Springs are for higher rpm stability tho, I have everything balanced to 10k. I'd like to be able to rev it to factory redline without worrying about valve spring issues.
I am 100% positive what was considered valve bounce by the dual valve spring advocator was actually turbo and boost control related not valve float related. If the intake valves where getting held opened by boost pressure near redline it would make the same tapping noise or worse than you are experiencing now.
I wouldn't really call 6250 rpm higher rpm.
If you are planning anything close to 10 000rpm remember these engines have a longer stroke than bore and unlike 90% of 2l 16v 4 cylinder engines it makes low end power and torque or it breathes well at low rpm, short intake manifolds and short stroke blocks breath better at higher rpm.
Your options for a turbo that will breathe to that high will spool around 6500rpm or would require variable trim housings on the compressor and impeller sides.
The F2T gearbox has pretty tall gears and even then, try shifting 1st, 2nd, 3rd and 4th at redline and note at what RPM you land, I have done the math but im not looking it up right now, it's above 4000rpm so from gear to gear you get about 2200rpm to accelerate with and need to shift. It doesn't matter where in the rpm band that 2200 rpm are (gear ratios are fixed) as long as it pulls the hardest for those 2200rpm it will be it's fastest.
Tall gears take torque to accelerate not horsepower.
Remember with all vehicles the force measured at the wheels by a dino (torque) drops and the mathematical equation (horsepower) climbs.
What moves and accelerates a car force exerted at the wheels or mathematical equations.
Was running 10w30 then tried 15w40 and it was a bit better actually. Recently there was a big temperature drop, also snow now...so the car is parked for the winter lol.
If you only run it in the summer than run 20w50, heck I run it all year long in some of my F2T's.
Today I spent a few minutes with my car. Friggen cold weather and no time lately.
Really quick I brought all 12 valves to TDC to check lifter movement and preload.
Preload is basically perfect, couldn't get a 0.03" feeler gauge in, and the rocker arms have slight drag when moved side to side.
I found one bad lifter with the engine cold so next time I will go around the block, and check again. I think I will maybe mod a spare valve cover if the issue persists...may help diagnose this.
I would be curious to know what the bottom of the cam bearings look like with all that pressure pushing against them. If they start to wear down you will get valve tap all the time.
You have a 91GT so there is no cam lobe for a mechanical fuel pump in the rear head housing, you can confirm this by pulling the distributor and looking in the housing hole it sits in. From what I recall the cam can be pulled by removing the cam gear and housing, the rocker assemblies and loosening the rear cam caps upper housing nut, then slide the cam out toward the cam gear end of the head.
Just how do you plan to modify a spare valve cover?
You do know the rockers squirt oil all over the place, If you made a plexiglass cap and cut opened the valve cover it would be about 2seconds before you cant see anything through the oil, if you just cut the top off you won't be able to get close to the engine bay when idling.
I doubt a collapsed HLA could be diagnosed visually when each rocker moves 6.25 times per second at 750rpm.
Are any of the rockers hitting the valve cover, you should be able to feel it with your hand, a tap on the valve cover that feels strongest where the rocker hits it. Otherwise it's the rocker hitting the cam, it should feel the same anywhere you touch the valve cover.
If a rocker is hitting the PCV baffle in the valve cover you can source a 91 probe GT valve cover, they have more clearance between the baffle and rockers. Or you can take a spare valve cover and sand down the upper level of the aluminum valve covers baffle chambers, this will move the baffle plate away from the rockers. A small palm sander will do the job. A light layer of gasket maker on the valve cover lips will do, use locktite on the screws you don't want them falling into the rockers and cam.