DEemon tuning issues, CANNOT get it to lean out, running wideband 02 whats wrong?

[Shawn MacAnanny]

I finally got around to remove my holley 750 and putting on my speed demon 750 dp. I cant get the damn thing to lean out. I am thinking it may be a power valve issue as i my laptop is in my truck at work and i can check vacuum but i am running the same power valve as before.

I disassembled the whole carb when i got it. metering blocks were switched but fine. Carb had a 6.5PV, 75 primary jets and 82 secondary jets. I could get the idle AF down to 12.5:1 or so with this setup. but WOT would peg at 9.6:1 and cruising would be around 11:1. I tightened the 4 corner screws all the way in and backed them out only 1 turn. They are barely out.

Today after fiddling around with the carburetor a little bit, i lowered the float bowls (to bottom sight glass marker) didnt do anything. I replaced the primary jets with 71s, and the secondaries with 75s and it now runs RICHER! i have on idea. I'm idling at like 11.8:1 at idle same 9.6:1 at WOT, and 11.5:1 on cruise (cruise seems to have leaned up a bit). I am testing this about a half hour apart engine at same 180 degrees, air temp is about 70-75 today i am at sea level.

The carb does have idle EZ which i am unsure of what it does so i am going to go fiddle with that for a while. The rockers i just adjusted today (re-torqued them down and thread locked them in place) didnt change anything but i just wanted to be sure.

What do you guys think? Before when i had the vacuum gauge on the car it read about 6psia vacuum which is about 12Hg. Any ideas as to what could be causing this carb to run so lean? All of the holes looked right in the metering plates, this exact carb was drynoed on a 530hp ford 331 before. I am just stumped.

[chicane67]

Sounds like you need an adjustment to the idle feed restriction circuit and possibly... you also may need larger idle air bleeds to increase the amount of emulsification.

One question... if you turn in the idle air screws all the way in can you get the engine to stumble/die ?? Or does it keep running... just rough ??

[Shawn MacAnanny]

Yes! Thank you! i hadnt tried going in more than 1 turn as my holley ran 2 turns out and i decided to try 3/4 turn and it idles just about perfect now! Of course i have alot more lean stumbling and such but i am pretty sure i need a 5.0 power valve and to adjust some of the pump cams but it definately worked! Thank you!

I still have one cylinder bank reading a a point different than the other bank eveyr now and then so im going to throw a set of new plugs in there as i am sure these have fouled out and then get my laptop home to get some readings on vacuum. I am also told i should change my timing from manifold vacuum to the timing port on the carburetor (i run a map sensor and distributor less ignition) and this will help get rid of some of my stumble too. They are saying i am losing my vac advance timing as soon as the throttle opens and defaulting to the 12 degree or so baseline timing instead of the 22 i have i vacuum advanced to.

[chicane67]

Unless it popping when you transition from the idle circuit to the mains... your accelerator pump might be too fat... not too lean.

I take it that this carb has four corner idle ??

[Shawn MacAnanny]

yeah has 4 corner idle new to me as well as a double pumper, everything ive had before was vac secondary. Accelerator pumps are 30 cc, 31 squirters, and pink cams on both sides. My engine SHOULD make somewhere around 500-550hp. I rev to 7000rpm, running a 97gph pump at 7.5psi. Just a pre-filter for right now but i do have a pint sized 10 micron filter to install before the carb next.

[Shawn MacAnanny]

FWIW i had to run a 50cc accelerator pump, red cam, #35 squirter and hollow screw to get the off-idle stumble to go away on my holley 750. LS1s require ALOT of fuel off idle.

[chicane67]

Four corner will normally run leaner on all four... about three quarters turn out vice that of the 1.5 to 2 turns out for the two corner idle.

Honestly... I'd try the old set up on the new carb as a starting point... that's IF... you had it tuned correctly with the last carb.

[Shawn MacAnanny]

Well the jetting was close at 72/76 on the old setup and it ran better with a 5.0 power valve then i put in adjustable metering blocks which gave a little stumble once again probably because of the longer distance the accel pump shot had to travel.

I'll be tuning it quite religiously this week if the weather stays nice until i get it exactly right. The last carb i would have said was 95% before the adjustable metering block then went down to about 90% and didnt mess with it because i wanted a double pumper carb since my car is so light. The carb is about 70% right now i would say. Its spinning my 30x14.50 MTs in 1st gear which the car has never done before probably because the secondaries sling open instantly. I still have 30psi in them though so i can always lower the pressure but still its an improvement.

[MonzaRacer]

problem one
opening the box labeled demon carb.
Havent seen a lot of good ones especially for the price.
Quick fuel make better for less from my experience but then I also have had good luck from HP Holleys

[kryptik]

problem one
opening the box labeled demon carb.
Havent seen a lot of good ones especially for the price.
Quick fuel make better for less from my experience but then I also have had good luck from HP Holleys

Agreed, my quickfuel is superior to my previous demon.

[LSXfan]

problem one
opening the box labeled demon carb.
Havent seen a lot of good ones especially for the price.

was helping a buddy troubleshoot carb problems he was having, missing badly, off idle stumble just running real ****ty everywhere, after trying everything I knew i removed the bowls to find them packed with metal shavings, this was on a 2 day old Speed Demon. He sent the carb back (after fighting with "tech" support) and they sent a replacement, I pulled the brand new carb out of the box and removed the bowls to find them packed with metal shavings. I told him I was done "helping" unless he bought a different carb...true story

[Skip Fix]

My 850 Demon was almost right on out of the box. Cruising the AFR gets fatter so it needs some bleed changes there. Worked good enough to make 695 HP on a different 500" Pontiac motor. Good BSFC numbers on it.

[jfaria78]

I have an 850 Mighty Demon and I never had any problems either. Been almost two years.

[chevyhector]

I have a 650 Demon, and I too have the "too rich" issue ! Any tunning web sites that you guys might know of?

[John Wright]

From my limited understanding of the Idle-Eze feature is that it is supposed to help help heavily cammed engines during idle by adding air to the circuit just like drilling holes in the butterflies did. The transfer slot is supposed to be a certain size and once you tip the butterflies past that you can't hold the idle down. I was never happy with the 650 that I had either, that carb had several issues right out of the box(milling machine shavings in the bowls were one of them). Find a good carb guy and let them work their magic...I hear that the carbs can make power if you get the bugs worked out of them. I didn't have patience enough to get to that point, so that carb is long gone.

[CarPlayLB]

Demon carbs get a bad rap...I got rid of mine years ago too! When the carb worked correctly, it was better than any carb I have used....BUT it didn't always work correctly! I had flooding issues due to a seconday needle and seat hanging open randomly. I tried 3 different sets with same results. Now that i think about it, it was probably 'cause it was full of crud from the manufacturing of the carb. I think you should take everything apart and clean it well...rebuild it...start your tuning from scratch with no questions about the unknown. I think you can take some variables out of the equation! Good luck

[DCx]

threw a holley 750 hp street on my carbed ls1 with FAST wideband and it corrected all of my demon rich and erratic idle issues.

sold that pos on ebay for nearly nothing.