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Old 08-12-2006, 11:13 AM
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Super lean part throttle on a demon - help!

Hey guys and gals,

I have done a bunch of searching on demons and other things I can think of, but I havent' found an answer to my questions. Sorry in advance for the long read - trying to give all the info I can offer to get the best help.

I have a SBC 406, 10.5:1, Dart Pro1 230 2.08,1.60, hyd. roller, 240/248@.050, 6 spd manual, 4.86 R/P. 18deg init. 36 total at about 2400, vac. advance (Crane adj. version)

Previously had a 750 Vac sec holley on it - it ran very well, very streetable. My neighbor had a 850 speed demon, double pumper that he was having issues with and so he took it off. I decided to give it a try to see if my motor would like it.

It certainly does like it. And so do I (once the main boosters start working), but it's almost impossible to drive (cruise). It has what I think is a dead lean spot as I open the throttle plates (squirters work fine, no play etc). ALmost like the transition slots aren't working somehow. A/F gage goes dead lean at this point. It's somewhat noticeable in the garage, but on the street, I can barely get the car to move until the throttle plates are open far enough to get the mains working.

THis is the exact problem my neighbor was having with it, too. I did go through it with air nozzle, and visually check everything I could. Tried the idle needles, but from what I can tell they only control the idle mixture, not the transition slots, and this has made little or no effect on my problem. Haven't changed jets - shouldn't have to - they are stock, and did work on my neighbor's car before the carb started having this problem.

I pulled the baseplate off and looked in the slots - clean, but almost like someone took a saw and tried to cut a bit deeper in the slots - the "cut" is about half as wide as the rest of the transition slot, but maybe only .020" deep. Maybe this is normal??

Car idles normal at about 1100 RPM, goes like crazy once the mains come in. If I just stab it, the throttle plates go past the trouble spot and no problem - but I can't drive it that way in traffic - that's for sure!

Help!!!

thanks . jer

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Old 08-12-2006, 04:14 PM
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It seems pretty simple, if the primary side is lean, throw some jet in it. Just because the jetting is stock doesn't mean it's correct for your application.
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Old 08-12-2006, 08:21 PM
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Rick,
I guess there really isn't any reason not to try some jets. Maybe I will. It' s just that the carb was taken off (by my neighbor) for a refresh (if I remember right) and after that it never ran right again. No jet changes were made. It would seem that something else changed to cause the condition.

I did install a new powervalve just for kicks. No difference.

And really, it's not lean once the throttle is opened past the transition slots. It seems to be just during tip-in.
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Old 08-12-2006, 10:45 PM
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Tip in then has to be in the pump shot circuit somewhere. Thought you were seeing it under part throttle condition. Have you verified the float levels?
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Old 08-13-2006, 07:27 AM
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Rick,

Sorry to be confusing. It is in part-throttle, but the plates have barely moved while in cruise speeds. I guess the actual tip in is not the problem since the car kind of starts to move them you actually hear a "sucking" noise from under the hood while the motor is starved for gas and the meter goes to 17:1 (which also was happening on my neighbor's 468). If I just drop the hammer she does not hesitate to motivate.

I have verified the float levels, they are just below the middle of the sight glass, which I've heard is a good level to start at. I guess I could try jacking it way up to see if there was any difference along with the bigger jets perhaps.

WIll try it and let you know what I find out. Truth be told, the sucking noise is what baffles me. Sounds like a vacuum leak, but I wouldn't know where that could be coming from. New carb mounting gasket in place.

THanks for the replies.
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Old 08-13-2006, 07:45 AM
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Not sure if its the same problem I had with my BG 750 mechanical but a simple re-gasket of the metering blocks did the trick for me... (same issue your having too...)
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Old 08-13-2006, 02:00 PM
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Thanks for the idea. Will try that along with a new baseplate gasket. It has not ever been replaced, and the carb is several years old.
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Old 08-17-2006, 06:34 AM
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Putting some jet in may help crutch the problem, but with the carburetor being larger than optimal for the combination there isn't enough air velocity to draw the fuel and atomize it properly at that RPM.
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Old 08-17-2006, 12:38 PM
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I have decided to order a 750 mighty with the annular boosters since then. BUt this carb was having this problem on my buddies 468 before he took it off and got him a 750 holley.

He took the nitrous plate off the car, and put the carb back on, and it's never worked right since. Can't figure it out.

I guess he'll be getting the carb back.

Does BG have a way of wet flowing this carb for him if he sends it in? he doens't want to just toss it out, but right now neither of us can figure it out.

thanks for any help!
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Old 08-17-2006, 12:54 PM
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Quote:
Originally Posted by hotrodf1

Does BG have a way of wet flowing this carb for him if he sends it in? he doens't want to just toss it out, but right now neither of us can figure it out.

Yes, that's not a problem...

From reading your post was it working OK before the nitrous plate was removed?
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Old 08-17-2006, 01:48 PM
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yes,

my neighbor took the plate off (don't know why really), and just set the carb back on and after that it had this problem.

Could it be the baseplate gasket perhaps???
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Old 09-17-2007, 12:13 PM
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Just a follow up to this topic if anyone runs across it looking for answers.

I fooled around with my neighbors 850 demon, and finally tried my 750 demon metering block on the primary side. The problem was gone entirely. So, something in the metering block is wrong - plugged up, or otherwise not working properly. And there's not much to them, so I dunno.

He was happy to find out what was wrong with it, although I didn't find the real root cause. Apparently when he had it apart, there was some junk to get in the passages, and despite all the blowing and cleaning we did, it just didn't come out. Or maybe one of the welch plugs is leaking.
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