MS,

        Steve's right on the money on the Demon, keep it. If you start comparing the pieces of each carb to the other, you'll throw that Holley away.  I run one on a 408" Windsor in my pro street Ranger. It's 3 tenths and a couple miles per hour quicker than a 780 Holley on the same vehicle. Once it's set it stays set. Love it.

                                                     Frank/Rebop          

Frank/Rebop

Bristol, In ( by Elkhart) 

Don't pitch the Demon señor - its basically an "upscale Holley" and easily capable of the same performance - maybe a little better. Size matters though - and these 525/625 cfm models are just about too much carb for a street auto-trans 292/312. The biggest advantage of the 390 cfm Holley is probably the greater venturi velocity.

I think the "four corner idle" is wasted on a 180°, dual plane, cross "H" pattern intake manifold - so setting the secondary idle screws back a bit more than BG suggests is just par for the course. The four corner set-up seems to be most helpful with a single plane "X" type manifold - where you really are feeding four corners.

Two more things to try - drop the float levels a bit lower than the center of the sight glass - and put the "plain" or black spring back in the vacuum secondary pot. The lower float levels take away any tendency of fuel to "pull" out of the boosters at idle - you still aren't lean enough if you have to jack up the advance the way you do. You ought to be able to make this engine "knock" by over-advancing it.

The engine can't use all of the secondary air volume this carb can deliver - so the quick opening allowed by the light spring - is giving you a false feeling of acceleration. The "cut-in" rate of the secondary opening should be smoother than we often think.

Good Luck! 

 
 Steve Metzger       Tucson, Arizona

Steve,



As soon as I turned a deaf ear to Demons recommendation that all 4 mixture screws be adjusted equally and took your advice my problem was solved. I started from scratch with all 4 backed out one full turn and went from there. It liked to idle with all 4 between 1/2 to 3/4 turn out so I took it for a drive and still had the dead spot. I then backed the secondary screws out a 1/4 turn and turned the primaries in for best vaccum (little over 1/8 turn). That almost completely killed the hesitation. I think I just need to keep messing with it in this direction and hopefully get rid of all of it. Before I (you, by proxy) had it running this well, I had been messing with the secondary springs so I am going to start from scratch on those (running a white now I believe).



The ignition curve you picked seems to work perfectly (light silver/blue/silver bushing). The spring selection helped, but the silver bushing really woke it up (royal pain to swap, though). I still don't hear a knock so I might go up to the red just for kicks. I would recommend this combo as a great starting point for the MSD. I am still running way too much initial (according to the light), but the car seems to like it this way. I have ordered a new light with the timing adjust feature and we'll see what that one says. If it turns out that it is advanced that far I will look deeper into the carb as the culprit or just get rid of it and buy the Holley.



I put the Demon on just to try something new, and wish I had gone with the 390 Holley that everyone says is great on these. I guess I am just a glutton for punishment.



Thanks for all the help

I'm running a 570 holley street avenger on a 1" aluminum open spacer.  I had the idle mix set real nicely with the help of my air/fuel gauge, but after reading this, I don't think the blades were closing all the way because i think my idle was around 600-700. - too high.  So I need to check that again.

I have to get headers/exhaust back on this week and then I can atleast start it again and do some tuning.  - John

 

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

John - You are going to have problems sorting out the spark advance if you can't "load" the engine. For now - try to figure out how to get your carb to idle nice and clean with the primary blades positioned correctly relative to the transfer slots. Do you have an isolator spacer under your carb? Couldn't tell from the photo.

I don't believe it is even close to normal to have an engine sitting there at idle with 20+ degrees of initial advance. To my thinking Mike's outfit is way rich and he is basically "leaning it out" with spark advance. With a fairly ordinary advance curve in there - operating the accelerator pump is dang near putting the fire out. Any time you can solve (or mask) an accelerator pump issue by adding advance - warning lights should come on.

Ford built about a zillion carbureted cars and trucks that had relatively small amounts of initial spark advance and lean idle settings. I won't say this arrangement was great for get-up and go - but it started OK, drove a lot of guys to work, wives to the grocery and went cruising on Saturday night. When I looked at the Demon web site for spec's on the Road Demons - it appears that the 525 and 625 models have the same throttle blades but differ only in the idle bleed, main jet and booster specifications. A 300 cubic inch engine at 500 rpm is only going to consume about 45 cfm at perfect efficiency (sucking every cubic inch of air it can). That's about 20-25 cfm at each venturi. Those Demon carbs were originally calibrated for modern v8 engines that ordinarily idle at higher rev's and have more cubic inches to feed. So the suction "signal" at the primary is going to be way different than the carb was wet tuned or calibrated at the factory. 

 
 Steve Metzger       Tucson, Arizona

At what point when I have too much advance will I notice the "knock"?  I'm not very experienced with the world of advance and don't know what I'm listening for.  I added about 8 degrees of advance when I installed my MSD timing control and the car ran much stronger.  I guess I probably need to add a little more.  I added the control so that I could fine tune +/- 7.5 degrees with ease.  This topic is going to be great once I get my car back on the road! Thanks in advance for the help! - john 

John: Lake Forest, IL



'57 Fairlane 500 - Looking better every day.

The reason for "spark advance" is to give the air fuel mixture time to ignite and burn. You are trying to (approximately) hit peak cylinder pressure at top dead center. The faster the engine is turning - the sooner you've got to light the fire. So much for the 5 cent science lesson.......

If the mixture is poor - and the resulting flame front is poor - you've got to light it sooner. Usually - cranking in the advance, makes a system systematically leaner - up to the point where it becomes explosive and knocks. This would suggest the carb is slurping a bit of fuel in there and the engine has to have all this advance to accomodate it. Does this Demon have the four corner idle? I had one of the 625 Road Demons on a Cougar - seems like you have to put a 1/4 turn on each of the secondary idle mixture screws for starters (?) Make sure the choke fast idle is out of your way (warm engine) - then you want to be sure the primary throttle blades are positioned so you can see about .040 or less of the transfer slots when stopped against the idle screw (do what you have to do to maintain this screw positioning).

When the above is true - the engine should run at idle and the carb ought to "answer" the two primary idle mixture screws. If it won't run - needs throttle opening - open up the secondary throttle blades a hair (I can't remember how this worked on the Demon) but not the primary blades. You should be able to see the results of primary idle mixture screw adjustment on a vacuum gauge (tune each for highest vacuum) - if not - first try closing the two rear idle screws to less than 1/4 turn. If that doesn't work - usually it means the carb is leaking fuel into the intake from an uncontrolled source. Often that source is a power valve. I have also had a valve that was marked 6.5 (the inches of mercury where it opens) replaced it with another 6.5 and it still screwed up the idle until I replaced it with a 5.5 (?)   

An open center carb spacer can screw up the way the primary mixture screws behave as one will tend to "dominate" and govern the system by providing idle fuel to both sides of the manifold. I think street engines behave better with a divided spacer so the the "180°" manifolds do what they are supposed to do (and cut venturi pulses in half).     

 
 Steve Metzger       Tucson, Arizona

Well, I set the timing, with a light, to about 12˚ initial (any lower and it would barely stay running). This is assuming that the last hash on the pulley is 10˚ as the manual descibes. I had changed to the light silver/blue spring combo already but had left the black bushing in. I took it around the block and the hesitation had gotten worse to the point of the car being almost undriveable. It would pop and spit under any load and stalled once. I put it back in the garage and bumped the timing by ear and then checked with the light. It looks like approx. 20-22˚ (the 10˚ hash is just a hair beyond the trailing edge of the timing mark stamping). Took it around the block again and it ran much better. It still has the hesitation but no knock and it does fine when I mash my foot in it. I will put the silver bushing in and move on to the carb as the source of the hesitation (I have heard Demons are notorious for this.



Why does this thing like so much initial?

Mike - I printed out the instructions (8 pages) for the 8383 off the MSD web site. They don't say it directly, but on the first page they have an explanation of total timing that implies that they are talking about crank degrees not distributor degrees. So I would have to assume (scary word) that the curves on page three are plotted in crank degrees. These curves would be added to whatever static timing you set at idle by clamping the position of the distributor. 

I would suggest trying using the second one down on the right side of the charts. Use the "silver stop bushing" for a 25° upper limit and (1) silver (light) spring and (1) blue (light) spring. Set the initial crank timing at 10°-11° (35°-36° total timing) and go from there. Hoosier suggested a bit more initial so you can easily adjust to that by moving the distributor clamp. This is for the mechanical so the vacuum pot would be disconnected and the carb or manifold source vacuum plugged while testng.

The first page of the instructions says the supplied vacuum pot will deliver up to 10° advance at 15 lbs of vacuum. This is a non-sensical value - I suspect it should have been inches of mercury. Its also not clear whether that is a crank degrees value or a distributor degrees value. The vacuum pot in my modified Ford distributor strokes about 5/16" at full vacuum - which translates to 15° at the point plate and 30° at the crank. I am hopeful that they are talking about distributor degrees in the write-up, as you may need a bit more to work with for cruising economy.

There remains a choice - to use live manifold vacuum on the pot - or the ported vacuum also available on most modern carbs. It will jack up the idle speed if you use the live port and you will have to accomodate it when tuning your idle setting. There are active proponents of both ports (one or the other) on this site. Often you will find one works better than the other just by trying out both tunings. My experience - ported vacuum will heat up the engine a bit more if it is allowed to just sit and soak. But it it less troublesome when sorting out the idle tuning.

Good luck - you will not usually get away wiith tuning the curve and not paying attention to the carb too.

 
 Steve Metzger       Tucson, Arizona

On a Thunderbird - the timing "0" is stamped into the pulley so if the key is good - the mark is good (not shifted). More likely - the goofy little stamping that was supplied as the indicator pointer is out of whack. I've used several ways to locate the "0" and indicator pointer on other engines - but not my Y-block. The best one of my experience was a stop bolt tool inserted through the #1 spark plug hole. With the other plugs pulled, you rotated the piston assembly up to positive stop before top dead center - then backwards to positive stop after top dead center. A guy told me that using a long dial indicator probe can do the same sort of thing (just don't do any more than "disturb" the needle at each point). By marking the position of "0" at both stops - and then splitting the difference to locate the indicator - you get pretty close.

Buy or borrow a "dial back to zero" timing light - a pretty handy gadget when working on this problem. When the engine is revved sufficently to stop the mechanical advance - dial back to the "0" mark to show total advance (vacuum disconnected). With a tach on there, you will also know when it reaches the full advance point. I think for a starting point - use 10° static on the crank and 13° in the distributor (26° when doubled at the crank). When this is set right it will dial back to zero at 36 total degrees. Change weight springs to get the full 36° at 2700 revs - then if it doesn't knock try 2500. If you have to go up to cure a knock, go to 2900 and so on.     

 
 Steve Metzger       Tucson, Arizona