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Old 07-24-2016, 10:38 AM   #1
Brian Boyer
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Default The mythical GX390/420 grenading flywheel

I'm not saying it does not happen, but i can not find any info or first hand reports of it actually happening. (Especially under 6,000 rpms)

My mud motor forum the guys are so hung up on the billet flywheel, and i'm talking on motors that are so far from built its not even worthy of conversation.

The rods too, i'm running 4,200 rpms on my stock rod now for extended periods (2-3 minutes) with no issues what so ever,.. Merely mentioning both my flywheel and rod being stock these guys are threatening to PM the site admin and get me banned. (Its really kind of funny, they get so defensive having laid down big bucks on the parts already,.. They refuse to even have a conversation about the parts even truly being called for at their rpm range)

Some of they guys describe the two components as "making power" by adding them.. And there is no reception to critical conversation. (Frustrating because new members come right behind them, read the same tripe,.. Buy a billet flywheel& rod,.. To spin 4,000 rpms )




Has anyone here had a big block flywheel give you issues, magnet fly off/ flywheel come apart,.. Anything?

And the stock rod, i'm currently 4,200, (i know i'm guilty of re-hashing this..) whats it good to?


Full disclosure, I did seize the rod to crank on the bench over-revving with a knock off tach that was set wrong.. Surprisingly un-eventful, simply slowed and died/ no real drama.
Guys if you have the $7 tach off ebay/ Amazon, the one with the S1, S2 keys, the manual it comes with is in Chinglish and useless,.. I finally looked at the Tiny Tach manual online and got the correct info,.. If you have this knock off tach your setting is "03"... But be aware, even with the correct setting there is some lag in readings, not horrible, but there is a little lag. With the wrong setting the first go round i was really really over-revving the 420, who knows what i was hitting, salvaged the crank thank goodness.
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Old 07-24-2016, 02:48 PM   #2
TX Foilhead
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Default re: The mythical GX390/420 grenading flywheel

I don't worry about it because it's on the passengers side.
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Old 07-24-2016, 02:55 PM   #3
Brian Boyer
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Default re: The mythical GX390/420 grenading flywheel

My title says 390/ 240, it was supposed to read 390/420



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Old 07-24-2016, 06:24 PM   #4
sho305
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Default re: The mythical GX390/420 grenading flywheel

If you go read at 4cycle.com they talk about them blowing, the magnet blows the coil off and the coil goes flying around in there causing lots of issues. Some of them are said to have broken the flywheel, some just blew the coil off and the coil broke stuff and flew out. On a gokart the engine is sitting there, the flywheel can come out and cut into the guy racing next to you, so that is why they get so wound up about it, and I can't blame them. Now this was a few years ago and clones may be better, however they are talking over 6K rpm this happens some say over 5K. But realize these guys run karts or minibikes and rev the snot out of them all the time, they see a lot of top rpm (whatever rpm that is depending on build). They say you can run it all day at 4K stock and it will have no problems. Vegas says at near 6K it will round out the rod and it will fail, others say at 6K and more it will not oil and will fail. Most say don't run the stock flywheel at 6K or above.

A billet flywheel will give you a performance increase in most uses because it will rev much faster. Though on a CVT or marine it may not as much because rpm does not change as quickly. A gokart will come out of the corners faster with a light flywheel.

It can be harder to start a clone with a light flywheel, it will not idle as slow either. They tend to be nicer on high rpm engines.

Most people with HD springs seem to run up to 5200-5500+ rpm in carts at top speed, but they don't run that speed very much. They get away with it with the stock rod and flywheel. But for harder use everyone says get a rod, any cam even the cheater you need a rod (because it will hit 6K no problem). Far as flywheel most run stock but I think most stay under 6K in a cart, with the CVT you don't need big rpm, it would be harder to drive and tune much like a sled engine runs. Its not common in GC. Likely that is why vegas makes the 460 with the cheater cam, its cheap and more than stock yet mild enough to work well in a GC. I don't know how much more power you would get from a marine/tq cam in comparison and still running say 6K or less rpm, in a 460 or similar. Most clone engine builds go for more rpm to get the power, but you need good parts to do it, and not sure what longevity is with them in constant use. Most trail riding you only buzz up to top speed and you let back off.
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Old 07-24-2016, 07:27 PM   #5
slonomo
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Default re: The mythical GX390/420 grenading flywheel

I agree sho, the issues that Vegas Carts has reported were above 5000 rpm with the rod egging out. The billet rod cures that problem but not the oiling issue over 6000 rpm. Not really sure anyone knows the actual oiling limit on these engines, just the theory.

The magnet issue I think was a pretty old issue on "some" of the clones, but not all. Just the ones that the magnet was glued/epoxied on. I'm not sure if any of them are like that now. If the magnet is screwed on, a little bit of thread locker will cure that.

The flywheel "grenading" is probably from the magnet coming loose, as sho said. I haven't heard any recent stories either. But, I will say that if I was using one of these engines for a go kart, I would definitely have a billet flywheel and rod, no brainer. But on a cart or a boat it might not be necessary.

I don't own a boat of any kind so I can't speak on how this type of engine would do in a boat, but if it was reliable continuous performance at high rpm, I would opt for the billet parts. Just me. Time will tell how well your engine will hold up Brian. I'm sure you'll keep track of the hours of use etc and you can report as time goes. Hopefully it holds up well for you.
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Old 07-24-2016, 08:32 PM   #6
lockman1
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Default Re: The mythical GX390/420 grenading flywheel

Quote:
Originally Posted by Brian Boyer View Post
My title says 390/ 240, it was supposed to read 390/420
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Old 07-24-2016, 08:47 PM   #7
Brian Boyer
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Default Re: The mythical GX390/420 grenading flywheel

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Originally Posted by lockman1 View Post
Hey thanks man!
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Old 07-25-2016, 01:22 AM   #8
sho305
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Default Re: The mythical GX390/420 grenading flywheel

The arc rod supposed to oil better. The open guys do run 7K rpm some more, but many say they are a BB and don't like rpm like the smaller ones do. But those guys also go nuts on parts, such as entire new HD block covers. The block tends to break over 30hp they say, lot of them getting the VC460 its good for more but I don't know how much. Otherwise you use a honda GX390 block its heavier than clones. Fortunately for us a 30hp+ 7K rpm clone is not ideal for a GC so we don't have to worry about it.
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Old 07-25-2016, 07:40 AM   #9
raydhd
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Default Re: The mythical GX390/420 grenading flywheel

The arc rod actually has an oil passage right in the splash fin.It leads to an oil grove in the big end of the rod.Into this are a pair of shell bearings that have drill passages in them feeding oil to the crank throw itself.As long as there is oil for the splash fin to pass thru it is going to pressure feed the crank throw and rod bearings.Also note the more the rpms the harder it is going to feed. ARC did a masterful job in designing this piece to eliminate rod failure due to oil feeding or strength issues.
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Old 07-25-2016, 11:36 AM   #10
Streetbu
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Default Re: The mythical GX390/420 grenading flywheel

My buddy raced cage carts with clone engines for years. Long before there were many performance parts available. He actually took a stock flywheel and machined it down on a lathe to lighten it. Dropped almost half the weight. Then sent it out to be balanced. Ran the same flywheel for 6 years, spinning a max of 8800 rpm. No issues.
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