Ok so here's the specs. I have a 94 k1500 pickup. Put a new longblock from jegs about 10,000 miles ago. Stock replacement with 193 heads and flat tappet cam. I replaced cam and lifters with sum k1102 but not springs. I have a custom tune. Intake, exhaust, plus new distributor icm and coil.
So the problem. I think I am experiencing valve float. Truck runs fine until Around 4,000 rpms. Then feels flat and sounds like crap too.
I have read to get "z28" in several places. But I am unsure of exactly which ones to get. And a little confused why they are just outers. I seem to remember having dual springs stock. Thanks in advance.

 

If you are going to change springs, I go with the ones that Summit recommends for that cam.

https://www.summitracing.com/int/parts/sum-174001

 

Thanks

 

WHAT LONGBLOCK??? New GM? Rebuilt GM? Rebuilt aftermarket? P/N, link, and specs, please.

I'd also be interested in what you call a "custom tune".

The springs that come installed on the swirl-port heads on the crate engine I bought are the SAME ones used on the pre-Vortec version of the HO 350 (300 hp) crate engine--spring P/N 3911068. GM says they're good for .550 lift. The crate engine I bought is now discontinued--it was the 12513151 '91--'93 Caprice LO5 with roller cam and 9.6:1 compression. Far as I know, they're a standard nothing-special spring installed on most if not all swirl-port heads.

They're a fine budget-performance spring that will go WAY over 4K rpm.

Something else is wrong. Remember that the 193 heads are approximately the worst ever produced for midrange and higher RPM power/torque.

What is your fuel pressure?
What is your cranking compression? Is it realistic for your altitude?
Does the throttle open fully when the pedal is pushed? HOW do you know?
Did you ever test for back pressure/plugged catalyst?
What does the data stream tell you at 4000 rpm and heavy throttle, when you connect your scan tool? O2 voltage? Coolant temperature? Short- and long-term fuel trim? Commanded spark advance? Knock sensor activity? Map sensor reading? TPS reading?

 

12568758 is the part number I think. It was new gm. By custom tune I mean at brian tbichips tune. My fuel pressure was 11 psi ?. do not own scan tool. Throttle does open fully looking into tbi. BTW truck is 1994 k1500

 

That engine should be 210 hp at 4000 rpm. Therefore, the valve springs are designed to support AT LEAST rated horsepower, and beyond.

Fuel pressure is 11 psi AT 4000 RPM? Or 11 psi at idle?

Still waiting on cranking compression, back pressure testing, and of course you'll want to come up with scan tool data.

 

At idle. I will have to get a fuel pressure guage I can route in cab to find out psi at 4,000 rpm. I have never compression tested the engine. I don't see back pressure being a problem. I have full length headers 2 1/2 inch duals with a crossover and they dump right behind cab. No cats with glass packs. What scan tool do you recommend? I'll have to get one. Is that something that can be used in future for tuning software???