Vacuum leak??

[Wicked69]

I'm tuning my mighty demon and have problems with idle. I have to open up the butterflys a lot to get it idle (1200rpm).  I have a LM-1 wide band hooked and it shows 13.3-13.7 AFR.

Mighty Demons 'idle-eze' has effect on idle AFR but not rpms. Then I go for a spin around the block and stop, engine idles 1700-2000 rpm. Not good... I have sprayed startspray intake and carb gaskets but couldnt find a leak. Opened up the distributor but its ok. Nothing stucking there.

Still sounds like vacuum leak to me, carb experts whadda you think?

[firefighter3931]

Where is your base timing set at ? It might need more timing at idle to increase manifold vacuum. If your idle speed is 1200 you are idleing on the man circuit and not the idle circuit. Do the idle mixture screws ahve any impact on idle quality ?

Ron

[deputycrawford]

Don't forget that there are two idle screws. One for each set of butterflies. Did you walk both idle screws down in small increments? Also, I agree with the idle timing. If you are running a mighty then I would suspect 9-12" of vacuum at idle. You might want to set base timing around 16-18 degrees and see if the idle tuning becomes easier. If you run an MSD distributor, it will be a cinch to change the advance bushing. I run the 18 degree bushing. That gives me 18 degrees at idle and 36 degrees total. With one blue spring and one small silver spring, all the timing is in by 3000 RPMs.
Of course, all this depends on your cam timing and your specific engine requirements, but it is a place to start.

[Wicked69]

Where is your base timing set at ? It might need more timing at idle to increase manifold vacuum. If your idle speed is 1200 you are idleing on the man circuit and not the idle circuit. Do the idle mixture screws ahve any impact on idle quality ?

Ron

My vacuum is 6.5 hg and 4.5 in gear. Power valve only in the primary side and its 2.5

Timing is 20 degrees initial and 38 total. I have lightest springs so 38 degrees comes at 1800 rpm.

[Wicked69]

Basic adjustment on butterflys was made when the carb wasnt installed. Right amount transfer slot is showing when idle screws are opened 1/2 rounds. On that setting car doesnt idle. I have to open up butterflys like 3 rounds to get it idle without keeping the foot on the gas.

Then it idles nice and wide band shows decent AFR ratios. I can fine tune the AFR ratio with 'idle-eze' but it doesnt do nothing on idle rpms.

[firefighter3931]

Where is your base timing set at ? It might need more timing at idle to increase manifold vacuum. If your idle speed is 1200 you are idleing on the man circuit and not the idle circuit. Do the idle mixture screws ahve any impact on idle quality ?

Ron

My vacuum is 6.5 hg and 4.5 in gear. Power valve only in the primary side and its 2.5

Timing is 20 degrees initial and 38 total. I have lightest springs so 38 degrees comes at 1800 rpm.

 


The timing looks like it should be in the "ball park". With only 4.5in of vaccum in gear i have to ask, what cam is in the motor and how much static compression ? The idle vacuum seems very low....The engine is either way overcammed or there is a sizeable vacuum leak.


Ron

[Wicked69]

Motor is RB 448. Compression 11:1 and cam is custom ground solid roller 258/268 @ .050 lift .638/.632 lift 112 lsa. Those vacuum numbers are pretty much what I expected from this cam

[firefighter3931]

Motor is RB 448. Compression 11:1 and cam is custom ground solid roller 258/268 @ .050 lift .638/.632 lift 112 lsa. Those vacuum numbers are pretty much what I expected from this cam

Yep, that's a lot of cam. 

I have a 264@.050 109lsa custom solid in my 10.5:1 446 but it makes a little more vacuum than yours. We ended up drilling some small holes in the butterflies to get the idle down into the 1000rpm range. You might need to do the same to get more air into the engine at idle. Start small and experiment with it. Have you fooled around with the idle air bleeds at all ?

The LM-1 is a valuable tool for this type of tuning. I need to get one. Fortunately mine was dialed in when we had it on the dyno. The carb is  a Holley with the Proform main body which is identical to the HP series race carbs. I'm wondering if yours would work better with a Race Demon as opposed to the Mighty Demon ?


Ron

[Wicked69]

You are right... Race Demon might work better. This carb is from my old combo. Its 825... Mighty Demons are recommended to cams up to 260 degrees .050. Maybe the vacuum numbers get better when I get these bugs sorted out. My old motor had 11 hg vacuum. With current 6 hg  I was suprised how little, if any, difference it made on my power brakes.

No need to drill holes to butterflys. Might Demons have that 'idle-eze' feature.... Theres air channel controlled by screw under the air cleaner stud. Takes out the guesswork what size holes you should drill on the butterflys.

To ensure proper oiling to roller lifters I shoot for 1200-1300 idle rpm. I have air bleed kit and drill bits but I'm not fooling with them before I got this idle problem solved.

Fuel curve needs to fine tuned... It starts with rich tip-in (11.4-11.8 AFR), slightly rich cruise 12.5-13.1 AFR, then gets leaner as rpms go up at WOT 13.1-14.0 AFR

I dont know can I trust these numbers when theres possible air leak somewhere...

I know everybody love pictures... So heres my Charger.

License plate tells that I'm not a guy from your block....

[firefighter3931]

You are right... Race Demon might work better. This carb is from my old combo. Its 825... Mighty Demons are recommended to cams up to 260 degrees .050. Maybe the vacuum numbers get better when I get these bugs sorted out. My old motor had 11 hg vacuum. With current 6 hg  I was suprised how little, if any, difference it made on my power brakes.

No need to drill holes to butterflys. Might Demons have that 'idle-eze' feature.... Theres air channel controlled by screw under the air cleaner stud. Takes out the guesswork what size holes you should drill on the butterflys.

To ensure proper oiling to roller lifters I shoot for 1200-1300 idle rpm. I have air bleed kit and drill bits but I'm not fooling with them before I got this idle problem solved.

Fuel curve needs to fine tuned... It starts with rich tip-in (11.4-11.8 AFR), slightly rich cruise 12.5-13.1 AFR, then gets leaner as rpms go up at WOT 13.1-14.0 AFR

I dont know can I trust these numbers when theres possible air leak somewhere...

I know everybody love pictures... So heres my Charger.

License plate tells that I'm not a guy from your block....

Wicked 69, fwiw i agree with the higher idle speed...especially with a solid roller cam. I'm still surprised that it's producing that low of a vacuum reading.....the cam is big, but not that big. I wonder if the fuel curve on the mighty demon is agressive enough for your build....i use holley's and have had good luck with them. The Hp series carbs work very well with big cams.

One other thing you might try ; increase the spark lead at idle and see if the vacuum picks up. With a cam that large it will probably want 20+ degrees at idle, or more.

Do you have another carb that could be barrowed.....either a race demon or Holley HP series ?


Ron