SLO SS K2

Can someone explain to me at what point they open? Or, should I say what cause them to open. I'm trying to find out if I change my PV correctly if my eye burning exhaust will go away. I've put more jets in this carb that the US Navy put in Iraq. Every time it's fat. I'm not sure if my cam causes too low of manifold vacuum or what the deal is. I have a mystery cam in the engine for now. And it's a 750 Mighty Demon on a 383. Any he'p would be appreciated.

Oh yeah, the 4 corner mixture screws are only about 3/4 turn out. This produces the highest hg (vacuum) reading.

Thanks,
SPK

XcYZ

And what is your vacuum reading?

SLO SS K2

14 whatevers of manifold vacuum at the correct place in the carb base. Not the one for the vacuum advance. I am NOT a carburater man by any means, but I can and will take it apart and replace parts or do any modification needed, within reason.

grygst76

Put your car in gear while it's warmed up and then check the vacuum, if it's high like you say, you need a PV that's rated 1.5 inches below the vacuum, How is the carb jetted? what size jets?
Is it a mechanical or vacuum secondary? What are the heads? what size valves? what is your initial timing? what is the timing at 2500RPM's? I ask this only so I can give you a better guesstimate as to what you should do..I have been working with holly's and Demons for longer then I know!! In order to get the best possible settings I need to know your combo, Do you have headers? if so what size primaries and collector? is it 2 1/2" dual exhaust? all these will come into play..

SLO SS K2

Put your car in gear while it's warmed up and then check the vacuum, if it's high like you say, you need a PV that's rated 1.5 inches below the vacuum, I will check into this

How is the carb jetted? what size jets? 78/86

Is it a mechanical or vacuum secondary? Mech mighty Demon 750 double pumper
What are the heads? Dart Iron Eagle, 200 what size valves? 2.02/160
what is your initial timing? 8-10
what is the timing at 2500RPM's? Not 100% sure about this because of inability to read the tape, but according to my buddy it does advance fine. I have a 6AL setup
I ask this only so I can give you a better guesstimate as to what you should do..I have been working with holly's and Demons for longer then I know!! In order to get the best possible settings I need to know your combo,
Do you have headers? Hooker competition headers
if so what size primaries 1.5, and collector? 3”
is it 2 1/2" dual exhaust? 2.5 with NO crossover. I have a POS MIDASS exhaust. Have any suggestions for that? I don’t want it too bee real loud but I do want power.
all these will come into play..

grygst76

Ok,
1. you are jetted too high for a 4 corner idle mechanical carb, especially the secondaries. Secondaries should always be 4 sizes bigger then the primaries.
start at the beginning again with the demon factory jetting.
go back to the idle mixture screws and start with them all 1 and 1/2 screws out.
start at the drivers side primary screw and go out half a turn, go to the passenger primary and repeat, go to the passenger rear and repeat, then the driver rear, It's a pain in the rear to do 4 corner mixture!! make sure they are all set the same.
Once that is set to a good vacuum recheck your idle setting and adjust it.
Now put your car in drive and then check the vacuum. Tell me what that is....

what type of manifold do you have??

SLO SS K2

I'm sorry. The jets are 75/83. I put the stock ones back in the carb. I have gon down as far as 70/72 with no apparent chang in fatness

angel71rs

14 inches of vacuum should keep the PV closed at idle.

Power valves are spring loaded diaphrams. The spring keeps it in the open position. Inside the carb, the "valve" part feeds extra fuel when open. The diaphram is exposed to manifold vacuum on the other side of the valve. The vacuum works to move the diaphram (opposing spring pressure) and close the valve at idle and under cruising conditions for better fuel economy (they used to be called economizers).

When you floor the pedal, manifold vacuum drops to near zero, the diaphram no longer pulls the valve closed, spring pops it open, extra fuel is fed to the engine.

It is possible for PV's to blow if engine backfires, but this doesn't happen much anymore since manufacturers started installing protection valves in the passage that provides vacuum to the PV diaphram. If it is blown, diaphram no longer works, PV is open all the time, dumping fuel in engine. To test, pull out the PV and suck on the diaphram side to see if valve closes.

Most PV's are 6.5 on new carbs, so 14 is plenty to keep it closed, so it's probably something else making it rich. First check the fuel levels in the bowls. On Holleys you remove a sight plug on the passenger side of each float bowl. Fuel should just be at the bottom of the hole. If it is too high, adjust inlet needle at the top of the bowl to lower it. If a river of fuel runs out, there might be trash in the inlet needle seat. Just pull it out the top for cleaning. Not familiar if the same goes for Demons.

Another reason for probable richness is over expose of transfer slots. In the primary throats there are two small holes that feed the idle fuel. These are located under the front edge of throttle blades. There is also a transfer slot in each primary bore near the idle feed holes. These provide fuel as the blades open up, exposing the lenght of the tranfer slots.

A lot of times the instalation of a cam with long duration requires that the idle stop screw be cranked up to get the engine to idle. But this overexposes the transfer slots and they dump in extra fuel. Fix requires drilling a small hole in the front half of each primary blade to provide extra air for engine idle. This allows idle stop screw to be backed off so less of the transfer slots is exposed (.040" or so exposed is ok). I like to start with 1/16 holes.

I run 71/76 on my 750 Holley, but I'm at altitude. I think 71/76 is what Holley spec'd for 750 vacuum secondary carbs when equipped with secondary blocks.

SLO SS K2

Thanks,
I have an RPM air-gap. (that was the rest of the answer to the question grygst76 asked and I didn't answer. Do ya' really think It's safe to drill the plates? (assuming I pull the carb, I know to do that.)

Thanks for all and any more of your help.

Freak

this is exactly what I was thinking.

take the carb off the engine and take a look to see how much of the slot is showing (dont mess with the idle speed setting when you do this) if there is a lot uncovered Id say thats your problem. did you have to turn the idle speed screw way in to get the thing to idle?

and yes drilling the blades is safe. just start small and do it slow. and make sure you take the time to reset everything each time you go bigger (if it doesnt fix it right away)

SDAKOVIC

i drilled the blades on my 1981 and this helped my problem, but i only had 6 inchs of vacuum at idle in gear (alot of overlap, .600 inch lift) my intial timing was at 16 all in by 3000. so i would say go ahead and drill away and make sure your floats are set right.

BIGBADBOWTIE

I 3rd the motion for drilling the throttle plates. It will work.

grygst76

drilling the throttle plates is always last resort check your float level and some of the things I explained and then get back to me on vacuum readings..if you screw up on a demon carb, make sure you have some loot to replace those shafts