Box Stock

Gx390 realistic rpm?

D

Dsx_fab

Member
Hey guys, new member but long time lurker. The knowledge on this forum is unparalleled and I may even go as far as to say intimidating. I have spent considerable time on this forum trying to find an already answered question that fit my “build” but no such luck.
I’m more of a fabricator, light engine building, but recently I built my entire vehicle around a gx390, so no going back. But man, it’s a completely different animal. Basically I pulled the gov and swapped the crank (it was tapered) and went for a ride. Was immediately disappointed by the rpm limit. After reading on here, I understand why I was disappointed. I know now these aren’t like the small blocks. So I ordered stuff and am building this thing, somewhat against my will (engine internals have always been stressful to me). SO, on to my question...
I want at least 5500 rpms, safely. Here’s what I have so far, but nothing is installed cause I’m waiting on my piston.

It’s a Ut1 390
Arc std length rod
Blue stripe (50 pound I believe) valve springs
Ut2 std short skirt piston and thin oem rings
Custom intake, 32mm tube with uni filter
Stock carb with some new jet sizes for tuning
Honda .010 head gasket
32mm header with a straight through muffler

Again, all I want is a strong, safe 5500. I’m leaving the balancer. This is just an off-road setup, not really for racing. Am I on the right track? My long term plans are titanium push rods, champion shaft rockers, and a 280 cam.
 
flattop1 said:
https://4cycle.com/karting/threads/...roken-a-honda-balancer-shaft-on-a-gx390.9017/
Search , balance shaft
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I’ve read quite a bit on that topic. And maybe it was misread, but I want to leave the balancer. I feel the only up side to removing it is freeing up power, which I’m not terribly interested in. Given my gear ratio and weight of the vehicle+me I’m already pushing the limits of my clutch. It’s fast enough for me, I just want it to not break (for a while at least)
Also thank you, I didn’t see the Honda section. Strange that it’s tucked away like that. I’m currently browsing all the 390 threads trying to learn everything I can
 
I agree . Though the faster you spin it , the more trouble it could present .
Flywheel is a concern also .
 
flattop1 said:
I agree . Though the faster you spin it , the more trouble it could present .
Flywheel is a concern also .
Click to expand...
I really wanted to do a billet wheel but I’m running a charging system. I’m seeing that with a Honda genuine I should be ok at a steady 6k. My sons small block bike with 18lb springs hits 5500 before it drops off. We wanted to ride together and I didn’t give it much thought, figured my 390 would also max at about the same. So assuming I’d be at 5500, I calculated my gear ratio so we would sit at the same top speed. Only I’d get there faster.
 
Dsx_fab said:
Curious why my Honda motor was moved to the predator section :unsure:
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The clone section is for questions and answers for that particular engine. I try to keep it just for the clone racing engine because it is an engine that has specific rules written around it for the two organizations that the engine participates in.
Several years ago I noticed the growth of predator and Honda question beginning to flood the clone forum. I asked Bob if we could have a forum for the predator and he put it on here. He also added the Honda forum.
Like the animal and flathead forums, both are governed by rules, the clone is kept separate for the new guys wanting to gain knowledge about that specific engine.
The clone is a 196/200 engine. All other engines get moved to the appropriate forum. I missed that it was a Honda
 
flattop1 said:
I agree . Though the faster you spin it , the more trouble it could present .
Click to expand...
Well I tested it out today. Got my 5500, and it keeps going, and at a certain point begins to feel a little scary. It’s loud, and vibrates a lot more than the small blocks.
I don’t plan on spending a lot of time at 6000 but it gets there, and it makes some noise I’m concerned about. It sounds like a very rapid pinging, so rapid it almost sounds like squealing. Maybe I’m hearing detonation? I’m just used to a car that’s more of a knock. Not this high pitch. It’s basically from about 5700-6000.
plug is a nice brown
4 degrees advanced
.042 jet
Maybe I’m just hearing some sort of resonating?
 
I don't see why you need 50lb springs. They should match the cam. Too much is as bad as too little. With .080" off the head, you might want to check rocker angle to see if you need shorter pushrods.
 
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IndyHarry said:
I don't see why you need 50lb springs. They should match the cam. Too much is as bad as too little. With .080" off the head, you might want to check rocker angle to see if you need shorter pushrods.
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I didnt think I needed 50 either but I couldn’t find anything lighter. Since then I found that Vegas carts has a 35 pound set, but it seems to run great with the 50s. Now when I do my cam and rockers I’ll already have the springs I’d need I guess
 
Dsx_fab said:
I didnt think I needed 50 either but I couldn’t find anything lighter. Since then I found that Vegas carts has a 35 pound set, but it seems to run great with the 50s. Now when I do my cam and rockers I’ll already have the springs I’d need I guess
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Yes the 280 cam needs the 50lb springs. I'm thinking stock springs are 32lb for the GX390. What did your piston in the hole measure out to?
 
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Dsx_fab said:
I didn’t measure it, figured with all stock Honda parts it shouldn’t be an issue.
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The .010" gasket used to set the piston to head clearance may not be enough to avoid contact is what I was getting at. That could break the piston and cause a scraping noise.
 
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