Street Demon 625 experience so far

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trebor75

trebor75

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I bought this carb about a year ago, brand new. Have not been doing much driving until lately though. This is my experience so far. Since it's a different design my old Moroso air cleaner hit the choke, so you either have to get a spacer to bring the air cleaner up a bit, or get the Street Demon air cleaner. The fast idle adjustment for the choke is accessed thru a hole in the back of the choke, facing the firewall. It's very hard to adjust without removing the carb.

Out of the box it has excellent part throttle response. The engine feels really happy and responsive. Easing into full throttle it pulls very hard. Great driveability so far! Returning to Idle it has some binding/sticking issue. I set it up to idle at 800rpm, I noticed it dropped to 600rpm sometimes. I removed my linkage and air cleaner and it's still there. This is after it has come of the fast idle cam. Pushing the linkage down with my fingers brought to it 600 rpm. I took the carb of to check the transfer slot and it was almost not visible. Set it to a square and opened and closed the throttle many times to get a feel and check for binding. It seemed alright but after installing it the problem remains. I have a fresh pair of return springs installed as manual suggest. So I need to take a closer look at this.

Driving it in a more spirited manner it has a bog when you stomp on it.
I borrowed my friends Innovate LM-1 wideband. The idle mixture was rich so I set it to around 14.7. That made it a bit leaner driving in low rpm's also. I get 12.5 on the highway at 2500 rpm. It has a bad lean bog going into WOT fast.

I don't have any jets or metering rods to swap as I just started tuning the carb. But I aim to lean it out a bit on the primarys. At WOT I got around 13, that's pretty good right? The Demon comes with 80 jets for the secondaries.

The secondaries have a air valve door to make the transition smooth. From manual: "The secondary air valve door is designed to dampen the transition from primary to secondary operation. Controlled by a torsion spring, the secondary air valve door is tunable to permit seamless primary to secondary throttle response regardless of the throttle rate opening. As an example, you experience a hesitation or backfire when the secondaries open. This means the secondary air door is opening to quickly, so it is necessary to adjust the door tighter (counterclockwise) until the hesitation disappears."

I've tuned this in small incremental steps to cure my lean bog going hard WOT. But the bog remains. So I moved the accelerator pump connection rod to the top position to give it larger pump shot. The bog remains. As I mentioned I dont have any jets, metering rods and accelerator pump nozzles to swap at this time.

I have a Pertronix Ignitor II & Flame Thrower coil. My initial is around 20-23. There is some lope in my cam so it's hard to say exactly. Total is locked to 36 and it's all in at around 2500 rpm. The motor seems happy, don't kick back, don't ping and don't get to hot.

I have checked float levels and they are spot on. I have checked the jets and nozzles for dirt.

So if I can get rid of the bog I will be a happy camper. Because I really like the throttle response of this Street Demon Carb. Compared to my Holley and Edelbrock I've had before this just feels much better for my combo. I'm not saying the other brands are bad in any way.

So I write this to get some tuning tips for the lean bog and just share my experience with the carb for anybody looking into getting one.

So this is my combo:
1970 Plymouth Duster
273 Commando - 1965
Compression: 10.5:1 (well this I dont know for sure, it's a true commando though).
Heads: Gasket matched and ported
Cam: Solid lifter Comp Cams Magnum 282S - 282/282 Lift 495/495 Beehive Springs
Edelbrock D4B intake / Street Demon 625 carb
PerTronix Ignitor II Billet distributer & Flame Thrower coil - Taylor plugwires
Hooker Headers
2.5" Dual Exhaust / Super 44 Flowmaster
727 auto / Shift Kit
10" Turbo Action Street "S-800" (17805SMED) converter (3200 stall)
8 3/4 - 3.55 - SureGrip
6 leaf - leaf springs from Espo
B&M Quicksilver Shifter
BF Goodrich 245/60-14 out back
 
Bought a Mighty Demon when they first came out BIGGEST piece of **** carb. i ever bought!!! would NEVER waste my money on one again!!!
 
It's basically an updated version of a carter thermoquad. I'm not an expert by any means, I'm just throwing out ideas. How far did you set the secondary airdoor spring tension? I know on my rebuild manual for my thermoquad it says to give it about 1 1/4 turn to set the initial tension of the spring. I'm curious to see how far you have it now and are still experiencing a bog. Also you could look at adjusting your metering rod step up spring. I had issues with my tq and I "richened" up the mixture by adjusting my metering rod tree. You could get a demon calibration kit and run a larger jet I think, but I'm not totally sure. Sorry if this wasted your time, just throwing out ideas. Love the car by the way!
 
Mighty Demon and the SD 625 are Holleys compared to TQ's.
The TQ "rod tree" can be adjusted for a global AF change, not normally tweaked but for high altitude areas.
check this link out Carter Thermoquad Information Page
"...Secondary Circuit Mixture Adjustment

The secondary circuit controls the fuel mixture throught the secondary throats, thus is only used under full throttle conditions. This mixture is controlled by two fixed jets which are screwed into the underside of the top cover. These jets look like two long brass cylinders.

Thus the only adjustment of the secondary mixture possible is replacement of these jets. Seat of the pants tuning is possible here - if it feels faster, the jets are probably closer to correct. Again, a run on a dynomometer will enable a more accurate result.

However, availability of alternative jets is limited: see my notes above regarding primary jets.

Secondary Throats Air-door Adjustment

The Thermoquad's secondary throttles are opened via a mechanical linkage to the primary throttles, beginning to open once the primary throttles are approximately 3/4 open, and becoming fully open at the same time as the primary throttles become fully open. There is also an air-door (similar to a choke butterfly in appearance) mounted at the top of the secondary throats, and clearly visible with the air-cleaner removed. This air-door is held closed via a spring, and will only open when the vacuum beneath it (created by the secondary throttles having opened) is sufficient to overcome the tension in the closing spring. This air-door enables a smooth transition to wide-open throttle in the following manner: because of the spring-loading of this door, it will be sucked open a short time after the secondary throttle plates are open; the secondary discharge tubes (visibly as two long tubes protruding into the upper sections of the secondary throats) are located above the secondary throttle plates, but below the air-door, thus are exposed to full manifold vacuum in the time between the throttle plates opening and the air-door opening: this ensures fuel flow through the secondary jets will be provoked by the vacuum effect just before the air-door opens and air starts to flow through the secondaries. In curcumstances where manifold vacuum is too low to begin drawing fuel through the secondary jets (eg: full throttle at low engine speed), the air-door will not open either, preventing any flow through the secondary throats, and preventing the engine bogging down through over-carburation.

In practice, there are two ways in which the air-door can be out of adjustment. The air-door spring can be under tensioned, allowing the door to open before fuel flow has started in the secondary jets; or it can be over tensioned, preventing the door from opening until sometime after fuel flow has started in the secondary jets. The first situation will result in a period of too-lean mixture, the second will result in a period of too-rich mixture. Both cases will cause a bog or stumble in the engine as the secondaries are opened.

Before attempting to adjust the air-door, ensure it is not jammed in any way. With the engine off, it should be possible to manually push the air-door fully open. It should move smoothly to it's full open position, and spring back to a closed position immediately when released. With the air-door held open, it is also possible to check the secondary throttles are working correctly. With the choke fully disengaged (push the fast idle screw to it's lowest position and hold there if necessary) look into the secondary throats whilst moving the throttle lever through it's full range. The secondary throttle plates should begin to open when the throttle lever is about 3/4 through it's range, and should become fully open once the lever is moved to it's limit. They should close smoothly as the lever is returned to it's base position.

SPRING ADJUSTMENT:

Eric can supply an invaluable tool for adjusting the secondary air-door

The air-door spring is located inside the carburettor, and is wrapped around the air-door shaft. Looking at the left side of the carburettor, the end of the air-door shaft is visible almost flush with the surface of the carb top cover. The shaft appears to ride in a slotted sleeve, and the end of the shaft itself is slotted. Spring tension is adjusted by turning the shaft itself. The slotted sleeve is actually the shaft lock. To adjust the spring tension, the lock is released by rotating it anti-clockwise, spring tension is then increased by rotating the shaft ANTI-clockwise, or decreased by rotating the shaft CLOCKWISE. When the desired tension is achieved, the shaft is locked by tightening the lock in a clockwise direction. Being spring loaded, of course, the shaft will attempt to spin itself fully clockwise whenever the lock is released. It is therefore necessary to hold the shaft in it's current position whenever the lock is released, or in the process of being locked or released. It's easiest to make these adjustments by using a screwdriver on the shaft, and the factory tool (or a copy of it) to release and tighten the lock whilst the screwdriver holds the shaft in place. It's difficult, but not impossible to just use two screwdrivers.

SPRING BASE SETTING:

Because the shaft will invariably slip out of your grip and spin down to the loosest position, a base setting for the spring tension is achieved as follows. Loosen the lock and allow the shaft to spin out (the air door will flop fully open). Wind the shaft in an ANTI-CLOCKWISE position until the air-door GENTLY closes. Wind the shaft an additional 1.5 turns anti-clockwise. Hold it in this position and tighten the lock.

FURTHER SPRING ADJUSTMENT:

It's easiest to adjust the spring from an under-tensioned starting point. If you feel (on the basis of symptoms described above) that the spring is overtensioned, reduce the tension (by winding the shaft CLOCKWISE) until symptoms of under-tension (ie: mixture leaning out) become evident when flooring the accelerator pedal whilst driving. Once it is obvious that the air-door is opening too soon due to spring under-tension, the object of the adjustment process will be to increase spring tension gradually until the symptoms are _just_ removed. This way, the secondaries will begin flowing air at the soonest possible opportunity, but not an instant too soon, thus maximising acceleration.

So: starting from a situation where the air-door spring is clearly under-tensioned, wind the shaft in 1/4 ANTI-clockwise turn increments, test driving the car in-between adjustments. As soon as the acceleration when the accelerator pedal is floored becomes smooth and forceful, without any bogging or hesitation, stop - dont, make any further adjustments.

Note: In practice, it is difficult to make a Thermoquad transition totally smoothly to full-throttle. Due to the extremely large size of the secondary throats relative to the primary throats, opening the secondary throats will always result in a dramatic slowing of air flow through the carburettor, resulting in at least a minor hesitation. Air-door adjustment is thus a matter of trial and error: starting from the base setting described above, first try reducing the tension on the air-door spring, if this makes the hesitation worse, try increasing the tension on the spring. Only add a 1/4 turn to the adjuster each time. Eventually you will find the position which results in the smoothest transition to full-throttle for your case - but it still may not be all that smooth. In my own experience, a fairly light tension on the spring gives the best result, but your case may be different.

Also, as always, it is assumed the carburettor is otherwise in good condition and correctly adjusted. Other factors can mask or worsen a hesitation on transition to full throttle. Eg: A carburettor that is running over rich may be smoother on opening the secondaries as the rich mixture will mask a momentary leaning out as the secondaries open.
 
dustadude said:
It's basically an updated version of a carter thermoquad. I'm not an expert by any means, I'm just throwing out ideas. How far did you set the secondary airdoor spring tension? I know on my rebuild manual for my thermoquad it says to give it about 1 1/4 turn to set the initial tension of the spring. I'm curious to see how far you have it now and are still experiencing a bog. Also you could look at adjusting your metering rod step up spring. I had issues with my tq and I "richened" up the mixture by adjusting my metering rod tree. You could get a demon calibration kit and run a larger jet I think, but I'm not totally sure. Sorry if this wasted your time, just throwing out ideas. Love the car by the way!
Click to expand...

Thanks :) The factory setting for the secondary air valve door is one full turn counter-clockwise after the air valve door is completely closed. I have many small adjustements of about ¼ turn at a time and then test it. I eventually ended up having it really tight. It still moves freely but it was very tight and that did not cure the stumble either. I might have missed a "sweet spot" during this so I'm gonna set it back to factory setting and try it again. Thanks you for the ideas!
 
pishta said:
Mighty Demon and the SD 625 are Holleys compared to TQ's.
The TQ "rod tree" can be adjusted for a global AF change, not normally tweaked but for high altitude areas.
check this link out Carter Thermoquad Information Page
"...Secondary Circuit Mixture Adjustment

The secondary circuit controls the fuel mixture throught the secondary throats, thus is only used under full throttle conditions. This mixture is controlled by two fixed jets which are screwed into the underside of the top cover. These jets look like two long brass cylinders.

Thus the only adjustment of the secondary mixture possible is replacement of these jets. Seat of the pants tuning is possible here - if it feels faster, the jets are probably closer to correct. Again, a run on a dynomometer will enable a more accurate result.

However, availability of alternative jets is limited: see my notes above regarding primary jets.

Secondary Throats Air-door Adjustment

The Thermoquad's secondary throttles are opened via a mechanical linkage to the primary throttles, beginning to open once the primary throttles are approximately 3/4 open, and becoming fully open at the same time as the primary throttles become fully open. There is also an air-door (similar to a choke butterfly in appearance) mounted at the top of the secondary throats, and clearly visible with the air-cleaner removed. This air-door is held closed via a spring, and will only open when the vacuum beneath it (created by the secondary throttles having opened) is sufficient to overcome the tension in the closing spring. This air-door enables a smooth transition to wide-open throttle in the following manner: because of the spring-loading of this door, it will be sucked open a short time after the secondary throttle plates are open; the secondary discharge tubes (visibly as two long tubes protruding into the upper sections of the secondary throats) are located above the secondary throttle plates, but below the air-door, thus are exposed to full manifold vacuum in the time between the throttle plates opening and the air-door opening: this ensures fuel flow through the secondary jets will be provoked by the vacuum effect just before the air-door opens and air starts to flow through the secondaries. In curcumstances where manifold vacuum is too low to begin drawing fuel through the secondary jets (eg: full throttle at low engine speed), the air-door will not open either, preventing any flow through the secondary throats, and preventing the engine bogging down through over-carburation.

In practice, there are two ways in which the air-door can be out of adjustment. The air-door spring can be under tensioned, allowing the door to open before fuel flow has started in the secondary jets; or it can be over tensioned, preventing the door from opening until sometime after fuel flow has started in the secondary jets. The first situation will result in a period of too-lean mixture, the second will result in a period of too-rich mixture. Both cases will cause a bog or stumble in the engine as the secondaries are opened.

Before attempting to adjust the air-door, ensure it is not jammed in any way. With the engine off, it should be possible to manually push the air-door fully open. It should move smoothly to it's full open position, and spring back to a closed position immediately when released. With the air-door held open, it is also possible to check the secondary throttles are working correctly. With the choke fully disengaged (push the fast idle screw to it's lowest position and hold there if necessary) look into the secondary throats whilst moving the throttle lever through it's full range. The secondary throttle plates should begin to open when the throttle lever is about 3/4 through it's range, and should become fully open once the lever is moved to it's limit. They should close smoothly as the lever is returned to it's base position.

SPRING ADJUSTMENT:

Eric can supply an invaluable tool for adjusting the secondary air-door

The air-door spring is located inside the carburettor, and is wrapped around the air-door shaft. Looking at the left side of the carburettor, the end of the air-door shaft is visible almost flush with the surface of the carb top cover. The shaft appears to ride in a slotted sleeve, and the end of the shaft itself is slotted. Spring tension is adjusted by turning the shaft itself. The slotted sleeve is actually the shaft lock. To adjust the spring tension, the lock is released by rotating it anti-clockwise, spring tension is then increased by rotating the shaft ANTI-clockwise, or decreased by rotating the shaft CLOCKWISE. When the desired tension is achieved, the shaft is locked by tightening the lock in a clockwise direction. Being spring loaded, of course, the shaft will attempt to spin itself fully clockwise whenever the lock is released. It is therefore necessary to hold the shaft in it's current position whenever the lock is released, or in the process of being locked or released. It's easiest to make these adjustments by using a screwdriver on the shaft, and the factory tool (or a copy of it) to release and tighten the lock whilst the screwdriver holds the shaft in place. It's difficult, but not impossible to just use two screwdrivers.

SPRING BASE SETTING:

Because the shaft will invariably slip out of your grip and spin down to the loosest position, a base setting for the spring tension is achieved as follows. Loosen the lock and allow the shaft to spin out (the air door will flop fully open). Wind the shaft in an ANTI-CLOCKWISE position until the air-door GENTLY closes. Wind the shaft an additional 1.5 turns anti-clockwise. Hold it in this position and tighten the lock.

FURTHER SPRING ADJUSTMENT:

It's easiest to adjust the spring from an under-tensioned starting point. If you feel (on the basis of symptoms described above) that the spring is overtensioned, reduce the tension (by winding the shaft CLOCKWISE) until symptoms of under-tension (ie: mixture leaning out) become evident when flooring the accelerator pedal whilst driving. Once it is obvious that the air-door is opening too soon due to spring under-tension, the object of the adjustment process will be to increase spring tension gradually until the symptoms are _just_ removed. This way, the secondaries will begin flowing air at the soonest possible opportunity, but not an instant too soon, thus maximising acceleration.

So: starting from a situation where the air-door spring is clearly under-tensioned, wind the shaft in 1/4 ANTI-clockwise turn increments, test driving the car in-between adjustments. As soon as the acceleration when the accelerator pedal is floored becomes smooth and forceful, without any bogging or hesitation, stop - dont, make any further adjustments.

Note: In practice, it is difficult to make a Thermoquad transition totally smoothly to full-throttle. Due to the extremely large size of the secondary throats relative to the primary throats, opening the secondary throats will always result in a dramatic slowing of air flow through the carburettor, resulting in at least a minor hesitation. Air-door adjustment is thus a matter of trial and error: starting from the base setting described above, first try reducing the tension on the air-door spring, if this makes the hesitation worse, try increasing the tension on the spring. Only add a 1/4 turn to the adjuster each time. Eventually you will find the position which results in the smoothest transition to full-throttle for your case - but it still may not be all that smooth. In my own experience, a fairly light tension on the spring gives the best result, but your case may be different.

Also, as always, it is assumed the carburettor is otherwise in good condition and correctly adjusted. Other factors can mask or worsen a hesitation on transition to full throttle. Eg: A carburettor that is running over rich may be smoother on opening the secondaries as the rich mixture will mask a momentary leaning out as the secondaries open.
Click to expand...

Thanks. It will take me a little time to go through all the information here. Very appreciated!
 
stroked340 said:
Demon brand is "garbage" good luck trying to get it run right..
Click to expand...

I dont get post like this. Everybody is entitled to their opinion. But your post really isn't helping me much either. I wont write a carb of as garbage just because one person is telling me it is. If you read my first post I have a lot of positive things to say about this carb. I just have a bog when I stomp on it to WOT. Many carbs untuned for the setup could have that. I would be suprised if it's impossible to cure taking the time tuning it.
 
trebor75 said:
I dont get post like this. Everybody is entitled to their opinion. But your post really isn't helping me much either. I wont write a carb of as garbage just because one person is telling me it is. If you read my first post I have a lot of positive things to say about this carb. I just have a bog when I stomp on it to WOT. Many carbs untuned for the setup could have that. I would be suprised if it's impossible to cure taking the time tuning it.
Click to expand...
Ignore it.
Are you sure its a lean bog?
What elevation are you at? Most factory setup are for 72*F sea level. If your at high elevation you are rich and need a tune kit. I always tune the secondary door spring in 2nd gear WOT pulls. I'm a TQ guy but the Demon is similar. I run a Demon 750DP and they are good carbs IMO.
 
I had a Demon carb on my 340. Seemed like a good idea. I looks like a nice carburetor. However, I never could get it to return to a consistant idle speed. Also had the same bog as you describe, and I could not get rid of it. My car runs great with an out-of-the-box Holley #3310 750 on it though!
 
mderoy340 said:
Ignore it.
Are you sure its a lean bog?
What elevation are you at? Most factory setup are for 72*F sea level. If your at high elevation you are rich and need a tune kit. I always tune the secondary door spring in 2nd gear WOT pulls. I'm a TQ guy but the Demon is similar. I run a Demon 750DP and they are good carbs IMO.
Click to expand...
Yes I am sure. The wideband pegs super lean at the bog.
Geographic coordinates of Stockholm, Sweden
Latitude: 59°19′57″ N
Longitude: 18°03′53″ E
Elevation above sea level: 28 m = 91 ft
 
69_340_GTS said:
I had a Demon carb on my 340. Seemed like a good idea. I looks like a nice carburetor. However, I never could get it to return to a consistant idle speed. Also had the same bog as you describe, and I could not get rid of it. My car runs great with an out-of-the-box Holley #3310 750 on it though!
Click to expand...
Thanks for sharing. Was this also a Street Demon carb? If so, how did you try to get rid of it?
 
trebor75 said:
Yes I am sure. The wideband pegs super lean at the bog.
Geographic coordinates of Stockholm, Sweden
Latitude: 59°19′57″ N
Longitude: 18°03′53″ E
Elevation above sea level: 28 m = 91 ft
Click to expand...
As a test, Fatten up the idle mixture screws and see if it helps with the bog. You could also increase (drill nozzle by .001) steps.
 
mderoy340 said:
As a test, Fatten up the idle mixture screws and see if it helps with the bog. You could also increase (drill nozzle by .001) steps.
Click to expand...
Yes that sounds like something I should try asap. Before I went home from the garage today I leaned over the carb with a flashlight. I could not see any gas from the pump shot. I really made an effort but I could not get a visual of gas coming out of the accelerator pump nozzle when opening the throttle. The linkage has no play. I need to take a closer look at this. I'd be really surprised if there was something wrong with the pump because throttle response is amazing. Just weird that I could not see it.
 
stroked340 said:
Bought a Mighty Demon when they first came out BIGGEST piece of **** carb. i ever bought!!! would NEVER waste my money on one again!!!
Click to expand...

That's because Barry Grant didn't know how to run a company. Holley has long since bought Barry Grant out so the product is much better.
 
69_340_GTS said:
Some of the parts from the kit are inside the carb now. So it is not a complete kit.
Click to expand...
I see. I need to ask you something since you have experience with this carb.
Before I went home from the garage yesterday I leaned over the carb with a flashlight. I could not see any gas from the pump shot. I really made an effort but I could not get a visual of gas coming out of the accelerator pump nozzle when opening the throttle. The linkage has no play. I need to take a closer look at this. I'd be really surprised if there was something wrong with the pump because throttle response is amazing. Just weird that I could not see it.
 
stroked340 said:
Whats got to do with a shitty carb.:icon_fU::icon_fU::icon_fU:
Click to expand...

The same as any of your nasty comments has to do with anything. Nothing. I just said it to be nasty. Do you need a safe space now? If you caint take it, stop dishin it out.
 
Last edited by a moderator:
If you cant see a stream of fuel squirting into the side of the venturi, you have a pump issue. I cant fathom (see last sentence) a plugged squirter as the pump arm would show no movement, but if you have a linkage issue, then maybe. The pump squirter is located right under the choke flap (front) and is a round dealy with allen head bolt. There is a physical black tube (see figure 25 in link) connecting the pump well to the shooter that may be disconnected inside the top plate. But a dead lean spike on acceleration and no spray is simply a non functioning pump circuit. The pump rod itself is spring loaded so if the rod is moving, that is not proof the the piston is moving in the bore.
http://static.speedwaymotors.com/pdf/91012017.pdf
 
pishta said:
If you cant see a stream of fuel squirting into the side of the venturi, you have a pump issue. I cant fathom (see last sentence) a plugged squirter as the pump arm would show no movement, but if you have a linkage issue, then maybe. The pump squirter is located right under the choke flap (front) and is a round dealy with allen head bolt. There is a physical black tube (see figure 25 in link) connecting the pump well to the shooter that may be disconnected inside the top plate. But a dead lean spike on acceleration and no spray is simply a non functioning pump circuit. The pump rod itself is spring loaded so if the rod is moving, that is not proof the the piston is moving in the bore.
http://static.speedwaymotors.com/pdf/91012017.pdf
Click to expand...
Alright. I will have it apart and take a closer look at it. I already had the nozzle of to see if it was plugged. Depressing the pump arm with my finger I can feel that it is spring loaded and it returns nicely. There is a needle under the nozzle, the pointy side should face down right?
 
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