scottyp99

Hi, all!
Not really Corvette specific, but this carb is going on my 'vette, so here I am. I bought a used Holley 4011 carb, LIST-r84021, spread bore, vacuum secondaries. Started taking it apart, and, lo and behold, there is a second power valve for the secondaries! I have only ever seen two power valves on double pumper carbs, and two accelerator pumps to go with them. This carb is a vacuum secondary carb, no secondary accelerator pump, just an extra power valve. What is the deal with that?

Also, on first examination, there doesn't seem to be a transition slot in the venturis. Any tips on where to bench set the idle?

OK, thanks,


Keep the shiny side up!
Scott

MelWff

Thats really not unusual. I have a 4150 series 3310-1 which has a secondary metering block with power valve and is vacuum secondary. The factory Holleys, like on the 70 Lt-1 had the same set up. Make sure the secondary throttle stop screw is set 1/2 turn more after contacting the tang on the throttle linkage. I would set the primary throttle screw 2 turns after touching the tang on the throttle linkage. Hopefully your carb is similar enough that what I'm saying makes sense.

scottyp99

Huh!! I haven't torn apart all that many carbs, and this was a new one on me. I guess ya learn something new everyday! I guess it isn't really much different than the enrichment system on the Q-jet secondaries, just uses a power valve instead of metering rods. So, now I can not only adjust secondary opening rate with the diaphragm spring, I can also play with when the enrichment comes in on the secondaries. Great, another tuning variable!!! LOL!! Oh, well, I guess I shouldn't complain about having options, right? Can anybody point me in the direction of how to tune for the secondary power valve?


keep the shiny side up!
Scott

MelWff

Same as primary, about 2 points below your vacuum reading at idle to make sure it doesnt drop open. It should have a 6.5 in it now. Unless you have a radical cam you probably can go up to a 8.5 both front and back. I think the only real way of calibrating it is on a dyno and watching the air/fuel ratio.

scottyp99

Yep, 6.5 front and back. Engine is 1980 L48, block hugger headers, 2 1/2" dual exhaust, Performer intake, stock cam and heads, pretty mild would be an understatement. I have plans to build a ~325 350 horse engine in the future, but it will have as mild of a cam as I can get away with, maybe a Comp Cams XE256H, but that is a story for another day.......

I am glad to hear that secondary power valve tuning is relatively simple, The only info I could find on the internet had to do with double pumpers, and it was pretty much "If you are racing, get rid of it, and jet up the secondaries, but if it's a street car, keep it." What I like to do, is to get the carb in a relatively good state of tune, and then drive it around, watching the vacuum gauge. I take note of the vacuum at which the engine feels like it is starting to have a bit of a load imposed upon it, and that is where I need the enrichment. So far, the 6.5 has always been close enough, but could maybe be kicked up a couple notches, like you said. Thanks for the info, bro, I appreciate it.


Keep the shiny side up!
Scott