Demon idle mixture tuning question

Posted By: ZIPPY

Demon idle mixture tuning question - 05/19/09 08:16 PM

Fired up the old rustbucket for the first time a couple weeks ago, started playing with the tuneup a little bit.

On my 4150 Race Demon (it's the downleg booster one rated at 1000cfm)I noticed I get the best idle quality with the secondary mixture screws about 1 to 1&1/4 turns out which seems fairly normal (from past experience).....but the primary idle mixture screws have me weirded out a bit.

Best idle quality/vacuum/rpm is with the primary mixture screws only adjusted out about 1/2 to 3/4 turn.

Assuming float level is correct, wouldn't that indicate a need for smaller or larger low speed air bleeds? Or am I overlooking something?

Thx

Posted By: Blown61

Re: Demon idle mixture tuning question - 05/19/09 08:42 PM

The only thing those screws control is the amount of air and fuel mixed that gets discharged at the idle slots, it does not adjust the mixture.

Posted By: Quicktree

Re: Demon idle mixture tuning question - 05/19/09 09:07 PM

open the fron and back the same amount, split the difference.

Posted By: ZIPPY

Re: Demon idle mixture tuning question - 05/19/09 09:15 PM

I tried splitting the difference between the front and back, and it makes over 1" of vacuum less than doing it the other way, and just idles a whole lot weaker.

Posted By: sixpackgut

Re: Demon idle mixture tuning question - 05/19/09 09:37 PM

i would think a little more transfer slot is showing on the primary side

Posted By: Quicktree

Re: Demon idle mixture tuning question - 05/19/09 09:55 PM

Quote:

I tried splitting the difference between the front and back, and it makes over 1" of vacuum less than doing it the other way, and just idles a whole lot weaker.

then adjust with the idle screws to get it back.

Posted By: ToddP

Re: Demon idle mixture tuning question - 05/20/09 12:37 AM

I have noted that for whatever reason,(seat angle?) demon metering blocks, idle mixture screw go fat after about 3/4 turn. Regardless of ifr or iab.

Holley,blp, quickfuel seem to be correct at around 1 1/2 turn.

I would say that this is normal for Demon carbs.

Posted By: solidcam

Re: Demon idle mixture tuning question - 05/20/09 12:43 AM

You are correct Todd.BG even mention this on their set-up DVD.I think there is a past thread here that also brings this up.My 750 Demon is around 3/4 turn on all four.

Posted By: emarine01

Re: Demon idle mixture tuning question - 05/20/09 12:57 AM

I am running a bg 1000 race also , Its a little strange on idle set up , I start with all 4 corners at 1 turn out , ball out the throttle plates around .025 at transfer slot and allways work my way in around 3/4 on the mix screws, My 1050 holley is 2 turns out and reacts normal ?

Posted By: dustyhof

Re: Demon idle mixture tuning question - 05/20/09 01:04 AM

My 850 Demon works the best with the primaries 3/4 turn out and the secondaries 1/2 turn out. I have tried it every way, and that is only way the motor won't die in gear. Someone had drilled some holes in the primary throttle blades though, so a little more fuel seemed right. I tried about 1 1/2 turns out, and it was extremely rich.

Posted By: ZIPPY

Re: Demon idle mixture tuning question - 05/20/09 01:26 AM

Quote:

I have noted that for whatever reason,(seat angle?) demon metering blocks, idle mixture screw go fat after about 3/4 turn. Regardless of ifr or iab.

Holley,blp, quickfuel seem to be correct at around 1 1/2 turn.

I would say that this is normal for Demon carbs.

Thanks, and also to the others who said they noticed something similar.

This is the only Race Demon I've had the pleasure (?) of working with. On all the Holley stuff I've worked with I was always able to get all 4 fairly close to the same, maybe all within 1/8 turn or so, but this carb is not having it....so I guess I better forget some of the Holley experience if it doesn't really apply.

I like the idea of maybe closing the secondaries a bit/opening up the primaries to see if that has any effect on it. Have not tried that yet

Idle quality is acceptably decent and all the way it is, I was poking around to find ways to make it a little bit stronger. It's 451ci and the cam is a 262/264 @ .050 on 110 so I'm not expecting any miracles.

Posted By: Crizila

Re: Demon idle mixture tuning question - 07/02/09 04:00 PM

Quote:

You are correct Todd.BG even mention this on their set-up DVD.I think there is a past thread here that also brings this up.My 750 Demon is around 3/4 turn on all four.

My 750 annular likes 1/2 turn on all 4. Per BG, tried going to .033 on the idle feed restrictors ( factory is supposed to be .036 ). Didn't seem to make any difference.

Posted By: 602heavy

Re: Demon idle mixture tuning question - 07/02/09 08:34 PM

The reason the throttle plate position varies from P to S is due to the throttle blades not fitting correctly , flip the carb over & take a look how bad they fit , loosen the throttle plate screws & 'jiggle' the blades in the bores , refit & see how different it is , even had to file the blades on a couple of em.

These carbs are speced on 'timing figures' , lean em out how they should be & BGs timing figures go out the window , is it just sales patter?

Posted By: ZIPPY

Re: Demon idle mixture tuning question - 07/02/09 08:41 PM

Quote:

adding bigger idle air bleeds will allow more turns on screw and with the bigger bleed will atomize much better.

That was what I was wondering about too on the first post, but being that I never experimented with air bleeds, really didn't know either way. Interesting, might have to mess with it some more then.

Posted By: solidcam

Re: Demon idle mixture tuning question - 07/02/09 08:44 PM

In my original post I ommitted the point that I have also stepped up to larger diameter idle feed restrictors( can't remember if it was 1 or 2 steps).The rich condition was removed but the idle screw postions still came out at 3/4 turn on all four.

Posted By: mopar dave

Re: Demon idle mixture tuning question - 07/02/09 08:53 PM

that does'nt seem right with the ifr. they flow fuel and if you go bigger you should get a richer condition. go bigger in idle air bleed and it will lean out.

Posted By: 602heavy

Re: Demon idle mixture tuning question - 07/02/09 08:59 PM

Increasing the IFRs is the wrong way to go , these are fitted with .037" & need taking down to .030 @ least , the IABs are around .067" , i had a Demon on 500" which pulled 16" vac , the IABs came out to .062" with IFRs @ .025 , the throttlke blades were also cleaned up , put a .050" restrictor in the trans feed also (.080 stock) , if you move the IFRs to the lower position in the metering block just watch out for any leaks between emulsion well & idle restrictor as there's not much material here , best thing is to keep the IFRs @ the top.

Posted By: Joshs68

Re: Demon idle mixture tuning question - 07/02/09 09:17 PM

Jesse at bigs told me to set up my/his 950 1 1/2 turns out on the primary and 1 out on the secondary. I always thought they should be the same. I just tried without question and I must say this thing idles fantastic for a 470 w a 265@50 cam. dont know if that helps but what I am trying to say is that try it where it runs the best even though it may not follow the conventional path.