212 predator hemi/big valve non hemi head

My Nibbi PE24 has a big ol throttle body, and the slide valve is huge too. I think you may be fighting with so much air rushing through by 3/4 throttle that the pitly trickle of gas getting through becomes no match at a certain point. And it may be that 5900 rpm happens to be the point where you notice it most. Go to a larger jet, but don't change the needle just yet. See if the bigger main settles your cough at wider throttle. Lift the needle one spot if the engine still feels like you're leaving something in the bottle ..(I've gone up to five, with a shim!).. If on the other hand it feels or acts too rich (sluggish getting to higher rpms) try dropping the needle one spot. That'll use the tapered shape of the needle to delay the increase in fuel just a scoche.

But again, this only after you find the best main jet.
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You said stock valves and retainers but then said it had big valves. Can you clarify what head your using? The fact that it happens exactly at 5900 rpm every time sounds like valve float but 26lb springs should go higher than that unless you're using the hemi specific springs on anon hemi head.
It is a non hemi head with 27/25mm valves and they are 26lb springs that I ordered for my hemi head
Check your pilot jet size while you're in there too. How much twist of the throttle is getting you to the problem rpms (5900)? Right out of the gates? 1/2 way? I'm thinking if you're meeting the problem pretty early in the twist, which is where the venturi effect is expected to really kick in and take over, you may also want to upgrade the pilot to help with that transition. Of course, you also want to be sure any bigger pilot still meets the air screw "1 to 3 turns out" rule.
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I want this build to do 9k rpm is it possible
Will it spin to 9000, maybe. With stock keepers and locks, probably not for long. Don't make too many changes at one time. What does the spark plug look like after it lays over? Kill the engine, let it cool and remove the plug. That should give you an idea if it's lean up top. Where is the fuel tank mounted? Are you running a fuel pump? Just a few things to look into...
The 26Lb hemi specific springs are made for a tighter install height so as not to coil bind on a hemi. Most places just sell the white stripe original springs and they are what you need. Do you have the ability to check the seat pressure? or the current installed height of the spring as it is installed?
I'm very interested in your focus here, Ole4. Sitting and trying to imagine what that float might feel like. Loss of compression? Backfiring? And I'd also think you might sometimes see such floating being triggered at lower rpms, too - such as during sudden or erratic revving...
MM I'm thinking it pulls hard right till falls flat on its face??? but what do I know, I just chimed in, so I don't lose the thread and can follow along with the wisdom of the Gurus... hopin Osmosis come thru!
Haha, @MB, same here on the osmosis. So yeah I'm wondering that too about the falling flat. And keeping in mind @Ole's scenario here wouldn't be a case of everyday 'float' because of springs being too boingy, but also just plain shorter, so a certain type of boinginess and potential float fail (in my expert opinion). And here, yeah, I guess you'd see it fall flat into something of an rpm rut?

Let's not forget though, @Wes Bros seems to say he can baby through it. So a lot goin on here! But yeah don't change too much at one time, like @Minibikin said. And for sure check those springs first.
Let me clarify, there were special hemi specific 26 lb springs with a shorter install height (I think Hemi stock is .815) and the standard white stripe spring which is a .850 install height. Now he mentioned a big valve head without providing anything about it. I have to make assumptions here, If he got SI stainless valves they come either .050 or .150 longer depending on size, some of the smaller valves you can get stock length. So Lets say he had hemi specific 26lb springs made for a .815 install height to give approx 26lbs of seat pressure and the new head has a .050 or .150 longer valve then the install height will be either .900 (about 20lbs of seat pressure) or 1 inch which is not even in the valve spring chart. Valve float will generally appear at a specific and repeatable RPM if the acceleration of the valve overcomes the spring pressure. It will make a funny noise, engine will get quieter, and not gain any more RPM. There is another kind of valve float caused by valve spring harmonics that can appear then go away and then the engine keeps accelerating again. He needs to measure the installed heightof the spring and that will clear things up.