Tuning a Demon Carb

[coupedup]

Looking for some help tuning my Demon Carb. I believe it is a 750 but I am not too familiar with carbs. What I do know is the car runs very rich. It will burn your eyes its so rich. It also seems to starve/or bog when I get in it heavy and it will blow black smoke I'd assume from the raw fuel. If I accelerate at a normal pace and at normal road speed it seems to be fine.I have only pulled a few plugs and they too show signs of running to rich. Where should I start? Any members in NJ willing to come wrench on this thing would be great as well. I have a fridge full of beer and Red Bull. LOL

[Mr Joshua]

if i remember right 1/8th of a turn out from lightly seated for the 4 corner idle screws
try that..
then find out what jets are in the main and secondary metering blocks.
looks for SN's stamped into the casting so it can be ID'd
plus whats your combo, CID, intake, cam, etc..

[coupedup]

Sorry. My combo would probably be a great help. It's a 306 with forged internals. It has a E cam and gt40 iron heads with 1:7 rockers. As for the intake I am not sure, I cant seem to find any castings or names to ID it. And the Demon 750 sits on about a 1" spacer. It also has a sumped tank with a Holley Blue pump as well as a Holley fuel pres. regulator. Fuel pressure is set at about 7psi. Spark comes by way of a MSD Dist, 6AL box and Blaster coil.

[Mr Joshua]

if it's a 750 demon it's way too big for your combo in my opinion...
heres the identification guide
http://www.barrygrant.com/demon/default.aspx?page=3
if it is indeed a 750 i'd sell it and get a holley/road demon/edelbrock 600-650 Vacuum Secondary for daily driving w/ auto trans Double Pumper for some street some racing..

[dy85merc]

I run a 650 Demon on mine, seems to do OK...but good luck tuning the damn thing. LOL. I think it took me about two weeks to get mine sorted out...what an ass pain. LOL. My dad has an 850 Demon on his, never could get it right. So he sent it to them (Barry Grant) to have it looked over...they said their carbs are rated 100 cfm higher than all other equivelant carbs. Example...Demon 850 = Holley 750...So if you have a 750, it is more like an 850...which is absolute overkill. I have no idea why they would do that, but they did.

[Eds82notch]

I have a 750 demon on a 357 windsor and i feel its to much for my combo. I cant seem to get it right, it runs pretty rich as well...Good luck

[coupedup]

yeah that seems to be the common response. Overkill! I am headed outside now to try and positively ID this thing. Thanks Joshua for the ID guide.

[coupedup]

Ok, so I'm confident that it is the Speed Demon series without the optional electric choke. However, there is nothing to identify the cfm's on the carb. I have downloaded the manual and I guess I will begin the trial and error portion of this thread. In the trouble shooting section it mentions lowering the fuel pressure. Is 7psi too much??

[Fast86GT]

Does it have a vacuam secondaries or mechanical secondaries?I always ran 6-6.5 psi (on a holley)when i had a carbd car w/no issues.

[Mr Joshua]

here is the other part to ID'ing your demon carb..
http://www.barrygrant.com/demon/default.aspx?page=38
and more specs
http://www.barrygrant.com/demon/defa...e=29&specs=29b
the manual just gives basic tuning techniques that's all
http://www.barrygrant.com/fromBarryG...n%20Manual.pdf
Barry grant is notorious for underrating his carbs.. flowing 75-100CFM over similar rated holleys. thing i ran into on my stock 302 w/ intake and exhaust.. even a 1" spacer it still didn't pull enough signal w/ a speed demon 650 DP
same thing w/ the 351 and the 650 mighty demon.
had enough fuel down low at 1/4 throttle or lower.. soon as it switched from the idle circuit to the main circuit it needed more fuel than it could deliver w/ the low signal.. the 4 hole 1" spacer helped improve the signal, but the carbs flowed too much for the combo. poor signal = slow airspeed at the venturi's
this selection chart is pretty much spot on for demon carb selection..
http://www.barrygrant.com/demon/default.aspx?page=5
near stock 302-306 wants a 525cfm Road demon or jr
or a 575cfm Speed demon for mild 302-306's

[coupedup]

OK. I looked a little deeper and it looks like it's the Speed Demon 750 w/ Ford Kick-down Linkage. Part # 1402010VFE. As you all have mentioned; I believe I have too much carb for my current setup. However, I am going to see if I can get this thing to lean out a bit with current combo. I am going to adjust the float levels and kick back my fuel pres. a little to see if that helps.

[11secGT]

I run a 750 speed demon mechanical carb on my 347 with a TFS Stage 3 cam so vacuum is very low(4-5")so I've had to change the factory power valve to a 4.5,put hi-flow #28 shooters and rejet it down to 70/75 if memory serves me. Also adjusted the 4-corner idle mixture with a vac.guage and no more overrich condition or sore eyes. Actually has lean surge now below 3000rpm so gonna finetune primary jets next spring to cure that.
If all else fails gotta work on the idle bleeds,Demons are renowned for running rich here too.
Keep in mind this is on a 347 and it's kinda big so if you have a 750 on a 302/306 it's gonna
be even harder.

[Mr Joshua]

with his combo if it's a 750cfm then it'll more than likely be lean as sin after 1/4 throttle, the more you open the blades the leaner it will be. as your volume increases and your velocity decreases..
as your venturi are too large to speed up the volume of air going over the transfer slot it causes the low signal which leans out the mixture as the air flowing over the slot isn't enough to draw the fuel out..
there is defiantly a point where a point you can reach in selecting a carburetor that it becomes un-tunable.. no matter what you throw at it.. you want your selection to be in the middle so you get the best performance with the least amount of fiddling..

[fordfreak300]

Couped up, if you do not have a lot of experience tunning carbs your in for a big headache with this carb. Your going to have to learn every aspect of tunning a carb and practice it all just to get your car to run right. I would highly suggest selling this carb and getting the appropriate carb for your car. For the current carb you will most likely need to rejet the primary and secondary, switch the accelerator pump to a smaller one and get a slower accelerator pump squirters, different accelerator pump cam, as well as using the vacuum gage to adjust the 4 corner idle screws. Even if you do get it adjusted in to run good, it is still too big of a carb and I think you could get it to run better and much more easily with the right sized carb. If you keep this carb you will need to buy a complete jet kit, various accelerator pump cams and squirters. I left out need to change the power valve to be the appropriate one if it is not already. I suggest you get a book or do some reading on the internet about how to truly fine tune your carb. With the current carb your going to be doing some serious tunning. I really think an appropriate sized carb will be much better though.

[fordfreak300]

with his combo if it's a 750cfm then it'll more than likely be lean as sin after 1/4 throttle, the more you open the blades the leaner it will be. as your volume increases and your velocity decreases..
as your venturi are too large to speed up the volume of air going over the transfer slot it causes the low signal which leans out the mixture as the air flowing over the slot isn't enough to draw the fuel out..
there is defiantly a point where a point you can reach in selecting a carburetor that it becomes un-tunable.. no matter what you throw at it.. you want your selection to be in the middle so you get the best performance with the least amount of fiddling..

While this may be true, if he by chance does get enough air going over the transfer slot he will be pig rich. The problem is with the wrong size carb he will end up with one or the other and cause the opposite trying to correct it, thus always having to tune it. If the condition you describe is true then he will end up trying to jet the primaries bigger to fix the low rpm driving and thus be too rich on the top end, if what i said is true he will be down sizing on primaries then be too lean on the top end. Of course if he is smart he will get the primaries set right for low rpm then set the secondaries for the top end, but then he will have to really play with the accelerator pump circuit to cover the tip in. Then there is the whole powervalve issue that will arise from the vacuum dropping when he opens the throttle all the way up. I know it can all be done but why do it with the wrong carb, when the right carb will be easier to tune and probably provide better power? Yes Joshua I agree with you, you want the carb you have to do the least fiddling with to tune.

[Mr Joshua]

While this may be true, if he by chance does get enough air going over the transfer slot he will be pig rich. The problem is with the wrong size carb he will end up with one or the other and cause the opposite trying to correct it, thus always having to tune it. If the condition you describe is true then he will end up trying to jet the primaries bigger to fix the low rpm driving and thus be too rich on the top end, if what i said is true he will be down sizing on primaries then be too lean on the top end. Of course if he is smart he will get the primaries set right for low rpm then set the secondaries for the top end, but then he will have to really play with the accelerator pump circuit to cover the tip in. Then there is the whole powervalve issue that will arise from the vacuum dropping when he opens the throttle all the way up. I know it can all be done but why do it with the wrong carb, when the right carb will be easier to tune and probably provide better power? Yes Joshua I agree with you, you want the carb you have to do the least fiddling with to tune.

thats just one scenario based on the two motor combo's w/ demons i've experienced, a different combo will more than likely result in a different situation.. in either scenario, it wouldn't be what i'd consider ideal.. he could defiantly sell the current carburetor for a sizable profit then re-invest in the proper sized holley or come out even if he can find a used 525 or 575 demon and be right in the ballpark out of the box..

[Mr Joshua]

The problem is with the wrong size carb he will end up with one or the other and cause the opposite trying to correct it, thus always having to tune it.

i certainly agree.
not a good scenario if he's not into getting that deep into tuning.

[dy85merc]

If I had it to do over again, I would have bought a Pro-Form or quickfuel...or something a little more familiar. Now that it's on and done, it's pretty good...and it sure does look cool. LOL.

[Mr Joshua]

If I had it to do over again, I would have bought a Pro-Form or quickfuel...or something a little more familiar. Now that it's on and done, it's pretty good...and it sure does look cool. LOL.

looks sweet w/o an air cleaner.. mine's always hiding
i went w/ an AED less bling
my engine builder said that AED and Quickfuel both make damn fine products
he just preferred to give AED the business as he'd been dealing with them longer. so thats who i went with..
after dealing with 2 demon's i didn't feel like having to deal with one again..
they give balls out performance if the correct one is selected for the combo ,cleaned out before bolting on and tuned correctly.
not a big fan of their QA/QC department..
usually alot of chips and casting flash in the passages that have to be cleaned out.

[dy85merc]

I normally have the 85 dual-snorkel air cleaner on it, rarely see it on mine too. It's hooked to the two lower air damn inlets, home-made ram-air...pretty sweet. I agree with cleaning out the demon before install, cause they don't do too good at the factory. And I will be purchasing/trading for another carb, hopefully soon.

[fordfreak300]

Hopefully by the time coupedup reads this he will decide to get the right carb for his motor. He will be way happier in the long run if he does.

[coupedup]

Hopefully by the time coupedup reads this he will decide to get the right carb for his motor. He will be way happier in the long run if he does.

Well, It's hard to argue with all you guys. I think I will do exactly that and get a smaller carb. My last car was a 93 coupe with efi so this is really my first stab at the carb thing. Hopefully I can find somebody close that would trade carbs. The car is actually for sale or trade and I hate the thought of letting it go when it runs this way. Hopefully I can get her ironed out for the new owner. Thanks guys for all your advice. I wish I found FEP sooner!

[fordfreak300]

We wish you had found FEP sooner too, you wouldn't be selling your car.

[Mr Joshua]

Well, It's hard to argue with all you guys. I think I will do exactly that and get a smaller carb. My last car was a 93 coupe with efi so this is really my first stab at the carb thing. Hopefully I can find somebody close that would trade carbs. The car is actually for sale or trade and I hate the thought of letting it go when it runs this way. Hopefully I can get her ironed out for the new owner. Thanks guys for all your advice. I wish I found FEP sooner!

why the sale?
i'd put it up in the classifieds here WTT: 750 Speed Demon for 600-650cfm holley 525/575 road/speed demon

[coupedup]

We wish you had found FEP sooner too, you wouldn't be selling your car.

No worries! I'm not going far. I'm just more of a coupe guy thats all. I will keep this one until the right coupe comes along.