Is this Demon just junk or am I just junk at dealing with it?

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threefortyduster

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I have a 650 Speed Demon. It used to be the primary carburetor for my car, and after getting tired of screwing with it, I changed it out. Since I've been getting some quarantine projects done I decided I'd try and get this thing reset to factory and see if I can at least make it a decently-working backup for the shelf.

Background: This was bought used, and it has never worked "right." I have never been able to get the car to idle without being stupidly rich with this carburetor. It has been rebuilt several times by people who actually know what they're doing (certainly not me haha) and it just never doesn't stink the place up at idle, and it always had an issue with wanting to stall after stabbing the throttle and quickly backing off.

Motor: 340 that's actually 340 inches, X heads, MP Purple Shaft .474 cam, headers. The jets that made this carburetor somewhat useful were 68s in the primaries and 74 in the secondaries. 727/3200 stall converter/3.91 gear. Car is just under stock weight.

When I was messing with it tonight, I pulled up the Demon setup page, just to verify what I knew it was supposed to be set at. There should be .020" of the transfer slot visible below the throttle blades, on both the primary and the secondaries. I can get the .020" set just fine on the primaries when simulating the throttle return springs in place. The secondaries are a totally different story. When simulating throttle return springs in place, I get about .060" of slot visible, and cannot for the life of me get any less visible transfer slot, even with the curb idle screw not even touching the linkage - they just don't close any more than this. There aren't any obstructions in the way, the butterflies do not appear bent at all. They just don't close.

Are the butterflies adjustable on the Speed Demon? If so, how? I tried removing the screw and locknut on the butterfly shaft, but stopped because I didn't want to break anything. Should I just throw this pain in my *** away? Anyone want to buy a pain in the *** Speed Demon? I really hope there's something absolutely stupid that I'm missing because I've hated this carburetor for so long and I'm blinded by my own rage.

primaries.JPG


secondaries.JPG
 
Sounds like you're ready to learn.

I see 4 corner idle. With that cam, the engine is probably making 1/2 decent vacuum at idle. So plan on using very little idle mix. In fact if you shut the secondary idle mix screws all the way, it might might be a good starting piont. Then crack them as needed after adjusting the primaries and fiddle with all four to get the best result. Best result means putting it in gear and getting the least amount of rpm and vacuum drop.

As far as secondary transfer slot goes. Yes there should be a small screw to adjust the secondary idle position. Holley put them so you could only adjust them with the carb off (like in photo). Barry Grant - I don't know. But you are correct, secondary transfer slots should not show. or at least hardly show.

Re: .020 on the primary. Barry Grant carbs have a rep for wider and longer t-slots than old school holleys. Regardless don't feel locked into the number. Some adjustment is OK. When its too far off, you'll notice a delay or a bog when gently pulling away from a stop sign or such..

There's probably more, but that's a good starting point.
Some barry Grants have 'idle eze' air bypass adjustment. The purpose is to allow the throttles to start in the .020ish position without resorting to drilling holes in the blades or cranking open the secondary blades. I think Bill Jones came up with the idea.
 
Mattax said:
Sounds like you're ready to learn.

I see 4 corner idle. With that cam, the engine is probably making 1/2 decent vacuum at idle. So plan on using very little idle mix. In fact if you shut the secondary idle mix screws all the way, it might might be a good starting piont. Then crack them as needed after adjusting the primaries and fiddle with all four to get the best result. Best result means putting it in gear and getting the least amount of rpm and vacuum drop.

As far as secondary transfer slot goes. Yes there should be a small screw to adjust the secondary idle position. Holley put them so you could only adjust them with the carb off (like in photo). Barry Grant - I don't know. But you are correct, secondary transfer slots should not show. or at least hardly show.

Re: .020 on the primary. Barry Grant carbs have a rep for wider and longer t-slots than old school holleys. Regardless don't feel locked into the number. Some adjustment is OK. When its too far off, you'll notice a delay or a bog when gently pulling away from a stop sign or such..

There's probably more, but that's a good starting point.
Some barry Grants have 'idle eze' air bypass adjustment. The purpose is to allow the throttles to start in the .020ish position without resorting to drilling holes in the blades or cranking open the secondary blades. I think Bill Jones came up with the idea.
Click to expand...
Thanks. To clarify a few more items: The Speed Demon has a curb idle screw that is adjustable with the carb on the motor, it doesn't have the screw on the bottom of the base plate. This one is pre-Idle-Eze, so it doesn't have the screw at the base of the air cleaner stud.

TBH, I don't intend on putting this carb on the car for a very long time because of how much of a bear it has always been to deal with. BG made it very clear in their instructions that all four corners had to be adjusted the same and all butterflies had to be adjusted the same with the transfer slots, so I was always stuck on that and didn't want to really venture out into changing stuff independently...maybe its time to jump out of the comfort zone.
 
threefortyduster said:
Thanks. To clarify a few more items: The Speed Demon has a curb idle screw that is adjustable with the carb on the motor, it doesn't have the screw on the bottom of the base plate. This one is pre-Idle-Eze, so it doesn't have the screw at the base of the air cleaner stud.

TBH, I don't intend on putting this carb on the car for a very long time because of how much of a bear it has always been to deal with. BG made it very clear in their instructions that all four corners had to be adjusted the same and all butterflies had to be adjusted the same with the transfer slots, so I was always stuck on that and didn't want to really venture out into changing stuff independently...maybe its time to jump out of the comfort zone.
Click to expand...


I can tell you BG is full of crap, especially if you are running a PCV valve (you should be). I never get married to all 4 corners set the same. Make it idle correctly. If it wants more fuel at a certain corner by all means, give it to it.

If you want to learn to tune one of these buggers, mattax will load you up with all the info you need.
 
What is your vaccuum at an idle in gear?
The purple shaft 474 is getting to that range that it has enough duration that would cause lower vacuum especially in an automatic car.
With that said, what power valve is in it? Power valves will hang open if its rating is to close to your idle vacuum creating stupid rich at an idle.
You can also try decreasing your shooter size. The large shooters can sometimes bleed fuel at a higher than normal idle.
 
D-Rocket said:
What is your vaccuum at an idle in gear?
The purple shaft 474 is getting to that range that it has enough duration that would cause lower vacuum especially in an automatic car.
With that said, what power valve is in it? Power valves will hang open if its rating is to close to your idle vacuum creating stupid rich at an idle.
You can also try decreasing your shooter size. The large shooters can sometimes bleed fuel at a higher than normal idle.
Click to expand...
Vacuum in idle in gear with the idle at 800-900 rpm is about 9". If I increase idle speed to 1200, I can get up to about 12" of vacuum, but I don't like having it idle that high. Initial timing is 16 degrees. When I turned initial timing to about 22 degrees, is when it idled at 1200 and also didn't like to rev up any more.

Just yanked the bowl off, and the power valve is a 65, so you may be on to something there! The currently installed squirters are 35 on the primary side and 37 on the secondary.
 
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The vacuum at idle is useful to track.
9" around 800-900 is fine. That's about what I figured. I ran a CC H280 for a long time. The MP probably has more seat to seat, so comaprable idle characteristics. As you clean up the idle it will probably improve.
But idle speed and vacuum in neutral can be misleading. An engine can be run very lean when all it has to do is overcome friction. The real test of power is in gear.

As far as the power valve, if its not leaking and the gaskets not split or something, leave it alone. Later, much later, you can test for opening point.
The squirrters are irrelevent. Later, much much later, pump shot can be looked at. It's there to cover for delay when quickly opening the throttle when the carb is supplying most of the fuel from the 'idle circuits'. 'Idle circuits' supply fuel any time the air speed past the boosters is relatively slow. That includes low to highway speed cruising. Hardly any power is needed to keep a car moving until interstate speeds.

You might find it useful to look at Chrysler Master Tech sessions 222 (1966) and 273 (1970)
Master Technician Service Conference - Chrysler's Training for Mechanics
 
D-Rocket said:
What is your vaccuum at an idle in gear?
The purple shaft 474 is getting to that range that it has enough duration that would cause lower vacuum especially in an automatic car.
With that said, what power valve is in it? Power valves will hang open if its rating is to close to your idle vacuum creating stupid rich at an idle.
You can also try decreasing your shooter size. The large shooters can sometimes bleed fuel at a higher than normal idle.
Click to expand...


This is just wrong. It’s was always wrong. It will always be wrong.

Idle vacuum doesn’t mean a thing regarding power valve operation. Piss on Holley for never having the balls to correct this.

Do NOT set your power valve opening by idle vacuum. It’s wrong. If you are going to do it, stop. If you are doing it, stop.
 
As far as the secondary throttle stop. Post photos because we can't see what you're working with there. Even if one of us had an early BG here, the fact its been reworked several times means anything is possible.
 
yellow rose said:
This is just wrong. It’s was always wrong. It will always be wrong.

Idle vacuum doesn’t mean a thing regarding power valve operation. Piss on Holley for never having the balls to correct this.

Do NOT set your power valve opening by idle vacuum. It’s wrong. If you are going to do it, stop. If you are doing it, stop.
Click to expand...
Link to a video where the power valve is closed with a manual vacuum pump.
Effect of an open or closed PV on idle
Paraphrasing FalconGeorge. Changing main jets has no real affect on idle. Not in most cases anyway.
 
Mattax said:
Link to a video where the power valve is closed with a manual vacuum pump.
Effect of an open or closed PV on idle
Paraphrasing FalconGeorge. Changing main jets has no real affect on idle. Not in most cases anyway.
Click to expand...

Alright, so I watched the videos. They were helpful to refresh me on the various circuits. I loved the part where the record had to be flipped. My how times have changed as far as media.

As for the secondaries, here's a pic. You can see how, even with the curb idle adjuster pulled completely off the linkage, the transfer slots still show about .060" below the throttle blades. There's just no way for me to see how I can get the blades to adjust further to only show the .020" that I can get with the primaries.

secondary curb idle.JPG
 
threefortyduster said:
Alright, so I watched the videos. They were helpful to refresh me on the various circuits. I loved the part where the record had to be flipped. My how times have changed as far as media.

As for the secondaries, here's a pic. You can see how, even with the curb idle adjuster pulled completely off the linkage, the transfer slots still show about .060" below the throttle blades. There's just no way for me to see how I can get the blades to adjust further to only show the .020" that I can get with the primaries.

View attachment 1715537087
Click to expand...
Thanks.
We can see the blades are not stick in the bores, or sure shouldn't be, so what's hitting?
How about the linkage? Could that be it?

Holley still has the pdf from when they took over and they certainly expected the secondary ilde/t-slots to be adjustable down to .020 or less.
Demon 1282020 650 CFM Speed Demon Carburetor

There's a slim chance someone filed the t-slots longer, but the geometry doesn't look like that's whats going on.
 
Thanks for the info. I read up on the power valve myth more. Wish I had of known that 35 years ago, when there was no Internet, and all you had was the holley tuning book.
Why in world wouldn't holley rework that info on Web sites?
 
The op needs to post a clear full picture of the base plate from the bottom, just to make sure it doesn't have both the set screw underneath
as well as the external adjuster.
 
Not meaning to overstep anyone.
I agree it looks too tight
Disconnecting that linkage and seeing if it comes back to fully closed will tell you if it needs adjustment. (I'd loosen it up anways)

If that doesn't help the only idea I have left is taking the pump cam off and making sure it isn't holding things up. ( like forced on backwards or something)
 
D-Rocket said:
Thanks for the info. I read up on the power valve myth more. Wish I had of known that 35 years ago, when there was no Internet, and all you had was the holley tuning book.
Why in world wouldn't holley rework that info on Web sites?
Click to expand...


Yeah, it’s bad. I’ve called Holley on it and even called Quick Fuel before Holley bought it and it fell of deaf ears.

How many jacked up, slow running, fuel wasting engines were out there because Holley and ALL the authors writing books about this stuff wouldn't fix that? How many carbs would Holley had sold if they TAUGHT the end user how they work even at a rudimentary level?

Fortunately for us here on FABO, we have guys like Mattax who can teach this stuff better than me, and who are totally down with helping not just on carb tuning but how ignition timing and the timing curve affect each other. And not only type it out, but provide incredible graphics and drawings and crap like that, that I have no clue how to do.

Lucky fo me, I have known a guy for my entire life who is a guru at the very least on all this stuff, and he is more than willing to pas it on. He has mentored me through this stuff, and when I get stumped he takes the time to walk me through it.

He’s not on FABO, but he goes by Tuner on a couple other forums. When he talks, I’m listening. And taking notes. So I’ve been very lucky in this.


That’s why I get so pissy when the topic of power valve timing comes up and what is said is the often repeated error of how Holley does it..

If I didn’t mention it before, FABO member @12many did extensive testing of power valve timing and posted his results somewhere on FABO. Interesting stuff he learned and he was kind enough to post his results.
 
Amen. If it wasn't for 'Tuner' I might still be stuck in the mess I got myself into following some 'hot tips' from Vizard. That was an expensive lesson! So anything I can do to further the cause (engines that run well!) is pay back for all the help I got.
There were a few others, on the Innovate forum in particular, that helped me learn. Of them Bruce 'Shrinker' Robertson stands out even though he made my brain hurt.
laugh2-gif.gif


I also remember how hard it was to follow the tech at work who had been involved in class racing when he was explaining things to me. This was years before - pre-internet. He knew his stuff but informationwise it was like getting hit by a machine gun. LOL.
 
Mattax said:
Amen. If it wasn't for 'Tuner' I might still be stuck in the mess I got myself into following some 'hot tips' from Vizard. That was an expensive lesson! So anything I can do to further the cause (engines that run well!) is pay back for all the help I got.
There were a few others, on the Innovate forum in particular, that helped me learn. Of them Bruce 'Shrinker' Robertson stands out even though he made my brain hurt. View attachment 1715537372

I also remember how hard it was to follow the tech at work who had been involved in class racing when he was explaining things to me. This was years before - pre-internet. He knew his stuff but informationwise it was like getting hit by a machine gun. LOL.
Click to expand...


I tell Turner all the time WRITE A BOOK but he thinks nobody would read it. I say he’s wrong. Plus, he can throw in a few stories. He has some great ones!
 
D-Rocket said:
Thanks for the info. I read up on the power valve myth more. Wish I had of known that 35 years ago, when there was no Internet, and all you had was the holley tuning book.
Click to expand...
I’m kind of a late bloomer myself when it comes to tuning carbs, but with forums like this along with books if one has the desire to learn its really a cinch, for me better late than never. Still absorbing the wealth of knowledge. I’ll tell you one of the best things of having the PV timed correctly for me is getting on the primary throttle only under various load scenarios and feeling that valve come into play and the power thats instantly “now” :thumbsup:
 
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