carb pro's please advise....race demon 750

[kmarccoco]

well, i checked my manifold vacuum today. it bounced from 4" to about 12" of hg. the power valves were rated 4.5". with that in mind i believed that the power valve was staying open all the time causing the idle and whole circuit to be rich(open power valves = about 8 jet sizes). mabey thats why the smaller #73 jets seemed to make the car run really good(smaller jets and open power valve equaled itself out.). i also was turning the idle mixture screws almost all in.

so today, i put 2.5 power valves in front and rear and left the #73's in. now it has a hard time staying running with butterflys set properly at about .020. and when driving, it stumbles on med-hard acceleraton and has lost some power. also, the idle mixture screws don't do a whole bunch. i understand that the power valves = about 8 jet sizes when activated, so should i jet up 8 sizes to #81's????

[Clois Harlan]

I think I am running 83 and 78 jet sizes , A lot of trial and error getting everything to run properly.

Clois

[Mike Simard]

Is your vacuum really bouncing around that much at idle? What size cam and engine do you have? It might be helpful to tape the vacuum gage to the windshield and see what the vacuum is at light cruise, that way you can get an idea of when you want the power circuit richened and match a power valve to that. Remember that there could always be other spark, valve or common engine gremlins present that may need to be dealt with before you start serious carb tuning. First work on getting the idle circuit set before worrying about jet sizes. The idle circuit handles most of the street driving and can make a car seem pleasant to drive or not. The main jets are relatively easy. A typical Cobra is probably overcarbed and needs tuning beyond adjusting the idle screws but can be made to work nicely with patience. There are restrictor 'jets' for the idle circuit on the metering block and air bleeds on top of the carb. The air bleeds for the idle circuit can be opened up to lean out the idle circuit and it's easy to change them. I've bought bags of undrilled Holley restrictors (or whatever they're called) and drilled them in .002-.005 increments to tune the air bleeds. As far as your main circuit goes, it sounds like your 2.5 power valve and main jets aren't letting enough fuel in. 2.5 is a pretty low number. What do you mean "hard time staying running"?

[Jerry Clayton]

You have been thru a lot---I have worked on several Demon carbs and they are basicly a upgraded airflow Holley but with very poor manufacturing processes causeing problems---

If the Demon issues have been resolved--maybe you have some bent intake valves or bad gaskets---you haven't said anything about trouble shooting the symtons

I use 5 jet sizes as a base line for power valve flow---why don't you just plug the power valve to take it out of the circuit????

Check some of my posts on the Demon carbs for some of the other guys recently

Just put a Holley 780 on the car and see how its supposed to run, then tune your Demon to similar specs---

Do you by any chance have all the plug wires on the right plugs????this was common after the FE engines came out--first tuneup and then the car ran like sh---

Jerry

[Dennis Kelley]

Switch to a Holley. I had two demons and neither of them would run properly. Went with a Holley and could not be happier. Demons have very poor quality control. .

[StanJ]

Originally Posted by kmarccoco

i understand that the power valves = about 8 jet sizes when activated

This is a fairly popular misconception. Holley/Demon style power valves are not metering devices at all; they are basically "on/off" devices that are referenced to engine load by way of intake manifold vacuum. The actual metering of fuel through the power valve enrichment circuit is controlled by the size of the passages – generally called “power valve channel restrictions” -- behind the power valve which lead into the main wells (vertical passages that connect the main jets to the main circuit discharge nozzles). To determine the correct increase or decrease in jet size necessary to compensate for use/removal of a power valve in your carburetor, a bit of simple math is necessary…along with a jet orifice size chart if you are using O.E.M. Holley jets or equivalent. First, measure the size of the power valve channel restriction (PVCR) passages in your carburetor metering block(s). For the sake of this example, I’ll use a stock Holley #4781 850cfm 4brl. The power valve channel restriction size in the primary-side metering block of this carburetor is .070”. Next, check the size of the main jet currently in use and determine it’s orifice diameter (in this case, a #80 jet, which has an orifice diameter of .089”). Now, compute the flow area for each of these two passages using radius X radius X 3.1416

(figures are rounded slightly for simplicity)

For the power valve channel restrictions:
.070” diameter / 2 = .035” radius
.035 X .035 = 0.0012
0.0012 X 3.1416 = 0.0038 sq in

For the main jet:
.089” / 2 = .0445” radius
.0445 X .0445 = 0.0020
0.0020 X 3.1416 = 0.0063 sq in

Now, considering that at full load conditions the power valve is open and the venturi has both the main jet and the PVCR available from which to draw fuel, it stands to reason that if we are going to replace the power valves with plugs and thereby eliminate the function of the PVCRs, then we must increase the flow area of the main jet by a like amount if we want to maintain the same WOT air/fuel ratio we had with the power valve in place. So, adding the area of both together:

0.0038 sq in
+ 0.0063 sq in
0.0101 sq in

…which is the area that we now need to convert to our new jet size. To do this, simply work backwards through the formula: Area (0.0101) divided by pi (3.1416) = radius squared (0.0032). Finding the square root of 0.0032 gives us a radius value of 0.0057”, which we simply double to find the diameter (0.1134) of the jet we need. Holley jet size numbers ARE NOT directly tied to their orifice diameter, so be sure to check the chart. Here we find that a Holley #93 jet has an orifice diameter of 0.1125”, and a #94 jet has an orifice diameter of 0.1155”, so you’re going to be right in that range; a 13 – 14 size number increase. Had we used a #4776 600cfm 4brl (using .043” PVCR diameter) or a #9375 1050cfm Dominator 4brl (using a .112” PVCR) the actual change would have been a lot smaller/greater respectively.

Note that you aren’t “locked in” to using a particular size PVCR; this is one of the areas that we carb tuners use to tailor a carburetor’s fuel delivery characteristics, particularly part throttle. Not saying that you can’t screw up here…you can create big-time problems if you get the available flow area too far away from what the venturi and booster actually want…just that this is one more area you can tune a carburetor for a specific engine application.

[decooney]

Ken,
Worst case, we'll pop my Holley 830 HP on there. I have it set a hair fat right now and this 107-109 degree heat is not helping much right now. If you try the jet and PV sizes I was recommending, it should be really close. If not, there is either a carb problem or something else still not right for whatever reason. Let me know, I'll be around this weekend.

[kmarccoco]

thanks duane..i'll give you a call in a bit....:>

stan, using your calculations, mine would call for a #87 or #88....orifice of .1035

[StanJ]

Give me some data about your engine combination, and I'll try to give you some workable baseline calibrations that will get you in the ballpark (assuming no fundamental problems with the carb itself). You should be able to fine-tune from there.

[kmarccoco]

thanks stan. completely new 428 bb, msd, edlebrock performer dual plane, roller cam/lifters. ported heads, matched intake manifold, 10.5-1 compression electric fuel pump. exaust mufflers have 3" inner diameter.

cam @ .050 duration is 248
valve overlap 28
timing 17 initial

thanks for everyones help... ken

[Jerry Clayton]

Ken

Does your carb have a 1/4 or a 5/16 aircleaner stud? The ones with a 5/16 stud have a idle speed adjustment valve in the center of the base plate accessable thru the stud hole---If you have a 1/4 stud you won't have the adjuster valve but can either send the base plate to Demon for mods or can machine a couple of grooves in an X pattern on the top side of the base plate where it mates with the main body and drill a 1/8 hole thru the center to allow extra air for idle. All od the Demon carbs that I've worked on have been full of machining chips in about every nook and cranny---I don't think BG has a aircompressor so they can blow the chips out before assembly--

Jerry

[Zoom This]

Don at 4secondsflat.com is a long time Barry Grant racer and is extremely knowledgable in setting up BG's to work correctly. I agree, BG has a lesser quality control for junk left inside new carbs. Once you're past that, the carb is really a great performer....BUT it needs to be set up right starting with your timing specs. Get in touch with Don and he'll set you straight. His manual on setting up BG carbs (as well as others) is worth its weight in gold, really.