Griff

Boost referencing shouldn't change anything at idle unless the vacuum is causing the PV's to open.
It sounds like something is off if the carbs are so sensitive to any adjustment of the 8 screws.

Have you tried adjusting the carbs in gear and tied to the dock??

BBYSTWY

I have adjusted them while in gear...I had someone drive while I was tuning just idling across the lake and I got it running perfect no surge 750-800 in gear A/F 12.5-12.8 and I thought awesome I got it...then I shifted to neutral and the surge was slightly there then went away thought cool another win...then I tried going back into gear and it died immediately....and I was back to square 1....I'm planning to pull the carbs tomorrow and check my blade location in relation to the transfer slots and hopefully find something there...This could be why my 8 idle screws are so sensitive? Possibly getting a "bad" vacuum signal due to the plates not being adjusted properly? just speculating but maybe?

I'm also going to pull the primary power valves and jet accordingly...I don't see a reason to keep them and have numerous people tell me to pull them...my secondaries have always been blocked put going to block the primaries as well

A/F's during the surge are hard to say...the gauge doesn't really react as fast as the surge or at least not at the same rythum at least but you def can see it change while its surging

SS496

Make sure your IFR's are in the low spots in the metering blocks as well.

BBYSTWY

Can you elaborate on that a little more?

MILD THUNDER

iTrader: (3)

A powervalve opening at idle, DOES NOT DUMP FUEL IN THE ENGINE. It is not a hose spigot that just dumps fuel in the engine. I dont understand why people keep blaming powervalves for their problems. The only time a powervalve will fuk your idle is if it is ruptured. End of story!

The power valve orifice simply allows fuel, from the fuel bowl, to enter the mainwell. From there, it flows from the boosters. It simply acts like larger jets were in the metering block when the valve opens.

Boost referencing on a marine supercharged setup is a waste. Sure the PV sees direct manifold vaccuum. Lets say you have a 4.5pv in there. What offshore boat is going to be up on plane, seeing 4.5" of vacuum in the intake? My fountain would be at 0" of vacuum at 3000rpm. If the pv was boost referenced, it simply would be open, the entire time the boat was up on plane. It would dump fuel at 4000rpm, because it would have already been open at 3000rpm, or whenever the pressure in the intake, was greater than 4.5" of vacuum. In a car, they work great. Because we cruise in vacuum in a car. In a boat, we dont cruise in vacuum, at least not a high enough vacuum to keep a pv closed shut. Mercury did not use boost referenced powervalves on their SC engines.

SS496

These are not my words below (from this thread https://board.moparts.org/ubbthreads...fr-tuning.html)...but explain it pretty well. How it impacts a blower engine vs. N/A...I dont know.....but I know my N/A (and blow thru turbo) stuff idles much more consistently (AFR stays much more steady) with the IFR in the lower position in the metering block.


If you look at any of the original Holley carbs they all had a submerged idle feed, either a press in restriction at the bottom of the block, a pressed in restriction in the idle well itself, or in the case of the early Dominators an idle feed tube that has the bottom of the tube crimped smaller with the restriction drilled in to the end.

The NACA paper #49 concluded that submerged fuel metering passages are free from instability and irregularity of discharge when the head is small. Head is referring to the signal used to draw the fuel in. This is the basis of why the lower position is best. And I can always make a car run better at idle and light throttle with a low idle feed.

So why do some think the high position is better? At WOT the emulsion characteristics are different between the two, how the circuits reverse to become a bleed into the mainwell is different. It may be they found on a particular combination it improved the fuel curve ON THAT combination. With the variety of emulsion positions available on todays billet blocks it's not necessary to sacrifice low speed and light throttle operation. And if you are not carb restricted booster selection is much better than it was in the past, improves atomization without resorting to emulsion changes that screw up the fuel curve. At WOT on an engine that has a limited RPM range you can do some crazy things that might improve HP a small amount, but on anything with a wide range of RPM and load it's a mistake.

MILD THUNDER

iTrader: (3)

https://www.corvetteforum.com/forums...gine-idle.html
Heres a test a guy did to prove an open powervalve has no effect on idle

BBYSTWY

Good read..thanks for the info!! Gives me more things to check!!

BBYSTWY

Read that article yesterday Mild....very interesting...I'm wondering if a ruptured valve could be my issue...I did put new PV's in when I set this all up when I had all the motor issues last season and due to the valve issues I was having I got some pretty healthy backfires through the carbs so I'm wondering if they popped in the process...After talking to a few people like I said earlier I think I'm going to pull them and block em off and jet accordingly...I know you've been an advocate for them in the past however if it's one less variable at the moment I think pulling them to get them out of the equation is my best course of action currently. Can't hurt to give it a try anyway.

BBYSTWY

Let me ask this..could initial timing(either high or low) present a surging issue?