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

 

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.

 

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...

 

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

 

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

 

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.

 

Thanks for the feedback guys.

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.

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.

 

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.

 

How does the IdleEze actually work?

 

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.

 

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.

 

you have a great carb. dont jump ship!

you just have to dail it in. BG is master sho-nuff if you can relate.

i think you made the same mistake a lot of people do. they watch the video and tune the carb for the engine in the video, not the engine in their car...."but thats ok too" .

my addvice when your stumped is to isolate the idle circuit. disable everything else . accel pump. choke. vac advance. big secondary spring. make shure the secondaries are close to close. i talkin force this thing to run in the idle circuit. then add things in as you go on your tunning adventure.

i think you can do it.

when it is idling , and you turn the idle screws in its suppose to kill the engine.

the good guys here gave some great tips.


-----------------------------------

i keep reading something that is setting off a red light in my head. you have a 350 , stock cam, and a RPM. why would the rpm mani be on that mill? not that you wont be able to get it running. i think there are way better choices, the performer for one.

 

i keep reading something that is setting off a red light in my head. you have a 350 , stock cam, and a RPM. why would the rpm mani be on that mill? not that you wont be able to get it running. i think there are way better choices, the performer for one.

Thats right, you need a Performer with your setup to run right with that Demon.

Now I have a 750 Speed Demon on a Performer manifold on a fairly hot motor and should be running and need a RPM... I'm willing to make a trade if you want? I'm in the Seattle area.

 

How does the IdleEze actually work?

The Idle-Eze alows you to set the butterflies to the proper position, and then use it to control the amount of air the engine gets to idle. In the old days you would have to drill holes in the butterflies in order to get the engine to idle properly. This eliminates the need to do that.

 

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.

Actually the Demon will make more HP.

 

Guys, I appreciate the feedback. Here's the latest on this issue.

I reset the butterfies to leave a square at the transfer slot, set the idle mixture screws at 1/2 turn out, dropped the primary float level to 1/3 up the window, set the EZ idle at 4 turns out from base, and plumbed in the advance can to manifold vacuum. After slightly playing around with mixture screws, it seems to run pretty decent now! I haven't been able to take to road test yet since it's raining out, but I'll report back later.

. why would the rpm mani be on that mill? not that you wont be able to get it running. i think there are way better choices, the performer for one.

Perhaps, but I purchased the carb and intake combo for a future 400SBC that will be fairly hot. The carb is a good match for the intake, maybe not ideal for my present application though.

Now I have a 750 Speed Demon on a Performer manifold on a fairly hot motor and should be running and need a RPM... I'm willing to make a trade if you want?

Thanks for the offer Jacks, but not only is this intake the polished Permastar one, it will be a better fit for my future 400.

 

O.K., I took the truck for a test drive with the settings from above... good idle, but pops though the carb on anything over light acceleration.

So... I transfered the vac advance from manifold to ported, and gradually kept bumping the base timing more advanced. The performance got better and better as the carb pop gradually disappeared, the final base timing was at 26 BTDC when the pinging started (timing marks are factory). Overall, the truck ran great like this. Problem is that it can't be started once shut off with this much advance.

So... I set the base timing back to 12 BTDC, I transfered the vac advance from ported back to manifold, and total advance at idle came out to about the same 26 BTDC.

Test drive once again... backfiring through the carb.

I'm thinking that I'll bump the base timing to 16 BTDC and put a stop on the vac advance it to limit it to 12 degrees.

Any thoughts or ideas?

 

Set your timing at 16-18 BTDC at idle then try one of, or a combination of, or all of the following in this order:

1)Bump the 4 idle mixture screws out an 1/8 of a turn more, test drive it, no good try 1/4 turn.

2)Rise the floats a bit at a time, test driving as you do.

3)Change your jets up one step.

Most important thing to do is document all of your setting before you start and then document EVERY step or change you make and the results, if you document you will be able to read back and figure what step where improvements and steps that made it worse.

I am no expert, these are step I took to get my 750 speed demon on my 400sbc to perform great.

 

It sounds like your mechanical advance is not working.

 

How does the IdleEze actually work?

http://www.competitionplus.com/04_15_2004/bg_idle_eze.html

 

The Idle-Eze alows you to set the butterflies to the proper position, and then use it to control the amount of air the engine gets to idle. In the old days you would have to drill holes in the butterflies in order to get the engine to idle properly. This eliminates the need to do that.

Was the drilling holes just a BG thing? I've never heard of this before...granted I am fairly new at this stuff but just curious.

 

Was the drilling holes just a BG thing? I've never heard of this before...granted I am fairly new at this stuff but just curious.

Drilling holes the butterflies has been a common tuning scenario for years on carburetors.

 

Thanks BG tech, learn something new all the time, I am still in the process of tuning my new holley, hopefully I don't have to break out the drill :sweat: lol

 

All right, major progress here. After a lot of tuning, and test driving, here are the settings I ended up with.

- Primary butterflies just a crack off from fully closed
- Idle mixture screws out ¾ from full in
- EZ idle out 1½ from full in
- Initial timing at 16 degrees BTDC
- Vacuum advance limited to 14 degrees and on manifold port
- Total idle timing about 30 degrees BTDC combined
- Changed one mechanical advance spring (lighter). Now the curve starts at about 1100 rpm and is all in (20 degrees) by about 2800 rpm

Results: good idle, no exhaust or intake back firing, crisp throttle response, strong acceleration. It could probably use a lot more fine tuning, but overall I am now quite pleased with this carb.

I’d like to thank all those who posted. Your help has been invaluable in resolving this issue.

 

All right, major progress here. After a lot of tuning, and test driving, here are the settings I ended up with.

- Primary butterflies just a crack off from fully closed
- Idle mixture screws out ¾ from full in
- EZ idle out 1½ from full in
- Initial timing at 16 degrees BTDC
- Vacuum advance limited to 14 degrees and on manifold port
- Total idle timing about 30 degrees BTDC combined
- Changed one mechanical advance spring (lighter). Now the curve starts at about 1100 rpm and is all in (20 degrees) by about 2800 rpm

Results: good idle, no exhaust or intake back firing, crisp throttle response, strong acceleration. It could probably use a lot more fine tuning, but overall I am now quite pleased with this carb.

I’d like to thank all those who posted. Your help has been invaluable in resolving this issue.

git r done .......... :thumbup: