Selecting power piston spring

[roch649]

I understand that I should use the strongest spring that still keeps the power piston seated at idle, but how do I know that it is? The spring I have in there right now is pretty strong. I have an assortment of others but I don't know if I need to change.

My engine
410 cu BBC
10.25:1
Comp Cams XE268H
.515/.520  224/230 110 deg sep
carb: 7040204
stock 290 cylinder heads and cast iron intake

I set the carb up using recipe 2. I made a HUGE difference. I had just bought the carb because it was the right one for my Camaro but was set up for stock engine. I found the idle tubes blocked so I drilled them out. Added bypass air. Got it to recipe 2 specs except I have .077 jets and .047 rods.

The car drives strong and I cannot put my finger on any particular issue but I am not sure I have the right spring in their....any advice from anyone?

[roch649]

To add....idles at 800 rpm pulling 11 inches of vacuum.

[Ethan1]

 Have you looked in Cliff's book? I believe it goes into great detail on selecting power piston springs.

[roch649]

I read the book carefully but I could determine was that the power piston needs to stay down during idle. How can I tell that it is?

[429bbf]

roch, i dont have that exact no.carb but you should be able to put a straw(coffee stir straw ) down through the front vent and touch the power valve . the instant you touch the throttle and loose a little vacumn the straw should rise. if it does not and everything is working properly you may want to install a stronger spring.hope this helps