tuning vacuum secondaries

[Jonstantine]

ok, well i bought a used 625 road demon off of ebay...
probably not the best idea, but i saved $75 or so

well, i got the idle, and butterfly positions pretty close i think
timing is set ok and the gas is about as low as you can go on the siteglass

ive tried the floats in the middle and and low parts of the sightglass - no difference really
the idle settings are pretty good - probably not perfect, but not bad either

so heres the problem - when i floor it, from about 2400 to 3500 it sputters a bit and just runs better (real fast) if I only press the pedal down 4/5ths untill about 4200 rpm
i called demon and they just told me to set all the basic stuff up, (and that the carb was too big) and i said i had pretty much done it already but i said i would work on it more
well im think the basic idle stuff, is about as good as im going to get it - for now

i think tuning the secondaries is the key - ill call demon back and get some recomendations later today
but, i want the real pro's opinions and know how

so how do i change when the secondaries open? i assume it has to do with the distributor, but i really dont know
ALL suggestions are welcome
thanks

 

[tweet66]

I just went though the same thing on my coupe. Same carb and all. To adjust the secondaries opening timing you need to change the spring that operates the vacuum diaphram. Look on the left, rear side of the carb. There's a "canister" hanging off the side that contains the diaphram and spring, on the Road Demons you'll need to remove the unit from the carb, on some others it will have an access cover and can be left on. Holley sells a kit with various colored springs with different rates that allow the secondaries to open sooner or later. I don't remember exactly the colors but the yellows are the lightest, plain (silver?) is standard and black is the heaviest. I only had to go down one lighter spring, but I don't know your motor combo and you'll have to test to see which produces the best results.

 

[ashford]

same problem here too changing the spring didn't get rid of the problem for me, it just gave me more low end torque. and no its not because its overcarbed. i let a friend borrow it to see if he liked it,and maybee buy one. he had the same problem same rpm range on a much more wildly built engine than mine

 

[ratio411]

Are you guys tuning the pump shooter?
You need to get the right size shooter in there and tune the entire pump linkage.
You really need to study the book to find the details.
I know how, but would make a long post to explain it.
I will touch some high points:
The pump arm should be adjusted against the pump diaphram, but not actually putting pressure. The pump linkage all the way back to the pump cam must be touching at each contact point, but again, no pressure when at idle.
I will throw out that I have found that a 330 cam with the screw in position 1 on the cam, and screw through hole 1 on the throttle linkage, gives the best action.
When the linkage is right, every contact point should be touching without pressure, then when you move the throttle arm, gas should instantly be put out of the shooter (squirter). Even with the slightest movement, fuel should flow accordingly. If there is any movement without fuel coming from the squirter, you still have slack in the linkage.
The shooter size depends heavily on your combo.
A small shooter, say 28, is for cars that don't rev fast or have highway gears.
A large shooter, say 45, is for light cars with steep gears that will transfer from idle circuit to main circuit in very short order.
It's all about duration of pump shot.
Your car will fall somewhere in the middle.
If you sputter when stabbing the gas, you are running out of fuel to make the circuit transfer somewhere in the pump system.
If you get a cloud of black smoke when you punch it, you are getting too much fuel, too fast from the pump system.
You need to shoot for instant response without the black smoke.
Get the super tuning manual for a better explanation.
Good luck
Dave

 

[Jonstantine]

ive played with the pump linkage, but i thought you tune the pump for best 60' time?

im going to the auto stores around 6pm and ill try to get the spring kit - ill let you guys know the results (especially if it doesnt work)

 

[one2gamble]

Ya it took me forever to get my road demon just to run. I think I need to go with a smaller squirter because I have the needle settings pretty lean though this is the first carb I have had to tune myself. I think I am going to try the different spring sizes for the secondaries just for fun. Anyway good luck.

 

[ratio411]

I just have to say:
I can't believe the guy told you the carb was too big! What a dope.
You are running only about 300 cfm in reality. The spring controls the rest. If you use the black spring in the secondarys, they won't start to open till some crazy high rpm, and then the never fully open.
Running the black spring will make your total cfm less than the stock 2v carb. How can he say that is too much?
I think you have a squirter issue in combo with a light spring. At the rpm you are talking about, the secondarys shouldn't even be in play. You describe more of a throttle response issue, which comes down to squirter size.
If the spring is in play that early, you surely need a spring change.
Also, watch the fuel bowl level.
I know you said you adjusted it very low on the sight...
If it is too low, it could stumble on a stab at the rpm you describe.
Dave

 

[Jonstantine]

im sorry, i realized i left out a critical detail

it doesnt mess up when i first floor it; it does ok. generally, i start at 2000 rpms
even when i start at 3000 rpms, it doesnt sputter till 3500 and gets going good at 4k

i tried a lighter spring tonight, i dont think it worked but i really cant tell because i need to eliminate some variables (and fill up the gas tank)!

ill know more in the morning

thanks again for your replies

 

[ashford]

ya id dont act like accel pump problem for me, its really consistant at 3000rpm to 4000 at wot, dosent matter what gear your in, and dosent doit right off the bat on punching it just more noticeable in lower gears. almost as if the secondary fuel bowl goes almost dry before the needle opens, but ive played around with the float level no difference. id like to know what this is also, im frusterated

 

[Great68]

I had the same problem, but fixed it with adjusting idle mixtures and float levels. I haven't had to change anything major one the carb since I bought it, so I guess i'm lucky.

 

[Jonstantine]

tweet, can you tell me your setup?
and great68, what do you have your float levels and idle screws set at?

well, all the info i previously had in this post is kinda out the window because of a bad gasket on the float adjustment
floats are 3/4 down on site glass right after last test (i thought i had set them in the middle, but after i turned off the car, thats where they were)
i have holley's plain spring in
(2k-4k test) the bog is not nearly as bad as it was (after leak fixed), mainly coming in two spots: 2.4k and 3.5k (car accelerates better at 3/4 throttle - fastest times ever)
(0-60 test) i cant really feel a bog but the car is a second slower than with edle carb

i had noticed with different springs, the car ran better with different amount of slack in the linkage - i think with the plain spring it wants a little bit of slack for best performance. i havent fully tested this theory but with no slack at all, off idle acceleration is just ok

last question - can i buy a holley tuning book because i think a demon book will be hard to comeby at the local store

i think tomorrows test will again be the brown spring
(ive tried it before along with the tall yellow spring, but i think the leak interfered with the test results)

 

[Great68]

tweet, can you tell me your setup?
and great68, what do you have your float levels and idle screws set at?

Floats are set exactly 1/2 way between the sight glass, primaries and secondaries. I set the idle screws using a vacuum guage for max vacuum at idle, so I wasn't keeping track of how many turns, but probably in the neighborhood of only 3/4 of a turn per screw.

 

[tweet66]

Jonstantine, I'm running a 0.030 over 1974 302, forged pistons, Lunati cam (.488 lift with 1.7 Crower rocker= .518). Twisted Wedge heads, 1 5/8" Flowtech headers, 3" pipe form the collector to the x-pipe and 2.5" out to the Flowmasters, Edlebrock Performer RPM mainfold, and the Demon 650 carb (vac. secondaries), a C4 with shift kit and 3.55 gears on the 8" rear. Runs good since I've played with the carb. But even now I still have some hesitation on full stabs of the throttle that last about 1-1.5 seconds then is gone. Just haven't had the time to play with it lately.
Ken

 

[Jonstantine]

well, i dont believe the spring has anything to do with my problem...

im going to put the purple spring in I think just because i probably dont need the full flow of the stock yellow spring (although since mine was used, im not sure the yellow spring is the same as the spring that came in the carb)

my new plan is to either change the squirter size or the pump size
i guess it would be possible to change the cam too... so i need suggestions

ratio knows his stuff but i didnt want to try this before because i dont know anything about it
so what should i do? get a larger pump, smaller squirter or different cam? what is the easiest to change and which would be the most effective?
basically, i think the pump is shooting its load too fast. With any suggestions please describe how i would do it.
Thanks,

*although i like the idea of going to a smaller squirter (thinking about it, this seems the best way to keep it from flowing too much gas too soon)
current setup:
squirter - 31
pump - 30cc
cam - 330 pink

** im going to put the screws in position 1 if they're not already there
id also like to state that i have highway gears and that on the highway at 2500 rpms if you floor it, it just bogs and doesnt accelerate at all
but if you put it 4/5ths... it goes good

 

[ashford]

well i wasted my whole saturday expirementing with my demon, so heres the low down.

problem- hesitation at wot 2500-3500 rpm, same condition all gears. its rpm dependant not accel pump. carb tested on 3 different engines ranging from 200-400hp roughly, little difference between them, with little or no help to fuel level or idle screw adjustments.

stock settings of carb- primary jets 70, secondary jets 78, stock secondary spring, fuel levels in center of glass

step one- isolated the secondaries, rendered them inoperable. result= ran fine like a small 2 bbl. conclusion- problem in the secondary.

step two-switched to black secondary spring, result= changed the rpm range of the problem up about 1200 rpm. conclusion- metering problem in secondaries.

step three- dissasemble, inspect, clean and change jets to 82 in secondary. result= slight improvement. conclusion- what the hell

step four- for the hell of it bumped the primaries to 72 jets. result=no change. conclusion- poor manufacturing/design in secondary venturis and or metering block.

step five- remove demon and reinstall old holley. result= slight loss in low and midrange power. conclusion- 625 road demon is a waste of $300.

proposed step six- call barry grant and see if there is a recall and or hotfix, if not the demon will sell to the first buyer of $100 or best offer.

 

[Jonstantine]

well, before you sell it; let me try a different squirter
i dont understand how you eliminated the accel pump as being the problem, but i also am not a carb expert by any means (just stupid enough to mess with em) *well, i think i do understand - but im going to try and hope anyway

and if barry grant tells you anything, please let me know
i dropped a weeks worth of pay for this thing thinking it would be flat out better than my edelbrock when the only problem with my edelbrock was the low end performance that would have been neutralized when i put a standard tranny in the car

lastly, im going to contact 4secondsflat.com

 

[Jonstantine]

another idea... could the vacuum advance not be kicking in fast enough for the demon? 4secondsflat.com mentions something about the demons being radically different in that department than your standard holley or ebrock. hopefully, i will get to talk to them on monday; and they might have some ideas.

my dad had my distributor replaced like 5 years ago, and i can almost guarantee he bought the boniest stock piece you could find...

*i know my ideas probably sound crazy, but i just want to try everything before i give up

 

[ratio411]

I know the answer to this question, but I will ask anyway, just incase...
Do you have your advance line hooked to the metering block or elsewhere?

31 squirter sounds large for hwy gears, IMO... 28s are decent for duration. Makes your pump shot last longer to cover up holes later in the rpm range.
Another:
I don't know about Holley 'clone' carbs, but Holleys have sintered bronze filters behind each fuel inlet that slows fuel flow. I know they can cause flow problems later in the rpm range, even if you have your float set at idle. What you have for fuel in the bowls at idle may not be the case under load... things like obstuctions in the fuel lines, dirty filters, inlet filters, substandard fuel pumps, etc can affect upper rpm responsiveness.
If you have good response at 80% throttle (4/5ths), then you just need to get fuel flowing for that last little bit.
You are running out somewhere IMO... your secondarys are opening and you have nothing to cover the momentary 'hole'. The time it takes for your main circuits to compensate for the new air introduced must be 'covered'.
The fact that you still have the hole when your secondarys open, no matter what spring you have, says something's not keeping up.
Spring changes just move the 'hole', not cover it.
Try a 28 squirter so you have some pump shot left later.
This may not be the issue, but it's an easy mod.
Good luck
Dave

BTW: Don't give up yet. You are learning much about your carb and will be very satisfied with your experience if you work it out.

 

[ashford]

i "freshened up" on carb theory again(if i learn one thing i forget another) sounds almost as if they used too big of a high speed bleeder or too weak of a booster, ill be calling b/g to see if the bleeds are changeable or even available

 

[ratio411]

Could very well be a defect in the carb.
The tech guy saying the carb was too big doesn't give confidence in there knowledge to help you though.
See if you can exchange it...
Dave