Calibration of 800cfm unit for mild 350 Chevy

[anvilhandsmagillacutty]

Hello guys. I have a mild 350 Chevy in my 55 sedan in front of a 700r4 transmission with 2500 stall lockup converter and 4:10 rear end gears. The motor is a dished piston Goodwrench short block with 58cc 1.84" 305 heads, a competition cams 268H, aluminum Quadrajet intake, dual snorkle Z28 air cleaner with tall lid, E-1500 K&N, and 1 5/8" mid length Patriot headers running into 2 1/2" super turbos. My question pertains to the primary jetting/rods to use in this Bigger 800 carb to make the small block fuel ratio correct. I have on hand many jets, rods and hangers. But, am conflicted about where to ballpark it.

[blazer74]

First step is to get Cliffs quadrajet book.

Also a description of what carb number and what's been done to it as far a you know first.

[anvilhandsmagillacutty]

I don't really want to get into much modification of the basic design right now. I just need someone with first hand experience using the large primary carb on a smaller displacement engine, to give me some starting point with jetting. In other words, what they are/were using. For example, what main jet number and rod will get me out of excessive richness, if any(that is, if the B.B. calibration is not already suitable). I don't have a problem tuning it once it's drivable, but sure would love to start off in the right direction. Thanks

[Frank400]

  Hopefully some people with more experience than me will chime in, but here are my suggestions.

A carb number would really help, I'm guessing you have large (.120") main air bleeds.  If so,  I'd suggest downsizing them to .070" (both), and going with a 71 or 72 main jet, along with Cliff's .044" full taper metering rod. 

    The idle/off idle definitely will need some attention too, I'd say as important as the primary main system. 

[anvilhandsmagillacutty]

Here are some various jet/rod combinations I have pulled out of swap meet carbs. These are all late 800 apt type carbs with Chevy style bellcranks. 454: 72 jet w/ 49M rods (DR rods/N hanger). 5.0L: 72 jet w/ 50M rods (CP rods/N hanger). 5.7L: 73 jet w/ 49M rods. Misc. 800: 74 jets w/55M rods (EH rods/E hanger). Out of all these primary combinations, which would suit the mild 350? Or, a combination thereof? Once again, we're only speaking if main metering here. Idle circuit, I'll delve into later on.

[carmantx]

You are getting good suggestions, but aren't helping yourself.  What carb are you working with?  What are the current calibrations? 
Build the entire carb for your engine is best thing to do, not just change some jets.

[anvilhandsmagillacutty]

Really?? So, buying the book will tell me EXACTLY which jet/rod combination to use with my particular engine/vehicle/drivetrain? And, the idle/off idle modification I've been advised of, will provide the ideal air/fuel ratio through the venturi/main system for all throttle positions, after idle/transition circuit has exceeded it's design limitations? Even at sustained W.O.T conditions...and provide my engine with the proper mixture to avoid lean out, potentially damaging detonation, or economy destroying/power robbing richness? Wow! I don't see what carb number, or original factory application, plays in jet/rod selection for use on a modified non original engine/vehicle, with far different fuel requirements than how it started out. For example, having none of the factory emissions equipment, EGR, AIR, THERMAC, CATALYST..etc. Seriously, I'm really only looking for replies from someone who has a known, proven combination, on a Chevy 350 or similar size Chevy small block with the large primary carb. All other advice is unnecessary. I'm not looking to get into web arguments, or serious debates over opinions either. My question is titled calibration, but I'm referring to main metering calibration. As I stated previously, I'm NOT going into idle restriction modification AT THIS TIME. This carb will be getting used on the 383 roller cam motor I'm currently building, which will also be using a 125 horsepower PowerShot plate system. That's why I'm not doing anything permanent with idle restrictions now. Thanks anyway.

[Frank400]

Primary calibration is not only determined by jet and metering rod.  A HUGE part of it is main air bleed size.  So let's say a 72jet/42 rod combo can be pig rich in one carb (carb number that is) and ultra lean in another one, same engine on both. 

   The idle system also is an important player here, as it's still doing work when the primary main system is working.   

  It's not as simple as "let me know which jets and rods".  The book will tell you HOW to get the correct recipe for your car.  Or just buy a complete carb kit from Cliff (NOT an auto parts store kit !) and if you're willing to tell him your carb number, he'll probably get you in the ballpark for what you want to do.  That is IF you're willing to give him your carb number.

[anvilhandsmagillacutty]

 Well alright then!! Consider me thoroughly humbled, AND thankful for that last reply. That explains alot. Especially, the inconsistencies with sizes in correlation to displacement of intended engine family. So, what youre saying here is, the AIR pickup in the air horn, will be sized for the intended engine. Therefore making one size jet, not always providing similar results in a carb with larger/smaller air pickup? BTW, the particular carb I am using is off a 425 Caddy. #17085239. I do not know of the original configuration, as it was gone before I got it. The carb is in very good shape, which is why I used it. I have, however...substituted the Caddy base with a Chevy base that has old type non "tamper proof" mixture screws.
Other than the lack of the third attaching screw, I don't see any problem. But, do you see an idle passage issue with this base?

[anvilhandsmagillacutty]

Wait a minute, now. It seems the identification information here is incorrect, at least with my carb. This says my carb is 1985 Caddy with manual transmission. If it was any GM car AFTER 1980, it would be ECM controlled, so I can't see that being right. Also, this carb number came up in MULTIPLE web search results, INCLUDING a GM parts book....as 1985 GMC/Chevrolet 5.0L federal manual trans!!! Looks like my question has been answered, by MYSELF! I will leave the idle circuit, and everything else as is...and simply run the 5.0L or 5.7L calibration. I guess GOD really does help those THAT HELP THEMSELVES!!! 

[tayto]

Canadian fullsized cars (b, d and g body for example) and trucks had non ecm controlled qjets right up until TBI took over.

[77cruiser]

Had non ecm carbs on trucks over here till middle 80's too.

[anvilhandsmagillacutty]

Please excuse my sarcasm, and understand it's not really intended to offend. Just my nature and off sense of humor. I thank you for your help. I think my carb is indeed a 5.0 liter truck carb, and already has suitable idle circuits as well as main air bleeds. I just wanted a suggestion on jetting for the non stock engine. With cam, headers,
 etc

[77cruiser]

The jets & pri rods are probably fine as is, but with that cam you may need to open up the idle tubes & down channels a bit. Similar carb I have on a 327 & I went to .036 on the idle tubes & .056 down channels.

[Cliff Ruggles]

Looking at the engine build the carburetor is going to need help in all areas, starting with the idle system.

Jet and metering rod requirements are based more on what you've done to your engine, and how the carb is set up from the factory than anything else.  Looking at what someone else did for a different application, or using a different carb number, or both, is useless information.

You need to identify the carburetor, most of the later carbs are single MAB, measure all the "players", then look at what you've done with your engine and proceed with modifications in all areas, starting with the idle system.

The idle system is the BIG player in this deal, as it brings in the required fuel for you application.  No stock carburetors is going to fill that bill, especially when you've chosen a larger than factory cam on a 110LSA in a small block engine.

My book provides guidance for setting up the carb in all areas, starting with the idle system and main jet/rod requirements as well, all based on idle and main airbleed sizes.  I'd start there and educate yourself as to what is going on.  From what I can see so far in your responses, you are setting the rules for building the carb and no one is going to be able to help you out.  Idle system modifications are going to be required trying to use a late model emission calibrated carb on a modified engine build.  You can't just come along and throw jets and metering rods at it and come up with a good end result.....Cliff