mongoose87

I am getting ready to install all the things necessary to do a vortec head swap from my stock 1972 L48 engine. Right now I have a stock divorced choke quadrajet, that runs ok, but as most know, Vortec heads do not have a heat crossover, so there are is nothing there on the Edelbrock vortec performer intake i want to use for heating up the coil for the choke.
As a result, I bought a used R4-M4MC quadrajet (17080213) off ebay that seems like it was converted to electric choke. I intend to do a full rebuild but I can't figure out how the secondary linkage works. There is a slot on the secondary cam linkage that prevents the secondary butterflies from opening (or so it seems). Not sure if I was stupid in buying this



If you see the pictures, there is a slot in the secondary linkage that allows the primary to open up all the way, while not opening the secondary at all. Is this how it is intended? Does vacuum open the secondary butterflies as well as the air valve on these later Q- Jet versions ?

lars

You have a broken actuation spring.

The 1980 carbs were electric choke carbs - that's the stock setup.

Should look like this, and the secondary throttle plates open fully with full opening of the primary throttle:









Lars

Edit 8:11 pm: Looking closer at your photos, your spring may not be broken. But the one end is disconnected from the actuation lever - I can see the loose end in the top photo. It needs to be hooked as shown in my photos. With that end loose, the secondaries won't open.

7T1vette

What he said.... But you should also know that the secondary throttle plates open more toward the completion of throttle linkage movement. So just be aware that the full throttle movement needs to be made for the secondaries to fully open. The electric chokes on the late C3 models eliminated many of the choke problems with heated and divorced choke systems. I'm not sure if you can 'graft' the later Q-Jet electric choke onto the earlier Q-Jets. Lars would be the best person to advise you on that; so you should continue this conversation with him to determine the best direction for you to go at this point.

lars

See my 8:11 pm edit in my post above. Your actuation spring is disconnected from your actuation lever. Hook it up.
Lars

mongoose87

Thanks for your replies! I feel stupid for not figuring this out.
With this being a carb from a different vehicle, I'm wondering what jets, primary rods, and secondary rods I should start tuning with. I know it would be somewhat of a guess, but just looking for a starting point.
Right now I have 43B primary rods, and I believe DA secondaries.

From what I've read, the cam I want to put in will only affect carb tuning slightly, but I still need to get the car running for the cam break in.
Hydraulic roller with Advertised Duration 266/270, Lift .485/.495 with 112 LSA
1 5/8 headers with 2.5" all the way back.

Any guesses?

lars

You can't use "B" series rods in an M4M carb - they're the wrong length. Jet the carb to its stock spec for the carb number and tune from that baseline if needed.

Also: Are you aware that that's a truck carb..? You can't use any primary rod with a .026" power tip in that carb or you will increase WOT fuel flow on the primary side by 16%, running the WOT mixture potentially down into the 10.5:1 range. I'd dump the carb and get a 1976 - 1980 Chevy passenger car carb. Keep the choke components and choke housing to retro-fit a hot air choke carb to electric.

Lars

mongoose87

Thanks for your response Lars! It is always great to get expertise from one of the best.
I knew the rods and jets are different from the earlier style quadrajets, but I thought all the rods would be easily available in all the online stores. I bought another used q-jet last night, a 17059504, which appears to be a 1978-1979 el camino, corvette (etc) carb.

I had no idea there were significant differences between the truck quadrajets and the passenger cars, but after doing a little research, it all seems related back to the rods not being available with the appropriate jets.

I am still unclear on exactly why a truck quadrajet will not work if the jets and rods are changed. I saw these from Cliff Ruggles site:
https://cliffshighperformance.com/Qu...-metering-rods

In the details it says they can be used on applications were the 0.026 rods were used, although the jets have to be changed. Not exactly clear how they can be used for all the K, J, L or P applications, but it I guess changing the jets does all that's needed?
Is there also something to do with the air bleeds between the truck and passenger carbs?

Hypothetically, could I sand down the M truck rods to 0.026 and buy some smaller jets and open them up to exactly what is needed?
I would never do this, but I have heard of old school hot rodders doing this.

7T1vette

The rods have specific shape and size to help regulate the fuel mixture as needed throughout the operating range of the Q-Jet. Just changing size (diameter) of the rod won't do what you want. Primary rods (and secondary rods as they progress thru airvalve movement) are designed for good transition as RPM increases. The correct rods and jets for your specific application are what is needed.

mongoose87

I get that the rods and very specifically made for smooth transition. I wouldn't actually sand down the rods on the truck q-jet, but i was throwing out a hypothetical to better understand what the difference is between the truck carb and the passenger carb other than rods and jets.

I bought the 17059504 carb last night, but with the vortec heads, cam, headers, and intake, I wonder if I will have to change the rods and jets anyway. If it's just the jets that are the difference between the truck and passenger car q-jets, won't I still be in the same place with 17059504 carb?

lars

The truck carbs have triple-booster venturies specifically designed for low-rpm fuel response. You cannot use passenger car rods or jetting specs in that carb. You also cannot use the early 4MV "B" rods in any M4M carb - the rods are different lengths. Why don't you just get the right carb instead of doing a Bubba-job on a truck carb that was never intended for your application? If you're going to piece that thing together with .026-tipped rods and custom jetting to compensate for the fuel flow difference, you're also going to have to start playing with high speed and low speed air bleeds. The only way you'll get all that right is to spend 4 hours of dyno time to get the fuel curve in the ballpark. I just don't see the point... I'd never do it, but if you have time and money on your hands, it sounds like an interesting project...

Lars

mongoose87

Thanks for all the advice. That passenger car 17059504 carb should be arriving this weekend, so hopefully it wont take much to get it working with my combination.

mongoose87

So i cleaned up the 17059504 corvette carb designed for a 1979 corvette, el camino etc. The one I got looked to have all the stock rods and jets according to Lars, tuning paper, although the paper I was looking at specifies "40," so I'm unsure which "40" primary rods have to be used.

Mine came with 40K rods, but when I finished up cleaning, adjusting and putting all the rebuild gaskets, needle, seat, etc. in, and I was putting the air horn back on, and I mistakenly bent one of the primary rods.
DAMN MY CLUMSY HANDS!
Personally, I think putting the air horn back on while holding everything in place is the hardest part of a rebuild.

I found some used 40L, 41L and 40J rods from quadrajetparts.com that I am hoping can replace the 40k rods. Unfortuneately, they didn't have any used 40K rods, or else I would have bought those. In the description these all look to have the 0.026 power tip, but not too much info on exactly their intended purposes or differences. I know the tapers are different, but how?
I am also thinking of buying jet sizes 1, 2, and 3 sizes larger (jets 73, 74 and 75) to help me tune the engine once I'm done installing all the new performance parts.

Can someone tell me if one of these will work?
Will the air bleeds need to be changed as a result?
APT setting changed?
Power piston spring change?
Is there somewhere that goes over the different rod letters and their exact taper differences and how that relates to the overall set up?

Sorry for all the novice questions.

mongoose87

So just an update:

I'm trying to get the new carb to work on my stock 1972 corvette before making all the modifications with the vortec heads and all the other goodies to ensure the motor will start and complete the cam break-in.

When I was reassembling everything for the 17059504 carb I noticed a crack on the throttle plate on the primary butterflies. I have read many things about worn throttle rods causing air leaks, so I took the throttle plate from the truck carb, 17080213 and attached it to the rest of the 17059504 corvette carb. I know, most will say this is a bad thing to "frankenstein" the quadrajet, but thought i would give it a try as an alternative to just searching and buying another suitable carb, at least for now (if this one wont work this would be 2 carbs i bought that wont work). I know there are differences in the available vacuum ports on some throttle plates, but not sure how different the other offices are that determine the air/fuel mixture. The 17080213 truck carb is a 800 CFM carb from what I can tell, but i measured the butterflies and they are the same as the corvette 17059504 carb.
I thought the idle air bleeds are in the main body anyway, no?

I put in the 40L rods into the main body of the quadrajet, kept the same 72 jets, which is stock, didnt touch the APT adjustment screw, and put the mixture screws at 2.25 turns out (same as my early 70's carb that runs relatively well).
I put it on the car and it did start up and run, but it is running lean with unstable idle. On my old carb there didnt seem to be any vacuum leaks, or they were small enough to for the motor to run relatively smooth with steady idle, so I don't think that is the issue. I know it's lean because I put my hand over the choke area, and the idle speed did increase.

I hooked up my vacuum gauge, and started turning the idle mixture screws out, but it didn't seem to make any difference because I went all the way out to 6 turns.
The idle speed adjustment screw does have an effect and would increase/reduce the idle.

I'm still pretty new at quadrajet tuning, if you can't tell already. ha

Can I make the truck carb throttle plate work on the corvette carb?
What are the differences between the throttle plates?
Do I need to do something with the idle air bleeds/APT?

When you compare the exhaust system and lack of emission equipment on my '72 compared to a stock '79, I would think my '72 would be a little bit less restrictive, but not sure I would see this at idle.

I'll try going up a jet size to see if that will allow the idle screw adjustment to do something.

Any advice would be appreciated.

mongoose87

Just a bit of an update;
I got the new carb running pretty good using the air horn and body from the 17059504 carb, and throttle plate from a 17080213.
I reduced the idle bypass holes to 0.63, opened up the idle down tubes to 0.036, idle down restriction to 0.06, and idle needle holes to 0.07. This is still using the 40J rods and 72 jets. I could probably reduce the idle bypass even further because it seems to run well at 3.5 turns out, and the idle speed in park is about 1000 rpms without the idle screw even touching the throttle lever.
I tried running 73 jets and turning the APT another 0.25 turn out, and it smelt more gassy; it still has good power, but I think 72 jets and original 3.5 turns out on the APT is better.
When i put in the new heads, cam, intake, 1 5/8" headers, and 2.5" exhaust, I'm hoping this will end up being close to what I need. When I put the new exhaust on it has a bung for the A/F meter, so that will help tuning this even better than just with a vacuum gauge.

At this point, this new carb is running far better than the older model Q-jet I had on there before (appropriate for the year, but a replacement carb). Throttle response is better, and so is WOT. I noticed this later carb has "transfer slots," and I'm not sure if these are part of the reason it is running so much better. Before, it wasn't really able to break the tires loose, but now the car can do a pretty solid peel out from a dead stop. (there's the butt dyno for ya. ha)
The retrofit choke on it is kind of crappy, and doesn't seem to open all the way unless i push it open, but I'll keep tinkering with it until i get it right.