carb question

[will scratch]

Demon 650 vac sec. if I adjust the idel screws (4 corner) in all the way then the motor seems to idel best. Holley video on you tube says the engine should die with idel screws in all the way or the power valve is bad. Do I need to replace the power valve? The engine seems to have no get up and go. I know Holley and Demon are different carbs. Some thing is wrong with this carb for it to be doing this isn't there?

[Marv D]

Don't know about the demon, but if you overjet a holley, and have a lot of signal (high idle or real mild cam) idle air will not want to adjust right. The Proforms are notorious about idle air bleeds being too big or too small, whatevere it would be to make the idle air screws not have a big effect. We have some really knowledgable carb guys on here. I'm sure they can help set ou straight. If they don't drop in here, add a post to the engine section.

[nxtruck]

Can you give us more info on the combo? Engine size, cam specs, converter stall speed, ignition timing, etc. There's alot of variables that affect how it will idle. In general, you are looking for the highest vacuum reading while adjusting the mixture screws. Baseline settings with a Holley are about 1 1/2 turns out from seated. BTW, don't seat the screws hard when turning them in. Just lightly seat them and then turn them back out about 1 1/2 turns and start from there. I would think that this would be close for a Demon carb as well.

[will scratch]

I messed with the float levels and got the thing running better. also changed two plugs that looked wet. the idel mixture screw on drivers side front will now kill the engine when turned all the way in, the other three are not producing the same response and can still be turned in all the way.
355ci flat tops
double hump heads 2o2 160
30/30 duntov cam; 480 lift .030 lash
2200 stall speed approx
rpm performer intake
hei
stock ex manifolds w/flowmaster type mufflers.
After today I still think this carb needs work.

[PGSigns]

Couple of things to look at. If the throttle blades are open to far you will loose the signal on the idle are screws. Set all for of them at a turn and a half. Then close the throttle blades all the way with idle screw and open it about a full turn and see if the engine will idle. You may also want to pull the carb and turn it over and look at the throttle blades to make sure when they are fully closed all for are centered correctly in the bores.
Jimmy

[nxtruck]

Is this a new carb? Could possibly need to be gone through. You said that you checked the floats. I should have stated that in my first response. That's one of the first things to check and it's good that you have. Do you have them adjusted to where the fuel level is right at the bottom of each sight hole in the bowls with the engine idling? You should be able to see a little fuel just trickling out of each hole while running.
As was stated by PGSigns, the next step would be to verify that the throttle blades are where they need to be in relation to the transition slots near the bottom of the throttle bores. With the carb off, turn it over and you should see 4 slots, one in each bore on the side nearest to the metering block. You'll want approx. .020 of that slot showing above the blade (with the carb upside-down). You can adjust the rear blades, also, to help get the idle speed where it needs to be and be close to where they need to be in relation to the slots.
Try these things and see what happens.

[prostreetC-10]

4 corner idle carbs can be tricky! In essence, each corner has it's own idle well, idle bleed up top on the main body, idle discharge hole under the throttle blade, an air/fuel needle controlling the mix flow to the discharge hole and then a trans slot which also provides fuel at idle. A 4 corner idle carb is really 2 primary venturis mated back to back. It is very easy to get a "shift" in the adjustment so instead of.....let's say for easy, each corner providing 25% of the mix you have a 15%, 35%, 30% and 20%. This is why one needle can have a greater or no affect on the idle.

Please look at both pages from C&S carbs here http://www.candsspecialties.com/tuning.html to get a better understanding. One thing that is nice about a Demon is that there is a secondary shaft position cage that is adjustable from the top. First make sure this is a carb that shouldn't be gone through FIRST. Then, as mentioned above, lightly seat all the needles and come out 1.5 turns. Next square up the trans slot on the front so the slot is as long as it is wide which will get you about .020 in length. I would just cover the rear slots as a starting point. Start it up and see where your idle speed is. If it needs to come up, you adjust the rears and not the front. Next you make adjustments on each needle in 1/8th to no more than 1/4 turn on EACH ONE. What you do to one you must do the same to each needle so they are always the same in or out. You do not independently turn each needle to "see what it does". This is how you get out of balance so each needle as a different effect on the motor idle quality.

The above is only applied if your timing/ignition is dead on. You never make carb adjustments if you need to dial in your timing. The timing must be done first so the carb tuning will be accurate. Hope this helps. I go through this just about everyday on 4 corner carbs.

[will scratch]

Thanks for the input.

I've felt it pull hard once enough to make me smile just after adjusting the float level in the front, before that it felt like a two barrel. Took the carb off and adjusted the butterflys, they were way off, turned the idles out 1.5 and reinstalled it. Started it but havent driven it yet.
It probably needs a kit it but I thought it might be ok.
Bought it used, let it set about a year and caught it on fire once.
Maybe I can drive it to the parts store Saturday.

[68 C-10 KID]

One of the biggest things I noticed on the Demons is that they have to run the fuel level much higher than a Holley. I had to run the fuel level half way into the sight glass.

Two of the main things that helps out when your are adjusting the A/F screws is a vacuum gauge and a timing light with a tach. It will tell you instantly if you are going the wrong direction w/ the A/F and the rpm window will allow you to continuously adjust your idle speed to were you like it.
The goal is to get the most vacuum out of the motor as possible. This is done with correct ignition timing and properly adjusted A/F screws.
Start taking them in 1/8-1/4 turn from your 1.5 turns out (going to be rich), and keep your eye on the vac gauge. If you start gaining vacuum your going the right direction.
If your front screws are out farther than your rears that's not to uncommon.
I would guess the fronts will end up being 3/4-1 turn out and the rears will be 5/8-3/4 turn out.
Once your all set with the most vacuum and idle quality you need do drop it in gear to get a vacuum reading and size the power valve's to 1.5-2 sizes bellow that reading.

After all that even when its cold the thing should fire right off after one pump of the gas when cold and a few seconds of throttle control and it should idle on its own.

[will scratch]

Thanks to everyone for your feedback. I had problems trying to order a rebuild kit for the out of business Demon and borrowed a 750 hp vs carb off my friends roundy round car. It seemed to work better but still not what you would of hoped. Finally took the Demon apart the other day and found a lot of metal shavings inside the carb, some of the holes were obstructed, took the metering plates to local speed shop and got some gaskets. I cleaned the thing up, put it back to gether and put it back on the blazer. Set the butterflys where they should be before installing. The A/F screws have never been happy past 1/2 to 3/4 out so thats where I left them. Took it for a drive and she was singing a new tune. Down low the thing is still a pig, but when easing up the on ramp in low gear, I could soon hear the growl of the back side beginning to open, whats that I thought lol, gave her the gas and we were off, say hello to 6500 rpm. Wow maybe there is some fun to be had with this lil ol thing. Now I'll go back to your post 68c-10 kid and try to make the vac gauge adj and select a correct power valve as needed. One other thing as you mentioned I was also told locally that the A/F adj. on these carbs are designed for low vacuum apps and if your making decent manifold vacuum at idle the screws can go almost all the way in. Thanks again!! P.S. I also had half 91 and half purple power in the tank which probably helped, anyways I'm still at it.