Demon carb issues. - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 26 (permalink) Old Apr 17th, 01, 09:07 AM Thread Starter
 
Join Date: Jan 2001
Posts: 367
Post

I have a Demon on my 383, and I took the jets from factory(76/83) down to 74/80's due to black smoke(rich) at WOT. The 74's up front really helped throttle response.
And with the cam I have, it only has 6-8hg of vacuum at idle,which is 800rpms.
So I changes the powervalve from stock 6-5 to Demon recommended 4-5. Then went it and changed the plugs again with new ones, and this last weekend was playing around and it still gives a big *puff* of black rich smoke when you initially hammer it. And a good puff at that.

What else could I be missing. Now I have a miss again, so I may have to change plugs again.

Any advice would be greatly appreciated.

It's the Demon 750 mechanical secondaries carb.

------------------
383 Speed-O-Motive stroker kit with Speed Demon 750, RPM intake,Proform HEI dist.MSD6a box,World Products Sportsman II heads, ported and polished with 2.02/1.60 valves,64cc,TRW forged pistons with 10.7:1 compression, Comp Xtreme 284 cam,Crane pushrods, Comp Cam 1.6 full roller rockers,ATI damper, 40 series Flowmasters, 4.11 gears with TH350 and Hughes 3000 stall.Powertrax NoSlip Locker

[This message has been edited by Tracy Focht (edited 04-17-2001).]
Tracy Focht is offline  
Sponsored Links
Advertisement
 
post #2 of 26 (permalink) Old Apr 17th, 01, 11:26 AM
Senior Tech
 
Join Date: Feb 2001
Location: Concord Cal. USA
Posts: 2,664
Post

You have an internal fuel leak of some flavor are the floats too high? did you get a bad power valve? check everything, that much fuel in the cylinders is not doing them any good.

oger is offline  
post #3 of 26 (permalink) Old Apr 17th, 01, 11:35 AM
Senior Tech
 
Join Date: Dec 2000
Location: Monroe, WA, USA
Posts: 312
Post

try a smaller squrter. both primary and secondary. if you have a too large a squerter you will be rich when you give it the hammer.

make sure you do not have too much transfer port showing, no more then .030" spec is .020". if you do this can cause a "puff" of black exaust. you are drawing from the transfer circit too soon. make sure your idle mix is ok too.

if you are not useing manifold vacuum you can get this too because the timing at idle is not advacned enough to give you good vacuum and you load up the motor cause you are not getting a clean burn. all so you have to open the butter fly's more to get it too idle with ported vacuum and the transfer slot sweet spot is very important with a demon carb, it is not as forgiveing as a holley is in this area. that is .020" to .030" spec i gave you before.

oger, demon recomend's this as a fix! that is why i posted it. i am not posting to hash over ported verses manifold ok. we do not aggree on this. so let's let it go at that.

i had a riped base gasket on my demon carb, you know the one between the body and the butter fly's. demon sent me a new one no question's asked.

i recomend to take the carb all the way down and clean it. i found alot of left over machineing stuff in mine, that is also how i found the bad gasket.

when you put it back together give all the gasket's a light coat of motor oil. it help's them re-seal, it sweel's the gasket a bit, and keep's em from sticking if you have to tear it down again.



[This message has been edited by ilbl8 (edited 04-17-2001).]
ilbl8 is offline  
 
post #4 of 26 (permalink) Old Apr 17th, 01, 12:33 PM Thread Starter
 
Join Date: Jan 2001
Posts: 367
Post

Thanks, right now it is a 12 degrees initial, and no vacuum advance at all.

------------------
383 Speed-O-Motive stroker kit with Speed Demon 750, RPM intake,Proform HEI dist.MSD6a box,World Products Sportsman II heads, ported and polished with 2.02/1.60 valves,64cc,TRW forged pistons with 10.7:1 compression, Comp Xtreme 284 cam,Crane pushrods, Comp Cam 1.6 full roller rockers,ATI damper, 40 series Flowmasters, 4.11 gears with TH350 and Hughes 3000 stall.Powertrax NoSlip Locker
Tracy Focht is offline  
post #5 of 26 (permalink) Old Apr 17th, 01, 12:47 PM
Senior Tech
 
Join Date: Dec 2000
Location: Monroe, WA, USA
Posts: 312
Post

Tracy,

i bet that is the problem. demon ,i feel, ment for there carb's to used with manifold vacuum. they like a strong signal, and manifold vacuum advacne help's do that at idle.
ilbl8 is offline  
post #6 of 26 (permalink) Old Apr 17th, 01, 02:28 PM
Team Member
John
 
JohnZ's Avatar
 
Join Date: Sep 2000
Location: Washington, Michigan USA
Posts: 7,728
Post

74 & 80 are pretty stout for a street 383; I've built three of them, and they ran like gangbusters with 68's and 76's and vacuum secondaries.

------------------
JohnZ
'69 Z28 Fathom Green
JohnZ is offline  
post #7 of 26 (permalink) Old Apr 17th, 01, 07:02 PM
Senior Tech
 
Join Date: Dec 2000
Location: Monroe, WA, USA
Posts: 312
Post

for some wierd resone a demon carb like's a slightly bigger jet then a holley. but on a dyno and in a car they flow less fuel to get the same result as a holley.......i think it has to do with the shape of the venture, and the fact that the air has a better shot into the carb then a holley. i am guessing at this tho. but i have heard if the same size holly like's a say 68 jet a demon will want like a 70 or a 72.

my road demon has a 70 in front and a 78 in back. it is rated at 625CFM, i belive it is a 650CFM that is under rated. the bore and butterfly size is the same as a 650CFM so i bet you can see why i think this.

i also think that the idle and transfer circit's are a bit fatter then a holley's. to deal with longer duration cam's and lower vacuum at idle.
ilbl8 is offline  
post #8 of 26 (permalink) Old Apr 18th, 01, 04:08 AM Thread Starter
 
Join Date: Jan 2001
Posts: 367
Post

Wow guys, thanks for the great feedback. I think I need to get the distributor recurved or go ahead and get the Crane adjustable canister and start playing with vacuum advance. It runs fine at 12 degrees inital and 36 total, but 16 inital would be great.

Now's the time I wished I would have gotten the MSD Billet distributor. Alot easier to tune the curve.

Thanks for the awesome advice. Looks like it's time to tune some more.

------------------
383 Speed-O-Motive stroker kit with Speed Demon 750, RPM intake,Proform HEI dist.MSD6a box,World Products Sportsman II heads, ported and polished with 2.02/1.60 valves,64cc,TRW forged pistons with 10.7:1 compression, Comp Xtreme 284 cam,Crane pushrods, Comp Cam 1.6 full roller rockers,ATI damper, 40 series Flowmasters, 4.11 gears with TH350 and Hughes 3000 stall.Powertrax NoSlip Locker
Tracy Focht is offline  
post #9 of 26 (permalink) Old Apr 18th, 01, 05:24 AM
Senior Tech
 
Join Date: Feb 2001
Location: Concord Cal. USA
Posts: 2,664
Post

ilbl8 I don't see how a vacuum advance will help at WOT but whatever. I admit to not completely reading the post it sure sounds like his pri. butterflies are open too far and it is idling on the main circuit and not the idle circuit. Using a vacuum advance with manifold vacuum will alow you to close the throttle blades enough to cure the problem so will opening the sec throttle blades alittle or drilling small holes in the pri blades

oger is offline  
post #10 of 26 (permalink) Old Apr 18th, 01, 09:57 AM
Senior Tech
 
Join Date: Dec 2000
Location: Monroe, WA, USA
Posts: 312
Post

oger,

manifold vacuum help's in off idle and transition on a demon better then ported/timed. his complaint is a over rich off idle performance problem not a WOT problem. thye are 2 diferant thing's. i am not gona hash over what is better because it depend's on too many thing's. just what demon recomend's and i have found out with my own demon carb. i had the same problem he has...and after understanding why i got the problem i fixed it. i am just sharing that info with a fellow that has the same problem.

now the secondary air door adjustment stuff and other stuff...........

on a holley it would work fine, but a demon is more picky about the transfer port. you have a 4 coner idle on a demon, this make's it very important about the tranfer port setting. on a standard holey you do not have a 4 coner idle, so you can open the secondary butterfly's more then a demon and not get the off idle stumble or over rich idle.

the demon is a more presise carb and respond's to air flow faster then a holley, this is how bary grant made it. i noticed that i had to adjust my idle very carefully over a holley. a 1/4 turn of the idle adjustemnt screw gave more of a responce than on a holley and the idle mix screws are the same way.

therorecticaly you should get better performance with a demon over a holley, this is because of how fast it react's to the flow of air. this is also why it flow's less fuel then a holley but get's the same or better performance out of the motor.

people like to say a demon is a holley clone, well in a way it is but it is an advacnement over a holley in the way it react's to air flow and meeter's the fuel.

i tried the old holley trick's on my demon and they did not work. demon's like the motor to have a idle timing of about 15*BTDC or better because of the air flow it creat's. the carb was not desiened as a emision's carb in any way, it is a performance carb. if you call demon and get a good tech on the line he will recomend manifold vacuum for the vacuum advance. this is because of the signal it will give the carb, and barry grant made the carb to use it more so then ported/timed vacuum.

it has a ported/timed port on it but i feel that was to make the fed's happy. even tho it is sold as a race/off road carb not a direct replacement carb it still has to have the ability to use ported/timed vacuum. if it dosen't the fed's and the CARB board get grumpy with you.

this is polatic's plain and simple, not performance. the EPA and CARB board dose not care about performance, only emisions and to de-tune what we have. thay have all but said performance kill's so we want to be big brother and protect you from yourself.

if you get over .030" of transfer port showing on a demon(primary or secondary) it will not "dial" in at all! i tried, every thing i mean that. the closer you are to the .020" the better. demon is very adimit about this, because it is what the carb was desined to use, too much and it get's too fat(rich A/F) in the idle/offidle circit's. holley is not as picky. i have seem holley's work fine with up too .050" of transfer port showing. this is because they do not react as fast to the signal from the motor as a demon dose.

between demon tech suport and my own resurch i figuared out the it was ment to use manifold vacuum. demon tech suport told me the carb was ment to have atleast 17" to 20"+ of vacuum at idle, more is better they prefer 20" but 17" is fine on a dyno. this is for the road demon and street demon, the race demon and king demon will work with less but still like's high vacuum at idle. but a dyno is not a rolling cahasie with all the varriable's. it is a tuning tool. what work's best on the dyno 99% of the time need's to still be tweaked in the car.

the race and king demon are sold as race only carb's. they have no vacuum port's on them. this is because a race car normaly has a very radical cam and has timing of around 17*BTDC or bettter to run and idle. also they idle in the 900 to 1500 RPM range. they do not use vacuum advacne. only mechanical, and are not set up for idle/off idle performance. they are ment to run at 3000RPM or higher. a race car is not set up for easy starting or hot starting just WOT runing.

i though the demon carb's where a holley clone with a few improvement's. after i got one i found out it is a diferant carb all to gether that just share's the same princiapl's as a holley. that is why holley sued barry geant over the "claw" carb's. he cloned a holley to well. he re-desined the "calw" carb's to make then diferant and better so that holley has no right's to sue him again.

oh just for a little trivia, the vast majority of patent's in the U.S. are for auto part's/improvement's. the is how barry grant got burned on the "claw" carb's. he now has pantent's pending for what he changed on the "claw" desine to make the demon carb's so that he can not get sued by holley again.

this is also why alot of people have not been happy with there demon's. they have not tried or taken the time to learn a few new trick's and to relize that a demon is not a holley.

they also get the wrong demon, like a race demon when a street/road demon is what they need , and do not listen to what demon say's about picking the right carb. thye also get a 750CFM carb when a 650CFM is all that they need. larger is not always better for idle/off idle performance, at WOT it is a help tho some time's. did you know that on a ZZ4 GMPP crate motor GMPP recomend's a 750CFM holley carb, well a 650CFM street demon out performe's a 750CFM holley on a dyno and in a chasie on this motor, properly tuned ofcourse. demon say's a road demon is good for up to 350HP well that has been proven wrong i have seen motor's make over 400HP and be drivable and very nicley tempored with road demon and kick A$$ on the strip. they sell the raod demon as a entery leval carb, but the only diferance between a road and street demon is the street demon has a quick change vacuum secondary pod on it, the road demon is not avalable in a double pumper, and the booster's on the larger street demon's are diferant. that is why i got a road demon over a street demon the price. at about $300.00 or $450.00, what would you do???? if you want to buy a demon carb with vacuum secondary's?

i do like my demon carb, and i am not a holley fan. i tolerate them as they do make good performance carb's. i feel after more people learn how to tune a demon you will see more of them in the race car's and street car's. they have better throtel responce then a holley and give the same or better performacne with better MPG. this is because they mix the fuel and air more efeciantly and react to the air flow changes faster then a holley.IMHO

oh well i did it again i posted a long winded one.. ..just trying to give you guy's all the stuff i have learned....


[This message has been edited by ilbl8 (edited 04-18-2001).]
ilbl8 is offline  
post #11 of 26 (permalink) Old Apr 18th, 01, 11:19 AM
Senior Tech
 
Join Date: Feb 2001
Location: Concord Cal. USA
Posts: 2,664
Post

ilbi8 all Holleys have 4 sided idle circuits on most the rear is not adjustable. He did complain of richness when he stepped on it which should be WOT. I have not said your way was wrong just given him an alternative method.

oger is offline  
post #12 of 26 (permalink) Old Apr 18th, 01, 11:29 AM
Senior Tech
 
Join Date: Dec 2000
Location: Monroe, WA, USA
Posts: 312
Post

oger ,

that is true but the non-adjustable holley secondary idle circit flow's very little fuel, compared to a 4 coner idle set up.

if you re read his post is is a "puff" of black smoke when he drop's the hammer, that is from the transition/pump shot area. the same go's for a holley here too.

i have also folowed his problem's, he used to have a bog/stumble too, again transiton/pump shot area.

like i posted. the demon and holley's are not the same. if you have a holley with 4 corner idle the transfer slot become's a more important thing, but not as picky as a demon is.

this is the area that a demon is very picky about, and it come's down to the amount of signal the carb get's and where the butterfly's are in realtion to the transfer port. holley's are more forgiveing in this area is all. they do not react to the air flow as fast as a demon dose.

i also stated that the idle and transfer circit's in a demon are fatter then a holley. this is becasue they are desined for a performance motor not a stock motor. most performance motor's like a bit fatter idle and transfer circit. this is from the higher lift cam and longer duration, with more air you need more fuel to burn right or you go lean.

[This message has been edited by ilbl8 (edited 04-18-2001).]
ilbl8 is offline  
post #13 of 26 (permalink) Old Apr 19th, 01, 04:56 AM
 
Join Date: Oct 1999
Posts: 217
Post

I know some of you guys made brief mention of cleaning the carb -even a new one, and I'd like to second that. I have a Speed Demon 750 that needed extensive cleanup. There was lot's of machining flash that needed to be removed. I noticed hanging pieces of aluminum when I went to change the power valve. Not only was ther metal obstruction the operation of the valve, but the threads for the jets were not cut cleanly. A very fine file and plenty of rinsing was the answer. As far as jetting, my 406 prefers small jets. I have a 70 in the primary and 78 in the secondary. Keep in mind that I live at 5000 ft though, so there is an extra reduction there.

------------------
Shawn Peterson
1969 Z28, 406SB
http://www.geocities.com/corsican69
Shawn is offline  
post #14 of 26 (permalink) Old Apr 19th, 01, 05:24 AM Thread Starter
 
Join Date: Jan 2001
Posts: 367
Post

Wow. great info being passed here.

Now I am carb illerate. So most of the internals are unknown to me. Setting idle mixture on 4 corners is one thing, but tearing into idle circuits and etc are confusing to me.

So for a good daily driver carb, that someone of my un-known carb knowledge maybe sell the Demon and get a Holley Street Avenger? I would have to admit, the electric choke is very appealing on the Avenger too. But how would it compare to the demon?

Thanks guys....you seem to know your carbs alot better than I ever will.

------------------
383 Speed-O-Motive stroker kit with Speed Demon 750, RPM intake,Proform HEI dist.MSD6a box,World Products Sportsman II heads, ported and polished with 2.02/1.60 valves,64cc,TRW forged pistons with 10.7:1 compression, Comp Xtreme 284 cam,Crane pushrods, Comp Cam 1.6 full roller rockers,ATI damper, 40 series Flowmasters, 4.11 gears with TH350 and Hughes 3000 stall.Powertrax NoSlip Locker

[This message has been edited by Tracy Focht (edited 04-19-2001).]
Tracy Focht is offline  
post #15 of 26 (permalink) Old Apr 19th, 01, 06:21 AM
Senior Tech
 
Join Date: Feb 2001
Location: Concord Cal. USA
Posts: 2,664
Post

It doesn't matter which carb you get your combo is a little shakey for a street car. You are going to have to mess with what ever you get. It would help if you could find a local person that is skilled in curing your problem. Otherwise you are going to learn the hard way.

oger is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome