Bunker82

Well I am trying to get things ready for a change in carburation. I have an Edelbrock (AFB) right now and have tuned it to run well, but I want to get some better economy and go for a more sleeper look so I want to build up a good non cc Q-jet. Core pickin's are slim so I ended up with an '86 Q-jet off of a Chevy pickup. So it has probably been severely corrupted by smog crap. I am trying to get information on what mods to do to it to feed my engine properly. I have some magazine aticles with some info on moding q-jets. I wanted to run this by some of the experts on here and get some opinions. One article lists some of Jet's specs as (all for 216 or more dur at .050 for the cam):
Idle discharge ports: 0.093
Idle tubes: 0.038-0.041
Idle air bleeds: increase 0.001 at a time
Idle air bypass: start 0.002 over and go up in 0.001 increments

And another article citing what Jones Performance used (no cam specs given):
Idle discharge ports: 0.093
Idle tubes: 0.036-0.044 (0.042 for street/strip)
Idle air bleeds: 0.055
Idle air bypass: ?

Do these sound about right for my application (see specs below)? Suggetsions? Comments? Anyone have their own numbers they use? As far as drilling stuff goes. There are some other tuning aspects. Reading through some old posts Damon suggests using the Edelbrock Performer RPM Q-jet vacuum diaphram.
And what about power piston springs? Would it be wise to get a set and play with that? I think I am under 15" of vacuum at idle, but I don't remember now. Not far from 15 though.
And what would be a good place to start with primary rods/jets and secondary rods/hangers?
Anything else I am forgetting?

Carb: 17085213 from a 5.7L '86 Chevy pickup
Engine: 383, Edelbrock heads, Edel. Performer intake, LT4 HOT cam (218*, 228* @ .050 and .525 lift), 1 5/8" shorty headers, 3" exhaust, 3.27 gears, 2000 rpm stall convertor.

Thanks, I appreciate any comments or suggestions. BTW I have Roe's book as well as the Haynes too.

SSC

If this carb truly came from an 86 truck it will have a dual capacity accelerator pump AKA MCS. This wont work without the temp switch on the t stat housing, it would be rich at all times. Ah most people change to older ones anyway so it probably isnt the original one. Id just start in stock form and not mangle the air bleeds or discharge ports. Id focus efforts more on the secondary side for economy pourposes, DR, CP or BA rods and a B hanger. A G hanger would be a tad better for economy.

Bunker82

Thanks for the reply. I grabbed if off of an '83 pickup but when I went to get a rebuild kit it came up as being on an '86. It doesn't have the electric dual cap. accel pump junk on it. But yeah who knows what it is really from. Thanks for the info on the secondary rods and hangers I have DRs CPs and B and G hangers so I have a place to start. I pulled it apart today and it has CN rods and an H hanger. Primary rods are 50Ms and it had 72 jets. What do you think of that stock primary metering combination? Good start or maybe a tad richer on the jets? I also noticed it is a 800 cfm casting. I hope that isn't going to impact economy much since I don't think I need a full 750 let alone 800 cfm.

SSC

I think just upping the jets to 76's or 78's and keeping the stock primary rods would be very well suited for your setup. If it is the 800cfm casting look carefully for all the plugs when you epoxy, some of these castings have plugs in the RF corner between the main body and the baseplate that tend to get overlooked.

Bunker82

Well it must have been rebuilt before since there is already a bunch of epoxy. Darn! that's the fun part all the gooey stuff. Are there any guidlines for when it's a good time to have the the throttle plate rod thingy (sorry for getting technical) bushed? I will probably just get it done to be safe, but I am curious.
Thanks

Bunker82

Oh throttle shaft was the term I was looking for. Whoops. Oh and does anyone have any info on doing the adjustable part throttle mod? Mine already had that plug removed from the air horn and the hole tapped. I was wondering what it does, if it is actually useful and how to adjust it. TIA

99Hawk120

As far as I know that's not the adjustable part throttle, that's actually the lean stop for the primary power piston.

Why they tapped the hole is beyond me. The stop itself is threaded. I guess you might do that to keep crap from falling in the carb?

Bunker82

Well the adjustable part throttle feature is found on Edelbrock Performer RPM Q-jets as well as the ones rebuilt and modified by JET (can be found at Summit). I have a magazine article that mentions it. I wish I could scan the pics that go with it but I can type out the captions that explain things alittle.
"The adjustable part throttle feature is accessible (as per JET) by knocking out the factory-installed soft plug in the air horn, threading the hole, and then plugging it with a pipe plug. Turning the threaded adjuster sets the height of the primary metering rod and establishes the swiftness of the transition from the idle circuit. Deeper metering rod settings boost part throttle economy, while shallow settings enhance throttle response."
That is for the air horn part. Then for the other part that is in the fuel bowl area just below the previsously mentioned adjuster:
"The JET man replaces the stock metering rod height limiter with a 10/32 Phillips head screw. This eliminates the need for a custom tool and allows adjustments with a common screwdriver. Once the plug in the air horn is removed (see above), both types of adjusters are accessible."
This allows adjustment of how far down the rods can go (rich limit I guess you could say) where as the air horn plug allows for adjusting how high the rods can go (a lean limit as you suggested). BTW that came out of Hot Rod July 2000. Keep in mind this is on a NON CC Q-Jet (and the '75 and later design only I think). I know from working on CC Q-Jets that they have a (semi) adjustable lean stop on them from the factory. But anyone with experience with this please let me know about it. Thanks.

99Hawk120

I don't have any idea why you'd want a rich stop. the diameter of the metering rods is the same at maximum travel regardless of the rod, so if you have too much primary fuel at WOT your jets are simply too big. If you drop the jets DOWN and then have insufficient part throttle fuel, change the rods.

In fact, the lean stop isn't that great either. As RB has said before, Qjets did just fine before they had that. All it does is limit the total amount of rod travel you have (basically reducing the amount of enrichment b/t idle and part throttle). Thinner rods fix that problem too.

Both the factory lean stop and the added "rich stop" are basically band-aids to cover improper rods and jets

None of the discussion in the Roe book points to anything looking like the Qjets I have as an APT.

Bunker82

Well I don't know if the apt deal is just a gimmick or not. Sounds like it could be but I was just trying to get some idea while I have the thing apart. Those captions I quoted seem to indicate some usefulness. I dunno. I will keep searching for info on it.

Bunker82

Here is a link so you can atleast see what I am talking about with the APT:
http://www.hotrod.com/techarticles/42590/index.html