Background: 56 TBird with 312 YBlock, ported ECZC heads, EZC 9425B intake, mild cam (Ford's highest lift from a '57 T-Bird, 275/.423"), dual-point Mallory YH distributor with dwell at 31, stock exhaust, timed at 18 BTDC.

A couple years ago I put on a a BG Speed Demon 575 cfm carb. The engine runs real strong, but I've noticed sooty black exhaust residue, and in doing my year-end checkup the plugs look dark. Also, no matter how much I try to adjust the 4-corner mixtures and the idle speed, I can't bring the idle down below about 1,100. (I previoulsy had a Holley on her and was able to idle it at 600.)

I'm thinking the carb's idle feed restrictors are too rich. They are 31's. I calculate that 28's would reduce the flow by 18%. The problem is that I've read the idle feed restrictors are pressed in on a Speed Demon although I haven't taken the bowls off yet. If that's the case I'd have to drill them out, tap the metering block, etc. Then I read that sometimes people try a single strand of stereo wire to cut down the flow, it's a lot easier. That makes sense to me, but what would I hook the wire to?

I also read that when reducing the IFRs, you should drill the 4 idle air bleeds in the carb's main body bigger. Should I do that? or will it mess up the onset of the main circuit?

Thank you. 

Does anyone know about the small wire inserted into the idle feed restrictors? Or how much (if any) the idle air bleeds could be drilled to lean out the idle circuit?

If you’ve ruled out the potential for vacuum leaks, then see if the secondary throttle blades can be closed further.  This may help you on your idle speed as your decription makes it sound like too much air is entering the engine.  But before doing that, be aware that some of the newer model Demons have an idle adjuster (compensator) located under the air cleaner stud that can be closed up to help restore idle control on the smaller cubic inch engines.

At this point, I’ll suggest concentrating on getting the idle speed down.  Until that happens, I don’t believe the engine will ever be happy with the low end fuel metering.  Don’t get overly wrought about the amount of idle transfer slot at this point and just get the idle speed down.  If necessary, close up the secondary side so it’s completely closed just to see if you can retain some semblance of low speed idling.  Closing the ‘idle-ease’ adjustment should have made a difference so I would recommend starting there again.

You’ll want to set the idle mixture screws for the best or highest idle speed.  If you’re trying to reduce the idle speed through the idle mixture screws because the blades are already closed, then the engine will be finicky at idle and especially when it’s cold.  If you can kill the engine by screwing in the idle mixture screws, then I’ll suggest going back and revisiting the throttle blade openings.

With the carburetor blades completely closed (idle speed screws completely backed off) and holding the carb up to a light, can light still be seen around the edges of the blades?  If so, then this will need to be corrected so no light can get around the blades.

Have done the wire trick in the idle feed restrictors on the Holleys in the past and it worked in those instances where there was nothing else wrong.  But if the engine is idling on the fast side, then fuel can potentially be coming in through the main circuit which will be misleading you on some of the other adjustments.

After testing 16 four barrel carbs of various sizes and manufactuer so far on the 312 dyno mule, there’s a world of difference in the various bases and some had vacuum leaks by lieu of the design of the carb base.  Some of these leaks were not obvious other than the engine just idling faster.  Spacers of varying designs had to be used in which to insure an adequate gasket seal at the carb base.  Examine your carb while it’s off the engine and make sure the carb gasket and/or spacer that's being used adequately covers or encapsulates the carb base.

That is exactly how I have it set now on my workbench. But actually, I thought that if idle is too rich, then exposing the rear slots is a good way to lean it out a little (open the secondaries to close the primaries a bit), as long as the rears are not opened too much. So, I kind of feel like I'm being pulled in two directions here. My gut reaction in situations like this has always been to seek the solid starting point first that was intended, and then make adjustments as needed later.

Previously, when I attempted to adjust the idle via the idle speed screw while engine was running, the throttle plates were obviously opened more than intended to be at idle. This (correct me if I'm wrong) causes a partial diversion of fuel delivery at idle, at which point idle fuel will instead be drawn through the path of least resistance. This becomes the more-exposed transfer slots, whose job is not to handle the idle, but to add fuel when the transition from idle to main circuit happens.

Yes, the butterflies are adjusted nicely so there's no light when they're closed.

But if I read you right, you suspect that an idle that won't come down below 1000 is indicative of a more basic issue than idle feed restrictors being off by a couple of thousandths. I know it's not a case of my cam having too much overlap, because the Holley used to idle beautifully.

I guess my next step is to re-check that the carb seats properly, gaskets align, reinstall it, and try again. I will let you know what happens.

If the level is creeping up or changing - the "pull-over" effect described in an earlier post can get you.

I've also found engines (more likely a camshaft timing issue) that wouldn't answer the idle screws right until I went to a stiffer power valve - like a 5.5. But......it could have been a 6.5 valve that wasn't quite right too. You might swap it just for comfort.  

Steve brings up a good point regarding the power valve.  Although your particular cam should have a lot more than 6½” of vacuum at idle, it would be good to put a vacuum gauge to the engine and verify the amount of vacuum that's actually present.  If it is indeed less than 12” at a reasonable idle speed, then you’re back to looking for some other problems.

The reason I say this.....I had a chebbie 402 motor with an "RV" cam that wouldn't idle right to save my soul - I fixed everything twice and was about to toss the carb (a Holley1850 x 600cfm) "over Fibber's Roof" (tavern next to the shop). A guy I worked with, looked over my shoulder and said - "change the power valve so the idle screws will answer". I couldn't see any reason to do this off the handheld gauge - but changing to a 5.5 made it work. might have made it work to change to another 6.5 but II didn't have one that day.

Thanks. I'm going to put in another 6.5 from my Holley that I know works. I've finished putting the Demon back together. Did the wire thing on the idle feed restrictors, and went down on the primary jets from 62 to 60. Also blew everything with the compressor, cleaned out the bowls, and added a manual choke setup while I was at it. Later today I hope to hook everything up and see how it goes. Will let you all know.

Thanks Nathan. I know when you close up the mix screws it shuts right down. I really think it's just a combination of little tweaks that need doing.

How do you like your Blue thunder intake?

Wasn't able to get to it right away, but everything is back together now. Hooked up the analyzer yesterday and at idle she was purring away at 600 rpm. Runs strong. I really believe it was a combination of better adjustment of throttle blades, getting a slightly smaller idle feed, and also ensuring bowls were set right and everything was clean. I didn't have to touch the air bleeds. The only issue I have now is with the manual choke. The choke rod keeps slipping off this little plastic cam. There's too much resistance from the throttle spring.

Anyhow, thanks much to everyone for all the great advice.