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 Qjet for daily 454 
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Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
So my plans have changed for my Malibu. I was given a 85 c20 with a 454. Turns out the engine is actually out of a 79 454 and had the stock 800 large smab qjet. The number is 17059520. It has at least had the idle mixture screws in covered. I plan on a full blown dual main air bleed carb, but to get it moving is there any mods to do? I do plan on changing to the "car" secondary linkage that brings in the secondaries sooner. Can anyone tell me the primary jets and rods it should have? It had 73s 47P rods and CV secondary rods. Not sure what a 47P is.


Sun Apr 02, 2017 9:30 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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Do you mean large primary venturi with single main air bleed, the one over the discharge nozzle, and you intend to convert it to dual bleed? If it is a single bleed carb the primary metering rods have .036" tip, dual bleed have .026" tip. There are several variations of rods in later carbs that have different tapers in the lean step, some taper a lot, .010" or so, others not so much. I can't say what is best for your engine, jet size or type of rod, the engine will have to tell you that. You can open the secondary earlier in the primary travel, but with a heavy rig, unless it is a hot rod, don't get greedy. A little moderation will help the mileage if the vehicle is heavy, because more weight will need more throttle opening to satisfy the need for speed.

What do you mean, "are their any mods to do?" (LOL??) Do I recall correctly, you have already done some QJets this way? Do you have a bone pile for parts, jets, rods, levers, hangers, etc. etc?


Mon Apr 03, 2017 2:53 am
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Joined: Tue Aug 23, 2016 10:38 am
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To be clear I am swapping this 454 into my '74 Malibu 4 door. I installed a larger 228/235 cam, weiand dual plane and headers. It is a a low compression (7.8 to 1) peanut port 454. In the break -in stand it had 15+ in/hg at 800 rpm.

Yes it is a large primary with single main air bleeds over the dis charge nozzle. I am going to leave the '79 in this configuration and slowly build a nice low mileage edelbrock qjet of the same configuration into the dual bleed.
The apt on the '79 carb was backed out 4-1/4 turns and it was using 47P rods with a .026 tip. I suspect it was changed so I had intended to put in a 50m (.035 tip) to get it close to stock specs.

I am thinking about using a 78 base plate with exposed idle air mixture screws, larger .067 idle bypass ports (vs .055) and the shorter "car" secondary arm on the primary linkage.

I guess since I am not going fuel blown carb recalibratiin I wondered if these tend to be stupid rich so I should start a little lean or vise versa? I know I should just get it running. Just easiest to do a little something now.


Mon Apr 03, 2017 6:15 am
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Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
To complement the 454 I am also adding a 2800-3200 converter and 3.23 gears with a LSD.


Mon Apr 03, 2017 8:30 am
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Joined: Fri Feb 08, 2013 2:37 pm
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The APT was probably backed out that far because .073" MJ would be lean at mid-pedal with the large venturi single bleed, and that might be why it has the rod with .026" tip, whoever was tinkering with it was trying to get it rich enough it didn't feel sluggish. The large venturi single bleed usually needs .076 ~ .077" MJ, but will be rich at WOT with the .026" tip.

No fault to anyone for whatever calibration parts were in it, at least they were trying. The carbs with that single bleed arrangement are goofy, trying to make them better in one throttle position or range will make them worse in another, whereas the original dual bleed arrangement has a more natural or desirable carb behavior where the desirable A/F characteristics over the full range are provided without what seems like needing a different jet size when the throttle position is changed.

Quote:
I am thinking about using a 78 base plate with exposed idle air mixture screws, larger .067 idle bypass ports (vs .055) and the shorter "car" secondary arm on the primary linkage.

Do you mean the Idle Air By-Pass or Curb Idle Discharge (where the tip of the mixture screw is located and the idle fuel discharges into the bore)? When swapping bases and bodies, take care that the base to body gasket used has the required openings and correctly seals the potentially different cavities. It's OK to swap bases and bodies, but you need to study close for potential leaks or blocked IABP. Some of the castings have variations in IABP and Evap Purge passage locations that can result in unwanted idle air (AKA vacuum leak .... ask me how I know ..... just yesterday .... cough ..)

You plan is to plug the single bleed and drill .046" orifices to mimic the dual bleed, is that right?

What are you using for distributor and how is it calibrated?


Mon Apr 03, 2017 10:42 am
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Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
I have 2 of these carbs, both large primary SMAB. The current one, 17059520 a '79 will remain SMAB just to get the car going once I get the 454 installed. I was hoping to get what the jet/rod was stock and start there but I am unable to find this in the limited information I can find online. Do you have a suggested starting point for this carb for the rod/jets leaving it SMAB?

I have a 17058222 that I am borrowing the base plate off of. It has larger idle bypass holes (.067 vs .055). I will probably drill the curb idle discharge ports to .086. It already has the shorter arm on the primary throttle for the secondary activation. It also has exposed idle discharge screws. I compared both of the old gaskets for each baseplate and they are identical.

Once I get everything going and reasonably lined out with the above carb. I will start on converting an Edelbrock q-jet I picked up to the dual main air bleed, move the idle air bleed, "tuner" secondary mod, etc.

Crazy how much of my free time a 18 month old takes up. I love her to death but I am learning my "play time" doesn't start till 8:30-9 and only lasts till 10:30. Makes it hard to get lots of big projects done.

The dist is probably going to result in a disagreement again.

The last time I built one of these low compression 454s it was a complete dog until right before the shift point. I finally got it to run much better by limiting mechanical advance by welding the slot and using about 26 degrees of initial resulting in ~42 degrees total (minus vac). It was a rocket.

The dist is the stock 79 454 dist. This time I bought a moroso advance kit for a HEI (they seem to have the nicest weights of the kits I have bought) and I bent the weights in the same way you bent the points/msd weights limiting the advance to about 20 deg mechanical. The springs right now are the light ones but they seem to be too light and I will move to the med springs. The vac advance can is a 10 deg unit that is usually specified in the advance kits. I had this on the shelf and it is fairly new so I will use it to get started. I pulled the dist apart and cleaned/polished it then greased it so everything moves nice. I shimmed it for only .010 of end play and put on the best gear I had in my pile. The engine is still hanging from my engine host after coming home from breaking in the cam on my buddy and machinist's run stand. Still waiting on $$$ to buy a converter.

Do you have any problem using 2 different springs for a curve between light and med? I try to filter my "internet experts" but one recently that seemed to have a clue claimed that 2 different springs would cause the weights to bind. I have never seemed to have this problem but I am just a simple mech engineer.

thanks for all your help

John


Mon Apr 03, 2017 12:02 pm
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Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
Well after searching for a while I find the stock jet should be a 72 and the rod should be the 47P that it has. Does that still mean it will run poorly with the 73 jet and this rod?


Tue Apr 04, 2017 11:16 am
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Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
so doing some simple math and using the "recipe" guys suggestion for the 850 edelbrock as an example this is what I find.

73 jet 47P rod has an area of cruising of .00245 and a wot of .003654

"recipe's" suggestions are
77 jet 52m rod. This has a cruising area of .002533 and a wot of .003695

Am I wrong in thinking either of these will work nearly the same?


Tue Apr 04, 2017 11:35 am
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Joined: Tue May 06, 2014 5:34 pm
Posts: 42
I don't think you can switch baseplates between 78 and 79 carbs. As far a I know and from the data I've collected over the years, 79 was the beginning of 3 screws holding the baseplate to the main body (vs 2 before that).

1979 was also the first year of the recessed and often sealed metric thread idle mixture screws. Not that being recessed and finer thread causes a problem, but the 3 screw vs 2 screw deal makes me kind of wonder if it will work. Just check carefully.


Sun Apr 09, 2017 7:56 pm
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Joined: Tue May 06, 2014 5:34 pm
Posts: 42
80427 wrote:
so doing some simple math and using the "recipe" guys suggestion for the 850 edelbrock as an example this is what I find.

73 jet 47P rod has an area of cruising of .00245 and a wot of .003654

"recipe's" suggestions are
77 jet 52m rod. This has a cruising area of .002533 and a wot of .003695

Am I wrong in thinking either of these will work nearly the same?


Your math and thinking is good but one VERY important variable is the main air bleed size. If the air bleed in the Edelbrock carb is the same as in the other carb, all is good. If not, then it's like comparing apples to oranges.


Sun Apr 09, 2017 7:59 pm
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