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Discussion Starter · #1 ·
Wanted to share my experience in swapping out my Qjet for a Street Demon 625. I was curious about this carb when it was released in early 2012 because of its similarities with quadrajets and AFB's. I ordered the #1901 carb, the version with the polymer fuel bowl, along with the throttle bracket, matching air cleaner and Demon's tuning kit, which contains an assortment of jets, metering rods and springs.

The swap was easy, as all I had to do was remove the power brake hose, vacuum advance tube, fuel line, throttle cable and the 200-4R TV cable. Demon supplied the necessary gaskets, studs and nuts to install the carb on my Edelbrock Performer intake. They also included the proper attachment ball that my TV cable snapped onto. Regarding the electric choke, I had to run the supplied electric choke wire to a switched 12V source. Good thing I got Demon's air cleaner too because it's specially recessed to clear the carb's large choke assembly. My old air cleaner from the qjet would not clear the Street Demon's choke housing.


UNFORSEEN DELAYS

I needed the following additional parts to complete the swap:
> 40 micron inline fuel filter (purchased Jegs #15040)
>> Demon requires an inline fuel filter with at least a 40 micron screen element.

> New throttle cable
>> The original throttle cable and bracket that was used for my qjet was not compatible with the Street Demon 625, as neither the length nor the attachment head worked with the Street demon's attachment point. Purchased Lokar's 36" universal throttle cable.

> New fuel hose
>> Because the Street Demon's fuel inlet is high on the passenger side, I needed to extend the length of the fuel hose.


FIRST TIME START-UP

Before firing it up, I checked if the new throttle cable was properly configured. After verifying that the throttle blades were at W.O.T. with the accelerator pedal floored, I followed Demon's guidelines of cranking the engine for about 30 seconds with the coil wire disconnected. This primes the fuel bowl. Then I connected the coil wire, pressed the pedal 3 times and fired it up. It stumbled a little, but to my surprise the choke kicked in an it stabilized around 1400 rpm. I gave it 2 minutes to warm up an pressed the pedal, and the carb came off of its fast idle cam, revving at 900 rpm (it's nice to have a functional choke again!).

Here's a video clip of the start-up, after coming off the fast idle cam:


After the engine reached its normal operating temperature, I adjusted the idle mixture screws for best vacuum. My engine needed 3.5 turns out.


FIRST DRIVE IMPRESSIONS

I spent nearly an hour driving on the streets and freeways, testing cruise, part throttle and heavy throttle acceleration, and even W.O.T. First thing I noticed was the feel of the pedal, - the qjet's throttle spring was much looser than the Street Demon's. I guess it shows how worn the spring was on the qjet? The acceleration feels more linear with the Street Demon. With my qjet, the power came on like a surge as the RPM's rised, with the Street Demon it's a steady, linear increase in push as the RPM's rise. Also, when the Street Demon's goggle secondary opens up, the roar of the exhaust is louder and deeper. The car feels stronger now at cruise, it has more pep when transitioning to a part throttle acceleration. Where it feels similar to the qjet is when I'm at heavy or wide open throttle.

Here are the results, with no tuning adjustments made except for the idle mixture screws:

> VACUUM @ IDLE: 14.5 = PARKED, 12 = DRIVE
> AFR @ IDLE: 12.7 = PARKED, 12.9 = DRIVE
> RPM @ IDLE: 900 = PARKED, 800 = DRIVE
> AFR @ CRUISE = 13.7
> AFR @ PART THROTTLE ACCEL: 13.1 – 13.9 (13.5 AVG.)
> AFR @ HEAVY THROTTLE ACCEL: 11.9 – 13.1 (12.5 AVG.)
> AFR @ W.O.T.: 11.5 – 12.1 (11.8 AVG.)

For comparison, this was the best tune for my qjet:

> AFR @ IDLE: 13.5
> AFR @ CRUISE: 14.4 – 16.2 (AVG. 15.3)
> AFR @ PART THROTTLE ACCELERATION: 14.2 – 15.8 (AVG. 15)
> AFR @ HEAVY THROTTLE ACCELERATION: 12.3 – 12.5 (AVG. 12.4)
> AFR @ W.O.T.: 12.1 – 12.5


WHAT'S NEXT?

For now, I'm going to enjoy the summer with this carb and feel it out. I'm impressed that out of the box, it's calibrated so close to my engine's needs. I might mess with idle rpms but that's all for now. I'm also enjoying the fact that my garage no longer wreaks like fuel, as the Street Demon does not have a vent tube in the front like my qjet did. It will be interesting to see how the car starts tomorrow morning. With the qjet I needed to pump 3 times, crank, pump 3 more times, crank and fire up. Will I get away with an easier start with the Street Demon? We'll see...
__________________

68 Firebird: 350 .040 over, #11 Heads w/2.05/1.66 valves, 8.8:1 CR, Tomahawk TC-03 cam, 200-4R w/3.36 gears, Street Demon 625 w/ Edelbrock Performer, H-Pipe dual exhaust w/Delta Flow 50's.
 

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Runs nice!!
Sounds good!
Idle's too high though. Should be able to turn it down to 650-700 rpm then your mixture screw will respond better. 3.5 turns is too much. You've uncovered the transition slot at that idle speed.
What's your timing set at?
And do all your hard work a favour and replace that "rubber" fuel line with a steel tubing one. Rubber will leak and you'll have a towering inferno.
 

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Discussion Starter · #4 ·
Excellent review, thanks- that carb was an unknown to me before now, as far as first hand experience.

Question: Where did the 2.05" intake valves come from? Do they seem to work well w/this combo?
Thanks for compliment! The 2.05" intake valves replaced the original 1.96" valves when the heads were reworked. Along with some mild porting, the heads were resurfaced to 85 cc to get 8.9:1 compression ratio. Since my block was a 1970 vintage, it was already chamfered to accommodate the larger valves.

As far as how it works with my combo, the engine was tested on a chassis dyno before and after the rebuild. The rebuild involved:

> Pontiac 350 .040 over
> 214/221 flat tappet cam, 112 LSA, .0450/.0465 lift w/ 1.5 rockers
> #11 heads, resurfaced to 85cc, 8.9:1 CR
> 2.05" intake/1.66" exhaust valves
> Edelbrock Performer intake with Q-jet
> Stock manifolds w/ 2.5" dual exhaust, H-pipe, Flowmaster Delta Flow 50's

RESULTS: 227 HP / 272 TQ
PRIOR TO REBUILD: 169 HP / 248 TQ

Here's the dyno chart:


 

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Good flat torque curve, nice. Not stellar numbers but w/the curve like it has I'd expect it to be a ball to drive.

Was any port work done to compliment the bigger intake? 3 angle valve job? I wonder if kicking up the rocker ratio to 1.6/1.65 would show good results?
 

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Discussion Starter · #6 ·
Good flat torque curve, nice. Not stellar numbers but w/the curve like it has I'd expect it to be a ball to drive.

Was any port work done to compliment the bigger intake? 3 angle valve job? I wonder if kicking up the rocker ratio to 1.6/1.65 would show good results?
Yes, there was mild port work done to compliment the 2.05" intakes, in addition to a Pontiac valve job. I'll acknowledge the numbers aren't stellar, but there was some HP left on the table because the quadrajet wasn't tuned after the rebuild. Also, I later found out the engine builder had wired the ignition coil incorrectly; it was an Accel coil that needed a full 12V, but he had not bypassed the ballast resistor, causing the coil to get only 10V.

It's possible I can get better results with higher rocker ratios, maybe with a different cam as well? Also, I still have the factory log-style exhaust manifolds on the engine, so it's possible with a header swap I might squeeze some more out of the engine.

But honestly, it's fun to drive it as it is. And with this carb, it feels more responsive than it did with the qjet.
 

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Biggest potential gains I see are long tube headers. The HO/RA cast iron manifolds aren't terribly bad, but the standard production manifolds leave a lot to be desired as you know.

I'd prolly stay away from more cam unless you're willing to go w/more stall speed, steeper gears, etc. But overall, I thing a crisp carb tune, a performance advance curve (this has been done I'm thinking) and good exhaust system will wake it up just fine. I don't know if the increase in rocker ratio would be much benefit, it would be interesting to see, though. Obviously the springs need to be up to the task, both in pressure and coil bind from the increased lift. And the pushrod holes in the heads will likely need opening up some...
 

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I recently installed the same carb on my 4.3 V6 in my chevy pickup.
It had a Quadrajet on it, but trying to tune it for the V6 was more trouble than it was worth so I just said forget it and ordered the street demon.
I'm satisfied with it thus far. Only con to it is it don't have enough vacuum ports. I perfer to set the idle mixture with a vacuum gauge and since the only manifold vacuum port it can plug into is the one going to the vacuum advance leaves no option but for me to set them by ear.
 

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I recently installed the same carb on my 4.3 V6 in my chevy pickup.
It had a Quadrajet on it, but trying to tune it for the V6 was more trouble than it was worth so I just said forget it and ordered the street demon.
I'm satisfied with it thus far. Only con to it is it don't have enough vacuum ports. I perfer to set the idle mixture with a vacuum gauge and since the only manifold vacuum port it can plug into is the one going to the vacuum advance leaves no option but for me to set them by ear.
or just install a T fitting, or use a port on the manifold.

FWIW I would also lean out the OP's idle adjustment just a hair, it won't make a huge difference, just a little cleaner plugs, a little better mileage, and a little cleaner oil- but the difference will be slight. It may help to bump your initial timing up a couple degrees but keep your total the same.
 

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Discussion Starter · #10 ·
TEST RESULTS - 1st Tune

My experience with the carb so far has been positive. Cold starts have been consistent, - 2 pumps on the pedal and the crank of the key. The electric choke kicks in and revs steady around 1500 rpm, until I nudge the pedal and it idles back down to 800 rpm. At full operating temperature I’m getting steady idle at 700 rpm. When the engine’s warming up and the choke flap isn’t fully open, my idle AFR is 12.5. When I’ve reached normal operating temps the idle AFR settles around 13.6. I have experimented with the idle mixture screws since my initial post and they're currently 2 2/3 turns out.

Hot starts require gently pressing on the pedal halfway while cranking. If I just simply turn the key after about 10-15 minutes, it takes awhile to fire up. Throttle response overall has been very much like my Qjet, with no hesitation, flat spots or dead spots in power while accelerating. Also, I'm not feeling any stumble when the goggle secondary opens.

I've made 2 changes so far:

1) METERING ROD SPRINGS

The stock metering rod springs were rated 6” Hg and were richening my part-throttle AFR too late. I swapped them with Demon’s silver springs, rated 8” Hg and experienced a faster transition to the rich step on the metering rod. As a result my part-throttle acceleration felt more responsive.

2) SWITCHED TO MANIFOLD VACUUM ADVANCE

This increased the vacuum readings at idle and smoothed out the engine's idle also. I had to reset the idle screw on the carb by closing the primary throttle blades further than before. Timing is now at 17 degrees idle, 34 degrees all in at 3000 rpm. The vacuum advance is adding 16 degrees, so with m.v.a. I'm at 33 degrees @ idle.

Here are the new results:

> VACUUM @ IDLE: 16.5 (PARKED), 14 (DRIVE)
> AFR @ IDLE: 13.3 (PARKED), 13.6 (DRIVE)
> RPM @ IDLE: 800 (PARKED), 700 (DRIVE)
> AFR @ CRUISE: 14.2
> AFR @ PART THROTTLE ACCEL: 13.7 - 14.1
> AFR @ HEAVY THROTTLE ACCEL: 12.2 - 13.5
> AFT @ W.O.T.: 12.2
> IDLE MIXTURE SCREWS: 2 2/3 TURNS OUT
> 60/52 METERING RODS
> SILVER METERING ROD SPRINGS (8" Hg)

Here are the previous results:

> VACUUM @ IDLE: 14.5 = PARKED, 12 = DRIVE
> AFR @ IDLE: 12.7 = PARKED, 12.9 = DRIVE
> RPM @ IDLE: 900 = PARKED, 800 = DRIVE
> AFR @ CRUISE = 13.7
> AFR @ PART THROTTLE ACCEL: 13.1 – 13.9 (13.5 AVG.)
> AFR @ HEAVY THROTTLE ACCEL: 11.9 – 13.1 (12.5 AVG.)
> AFR @ W.O.T.: 11.5 – 12.1 (11.8 AVG.)
> IDLE MIXTURE SCREWS: 3.5 TURNS OUT
> 60/52 METERING RODS
> GREEN METERING ROD SPRINGS (6" Hg)
 

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My experience with the carb so far has been positive. Cold starts have been consistent, - 2 pumps on the pedal and the crank of the key. The electric choke kicks in and revs steady around 1500 rpm, until I nudge the pedal and it idles back down to 800 rpm. At full operating temperature I’m getting steady idle at 700 rpm. When the engine’s warming up and the choke flap isn’t fully open, my idle AFR is 12.5. When I’ve reached normal operating temps the idle AFR settles around 13.6. I have experimented with the idle mixture screws since my initial post and they're currently 2 2/3 turns out.

Hot starts require gently pressing on the pedal halfway while cranking. If I just simply turn the key after about 10-15 minutes, it takes awhile to fire up. Throttle response overall has been very much like my Qjet, with no hesitation, flat spots or dead spots in power while accelerating. Also, I'm not feeling any stumble when the goggle secondary opens.

I've made 2 changes so far:

1) METERING ROD SPRINGS

The stock metering rod springs were rated 6” Hg and were richening my part-throttle AFR too late. I swapped them with Demon’s silver springs, rated 8” Hg and experienced a faster transition to the rich step on the metering rod. As a result my part-throttle acceleration felt more responsive.

2) SWITCHED TO MANIFOLD VACUUM ADVANCE

This increased the vacuum readings at idle and smoothed out the engine's idle also. I had to reset the idle screw on the carb by closing the primary throttle blades further than before. Timing is now at 17 degrees idle, 34 degrees all in at 3000 rpm. The vacuum advance is adding 16 degrees, so with m.v.a. I'm at 33 degrees @ idle.

Here are the new results:

> VACUUM @ IDLE: 16.5 (PARKED), 14 (DRIVE)
> AFR @ IDLE: 13.3 (PARKED), 13.6 (DRIVE)
> RPM @ IDLE: 800 (PARKED), 700 (DRIVE)
> AFR @ CRUISE: 14.2
> AFR @ PART THROTTLE ACCEL: 13.7 - 14.1
> AFR @ HEAVY THROTTLE ACCEL: 12.2 - 13.5
> AFT @ W.O.T.: 12.2
> IDLE MIXTURE SCREWS: 2 2/3 TURNS OUT
> 60/52 METERING RODS
> SILVER METERING ROD SPRINGS (8" Hg)

Here are the previous results:

> VACUUM @ IDLE: 14.5 = PARKED, 12 = DRIVE
> AFR @ IDLE: 12.7 = PARKED, 12.9 = DRIVE
> RPM @ IDLE: 900 = PARKED, 800 = DRIVE
> AFR @ CRUISE = 13.7
> AFR @ PART THROTTLE ACCEL: 13.1 – 13.9 (13.5 AVG.)
> AFR @ HEAVY THROTTLE ACCEL: 11.9 – 13.1 (12.5 AVG.)
> AFR @ W.O.T.: 11.5 – 12.1 (11.8 AVG.)
> IDLE MIXTURE SCREWS: 3.5 TURNS OUT
> 60/52 METERING RODS
> GREEN METERING ROD SPRINGS (6" Hg)
I would attempt to lean the WOT and cruise to help fuel economy but overall not bad. Thanks for posting the follow up.
 
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