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Brand new Demon carb experiencing major tuning problems

38K views 25 replies 12 participants last post by  sel2real 
#1 ·
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.
 
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#2 · (Edited)
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.
 
#3 ·
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...?)
 
#4 ·
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.
 
#6 ·
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.
 
#7 ·
MadMaggot,

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.
 
#10 ·
Mad Maggot said:
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.
 
#11 ·
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.
 
#12 ·
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.
 
#13 ·
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.
 
#14 ·
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 you’ve mentioned that you can’t keep the engine at a steady RPM it should be easier to diagnose. I’d 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 what’s 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.
 
#15 ·
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.
 
#16 ·
Mad Maggot,

I'm not wanting you to change the bleeds just yet, just to see if we can make a difference to the problem you're having. From there we can work with you to help dial the carburetor in.
 
#17 ·
Ok I checked for vacuum leaks with the carb cleaner, and there are no problems to be found. So now that that is rectified, I went ahead and performed those diagnostics you mentioned. First I set the idle so it was stable and not too rich, then pulled on the throttle until it went lean and wanted to die. Pumping the accelerator pump while not moving the throttle at this point definitely made a huge difference, it increased the RPM substantially and eliminated the sputtering, then after a couple seconds it returned to the dead lean state again. I also tried this starting from a higher RPM point and dropping the throttle to the lean condition, and it basically did the same thing. The wierd part is that plugging the air bleeds (the bigger ones on the outside) didn't really do too much to help the problem, although the best I could do plugging it was two small hex keys, so I doubt if I sealed the bleeds off entirely. The choke horn was in the way and I couldn't get my fingers to cover the holes too well. If I need to totally seal the bleeds to have an effect, I will come up with a better way to do this next time.

Also, I fixed the old 650 Holley that this Demon is replacing, and I put it back on for a bit and it worked great - so no vacuum leaks for sure. If for some reason I really need to get deep into the Demon to find the problem at least I have a backup so I can drive my car :) I bought a rebuild kit for the Holley and I still have two bowl gaskets and two metering block gaskets left over so if I need to tear into the Demon repeatedly I at least have backup gaskets.

Thanks for the help so far, and if there's any other suggestions please let me know.
 
#20 ·
Well, curiosity got the best of me, and I decided to take the carb apart and do a more thorough check of all the parts - against my better judgement seeing as how this carb is still under warranty. It's a damn good thing I did, Once I took off the metering blocks, I found metal shavings embedded in the metering block gasket where it mates to the main body, and some of those shavings came loose once the gasket was removed. The same thing happened to the secondary side as well. Come on, Barry Grant. :spank: I know you can do better than this - better give the quality control guys a smack upside the head. What if one of those shavings found it's way into a main bearing? ... :pain: ... It looks to me like they came from where the entrances to the main passages to the boosters in the main body were reamed out to make the funnel shape, as there was still some small shavings still not quite detached from there.

I had some time today so I got a hold of an air compressor and an air nozzle with a fine rubber tip, and went to work blasting all the passages with 120 PSI of compressed air, to make sure none of the metal shavings made it into any of the orifices. Once I was sure the metering blocks and main body were free from debris, I decided to have a look at the idle circuits to see if I could find anything that might be causing me trouble. At first I did not see anything wrong, so I decided to blow air through the jets to see where the fuel was able to go. It turns out after some deductive reasoning and some time blowing air and blocking various orifices with my finger that I could not get any air to come out of the orifice leading to the transfer slot or idle discharge port (the part that is regulated with the idle screw). I tried blowing air backward through this orifice and it would not go anywhere. This led me to believe there was no way for the fuel to get to the transfer slot or idle discharge port. I didn't seem to have this problem with the secondary side. After some looking, I discovered a potential problem with the primary metering block, as shown in the pictures I have attached.

The secondary metering block has a hole drilled in the orifice shown in the red circle, while the primary metering block does not. My guess is that the hole is supposed to be where the fuel goes from the main well into the idle circuit, and if that is the case, the entire fuel supply of the primary idle circuit is cut off. This would certainly explain my dead lean problems. I would appreciate it if you would confirm if this is in fact what is causing the problem. If so, I'm going to need a replacement metering block under warranty, unless there is some way I can drill the holes myself (though I don't have much confidence in my ability to be that intricate with a drill). I would also need a precision drill bit set to have the proper size hole, and that could be costly. Please inform me on how I should proceed.
 

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#21 ·
Hello hello out there.

Just curious if this problem was ever fixed for you! I am having the same issues with a demon I just acquired from my neighbor. He was having an off idle stumble really bad like you've described.

He never did get it to work again on his car. Originally, the carb he worked well for him. THen I think he put new gaskets in and it has never been the same since. I did notice that the new gaskets are blue. Does this mean they are HOlley gaskets??? Will they work?? :confused:

Any more help from BG TECH would be much appreciated. :D

THanks in advance for any help you can give!

jer
 
#22 ·
Yes I managed to fix the problem by drilling out the two holes that should have been drilled from the factory. (Shown in the pictures above) It idles great and light acceleration is responsive but I still seem to have a hesitation when I punch it off the line. I haven't played with the squirters yet so I suspect that this should fix the problem, though I will probably need to go with the big 50cc pump. As for the problem you described, I noticed that when I put the metering block gaskets from a Holley and Demon together they don't always have the same hole alignment. I'm not sure if I mismatched models when I did this, but it wouldn't hurt to check if all the holes in your gaskets line up with the holes in your carb. Also don't forget to check if there are holes that are completely covered by the gasket. It's possible that something is blocked off and you are not getting the right amount of fuel.

Another thing to try is adjusting the butterfly positions on both the primary and secondary side. My experience with Demons has taught me they can be very finicky if they are not set just right.
 
#23 ·
Thanks for the info.

Good to hear that your demon has cast away it's demons. he he.

Mine is still possesed. I did check the new gaskets visually, and I can't see anything at all wrong with them. I have even seen where Tech@BG has posted that the gaskets are interchangeable. A smart forethought in the design no doubt.

Maybe I'm going to the wrong people. I need an exorcist. :evil:
 
#24 ·
LOL @ the religious metaphor :thumbup:

There's quite a few reasons why you could be getting an off-idle stumble, Demons seem to be more prone to this. It's probably in need of some tuning. What engine/tranny/carb size/gear ratio do you have? I know that a heavy car with an auto tranny and highway gears can have a stumble with just about any carb, sometimes the squirters and pump cam need to be adjusted to cover this up.

Does the car idle well? Does it run smoothly at 1200-1500 RPM, or is there a constant lean condition (white smoke, choppy sound) at these RPM? Are your butterflies set to the proper positions? If it idles well, and runs well at a constant RPM, but hesitates or stumbles only when accelerating, then it's likely in need of some squirter/pump cam tuning.

I don't claim to be a carb wizard, but I've tuned my fair share. Here's a few great guides that can help with further tuning (written by experts, of course). They talk about tuning Holleys, but Demons work in pretty much the same way.

http://www.bob2000.com/carb.htm
http://www.jason.fletcher.net/tech/carbtuning/carbtuning.htm
http://www.holley.com/data/TechService/Technical/Carburetor Tech Info.pdf - This one needs Adobe Acrobat
 
#25 ·
yup

I had the same problems tuning my carb...one question I have is...what type of intake is it?...and are you running a spacer and what type if you are?...I was running a single-plane intake and an open spacer...this made the demon run like *** when my 650 dp holley ran ****...so these are key questions in the tuning process of the carb for sure...good luck...ps when I took the thing apart...I had all kinds of flashing and metal in the circuitry...so I would def. rip it down and look for that type of stuff too...once we put a "cloverleaf" spacer in and cleaned all the metal out of it...we vaccum tuned it and it ran mint ever since!!!
 
#26 ·
in the past 7 years, and 7 demon carbs purchased, 6 of the 7 have had issues (debris inside, idle feed circuits too big, etc) and 1, the ONLY road demon i bought, worked out of the box (minor tuning, of course). all in all, once all of the issues are worked out, the carb ran very good, if not, better than most holleys i've ever dealt with. there's a lot of information about people's findings with barry grant's carbs on this forum... all you need to do is search for it!

my 0.02
 
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