Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: old cars on March 30, 2020, 06:00:15 AM
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Hi guys, Im new hear. My question is about jetting these truck carbs that have SMAB of .050". Can we treat them to the same rules in the recipes of Cliffs book or do we look at them differently because of the SMAB being only .050". Thanks in advance
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I do have the book and read it a zillion times, I've built scores of Qjets but I admit I never built a single main air bleed, but I'm quite sure he gives some recipes for them in the book, I just don't have it with me now. I may be wrong, though.
I know for a fact that they use M style primary rods with .036 power tip, but I'm sure you know that already.
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Actually these truck carbs have early style castings. original jetting was #63 mains 40B rods and CP sec rod
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Ok sorry, I thought it was the alter style of SMAB carbs with external (from airhorn) APT.
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First recipe in Cliffs book shows SMAB .045 to .060 use .071 to .073 jet. With the original jet size on this carb being .063 and 40B rod and .050 SMAB this would seem like quite a jump. Maybe I am missing something here
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Yeah, those are SMAB type with "b" rods...they use the base plate APT system.
Main bleed .0465
IDCR .0465
Lower idle bleed .0595
Upper idle bleed .073
Idle air bypass .089
Secondary POE .043
Primary jet 63
Primary rod 40b
Secondary rod CP
Hanger M
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What are you using the carb for?
The 1977-78 truck units with the SMAB set-up are emission calibrated and used very small main jets with "B" series primary metering rods.
They calibrate similar to the later SMAB units but being smaller castings woln't need quite as much jet unless you open up the MAB's some (not recommended).
I would recommend if you are building one to free up the APT screw in the baseplate and get one of the external APT screws with a spring on it that I make here. I can also supply the correct jets and metering rods exactly for the application if other than "stock".
Those are actually excellent units and I much prefer them to the 1970-78 large main airbleed divorced choke models, especially if they are being used on a "high performance" application.
I actually have a good supply of those carbs here and save them to replace early Chevy divorced choke units especially if the customer is missing a big block carburetor and can't locate an original.
Another very good starting point for Chevy divorced choke models is to find a 7029207 Service Replacement carb. They were made in great numbers and plenty of them still around. I either use those or the 77-78 SMAB truck units for most of the time when someone is looking for a Chevy divorced choke unit for an original set-up and have "warmed" over the engine a bit......Cliff
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Im setting up the carb for a mild 350 sbc approx 350 horse , about 211 duration @ .050
This would probably be first recipe in your book. Thoughts on jetting size?
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In response to mcx. All of my measurements seem to coincide with your post. Ill be opening up the holes under the idle mixture screws a bit.
My initial thoughts were to use 64 main jets jets with 36B rods . This would change the jet area from 1.8606 to 2.1991
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That's not what I would use in it.
First get the APT screw out of the baseplate, then call me at the shop and we can go over specifics and get what you need for it.
As mentioned those are excellent units, they just need a little help in a few areas, jets, metering rods, PP spring, secondary metering rods, larger idle tubes and a few minor mods to get it set up for what you are doing.......Cliff
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So let's try this again. 7029207 carb was used on 1969 chevy 350-325hp vette #66 main jet 36b rod and correct if I am wrong had small air bleed of aprox .050" main body and .050" air horn. This would reflect the use of small jets. ( smaller main air bleed / smaller jet). In 1970 the chevy carbs starting using Larger main air bleeds . This is why we suddenly see larger jets .Example: 1970 chevelle/corvette 350/300hp 7040202 carb with #76 main jet 44b rod. Main air bleed aprox .112" main body and air horn.
The truck carbs of 1977/1978 used early style main body castings with a SMAB of aprox .050"
The question is (and I will try to put this politely) In first recipe of Cliff's book we have
Main air bleed (main body) .070 inch
Main air bleed (air horn) .070 inch
or
Single main air bleed: .045 to .060 inch
Main jet*: .071 to .073 inch
*For late-style single main air-bleed carbs with .060 to .080 nich main bleeds, use .074 to.076 jets
Being the 17058213 carb with SMAB .050" originally had #63 main jet and 40b rod going to .071 to .073 jet would be a huge step
So is this a typo in the book?
I am not looking for parts. I hope the is a discussion for learning purposes.
Than you
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Your answer is in my first response.
Some of (not all) of the 77-78 divorced choke non-CA emissions Chevy carburetors used an early single main airbleed design. They are somewhat rare so I didn't spend time covering them in my book as Car Tech limits both word and page count and I didn't want to waste a lot of time there. Bottom line is that they are excellent units. Downside is divorced choke and APT screw in the baseplate. For anyone building one I would leave the main airbleeds at the stock sizes.
I've also found virgin units with 64 and 65 main jets in them, so the factory may have been experimenting some with them during their brief run.
Interesting but the 7029207 carbs also showed up with main jets as small as 64 and as large as 66, but posted literature on them doesn't show this. So NEVER believer everything you "google-up" on the Internet.
Anyhow the 77-78 SMAB divorced choke models calibrate similar to the later design but having a smaller bore and different booster it's best to leave the small main airbleeds as-is and calibrate them per application. This is IMPORTANT because they use pretty large upper idle bleeds so already killing the transfer circuit some requiring improved fuel from the main system as low throttle angles/light engine load. As mentioned I highly recommend removing the APT screw in the baseplate and installing one of my adjustable APT screws instead. This makes they much easier to tune like the later SMAB models with the APT system in front of the power piston.......Cliff
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Thanks Cliff. Great info. So if I was building the carb as per First recipe in your book (mild 350 / 210 duration cam at .050" lift) would #64 jet and 36b rod richen it up enough or would #68 jet and and suitable rod get it closer?
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By the way I'm up in Canada (ontario) . Until this winter we were limited to 5% ethanol in gas. This spring new rules limit Ethanol in gas to 10%. I believe regular gas being closer to that limit. Our premium fuel I'm told is still around 5% ethanol. Wandering how that compares to other areas
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Don't be the first bit afraid of ethanol in the fuel. So many misconceptions about it on the Internet, like many other topics, and most of the folks scaring the chit out of the general public need to keep their day jobs and quit regurgitating inaccurate information on the subject.....IMHO.
I use it in everything here and have for quite a few years with zero issues. Just make sure that you address any potential long storage of it in a vented system as it will soak up some water. Also be aware that it doesn't like rubber, nor does it like "soft" blue accl pump seals, and will KILL buna rubber or neoprene seals (still being sold in many kits) in less time than it took me to type this. Hopefully you bought or are going to buy one of my rebuild kits because nothing else out there is going to hold up in that area. My kits are also more complete and much higher quality components than anything else out there. Not to mention I substitute high performance parts, such as smaller floats, larger fuel inlet seats and shorter accl pumps per request at no additional charge.
Ya, I know, sound like I'm "selling", which is true. I have to make money to keep the website open and feed my family, but also important is for folks to end up with good results with these things. I sell more accl pumps than I can count, and nearly every customer has already bought into the BS that the blue seal they got in their over the counter kit or from another vendor is "ethanol resistant". Mine is WAY more than that, it is lifetime warranty and to date I've never had to replace one.
....continued
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Anyhow, as far as tuning goes I don't like to make specific jet/rod recommendations and for several reasons. Number one folks just "google" up the sizes and look elsewhere for the cheapest possible price on them, or they dig thru boxes us used parts to find them and in almost all cases they used jets are "egged" out some from use and the metering rods worn down from many thousands of cycles. Currently there are 4 different profile jets being sold for the Q-jets and 3 of them don't work as the depth of the orifice and angle leading up to it is incorrect. So I give up details, a guy runs out and buys the parts elsewhere, gets poor results and doesn't know exactly why?
Good results start with the right parts and parts that are within spec, so I try to help out there as well. For all the early APT style carburetors I push folks building them to install our custom machined external APT screw. Then we can supply jets with tapered metering rods which will give them full control of part throttle A/F from rich to lean without having to take the carb apart and change parts.
I also have specific power piston springs custom wound for any application and we supply one with the tuning parts. I insist on a new secondary cam/spring and supply them in our kits as well (except the "basic" kit). So basically the service I provide here is to have customers supply all their engine/drivetrain/vehicles specs, tell me what carb they are going to use, then I supply them what they need so it's basically "plug and play".
Also be aware that I REQUIRE all the engine specs to make accurate recommendations. Things done with the engine build to improve engine efficiency may require LESS fuel. These include higher compression, tighter quench, better combustion chamber shapes, improved port flow, and good cam selection. So if anyone reading this wants specifics for tuning, be ready to supply specifics on your engine and I'll nail all that down for you........Cliff