Start with the motor
412ci sbc (4.155" bore x 3.8" stroke)
11.5 : 1 compression
Afr 220s
Solid Roller 264/267 @ 50, .672 .672 lift, 106lsa
Initial timing is about 16
All On 91 Pump Gas


Now the carb in question is my 850 Mighty Demon w/ Annular Boosters
It hass run pig rich since day one. Never got around to tuning it correctly so just always opened the butterflys more than they should be open and left it at that. Reason it didn't bug me too much is because I barely drive it on the street and pretty much just trailer it to the track. Been sitting for a while but I know it didn't fix itself so I want to cure this problem once and for all. At idle, about 1000 rpm, doesnt like going under that at all, it pulls about 4.5" of vacuum. The power valve w/ this carb is 6.5 on the primary side and blocked off on the secondary so I already know that is contributing to the rich condition. Jets this thing came with are 80/88. I plan on blocking off the primary pv and just jetting my primary jets up to 88 as well because like I said, pretty much a track car and light load isn't a real concern for me and im guessing it will be easier to tune with out having to worry about the pv.
I was just stateing that because even tho it is rich through out the entire rpm band i am just asking how to get my motor to idle decent. Everything else i can sort out l8r

So for the questions, while I know the power valve is open the entire time could that be why its so rich at idle? If the butterflys are set right with just a little of transition slot showing even w/ the power valve being open it shouldn't be pulling any fuel in...should it? If so could that be enough of a reason for it to be running so rich? I mean the plugs are black as hell, exhaust burns the cr*p out of my eyes, the idle mixture screws don't do anything, unless I screw them in all the way then the idle climbs like crazy. The Idle EZ...don't get me started on taht thing. So my guess right now is the continuosly open power valve is dumping tons of extra fuel and it SHOULD clear up the problem. Then again, you guys might tell me it plays no roll what so ever in idle quality which leads me to just playing with the idle air bleeds. .070s are in there right now, how much bigger at a time should I go to lean this thing out? Also the thing is at 7psi and fuel level is mid sight glass in both bowls. Thanks in advance fellas

-Rex

 

Well, I can tell you that after a friend of mine put a different power valve in his, the idle improved. His car used to burn my eyes really bad.

 

thanks chief

anybody else? im blockin off the power valve regardless, just wanted no surprises

 

Running a cam like that you will probably always be a little rich at idle but it sounds like yours is extremly rich. Theres some things you can do to make it better though. With that cam I would run more timing 20-25 initial and around 36-38 total. I would also consider just locking the timing down at 36 thats what I did with mine and it works great as long as you have a timing retard for startup. More timing should improve your idle quality a lot. I would also set the float level lower, about even with the bottom of the sightglass or maybe 1/4 full. Once you do that check your vacuum again and change out the power valve for one thats lower than your vacuum, probalby a 3.5 or 4.5. Then reset your idle mixture screws and see how it does. I would try keeping the power valve if you are worried about being too rich at idle, most cars I've seen with the power valve blocked off are extremly rich at idle and race only type cars that see no street time. I wouldn't mess with the air bleeds unless you have too. Hopefully that helps a little, good luck.

 

i'd lock the timing out if you can and install an ignition switch. So you'd basically start cranking the motor over then flip the switch and the starter shouldn't buck back against the load and the motor should fire right up. This helped my super rich idle condition on my motor. Also, drop down to a 2.5-3.5" power valve if you can and that should help matters as well.

 

lock it out huh, so basically kiss pump gas good bye?

 

what would locking out the timing do to eliminate pump gas? With a cam that large i'm assuming you're running a bigger stall 3000+ at least, or a manual trans that you don't "lug" the engine down low. The motor shouldn't have a problem at all with a locked out curve with that much duration bleeding off pressure at lower RPMs. I don't see detonation due to locked out timing being a problem.

 

ok, i figured the slow engine speed would make it ping...im no expert tho so that's why im askin. T-5 believe it or not , trapped 129.89 pig rich last time at the track now onto the better question..how do i lock it out

 

well first just set the timing to 28-36 at idle only to see if it immediately clears up the richness, don't rev the motor because the timing curve will still exist, but see if it runs better. My motor started running and sounding much stronger as the curve was advanced at idle. I did have to adjust the primary and secondary idle speed screws since the advanced timing allowed for the transition slots to be closed even more, which helps in leaning out the mix. Next readjust idle mixture screws for best vacuum/highest rpm off a tach or guage and see if you can breath again. If this combo helps, you would have to weld up the slots on the distributor to either run a smaller curve...ie 12 degrees of mechanical travel out of the slots or just weld it solid so the curved will be locked out. F'bird88 had a really great post I can e-mail to you that I saved a long time ago, about someone else's combo which was FAR more tame than yours or mine and it cleared up his issues, which is why I choose to follow his advice as well.

 

Please do

USMCJennings@hotmail.com

Appreciate it

 

sent

 

recieved, thanks a ton

 

well guys, blocked off the power valve, went from a 80 to a 86 in the primarys and left the secondaries at 87. Locked out the timing like you guys suggested at 36 degrees and well...she actually f*ckin idles now with the transittion slots .020 open like theyre supposed to be with out too much trouble. The idle mixture screws actually work now, thanks a ton fellas couldn't have done it with out yawll

Still rich when i give it gas tho , guess ill fine tune the jets at the track. Once again, thanks a ton guys

 

i'd unplug the pv and just buy the proper one instead. Unless this car will never see the street then you can leave it plugged.

 

1. Demon Carbs are designed to run with a power valve in the primary side.
2. You have too much duration for a Mighty Demon...refer to BG tech manual
3. Carb is too big for the application, refer to BG tech manual.
4. Locking out the distributor is not always the cure, it may be a band-aid but not always the correct solution.
5. 11:1 compression on Al heads is border line but you'll have to slow the timing down to avoid detonation on pump gas or run it way rich to cool the combustion chamber...either way you'll be sacrificing optimum power. If those are steel heads you will not be able to tune the combination to run pump gas without compromising power. Correct fuel octane levels are critical to optimum power.


IMO.....and I've only been doing this for 40 years and I've only set up 300-400 Demons a year for the last 14 years. I only curve 500-600 distributors a year so my experience is somewhat limited. :-)
{Edited by Moderator}