Demon carb problem

[DAVID GAGNARD]

I've been running a 650 Demon DP for about 5 or 6 years now with practically no trouble or adjustments.....Lately it has not been running well when cold and not much better after it has warmed up....

Basically, once you rev it a time or two, whether in gear or not, it doesn't return to it's normal 900rpm idle, instead idles about 1400rpms. You can "stab" the throttle or pull up the pedal with your foot, doesn't seem to make any difference, every now and then, it will return to it's normal idle.....adjusting the four corner idle circuits makes little to no difference,in fact the primarys can be screwed all the way in and it runs the same..........

So today, got home early from work and decided to pull the carb and go thru it. Everything looked good, no trash it in or anything to lead me to the problem. Took it completely apart, including the baseplate and checked out everything, blew out every little hole/port with compressed air, thinking maybe a piece of trash was the culprit, didn't see anyhting....

Re-assembled the carb with new gaskets and put it back on the motor and re-set the floats, cranked it up and it's the same story. At one time I had all four of the idle adjusting screws turned in all the way and it made NO difference....I read and re-read the manuel that came with the carb, just can't figure this one out!!!!!!!!!!!!!!!!

The motor will rev just like normal and once past about 1500rpms, runs great, just under 1500 rpms is where the propblem is......

BTW: I've rebuilt a whole bunch of Holleys and I'm kinda handy with them, Demon is basically the same carb....but this time I'm stumped big time....

Any ideas/suggestions other than getting another carb?????

As I said earlier, for 4 or 5 years this carb ran trouble free, this just started a few months ago and has been getting a little worse....

David

[scootter]

Did you check to make sure your dist advance isn't hanging up?

Just an idea. Mine used to all the time.

Scott

[DAVID GAGNARD]

Originally Posted by scootter

Did you check to make sure your dist advance isn't hanging up?

Just an idea. Mine used to all the time.

Scott

Yes, and it was at one time, would advance realllllll slowly and return even slower... Cleaned up everything in the dist. and added a couple of drops of light oil on the weights, they move freely by hand, back and forth.......

I'll re-check that tomorrow before going back into the carb...

What did you do to fix your dist. from hanging up?????????

David

[Jerry Clayton]

sometimes you need to take the shafts apart to oil them---it isn't the weights hanging up but the shaft assy

[CobraEd]

I bet you developed a vacume (air) leak in one of your intake ports where the manifold mates to the heads. That will cause exactly what you are experiencing.


.

[4into4]

Make sure the throttle is returning all the way. When the idle is up, pull the linkage on the carb and see if it closes, returning back to your normal idle. I had a Demon carb that would do this unless I kept the throttle shaft and linkage well lubed, or put a really strong return spring on it.

[Rick Parker]

Dave what about wear in the baseplate with the throttle shafts? Sometimes I have seen the return spring on the bottom of the pivot linkage with the accelerator attached at the top. This put the shaft in a situation where the tension of the return spring is pulling the shaft foreward into the baseplate. The better design is to put the spring in line with the accelerator cable (attached at the same point) so it pulls directly opposite of the accelerator cable. It will be a vacuum leak in the carb or a timing issue as previously mentioned. Now, if you had Webers............

[Jac Mac]

Now if you had Webers...................Double the trouble

Back to the Demon-- I have found several where the throttle shaft is 'loose' where fitted to the linkage arm. Since the arm has the idle speed adjustment, the shaft & throttle blades have a mind of their own with a random idle speed resulting.

[Gaz64]

David,

Is your 650 a Mighty Demon and if so, does it have "idle eze"?

[vector1]

you're sure it is the carb?

no vacuum leaks?

dist. mechanical/vacuum advance working properly?

i'm thinking maybe at some time the idle circuit got gunked up and you opened the butterflies?

or, at some point did you turn the idle adj. screw and open the butterflies uncovering the transition slot which means the idle circuit can not be used to tune the idle a/f mixture? there should be only about .040 opening at the primary butterflies.

the no adjustment at idle would indicate fuel is somehow getting into the intake via the secondary circuit.

and you could have two problems here, in searching for one you've created another, or fixed one and created another.

pcv valve?

[DAVID GAGNARD]

Well, I now have a lotta things to double check thanks to all the suggestions here....Let me back track and answer some questions....

The motor is my 331 cu in race motor, the carb is a 650 Speed Demon DP.I've been running this set-up for 4 or 5 years now ( no problems). Took the motor out to freshen it up and it's on a run-in stand now, getting broken-in and tuned before going back in the race car.

The last 2 races before pulling the motor is when this "problem" developed. It wasn't that much of a problem cause once I get it warmed up and I'm on the track and the motor probably doesn't go below 3000 rpms while "on track".

I bet you developed a vacume (air) leak in one of your intake ports where the manifold mates to the heads. That will cause exactly what you are experiencing.

I don't think it is a manifold air leak cause it was doing this before I tore the motor down and doing the exact same thing afterwards, but I'll check that anyway.

Make sure the throttle is returning all the way. When the idle is up, pull the linkage on the carb and see if it closes, returning back to your normal idle. I had a Demon carb that would do this unless I kept the throttle shaft and linkage well lubed, or put a really strong return spring on it.

Once started and idling correctly,everything seems to be fine, rev it a time or two and the idle rpm stays high, first thing I do is push back on the throttle linkage thinking it is not returning all the way, it does move, but only a very,very tiny amount, maybe a few thousands, I have the double throttle return springs on it, same as I always have had on it. I don't think the springs are the problem as they are very stiff..When the carb was off the motor, I turned it upside down and slowly worked the linkage from closed to wide open and watched all four butterflys to make sire they were not "rubbing" on anything or touching anything causing them to hang up,there was no problems there, all worked freely as they should.

or, at some point did you turn the idle adj. screw and open the butterflies uncovering the transition slot which means the idle circuit can not be used to tune the idle a/f mixture? there should be only about .040 opening at the primary butterflies.

When I had the carb apart for cleaning as I re-assembled it, I checked and double checked that and set it as per the directions from the Demon tuning papers that came with the carb.I have not touched the secondary idle adjustment, only adjusting the primary. Sometimes I have to turn that screw 1 to 1.5 rounds open to keep the motor running and then I think I'm out of the idle circuit by then....

pcv valve?

No pcv valve, each valve cover has a large clear plastic line going to a vented catch can. the large pcv port on the rear is blocked, one of the two small vacum ports on the side is also, the other vacum port goes to a vacum gauge on the dash.

i'm thinking maybe at some time the idle circuit got gunked up and you opened the butterflies?

Probably right, but since then I have had the carb apart twice to clean it out, I thought I did a good job of cleaning everything out and blowing it out, but that didn't cure the problem.

the no adjustment at idle would indicate fuel is somehow getting into the intake via the secondary circuit.

and you could have two problems here, in searching for one you've created another, or fixed one and created another.

I'm thinking the same thing, when it is idling fast, it should be out of the idle circuit, looking into the carb, there is some fuel "dribble" from the primary boosters, but that is at at least 1500prms, when back to normal idle speed there is no dribble of fuel from the boosters....

Dave what about wear in the baseplate with the throttle shafts? Sometimes I have seen the return spring on the bottom of the pivot linkage with the accelerator attached at the top. This put the shaft in a situation where the tension of the return spring is pulling the shaft foreward into the baseplate.

Back to the Demon-- I have found several where the throttle shaft is 'loose' where fitted to the linkage arm. Since the arm has the idle speed adjustment, the shaft & throttle blades have a mind of their own with a random idle speed resulting.

This is about the only thing I DID NOT really check!!!!!!!! guess what I'll be checking now?????I didn't give this part any thought since the carb is on my race car and doesn't have that much running time on it, never thought the throttle shaft or baseplate could be that worn in that short of time!!!!!!! but I'll defiently be checking this out later today.
I've got to re-jet the carb up a notch or two, plugs looked a little too lean for me.Once it is off and apart, first thing I'll do is check the throttle shafts for "play" and where the linkage arm attaches to the shaft, never thought to even check this before....
The return springs are on the bottom of the linkage and the accelerator linkage is connected at the top, so that makes sense.

sometimes you need to take the shafts apart to oil them---it isn't the weights hanging up but the shaft assy

definetly will do this today!!!!!!!!!!!

At times like this, I kinda wish I had a Holley, been running the same old 20+ year old Holley 600 DP on my street car for over ten years now and I don't think I have had to touch it in the last 5 or 6 years!!!!!!!!!!!!!!!!!!

Thanks for all the responces, I'll be in the shop later today working on that and a few other things, hopefully later this afternoon, it'll all be back together and running "properly"........

David

[dont]

I have the 850 BG and have been fighting this problem for a couple of months. I thought for sure it was in the carb. Changed out my Accell dist. last week due to excessive play in the rotor shaft for a MSD and the problem is gone. 500 miles so far and the idle returns to 850rpm every time.

[DAVID GAGNARD]

Ding,ding,ding, I think I've found the problem...........naturally, the one thing I DID NOT CHECK.............

Took off the throttle return springs and worked the throttle shaft back and forth, primary shaft has what I would consider a lot of play in it,not as much front to back, but a lot of play up and down.
Secondary shaft has about 1/3 that amount... I would consider this normal for the secondary shaft, but the primary shaft is way lose in the base plate, looks like a wear problem....

I checked some old Holley carbs I have in the shop and NONE come close to having this much play in the throttle shafts and some of these carbs are 20 years old!!!!!!!!!!!!!!!!!!!!!

I have a bunch of parts/spare carbs, thought I could swap out a baseplate, no go, most are vacum secondaries and they don't have the correct mounting for the mechanical secondaries, the primaries look to be identical.....

Guess next thing is to get with Demon and see what they say or can do, I imagine a new baseplate would be the quickest/easiets fix...

I do like the Demon carb, but the fact that it has this much wear in a short period of time ain't good........

David

[scootter]

Hey David,

I noticed my 850 Demon has more play in the primary throttle shaft then I would think is normal. I put extra springs on it at one time to try and get it to come back farther. After d$cking with that for a while, I took them back off, went to two normal return springs and readjusted the carb. It works a lot better now.

I have the MSD small diameter dist. I completely took it apart and lubed it up. Then replaced the two small plastic pads under the weights. It worked a lot better after that. Besides, one of the plastic pads was missing!!

Scott

[DAVID GAGNARD]

Originally Posted by scootter

Hey David,

I noticed my 850 Demon has more play in the primary throttle shaft then I would think is normal. I put extra springs on it at one time to try and get it to come back farther. After d$cking with that for a while, I took them back off, went to two normal return springs and readjusted the carb. It works a lot better now.

I have the MSD small diameter dist. I completely took it apart and lubed it up. Then replaced the two small plastic pads under the weights. It worked a lot better after that. Besides, one of the plastic pads was missing!!

Scott

Scott;

the secondary shaft has some play, but I would think not enough to cause my problem, but the primary shaft is reallllllllll lose. I checked a couple of old Holley's I have laying around and none had as much play in the shaft as the secondary shaft, much less the primary shaft!!!!!!!!!!!

I ordered a new base plate, the one with the e-z idle thing on it........

Since my cousin has a machine shop at his house, I'm gonna take the old base plate over there and we'll take it apart and drill the shaft hole and make some brass bushings for it and I'll keep it for a spare... Gotta get this motor going and back in the car, cause we're going to the 45th Mustang bash in Ala. in less than 2 weeks!!!!!!!!!!!!

It has been said to me by others that the aluminum the base plates are made of may be a "soft" aluminum for CNC purposes, that with the 2 return springs puts a lot of pressure on the throttle shaft and may have caused the wear problem.........

Gonna go thru the distributor also.............

David

[Jac Mac]

Most Holleys have very thin teflon/nylon strips/bushes on the shafts where they pass thru the base plate. I wonder if the Demons use the same trick or simply run the shaft straight in the base plate.

[DAVID GAGNARD]

Originally Posted by Jac Mac

Most Holleys have very thin teflon/nylon strips/bushes on the shafts where they pass thru the base plate. I wonder if the Demons use the same trick or simply run the shaft straight in the base plate.

I took apart a 15 year old Holley today, thinking I could make that base plate work, no go, and it did have the teflon/nylon bushings your talking about......

From what I can see on the Demon, there is/was no bushings of any kind, steel shaft going right thru the aluminum base plate, maybe explains the wear problem.... All though the carb is 4 or 5 years old, it has spent it's entire life on my race car, probably 10 to 15 running hours per year, so, that's not much use.

I figure I've put the same total amount of running time on my 65 fastback/street car per year. Been driving it since the mid 90's and the third hand Holley on it now has been on it for at least ten years and probably 10 years before that on a 67 fastback before I got it. I know I have not touched the carb in the last 5 years!!!!!!!!!!!!!!

David