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Quadrajet Tuning (SMI Rebuild)

trevmountain

trevmountain

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Kind of an update thread here. So as some of you know I sent off my older Edelbrock Quadrajet to Sean Murphy Induction or SMI for a custom rebuild to fit my application. Motor is also fresh and I am in the process of getting everything adjusted correctly. So far everything runs pretty good and I am trying to get as much info as I can regarding the fine tuning of these quads.

I started by adjusting my idle mixture screws to achieve maximum vaccum at the lowest possible smooth idle. I believe I have this as close as I am going to get it. I used a vaccum gauge and fiddled with it a few different times. With this being the only adjustment the truck runs pretty good but certainly does not feel optimal. At cruising speed I experience a slight uneven feeling or stutter. It is very slight, not a full on stumble. Basically she is just ain't purring like she should. Soooo should my next move be to start messing with the primary cruise mixture? Does this sound like it's running fat or lean?

Next symptom im experiencing is with the secondaries. When I mash it to the floor their is a pause or jump and then they open up. Like a wha-pause-whaaaaaaaa. I haven't started to mess with the secondary air door adjustment yet but I assume this would be my next adjutment? Other ideas for maximizing secondary opening?

I guess what I am looking for is a general step by step of adjustments to make and also what kind of response to look for when it's adjusted correctly.

Thanks
 
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On the secondaries, there is an adjustment for the spring that holds them shut, you can make it softer so they open earlier, or less vacuum, or you can make them stiffer so they open when you have higher RPM and could kick in only when you are driving aggressively but save gas most of the time.
trevmountain said:
Kind of an update thread here. So as some of you know I sent off my older Edelbrock Quadrajet to Sean Murphy Induction or SMI for a custom rebuild to fit my application. Motor is also fresh and I am in the process of getting everything adjusted correctly. So far everything runs pretty good and I am trying to get as much info as I can regarding the fine tuning of these quads.

I started by adjusting my idle mixture screws to achieve maximum vaccum at the lowest possible smooth idle. I believe I have this as close as I am going to get it. I used a vaccum gauge and fiddled with it a few different times. With this being the only adjustment the truck runs pretty good but certainly does not feel optimal. At cruising speed I experience a slight uneven feeling or stutter. It is very slight, not a full on stumble. Basically she is just ain't purring like she should. Soooo should my next move be to start messing with the primary cruise mixture? Does this sound like it's running fat or lean?

Next symptom im experiencing is with the secondaries. When I mash it to the floor their is a pause or jump and then they open up. Like a wha-pause-whaaaaaaaa. I haven't started to mess with the secondary air door adjustment yet but I assume this would be my next adjutment? Other ideas for maximizing secondary opening?

I guess what I am looking for is a general step by step of adjustments to make and also what kind of response to look for when it's adjusted correctly.

Thanks
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Yes I have a tach. Sorry but I am not clear on exactly what you mean with high and idle rpm on the sticker.
 
trevmountain said:
Yes I have a tach. Sorry but I am not clear on exactly what you mean with high and idle rpm on the sticker.
Click to expand...


There should be an emissions sticker that has the instructions on setting idle rpms. There are 2 to settiengs. High idle is when you start a cold k5. Then the chock spring heats up or the electic choke takes it off high idle to idle rpm. You set high idle rpm on the drivers as side screw that is not on the front of the carb.
 
It sounds like when cruising, it's running a little lean. I wouldn't change the primary jets, just go one size smaller on the primary metering rods.

The secondaries don't have an accelorator pump, so the door flaps (the flaps you see when looking at the carb from the top, the secondaries are below them) are opening too quick and causing a lean condition.

On the passenger side of the carb, next to the flaps, is a small screw and hidden underneath that area is a small allen set screw. The allen set screw is hidden and very hard to get to while mounted on the engine.

You loosen the allen set screw, and turn the other screw to 3/4 turn past "no tension". Less tension makes the flaps open sooner, more then 3/4 turn makes them much harder to open.

You can PM if you wish, it can be a long winded story on getting all the tuning right. But Q-jets are an excellent carb, when set up right.
 
I got my Q-jet done by SMI several years ago and the only things I initially touched were the idle mixture and choke. The secondaries were spot on for me, but I can tell you that if you call Sean, he will help you. I am NOT a carb guy but it sounds to me as you need a small amount more tension on the air doors for the secondaries. I can definitly tell you go maybe an 1/8th of a turn each time on the screw. I set my idle and high ildle speeds to what worked for my preference because I personally don't like an engine to rev much when cold, but still want to oil the cam decently. I eventually leaned my primary jets 2 sizes after looking at it with an O2 sensor.
 
Thanks for the responses

So what about the primary cruise mixture adjustment. I believe it is right on the front top of the carb below an allen screw plug.? Would I want to mess with this adjustment first before changing my metering rods? I figured it was probably a lean problem but wasn't sure.

Got the idle stuff figured out and thanks for that.
 
You could try adjusting that and see what happens just remember to write dowm what you do so as to know what to return to previous setting. I had a small stutter with my roller-cam 406 that I was able to cure with the vacuum advance. It was getting too much timing at cruise. Can you get any Idea of the mixture from the spark plugs?
 
6872xtc said:
You could try adjusting that and see what happens just remember to write dowm what you do so as to know what to return to previous setting. I had a small stutter with my roller-cam 406 that I was able to cure with the vacuum advance. It was getting too much timing at cruise. Can you get any Idea of the mixture from the spark plugs?
Click to expand...

What exactly did you do with your vacuum advance? Just have mine plumbed accordingly. My cruise stutter could very well be ignition, I have similar roller cam. I had the motor built and dyno'd. I just put it in the truck. Haven't done any timing as it was already done on the dyno. I don't have the paper work with me at the moment but I know the timing was set quite advanced. Maybe 15-20 degrees? Honestly I can't remember. It actually may be more like 30 degrees.

Yes I did pull a plug and forgot to mention this but they smell like gas. Not sure what this means but I sure it ain't good. Kind of forgot about that, I was so focused on just trying to get the carb adjusted I forgot about mentioning what my combustion looked like.
 
All I have done so far with the vacuum advance is unplugged it!! I have an adjustable unit that I am going to try but...SQUIRREL!!... when I get back to it I'll have to get more indepth. That was enough to let me know it had apparently too much timing at a cruise RPM of 1200-1300 in town.
 
Wanted to update this thread.

I got the carb back from SMI and I've been running the truck trying to get it broke in and attempting to get it all tuned and adjusted. I've had some issues with the engine just not running that great and not really feeling the power. I've got about a total of 300 miles on it with the break in oil. I recently took the truck to my mechanic to see if he could get it to run better and to look into any other issues I may have missed. Apparently the floats were off and the timing was way to far advanced. It runs better after messing with the timing for sure but it's just not all there. Don't get me wrong the motor runs decent, it's smooth, idles nice and does have power but I certainly ain't feeling this dynometer'd 500+ hp monster that it's suppose to be.

I don't have the build of the motor in front of me now but again dyno showed roughly 550 hp at around 5000rpm and like 420 at 3500 or so again don't have the paperwork in front of me. Torque was pretty much right on 500 all the way up the rpm band. I upgraded to a hyd roller cam, rpm intake, port work on the 049's ect. Wasn't really trying to build a neck breaker but more of a solid streetable engine. The results actually surprised my engine builder as he predicted the build to be about 80 to 100hp less.

I got 3:73's in the truck now. T 400 with stock stall, heavy duty converter, gear vendors.

One detail that may be contributing is the fact that when I sent all my info to SMI for the Q jet build I actually stated that I have 4:56 gears. I know he super custom builds these things so maybe this is part of the issue.

I know I am way to long legged with the tires, gear vendors ect and my plan is to put 4:56's in it soon. Actually got some set up to get just gotta do it. I would also potentially mess with my stall if need be.

Another issue I'm having is a finicky kickdown. I actually inspected the switch beneath the pedal and noticed that the switch wasn't being fully activated when mashed to the floor so I pulled the linkage out and welded a spacer on so that when mashed to the floor it would fully engage the kick down switch. This helped but not completely. More than half the time while cruising at low speed if I mash it it won't kickdown and it's just bog city. Sometimes it will but it's super inconsistant.

How much of this could just be gearing? What about the q jet deal? Is a pro carb builder really going to make huge changes to the way he builds a carb based on your gearing? What about break in oil and the whole break in procedure? Is the motor going to come to life when I throw regular oil in it. What about plugs? Engine builder put champion race plugs in it, can't remember the code. I know I've read many other posts regarding this debate.

Any info is always appreciated. Learned a ton from this forum and I thank you in advance.
 
I'm no pro but I did rebuild my own qjet and got it running ok on a stock engine. Having the float not set right could starve it of fuel. the float allows fuel to get into the bowl.
 
I am definitly not an expert, but I am wondering if maybe your expectations as far as seat of the pants feel is "out of calibration"?? I know I personally have had my expectations a little too high. It seems hard to factor in drivetrain loss as far as how it feels. I honestly don't think that the gears are affecting the carb as much as what you feel. I would just keep driving it and playing with it. Maybe it will grow on you and you will find the trick or someone that helps it really put a smile on your face! I have 4:56's in my truck with little 33's and I have fun with only 400 HP on the engine dyno.
 
I'm just wondering how much 4:56's will change the power curve. For example if you were to swapyour 4:56 to a 3:73 what difference would you expect. HUGE difference in get up and go grunt or suttle difference.
 
...about 20% difference
 
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