Plugs reading is my answer for lean. Checked them after I ran her to cove at Elsinore, I did check my floats today and level was really high. Poring out sight holes. Plugs are new and will double check gap on plugs. And will follow wire from distributor and make sure there isn't anything funny going on. I had timing set at 10 and 34 advanced. gonna double check that again tomorrow. And some one said previous possibility of valve springs or valve not closing. Hoping not, but that would be my last resort. And I'm running 3/8 fuel line all the way from tanks to filter to pump to carb. I did order a 4.5 powervalve today and 32 squirters to try out. Trying to get her squared away before Parker float but if not I can run her the way she is to put around.
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If you are running the springs that came with the heads new... the eddy hydraulic spring is going to be sufficient for the cam you are running... they are roughly 115-120 on the seat and in the neighborhood of 290-300 at about .500 lift (off the top of my head) Its a generic spring that a BUNCH of cam companies and head manufactures use. So in other words... plenty of spring for the cam and intended rpm range. It is
possible you may have a valve not closing, or cam going away, or several other possibilities such as severe valve train harmonics or related issues... and I would for sure not rule any of this out... but it would typically be very obvious as the thing would just plain run like shit all the time... rough idle, noisy valve train, etc... something that would allude to this sort of issue... and other than popping at 4k I do not get the impression from you that is the case.
As for the power valves and squirters... I do appreciate the fact that your willing to try anything at this point... but these are fine tune issues... and you have a big tune problem... much akin to stressing out about a splinter when your leg is cut off. Squirter is going to help/hinder the transition from idle to main circuit when you whack the throttle... if you have a stumble or it falls on it's face when you hit the throttle, then this is the area you make adjustments by either going up or down or squirter and/or accelerator pump size. It does not effect at all, what so ever the rpm range or rich/lean tune of the engine at idle/part/full throttle. As for the power valve... check this out... it is a fine tune/part throttle cruise tuning related issue... you adjust your power valve AFTER you have gotten your WOT tune/jetting set, idle mixture, your timing, and anything else that may affect vacuum at part throttle. Then, and only then, does any adjustment to the power valve have any value... as it provides enrichment under part throttle conditions all the way up to where the main jet takes over. Btw... here is something that may have not been mentioned... even IF you did need a lower than 6.5 power valve (which is doubtful that your gonna need to venture far since that cam is still going to make decent vacuum at cruise rpm)... but lets say you do... and a 5.5, 4.5, 1.5... whatever, is what you actually need... the fact is... the 6.5 would cause it to run RICH not lean as it would be open more often and a lot sooner... a 6.5 takes more vacuum to hold closed (in lean position) than lets say a 4.5. The point is... if you are in fact lean... this is not why. Now a power valve that is too low in relation to the vacuum.. like needing a 10.5 and having a 6.5... will cause a lean condition when in cruise rpm... but the second you whack the throttle and go to WOT... unless you have a super small cam or crazy small carb... something that will create
a lot of vacuum even under WOT condition... even then the power valve will open and richen up the system. So... no offense... and truly not trying to be a jerk or anything... I just think you have a bigger problem to sort out before worrying about fine tuning... and if you focus on these things... you're not gonna get anywhere near your bigger problem.
So... personally, if it were me... I'd just keep running down your check list and verify each and every thing is right before tearing into it.
*Compression test? if you have a bad seal... or valve holding open... or cam going to shit... it will appear here.
*Vacuum check? Steady? Bouncing? Dropping? etc... (this actually can tell you quite a bit about the condition of the insides of your engine sealing and running state)
*Verify timing? Steady? Bouncing around? Of course (I hope) you set this w/o the vacuum advance as it doesn't work in a boat.
*Ever check those plug wires? Yes, they may ohm ok... but that doesn't mean they aren't acring somewhere, which WILL act as a rev limiter
*Fuel pressure AT CARB under WOT?
*Tach working properly? Besides not reading or not reading correctly... I have also see where they were shorted out inside and acted like a rev limiter
*Full interrupted 12v to the HEI at all times? having the voltage drop under power will cause ignition issues under load and for sure not allow the engine to rev.
*Plug gap? I'd say with an HEI and moderate compression... a gap of .035ish should be just fine and a reasonable place to start.
*Cap/rotor/module/coil all ok? I believe you said it was new... but have you physically checked all this? I've seen "new" stuff from places like Summit actually been returned items and had bad parts/broken inside... it sucked huge... but they always make it right, after you find the problem.
*As weird as it may seem for me to ask... are you sure your timing mark is correct to TDC? Harmonic balancers can spin... especially 400 external balance outer rings... just a thought. May want to verify this.
Etc...
You're gonna find it... and it's gonna be something that you're gonna go, "REALLY?!?!?"

If you were a little closer... I'd say bring it by and we can throw an extra set of eyes on it and see if there is something you've over looked.