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Old 03-31-2013, 02:00 PM
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Demon carbs lean at transition

I got my AEM Failsafe hooked up and it works great. Easy to read display even in sunlight. Anyway, have some questions:
I am running a 6-71 blower with 2 Demon 650 double pumper blower carbs.

Idle AFR is 13.0 and nice and smooth at 1000 RPM

>>> In Neutral <<<

Bring the RPM up to 2000 and it reads 12.5
Bring the RPM up to 2500 and it reads 15.5 and the engine is missing
Bring the RPM up to 3000 and it reads 12.7 and smooths out again
Okay, where do I start? I am very good with carbs so lay it on me. I am thinking idle air bleeds are too big?

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Old 03-31-2013, 03:10 PM
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If when you set the idle mix screws at idle if the throttles are too far open at idle, this results in a off idle transition flat spot ( lean.) usually from a lack of idle timing.
remove both carbs and flip them over and look at the T slot exposure at idle. Pri and sec.
reset .020" ish on all 8 barrels. I bet they are un even or out of position at idle.
The at idle position of the throttles, re T slot exposure at idle is critical.

Do not set the idle using a AFR gauge, or attempt to set to any predetermined AFR at idle. Set the idle mix using a vacuum gauge for best idle quality. @ idle "best idle " is what matters, not the idle AFR. It needs what it needs.

Get this basic set up corrected first before playing with the idle air bleeds.
Very small changes in the idle air bleed have a BIG effect.

Be sure th power valves are closed at idle.

usualy blower motors with best with the timing locked out as more idle timing is better.

use a mirror to look down the carb at 2500 rpm to see if the boosters are flowing fuel, or...

What cam is in this motor?

Last edited by F-BIRD'88; 03-31-2013 at 03:16 PM.
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Old 03-31-2013, 03:38 PM
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Here's the deal. Had to lean the idle out to stop the blower surge. Screws are only out 3/4 a turn on all of them. Transfer slots are correct. Timing is locked at 28 degrees. Cam is 234/244 @ .050. Idle vacuum is 10 inches.

I get the feeling that if I could richen the idle I could solve the part throttle miss/surge but THEN the blower surge would come back?

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Old 03-31-2013, 05:14 PM
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You only need to richen it up a bit. It won't surge.
Do not attempt to target a particular AFR at idle. Give it what it needs.
Surge at idle is real fat or real lean.

Do yu have a boost retard box? if so set the timing to 34BTDC and take timing out as boost rises, with the box.
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Old 04-02-2013, 06:23 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
You only need to richen it up a bit. It won't surge.
Do not attempt to target a particular AFR at idle. Give it what it needs.
Surge at idle is real fat or real lean.

Do yu have a boost retard box? if so set the timing to 34BTDC and take timing out as boost rises, with the box.


Here's the fun for tonight (I actually do enjoy carbs, they make me THINK!)

Took some 18 gauge wire and stripped it to individual strands (tiny). Put one strand in each idle air bleed and bent over the edge of carb so they would be held in by the air cleaners. Started car and warmed it up. Car idled rough so I had to back all 8 mixtures screws out 1/8 more. No blower surge. Checked the AFR and Aha! A difference, although only .4 AFR. But the misfire at 2500 was reduced. Added one more strand to each bleed and now the IFR is down to 14.9 @ 2500.

So I ordered the Proform kit that has bleeds from 65-75. Bet I'll be using the 65's but it is just a hunch.

I could probably up the primary jetting but I already have 79's in there and a 6.5 power valve. Hard to believe 650's would need more than that?
When I power brake it against the convertor the AFR drops to 10.5 so I am plenty rich already!
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Old 04-02-2013, 09:16 PM
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Did you try just richening up the idle mixtures screws, First.?

Did you do the mirror test to see the rpm that main jet fuel flow starts at? It my be starting too late?
(high speed air bleed too big. (It doubles as a kill bleed)

The blower case must be warm before making any tuning evaluations.

( fine tune idle air bleeds by hand by soldering a pair up and drill with a 1/16" .062" drill bit, then hone by hand with the bit, as required, to fine tune the idle AFR.)

First thing to do is verify no vacuum leaks and No exhaust leaks. Leaky headers/gaskets will really screw up the idle/afr.
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Old 04-03-2013, 08:14 AM
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I just looked out the metering orifice sizes for that 650BC carb. To me it looks like the idle air bleeds are generous and the idle feed restriction is a bit stingy , on that carb. But I am not ready to tell you to start drilling etc yet.
(I am not that familiar with that specific carb model)

You can manipulate the idle air bleeds by hand while it is running by blocking /restricting flow with your fingers.
and read the AFR gauge @2500 and 3000 rpm. Then try the same with the high speed bleed, using a mirror too.
to see the main start up rpm.

Again verify no leaks (intake,blower and exhaust gaskets) and correct idle throttle position T slot exposure @idle FIRST
and correct base idle mix adjustment (not based on the AFR reading @ idle.)
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Old 04-03-2013, 09:42 AM
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Your idle air bleeds are causing a rich miss at 3000rpm?
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Old 04-03-2013, 04:12 PM
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Quote:
Originally Posted by spinn View Post
Your idle air bleeds are causing a rich miss at 3000rpm?
Nope. This Mallory 685 box will fire a bucket of fuel.
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Old 04-03-2013, 04:19 PM
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[QUOTE=Blown Camaro;1662635]

Car idled rough so I had to back all 8 mixtures screws out 1/8 more. No blower surge. Checked the AFR and Aha! A difference, although only .4 AFR. But the misfire at 2500 was reduced. Added one more strand to each bleed and now the IFR is down to 14.9 @ 2500.

QUOTE]

Okay, I must have misunderstood this comment.
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Old 04-13-2013, 07:08 PM
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Well thought I would give you guys an update. I tore into the carbs today and used a borrowed set of numbered drill bits on the IFR's. I went one step larger, but I do not know what size. What I did was take the largest bit that would just slide through and then went up one size. Honestly, my eyes can't read numbers that small anymore. Started the car and the idle was of course a lot richer. Went about 1/8 turn closing on the 8 idle screws and got a 12.5 AFR smooth idle. At 2,000rpm now have 14.0 cruising and at 2,500rpm it is 14.7! Man I am damn happy now. Tip in and it goes to 13.0 and a little more throttle and it is 12.5 . I think this is just about dead nuts for a blower motor.

Thanks for everyone's help. Now I just have to get it to the track so I can do the jets and HSAB's. I think it's close, just on the rich side.
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Old 04-13-2013, 08:37 PM
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Good stuff...

Adding a boost retard box will allow more idle and part throttle timing when not under boost
which will idle and drive nicer.
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Old 04-14-2013, 04:56 AM
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Quote:
Originally Posted by F-BIRD'88 View Post
Good stuff...

Adding a boost retard box will allow more idle and part throttle timing when not under boost
which will idle and drive nicer.
Right now I have a switch that cuts 4 degrees but I have to remember to flip it! I need to hook up a MAP sensor to this Mallory 685 box.
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Old 04-14-2013, 11:12 AM
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Quote:
Originally Posted by Blown Camaro View Post
Right now I have a switch that cuts 4 degrees but I have to remember to flip it! I need to hook up a MAP sensor to this Mallory 685 box.
You really need to benefit of a progressive boost retard.
The summit/Bosch 2 bar map sensor is half the price.

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Old 04-14-2013, 11:45 AM
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Why not employ a WOT throttle switch to switch on the retard at say 1/2 to 2/3 throttle
Or a simple hobbs manifold pressure switch say 3psi so you don;t have to "remember"
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