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Old 08-01-2006, 06:05 PM
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BBC Timing and Carb setup

Hei!
I have some problems with adjusting my new 454 BBC. It has Performer RPM rect. port heads; Performer RPM Air-Gap intake; Long headers, open exhaust at the moment; Full MSD igniton - Digital-6 box, Pro-Billet dist with mech. advance.
Camshaft is Crane hyd. roller, #139651, 244/256@.050.
Carb is 850cfm Mighty Demon, #5563020GC, annular boosters, 80-88 jets, 6.5 PW.

This is first time for me to "play" with BBC and Demon type carb. So there are too many new toys to play with and it gets confusing.

So .. so far I have done.
Initial is set to 20, mech. advance is 18 so total would be 38 - 1 light silver and 1 heavy silver spring and black bushing. I played with initial a bit - I got maximum manifold vacum with 24 initial. But that ruins total timing? I set it back to 20.
Carb is set up by manual. Fuel pressure is 7 psi. Floats are normal or rather low. Butterflies are set right. Fuel bowls were clean. Idle screwes are 1 1/2 turns out.
Engine starts fine. I let it warm up to 160F before adjusting. Idle and overall fuel mixture are little rich. Engine runs and revs great. But I can't get it idle. I was told that it should idle around 900-1000rpm? I have to press the pedal to keep engine running. If I let the pedal go it just dies. I adjusted idle mixture screwes, nothing changed. I played with Idle-Eze from closed to full-open, it started to suck air but nothing changed with idle. If I opened up primary and/or secondary butterflies I got the engine idle. But when I adjusted idle mixture after opening butterflies the engine won't run fine anymore. When I hit the throttle slowly it starts to stumble around 2000rpm and does not want to go past that. When I hit it hard it goes fast over 2000rpm but engine still doesn't work as good as it did before opening butterflies and adjusting idle mixture.
When I opened the butterflies then Idle-Eze changed rpm, but very little. I have checked timing many times. Vacuum is around 8.5"@20 initial. All carb and intake vac. ports are plugged. There is no visible fuel dripping to carb @ idle.
Same thing happened when I manually opened only secondary butterflies.

I would be very happy about every idea and help you can share!
Thank you!
Madis
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Old 08-01-2006, 06:39 PM
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BBC Timing and Carb setup

I also have a BBC,solid Comp roller cam ,big lift,lots of duration,etc.,and I only have my initial timing set around 13-14 degree's BTDC,so you might want to try setting your initial timing a little lower and see how that works for you,it will make a big difference in how your carb works at an idle.
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Old 08-01-2006, 06:47 PM
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I was told to use 20 initial and Barry Grant advises the same. But I'll try to get initial lower. Thank you!

Last edited by camaro80; 08-01-2006 at 06:53 PM.
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Old 08-01-2006, 06:55 PM
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BBC Timing and Carb setup

Something else while I think of it,mine seems to work better with the heavier springs in the distributor.Also,if you are using a basic stock type HEI with aftermarket vaccuum advance kit,make sure the weights arent slipping off the bushings,after much experimenting I found that factory weights and bushings with aftermarket springs worked the best.One other thing,and this usually gets a lot of negative responses,un-hook the vaccuum advance hose and the motor responds a whole lot better.Big blocks are hard on plugs anyhow,and doing this doesent seem to make any difference,just put new plugs in every year.I just took mine out the other night for a look-see and they are burning perfectly!I have put quite a few miles on mine so far this summer so this is a good indicator.Dont have any experience with a Demon carb yet,but I just ordered a Mighty Demon # 5423020GC 800cfm Annular design which was recommended by Tech@BG guy here on the forum.
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Old 08-02-2006, 07:32 AM
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Camaro80,

The first thing we need to do is to see what the engine wants to idle properly. A combination like yours will normally like between 16 and 18 degrees initial timing. From there 1-1/2 turns out on the mixture screws is probably a little rich, try going in ¼ turn or so on the screws. Now start the engine, and bring it up to whatever RPM you can keep it running at. Once you’re at temp, try lowering the RPM. The engine will want to die, as it does press on your accelerator pump arm without moving the throttle linkage. This will add fuel without changing the amount of air the engine is seeing. If the engine wants to run you’re too lean (come out on the mixture screws) if it immediately dies your too rich (try going in further on the mixture screws). It also sounds like we have too much of the transfer slots showing right now if you can go all the way in the mixture screws and the engine still runs. I’d remove the carburetor real quick, and re-baseline the butterflies. Close the Idle-Eze, and set both your primary, and secondary butterflies so that at idle you see about .020” of the transfer slot below the butterfly (it will look like a little square). Then restart from there.
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Old 08-04-2006, 03:13 PM
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Hei!
I tried these things what you suggested! I readjusted carb by the manual, turned idle mixture screwes in 1/4 turns and closed the Idle-Eze. I let the engine warm up to 160F but it still does not idle. It just dies when I let the throttle go. Otherwise it works and revs great. It did not die when it was cold .. it ran around 600 rpm.
I also pushed acceleration pump and engine almost died. I also played with the initial from 14-26. It seems to work better with 20+ initial. Manifold vacum was also higher with more initial. But it still did not idle.
May there be some missing parts? There were primary jets missing when I got the carb .. it is brand new carb. I installed new #80 jets from the Holley jets kit. Air bleeds and PW are ok.
Timing tape is also ok.
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Old 08-04-2006, 03:16 PM
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If it will idle when its cold, but not once she's up to temp she's most likely too rich. Try going in a little further on the mixture screws and see what that gives you.
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Old 08-06-2006, 05:42 PM
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Hei!
I got the engine to idle around 700rpm. I lowered float levels to lowest position on the sight class. Idle mixture screwes are 1 1/8 turns out. A/F shows still little rich idle mixture. Fuel mixture is little rich through the entire rpm's. But it goes stroich, when I let off the gas after reving. Initial timing was 20-22. It still seems to work better with 20+ initial. I turned Idle-Eze out 2-4 turns after I got the engine to idle, but almost nothing changed. It started to suck air through air cleaner stud hole?!?
How fuel mixture changes during engine warmup? Is it richer or leaner when engine is cold?
Thank you!!
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Old 08-07-2006, 08:43 AM
Technical Support Barry Grant
 
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The Idle-Eze pulls its air from just above the throttle plates in the baseplate, so the air cleaner stud (or bolt) must be in place in order for it to work properly.

The colder an engine is the more fuel it is going to want. It sounds like you may still need to go in a little on your mixture screws at this point.
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Old 08-07-2006, 04:02 PM
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Hi!
Engine started very badly today. It ran good last night when she was at working temperature. And she ran very lean when it was cold so I turned idle mixture screwes back out to 1 1/2 for warmup - headers were starting to glow. It also started better when mixture screwes were 1 1/2 out. Exhaust gases also smelled better when screwes were more out. But A/F sensor still showed little rich. It is a new gauge and sensor.

When engine was warm, then when I let off the gas after reving, then rpm dropped to 600-700rpm but continued to drop very slowly.

I have start retard option in igniton, but it is switched off at the moment. Should I use it?

Thank you!
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Old 08-08-2006, 08:15 AM
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Retarding the timing should if anything make it worse. There isn't enough heat in the engine to vaporize the fuel. It would "richen" the engine for intial start, but not something I've ever tried. How did it run once you got it back up to temp?
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Old 08-08-2006, 01:02 PM
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Hi!

It ran as it did last time when it got it warmed up. Idle mixture screwes are 1 1/8 turns out. ~700rpm at idle. Idle-Eze closed. Revs good from 800-900 rpm. When I tried to rev it from 700rpm then it stubled for a moment.

Now I have to get idle rpm higher(~900??) and get it work properly when she's is cold. Idle-Eze didn't change much. I turned it 2-4 turns out and closed the air filter stud hole.

Can I lower float levels more? They are set to lowest sign on the sight class.

Thank you!
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Old 08-08-2006, 02:27 PM
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glowing headers is a rich mixture and/or retarded timing.

BBC's like 40 to 42 degrees total timing due to large diameter pistons (long way for the flame to travel).

Your cam needs at least 1000 rpms to idle. That is not a cam for a low idle.

Nothing wrong with 20 degrees advance at idle unless starting hot becomes an issue.

Sounds like you are past the transition slot and will need to do some thing about it to get a consistent idle. Try adjusting the carb (idle speed and idle mixture) with one manifold vacuum port on the carb open (make a vacuum leak). If it runs better then drilling the throttle blades may be needed.

A 6.5 power valve is not low enough with 8.5 inhg at idle.
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Old 11-08-2006, 01:56 PM
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It has been a while I haven't played with my Camaro but I tried some things today and it seems I got it working better than before. I took of the carb and adjusted it by the manual - all butterflyes open .02", idle mixture screwes 1½ turns out, Idle-Eze closed. Engine started fine and I adjusted Idle-Eze to get it idle at 900-1000rpm. Initial timing was 20-22 and total 38-40. It idled fine and returned to idle after revving. Only thing was that when I hit the throttle really fast then it stumbled and backfired throught the carb at ~1500rpm. I adjusted initial timing to 24-26 and it got better. So, there comes the question. How should the timing curve change throught the rpm. How much should it be at ~1500rpm? Ok .. after I tried with 24-26 initial I changed it back to 20-22 but I changed distributors springs(1 silver light and 1 blue light) to advance the timing curve so the timing was 30-32 at ~1500rpm. How much can I advance timing curve without running into detonation?
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Old 11-08-2006, 02:35 PM
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Should be more than enough timing. Try a bigger accelerator pump squirter.
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