bg demon woes, help please
Thread Starter
Melting Slicks
Joined: Dec 2004
Posts: 2,161
Likes: 0
From: pr, frederick md.
bg demon woes, help please
im running a demon carb. my problem is with the little idle air flow device that sits under the aircleaner bolt/allthread. no matter what i do i cant stop my car from bucking/missing/jerking when i let off the gas.( i dont believe it to be spark related as i have changed distributors as well, but do have cutback electrodes on plugs. ) even if i just hold gas pedal at one speed it will buck. and its really bad. at first i worried about my rear gears it was so bad. but i have tuned it down to only a P.I.T.A. it only happens when im tilting into the throttle or out. if i go all the way in or let off all the way its not as bad. i can make it worse by turning the screw under the bolt. but never can make it go completly away.ive played with this for months, changed aceleration pump cam, and cant get it. ....and another funny thing is, when i turn my mixture screws in to make the car stumble so ill know where to set my mixture, it DOES NOT stumble.... it IDLES up??!! help please 
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Your idle circuit is extremely rich. Mine came this way and the idle mixture screws run best at 1/4-1/2 turn out. The solution is to increase the size of the idle air-bleed, thus delaying the idle circuit.
I had a similar problem with it loading up on fuel, a disassembly found a piece of aluminum stuck in the secondary needle and seat. I suggest you tear it down, and clean/blowout everything thoroughly, re-adjust your butterfly to .020, set idle screws at 3/4 turns out and have another go at it.
Also, check your power valve. They make a handy little tester that will tell you if it is bad or not.
I had a similar problem with it loading up on fuel, a disassembly found a piece of aluminum stuck in the secondary needle and seat. I suggest you tear it down, and clean/blowout everything thoroughly, re-adjust your butterfly to .020, set idle screws at 3/4 turns out and have another go at it.
Also, check your power valve. They make a handy little tester that will tell you if it is bad or not.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Bucking and jerking at cruise light throttle is indicative of an overadvanced timing condition. Check your total combined timing with full centrifugal slammed all the way in and full vacuum advance pulled in. If you're at more than 54 degrees, the car will bucj and jerk.
For an easy test, pull the vacuum hose off your vacuum advance. If the jerking stops, you have an overadvanced condition.
For an easy test, pull the vacuum hose off your vacuum advance. If the jerking stops, you have an overadvanced condition.
Drifting
Joined: Dec 2004
Posts: 1,321
Likes: 0
From: Edgewater (20 miles south of Daytona) Florida
Have you thought of calling BG and asking them what to do? They have a great tech assistance. Give that a try if the other suggestions are not what you are looking for. I've got 2 new BG 750 Mighty Demons and except for a slightly overrich condition at idle,( which I'm about to address) they work great. Hope you get your's sorted out ok.
Thread Starter
Melting Slicks
Joined: Dec 2004
Posts: 2,161
Likes: 0
From: pr, frederick md.
ive set the timing at 36 total, and have played with different setting on my old distributor and my new one. the problem im having directly relates to the carb, as i can make the problem worse at will by turning the screw under the air cleaner bolt, and/or playing with the mixture screws. i made an unbelievable improvement when i idled the carb on front and back idle circuits, instaed of just the front one. before that i feared for my rear and trans it bucked so bad. as for the timing, my car starts easily, pulls like hell, and does not ping on pump gas. ever get around to maryland LARS?!
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6th Gear
Joined: Mar 2004
Posts: 6
Likes: 0
From: South Carolina
BG Demon fix
One other adjustment that you may want to try is the carb butterfly adjustment. Talk with BG or review in manual that came with the carb. My '69 was doing the same thing. I adjusted the lean/rich screws and the butterfly adjustment and it worked perfectly. I have a 650 and a 750 Speed Demon.
Thread Starter
Melting Slicks
Joined: Dec 2004
Posts: 2,161
Likes: 0
From: pr, frederick md.
i believe im running way to rich as i have my mix screws about a turn and a 1/4 out. turned them down to half turn out and it seemed to pull better, but the problem is worse. will trty slowly turning out some more tommorow. im thinking of returning or replacing with a holley as this carb was funky since day one. the mechanical secondary linkage was binding and the front float bowl was loose so bad it leaked! thanks barry grant.
