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Old 08-18-2005, 05:29 PM
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New BG Speed Demon installed – Disappointed

I recently installed a BG Speed Demon 650 VS carb on my 350 (specs below). Initially the truck ran OK, but very rich. I thought maybe the PV was ruptured due to a few backfires, and that was the cause of the richness. BTW, the machining particles left inside of this carb from the factory leaves a lot to be desired in the quality department. I changed the stock 6.5” PV to an 8.5”, but that made no difference. The motor was running so rich that the sooty wet plugs began arcing internallyand I had trouble starting it.

I installed new plugs to resolve the ignition problem, but the only way I can get this motor to run is to vent the main baseplate vacuum port to atmosphere. I’ve tried repeatedly to get tech support from BG but I end up waiting, paying LD charges, then get transferred to a recording. I starting to think that I should have rebuilt my old Holley 3310.

The existing main jets are 70 primary and 78 secondary. Should I drop down 2 sizes on both and try it again. Or is something else wrong here. Any help is appreciated.


- 350 SBC (’72 C-10 P/U)
- Stock cam
- Edelbrock RPM A/G intake
- MSD HEI, initial @12 degrees BTDC
- Vac advance plugged off for now
- Vacuum = 17”

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Old 08-18-2005, 05:43 PM
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i had the same problem with my Demon 850 being very rich on the 500. I started out with the gas levels just above the sight bowl with the 4 corner adjustements turned out 1/2 turn each. I also turned the choke quite a bit clockwise to lean it out. Dont know if this is all correct but it worked for me anyway.

great carb once its dialed in, a real pain to tune !!! tech support guys (in my experience) are rude bastards and seem only to work a few hours a day 2 days a week
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Old 08-18-2005, 05:43 PM
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There is a BG tech that hangs out here often, hopefully he'll be able to steer you in the right tuning direction.
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Old 08-18-2005, 05:45 PM
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Quote:
tech support guys (in my experience) are rude bastards and seem only to work a few hours a day 2 days a week

Come on now, don't hold back!!! Tell us your real opinion! ROFLMAO~!!
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Old 08-18-2005, 05:59 PM
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I have heard nothing but good about BG carbs. My only complaint is they are too expensive.

Like Holleys, you have to really now a lot about them to make them work right. Tuning is very difficult to do well. It takes years of experience and lots of reading to get the best results.
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Old 08-18-2005, 06:00 PM
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I had the same problems with QC on my Road Demon 525... there were a lot of aluminum chips from machining left in the passages of the metering blocks, and not only that, it didn't even have the right jets in it from the factory! It had the standard jets for a 625...
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Old 08-18-2005, 10:14 PM
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Bump!


Kinda like to hear from the BG guy on the jetting questions.
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Old 08-18-2005, 10:27 PM
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if you're getting that rich you have a problem that's way beyond a couple of wrong sized jets. 70's on a stock 350 are in the right neighborhood. Might not be perfect, but you won't drown with those.

You've either got a float problem, or your choke is not working properly. A big clue is that you've got to get more air into the motor to get it started. That tells me you're rich from the get go.

I would talk to BG tech on here, he's a really good dude and will take time with you to help you get your machine running right.

Check your choke, fuel pressure, idle circuit and float levels. Another possability is that your floats or needle and seat assembly are stuck because of the trash you metioned earlier.

You do not have a tuning issue, you have a problem-

K
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Old 08-19-2005, 12:43 AM
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Horsepucky

Sorry to be the one who delivers some bad news.

The Speed Demon 650 is not suited for your engine!

The Demons are calibrated for a specific camshafts.

The Speed Demon are for camshafts with durations between 220 and 240 deg. at 0.050" lift.

The Speed Demon provides more fuel at idle for the larger cam duration. This makes it run rich at idle with a smaller cam.
Which explains why it will run when you introduce more air through the vac. port, -then you lean out the too rich mixture so that it becomes burnable....

Personally I would have chosen a Road Demon 525, or 625.
BG has a chart that helps you select the correct carb. for your application at their website.

When installing the demon you should also reconsider your entire ignition timing, especially when you have the possibility with the MSD distributor.

The correct timing makes a world of difference!

For the street you should be running vac advance.
It will help on the fouling problem, as you need more advance at idle to burn the mixture properly, and also for cruising you will need more advance to burn properly.
Manifold advance provides that.

The jet changes are the last thing that I do....

My best advice: sell the Speed Demon and get a Road Demon.

You will have a hard time getting the SD to run on your engine, and after messing around with it for a long time still not running right, you probably decide that "Demons are C.... and you will never buy a Demon again....."

You will really get the engine to run right a lot faster by switching to a Road Demon

Just my 2 cents.....

Hope you get it sorted!

/59Vette
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Old 08-19-2005, 01:52 AM
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Demon Carbs are supposed to be wet flow tested at the factory, so when you think about all the shavings being inside it when the flow tested it, by cleaning it all up, don't you think the flow numbers would be off? I recently put a SpeedDemon 650 DP on a built-to-the-hilt sbf. It ran like crap! Took it leaked fuel everywhere, sputtered, coughed, backfired... you name it, it did it. Took it apart, found all the nasty metal shavings, put it back together with some Holley blue gaskets, and it holds fuel now, runs decently at idle and seems snappy. But when driving it, there's a huge dead spot if you get on the gas hard, caused by it dumping way too much fuel - and that's not even getting into the secondaries! Adjusting the pump shot to be lesser didnt help it; I'm convinced that when they drilled out the passages for the squirters then flow tested it, the particles slowed the flow to let it "pass" their flow test. After cleaning it up, it just flows way too much fuel. I have an older SpeedDemon 650 DP, circa 1999-2000, that had idle problems because they machined the idle circuit too big (was WAY too rich at idle, the kind that burned your eyes, and wouldnt idle lower than 1400rpm with the screw holding the primaries open turned all the way out). Sent it back the BG and they sent me another carb with a correct idle circuit drilled. The older SpeedDemon runs flawlessly, and on this new motor I just built, runs hard (dyno tested 400hp to the wheels), runs efficient (20+mpg with an overdrive manual tranny), and doesnt burn my eyes at idle. Personally, I like BG's carburetors - when they're machined properly. It seems like their process has taken a serious turn in the wrong direction, as far as quality control.
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Old 08-19-2005, 05:24 AM
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Thanks for the feedback guys.

Quote:
The Speed Demon are for camshafts with durations between 220 and 240 deg. at 0.050" lift.
True, my intent was to move this carb to a 400SBC with a hoter cam in the future. But that won't be for a few years. Since the design is simliar to a Holley, I wonder why a Holley is compatible with a wide range of camshaft profiles. You may be right on here though.

Quote:
Check your choke, fuel pressure, idle circuit and float levels. Another possability is that your floats or needle and seat assembly are stuck because of the trash you metioned earlier.
Choke is wide open. I haven't checked fuel pressure, but float level is good at 1/3 to 1/2 up the window. Idle mixture screws make no difference.
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Old 08-19-2005, 05:41 AM
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Carburetor Tuning

This may not be the perfect combination, but we should be able to get you going in the right direction here. If you can go all the way in on the mixture screws without it making a difference the butterflies are open too far. You're best bet is going to be taking the carb off real quick, and resetting the butterflies to the baseline and restarting. Running an AirGap intake on a stock engine generally requires more ignition timing at idle which will also help clean up the idle. Start with your mixture screws about 1/2 turn out from seated, and turn the Idle-Eze out about 4 full turns from seated. 1/3 on the float levels should be a good starting point.
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Old 08-19-2005, 06:12 AM
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How does the IdleEze actually work?
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Old 08-19-2005, 07:32 AM
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Plug that vacuum advance back in! Makes a huge difference in throttle response and fuel mileage.

I'm running 68's up front and 74's in the rear on both my 3310 holley's. One on a mild BBC and the other on a hot SBC. Read a plug and change a jet. However, 70's and 78's are not that much off to make it run that rich. You need to look for other issues.

If the carb is pulling vacuum on the ported vacuum port at idle then the carb throttle is oppened too far at idle and is out of the idle circuit. If this is the case, you may need to drill a 0.100 holes in each front throttle blade so the blades can be closed a little. I had this problem and it soaked the plugs in gas, but the two little holes fixed it.
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Old 08-19-2005, 07:46 AM
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if i were you i'd switch to an edelbrock cam and carburetor. They're dyno matched to your intake and even together they probably cost less than your BG carb.
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