advice but id think the billet rod with a bearing is a good thing for dependability wouldnt you say?
may just save some parts and re do it so i no what i have

anyone no how many rpm,s the stock flywheel is good for?
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Unread 04-19-2013, 09:38 PM   #5
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Default Re: gx390 perf/upgrade questions

The stock flywheel is good for alot more rpms than stock camming and timing will allow. Your next limit is the valvetrain. Then you can start messing with ignition timing, cam lift, and cam timing. Once that is overcome, then you can worry about the flywheel.
Are you sure the knock is coming from the motor, and not the primary clutch?
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Unread 04-19-2013, 10:28 PM   #6
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Default Re: gx390 perf/upgrade questions

yeah im sure..i wish i wasnt sure but i have that mechanical ability all my life,just never messed with small motors much.
i got a video but cant get it from my computer to e mail or nothing..darn new camera dont have low avg settings like the old one i guess..cant even get it to photobucket
dang i wish it was something else but it really sounds like a rod..all i did was fill with new oil when i got it,,ran it maybe a hr and changed oil with 10w40 penzoil..
then upon start up ever since i hear a slight deep knocking sound..ran it hard kinda earlyer and it never ran so good ever..dont knock any worse,just has it and i hear it at idle and upon slowly givin it gas..

so,i guess i will start savin cash for billet rod w/bearing,stock stainless valves for dependability,and the good retainers and keepers vegas carts hasand better rocker arms and maybe push rods?and remove the gov and oil sensor inside the engine.
prob go ahead and get new rings "oversize"if poss so i can file fit them maybe.and hone the stock cylinder.
maybe i will price out the crank and cam side bearings if thats whats in there.
maybe use a thinner metal head gasket for a lil over stock compression?
hopefully the crank will be ok from me driving it..ive had lawn mowers knock a long time and much worse than what mine is now,,maybe all will be good a while
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