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Mad Maggot 07-14-2006 09:36 PM

Brand new Demon carb experiencing major tuning problems
I just bought a new Speed Demon 750 carb with vacuum secondaries, annular boosters, and an electric choke. It is replacing a tired 650 Holley double pumper that blew a needle and seat (and with it my patience :rolleyes: ). I simply unbolted the Holley from the manifold and installed the Demon, and to my surprise it did not function very well with the factory adjustment. I've bought Demons before and they basically bolted on and worked great - this one is an exception. Here's my engine setup for reference:

Chevy 355, decked to 5/1000" about 11:1 compression with flat tops (hi-octane fuel)
Ported 305 heads, 58cc chambers, comparable to stock Vortec head flow numbers
XE268 Comp Cam, 224/230 @ 0.050, .477/.480 lift, 110* LSA
1-5/8" headers into 3" single exhaust
Edelbrock RPM Air Gap manifold
Stock stall 700R4 tranny with 3.27 gears.

Basically this is a common 350 buildup, and I don't understand why the factory settings on the Demon shouldn't at least be close. There was nothing wrong with the motor (no vacuum leaks) before the Holley blew and I haven't changed anything since besides the carb itself.

After I let the choke disengage and the engine warm up at idle - a pretty choppy idle - I gave the engine a few revs and discovered that if I lightly tipped the throttle or let it rev just above idle (about 1000-1300), the engine goes very lean and it struggles to keep running. I even had it die a few times. If I punch it hard it doesn't seem to have a problem as the accelerator pump compensates enough for the lean condition. Also, as the main circuit starts to take over after about 1300, the lean condition improves and by 2000 everything seems fine, there's no more white smoke or sooty smell from the exhaust, and the engine is very throttle responsive. It could still be a little lean at this RPM, but it's hard to tell. Also, it doesn't seem to have any problems at idle, with the screws set about 2 turns out. The problem is just after idle speed, i.e. transfer slot territory. If I turn the idle screws out all the way to try to compensate for the lean condition just after idle, it helps a bit, but it is still lean, and the idle is stinky rich. I tried adjusting the throttle blades so the secondaries are closed all the way so I could open the primaries a bit more but it did not help. I tried raising the floats but it did not help either. Both floats are at the top of the sight window instead of the middle now. I took off the carb and checked to see if anything was plugging the transfer slots or idle mixture discharge orifices, but everything checked out. I figure the only other thing I could adjust is the jet sizes, but I thought I better ask here first before I spend the $$$ on a jet set (also Barry Grant Tech Support is off for the weekend). I want to make sure it's not something else, because I can't imagine a brand new Demon carb being this far out of tune from the factory.

If there something else I can try that I may have missed, I would appreciate someone pointing it out to me. I'm going to try farting around with it a bit more tomorrow morning, but if I can't get it right I'll go a buy a jet set and try changing those.

spinn 07-15-2006 08:10 AM

11 to 1 with iron heads might be a issue.

what is your idle ?

is the timming set right ? total ? vacuum ?

tip in problems with a demon are common. very tuneable. the pump shot is enough in volume, but how about duration? shooter size will effect tip in as well. the 750 demon is a great carb, the factory jetting should be close enough not to cause major problems on the right application. i would try and set the carb near stock and start your tunning process over. floats in the bottom or middle of the window. screws about 1 1/2 turns out. return the secondary throttle plate position.

Bumpstick 07-15-2006 08:38 AM

I had much the same problems with my mechanical 750 speed Demon... 2 things cleared it up... 1st I had to replace the gaskets which had been used in several jet changes so yours may just need to be tightened since its new...? (mine were compressed to many times) Also I had a blockage in the air bleed tubes that might have been due to sitting alot in the garage...? Yours may have something blocking them (something to check...?)

Mad Maggot 07-15-2006 11:01 AM

My idle is set to about 750 and once the mixture is set right it idles surprisingly smooth for my cam size. As far as the squirters go, it seems that if I open the throttle blades fast enough for even a small amount of fuel to exit the squirters, the lean spot is covered for a while. As far as timing goes, this engine has been working great for two years with my current timing settings (which have not changed), and even with the high compression I have not had any detonation problems - I use octane boost in my fuel to help with this. Unfortunately changing the jets will have to wait, all the parts stores are sold out because of three weekends of drag racing around here and they don't have any more in stock - d'oh!

Would changing the jet size have anything to do with off-idle mixture, or would changing the idle feed restrictors deal with this? I'm going to try changing the throttle blade positions again, but it seems like they both need to be closed more than factory or it idles too high. Maybe my high compression and synthetic oil is causing the motor to idle higher on less fuel and air and this is throwing off my carb calibration? If I try to use ported manifold vacuum for my distributor advance then it gets lazy off the line until the ported vacuum port is exposed to manifold vacuum - kind of a delayed response. I need to keep the distributor on direct manifold vacuum and I realize this could make it idle higher.

I'm going to try checking all the orifices again and resetting it to factory spec and start over. If I can't get it to work this time I'm going to have to contact Demon support on Monday.

kain13 07-15-2006 01:44 PM

i have noticed when changing carbs on my engine between demon and holley or whatever timing will change mabe you should check you timming and see what it is

Mad Maggot 07-15-2006 03:30 PM

I tore the whole carb apart and checked every orifice in the baseplate, main body, and both metering blocks and I could not find anything wrong with them. I made double sure that all gaskets were properly sealed and tight when I put it back together to make sure there were no vacuum leaks. I played with the throttle blades, mixture screws, floats, and timing and I simply cannot get the light throttle lean condition to go away. I even checked the power valve to make sure it was not stuck, but I can't imagine the power valve being the problem at very light throttle. I also don't see how changing the jets will help the idle or transition circuits. I'm totally at a loss as to what to try next. I can't find anything visually wrong with the carb at all, and it doesn't seem to matter what tuning combination I have dialed in. I also checked the idle-eze screw in the baseplate to make sure it was not letting in any additional air. Somehow the carb is not supplying enough fuel during the transition period, but it is still supplying enough at idle and cruise. It's looking at this point like I will have to get a hold of BG Tech, and if they can't help me I guess I will have to tow the car to a speed shop and let the pros handle it.

wildman926 07-15-2006 09:38 PM


I just got a 650VE on a trade, and it only had 4 hours of run time on it. It looks spankin brand new.

However, while going over the unit, I realized that the primary pump arm is not returning to its normal position everytime, and sticks open. If I had installed it as is, it would give the same symptoms as you are experiencing.

I also found on mine the c-clip was missing off of the vacuum secondary pod link to the secondary throttle shaft. So the secondaries were flapping open as soon as he would give it gas. He had leaned out the secondary screws to 1/4 of a turn out to compensate, I guess.

Needless to say, I have bought a rebuild kit and will freshen it up as soon as it gets here on Monday. I am looking to make it right, as I had to do with an 850ve that I got in trade as well. That thing is awesome.

pepi 07-15-2006 09:47 PM

learning more about the demons and i am starting to think they are properly named, not pictorially impressed with the tri-power either. think I will do the dual quad thing by eldo

302 Z28 07-16-2006 06:16 AM

Replace it with a Holley.


Frisco 07-16-2006 07:59 AM


Originally Posted by Mad Maggot
As far as the squirters go, it seems that if I open the throttle blades fast enough for even a small amount of fuel to exit the squirters, the lean spot is covered for a while.

This appears to be where your problem is. When the throttle blades barely move you should see fuel coming from the squirters immediately. How fast they open should not have anything to do with fuel coming from the squirters. You may need to adjust the accelerator pump linkage or increase the squirter size.

Tech @ BG 07-16-2006 09:18 AM

An off idle stumble under very light acceleration can happen before the accelerator pump (Squirter circuit) is active. If you can accelerate hard, and the engine responds properly that usually means the squirter sizing and adjustment are OK.

You should double check your ignition timing first before getting too deep into the carburetor, since different carburetors will want different settings even on the same engine.

Once you've gotten that done, we need to see what the engine wants. If you're dealing with a fuel problem it's either too rich or too lean. From what you're saying it sounds like you're too lean. Most likely changing your jets, and or PV will not help with the way you're describing the problem. The first thing to do would be to come out on the primary mixture screws (even if this makes the idle too rich for now) to see if that gets the off idle transistion taken care of.

Usually there are a couple little chages that will help with an off idle transition problem. Opening the primary butterflies a little will help pull more fuel out of the transition circuitry. Try closing the rear butterflies as much as you can, and then get your speed with the primary side. This should help, and give you the type of idle RPM you're looking for. From there, coming out a little on the primary mixture screws, and raising the primary float level will help.

Mad Maggot 07-16-2006 02:46 PM

I did some more experimentation to follow the suggestions I have received here, and unfortunately the problem has not gone away.

I adjusted the screw on the primary accelerator pump arm so if the throttle blades move even just barely some fuel will come out of the squirters. It wasn't bad before but now I removed ALL the slack from the pump arm against the linkage. While I might notice a hair more responsiveness with a light throttle tip now, it still does not fix the dead lean condition once the pump shot has been used up - let me clarify this: It does not matter how long the pump shot is, if I tip the throttle gently and hold it there, so as to keep the engine revving at about 1200 RPM, it is dead lean and stays that way until I either gun it or the engine dies.

I played around with the timing, remembering my old setting, and just experimented with retarding and advancing the initial timing as well as the vacuum advance on the distributor. It seemed that my original settings are still about optimum for this new carb, because any change in the timing seems to make it run worse, especially if I advance it more.

As far as the mixture screws are concerned, I can have all four screws all the way out to the point where they are about to fall out of their holes, and the off idle lean problem still exists, although not quite as bad. It is able to run at 1200 RPM with the screws all the way out, but it is very choppy sounding and there is still white smoke from the exhaust.

I played with the butterflies a lot, starting with having the secondaries all the way closed and adjusting the idle from the primaries only, to having the primaries closed all the way and adjusting the idle with the secondaries, as well as pretty much every imaginable combination in between. My conclusion is that it doesn't seem to matter where the butterflies are, it still is not getting enough fuel off idle.

Also, I raised the primary float so high you can't see the top of the fuel level in the sight window, and it STILL is not getting enough fuel off idle. The idle itself is extremely rich where the exhaust burns my eyes and nose, but once I slowly increase the revs it goes dead lean, and this confuses me greatly.

I've never seen a carb to this day that has one single circuit that can't get enough fuel, while the rest of the circuits are fine :smash: I'm going try changing the primary jets from the stock 67's to some 74's I have in another carb here, just to see what the heck will happen. I know it will be rich as hell, but if the transition circuit is still lean after I do this then I will have to look again for a blockage or something that I may have missed before.

Tech @ BG, if you read this post, what would qualify a carb to be considered defective where it must be replaced under warranty? Does it have to be something obviously visually wrong with it such as a main body crack, or can a problem like I'm having be considered a possible defect? I've never had a problem with a Demon carb until this bugger came along. Heck, I briefly ran an 850 Mighty Demon double pumper on this same motor and it worked great, even being badly oversized, yet this 750 with vacuum secondaries is having these kinds of problems.

Mad Maggot 07-16-2006 08:09 PM

Oh, one more thing I should bring up, what is a good way to check for vacuum leaks on an engine? I know my engine was working great before I changed carbs, and I haven't touched anything BUT the carb, but over the last three and a half years my car has been a great example of Murphy's Law. I really can't rule out the possibility that a vacuum leak somehow formed by way of divine intervention or something.

Tech @ BG 07-17-2006 06:57 AM

Mad Maggot,

It sounds like all of the circuits are working, but not calibrated properly for what you're combination is needing since you're able to make changes that are at least changing the way the engine is responding.

Now that youíve mentioned that you canít keep the engine at a steady RPM it should be easier to diagnose. Iíd bring the engine to whatever RPM you can keep it running at steady (say 2000) then lower the RPM. Now when the engine wants to die I want you to press on the accelerator pump arm without moving the throttle linkage. This is going to add fuel without changing the amount of air the engine sees and will confirm that you are too lean since it will help the engine to continue to run. If that is the case then take your fingers and cover the outside (idle) air bleeds in the front of the carburetor. This should add a bunch of fuel through the idle circuit and help get the engine to continue to run at this point. Where not looking to immediately fix the problem just yet, but to determine exactly whatís causing it. You can even use a couple of tooth picks to plug up those bleeds for the moment to keep the engine running. Once we see what this does we can determine what direction we need to go to get this problem rectified.

As far as your vacuum question a simple way is to spray around the carburetor, and intake with some carburetor cleaner while you have the engine running. If there is a vacuum leak the engine tone, and RPM will change.

Mad Maggot 07-17-2006 07:35 PM

Tech @ BG, thanks for the tips - I have not tried either of those diagnostic methods before and once I get some time probably tomorrow night (I'm very busy during the week) I'll let you know how it goes. The one thing that concerns me is that you mention playing around with the air bleeds, and since I have a Speed Demon instead of a Mighty Demon, the air bleeds can't be changed out, so I hope this doesn't turn out to be the problem. I already bought some carb cleaner and I will make sure I check for vacuum leaks prior to performing more carb diagnostics.

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