Hello gentlemen,

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. 

Joey
Here is a thread that has a lot of good info on carbs. Tim talks specifically about shutting down the four corner idle on your carb. Hope this helps.  Mike

http://www.y-blocksforever.com/forums/Topic32429-3-1.aspx

1956 Fairlane Victoria (ORREO)

 

Overlooking Beautiful Rimrock AZ

Thank you Mike. I read the thread, that helps.

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?

Sounds to me like your carb is dripping fuel while its running,probally caused by float being too high or dirty needle and seat.18btdc sounds like too much timing,try 10.

55 fairlane gasser 312 /c4, 57 f100 292/c4, 52 mainline gasser 302/t10 4speed

That carb should be close out of the box in regards to the air bleeds.  Demon carbs typically like more initial ignition timing so don’t be too alarmed if you’re needing considerably more ignition advance at idle for the engine to run cleanly at idle.

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.

Lorena, Texas (South of Waco)

Hi Ted. Yes, I have been across the timing spectrum and back many times with this setup, and it does need more initial timing.

The BG Specs chart for this Demon says the idle air bleeds are .070 out of the box. If you say that's appropriate, then that's good enough for me.

Yes, I am sure there are no vacuum leaks.

I have the carb on my workbench now. It's not a double pumper, but a "VE", so the secondary blades are adjusted by the same idle set screw as the primaries. Anyway, initial setup asks that .020 of the transfer slot be exposed at idle. I noticed that this was not the case when I took it off the manifold. And I know the reason for that was, in my attempt to get it near a reasonable idle speed, I played with the idle set screw after I got it on the car. It didn't matter what I tried, I couldn't get the idle to set down like it did when I had the Holley 4160. To get it down as far as about 1050 rpm I had my 4 idle mixture screws at just 5/8 of one turn out from closed.

Yes, this carb has the Idle-Ease. I found that adjusting it didn't have any effect whatsoever. I have read several theories on different websites that try to explain this. Anyway, I guess my main questions are about jetting--

Idle feed restrictors: Are the stock .031's too big? Should I try the speaker wire strand idea to pare it down? Have you ever done this?

Also...primary main jets: are the stock 62's too big for a 312 with this cam/intake etc.? Should I consider backing down to 60's? Although I can deal with that later.

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.

Lorena, Texas (South of Waco)

Ted, thanks for your reply. The BG manual says: "On engines that idle below 1000 RPM set the primary butterflies open to the .020" (square) as described, but the secondary butterflies should be set to the bottom of the transfer slot. In other words, at idle speed the secondary transfer slots will not be visible when viewed from the bottom, but any movement of the secondary butterflies will expose the shaft."

 

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.

Joey,while the engine is running take a look down into the carb and look for gas dripping from the ventris(not sure if I spelled that right).your engine is probally getting fuel from somewhere else other than your idle circuit.How is your float setting?It should be in the middle of the sight glass.If its too high then it will draw fuel from the the main circuits.If its not dripping then take a can of brake or carb cleaner and spray around the base of the carb while its running to check for a vaccum leak.I had the exact problem with a Holley four barrel that had a bad needle and seats.Ive also had similar problems with vaccum leaks around the carb base.I had to use a spacer to seal up the old iron four barrel intake around the carb base.The problem was gone when I switched to the Blue Thunder intake.If you can close off the idle mixture screws and it still runs ,its getting fuel elsewhere.

55 fairlane gasser 312 /c4, 57 f100 292/c4, 52 mainline gasser 302/t10 4speed

Mine (525 Road Demon Jr.)............. had to have new steel needle and seat assemblies before it wowuld settle down and have a consistent float level. Whatever brand of "Viton" needle and seat BG used as standard equipment wasn't happy with my local gas.

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 Metzger       Tucson, Arizona