Qjet 17057253 on a 380 Olds stroker

[Cliff Ruggles]

A "stock" idle calibration woln't work well for that engine combo. 

Also consider that the idle system feeds the main system thru transition via the transfer slots/idle mixture screw holes. 

So at low throttle angles it needs a percentage of the fuel to the engine from those areas. 

I never saw any details on exactly what was done for idle fuel, but it should be fine with idle tubes at .038", .055" or so DCR's, and .070"/.070" idle airbleeds.  It will also enjoy some bypass air, probably .100" or so to keep it off the boosters.

Bypass air is important for these set-ups but how much you need can vary some carb to carb, as initial timing, cam specs and ICL, engine compression ratio, etc will effect how much signal there is to the engine at idle speed.

I provided a few basic "recipes" for idle fuel in my book and the third recipe usually works fine for engines with moderate size camshafts in them making around 12-15" vacuum at idle speed.  Even with that said each engine will be a little different as there are many variables that effect the signal to the carb at idle speed.  So basically as far as tuning goes we recommend to get the engine happy at idle first, then move on to the main system.

Far as the main system goes, the 1976-up Pontiac and Oldsmobile units are excellent and don't need a lot of help in that area.  I typically stay close to stock for jet size and seldom if ever use "K" metering rods in them as they simply lack a lot of room for tuning with the APT system.  We use our .044" tapered rods in stead as they taper gently on the upper section and also tapered between sections rather than "stepped".......Cliff

[Seff]

I haven't run the car with the stock carb yet. I trust you're right about it being lean.

With the above in mind, these are the measurements I found on the carb. I changed the air horn after a crack was discovered in the original, and I forgot to increase the size of the Upper MABs.



Upper Idle Air Bleeds: 0.070"
Lower Idle Air Bleeds: 0.070"
Idle bypass air: 2 x 0.075"
Upper MAB: 0.065"
Lower MAB: 0.070"
Idle tubes: 0.040"
Idle down channel: 0.055"
Acceleration discharge holes: 0.075"
Rods: 44 tapered, from Cliff.
Jets: 73




Running like that it provided 13" or so at idle, but off-idle was very lean, leading to stalling.


I have fresh idle tubes spares that I can install if we think it'll make a difference.

[Cliff Ruggles]

I'd check to make sure the donor airhorn is sealing well at the gasket. 

You should have plenty of idle fuel for that engine combination and it shouldn't be going lean off idle or anywhere else when you increase the throttle angle/engine load......Cliff

[Seff]

Air horn is crooked, body is crooked on upper and lower mating surfaces., baseplate is flat.

[Cliff Ruggles]

A little warp is normal and fine provide the parts seal up effectively.

Considering the symptoms and issues you are seeing, I suspect a fundamental issue with the carburetor someplace not related to how it's set up.

For example, you mention increasing throttle angle/engine load and going lean. By design that scenario will bring in MORE fuel. 

Next time you have the engine running remove the air cleaner and get up over the carb enough so you can see into the primary side.  Move the throttle quickly and you will see a nice shot from the accl pump and well atomized cone spray of fuel coming from the venture areas.  This is adding more fuel, not less. 

The same thing happens when you drive the vehicle, but it also brings in additional fuel at low throttle openings from the transfer slots above the mixture screws.  There is no scenario that I've seen where less fuel would be delivered with any significant increase in throttle angle and engine load.

I also use basically the same carburetor (1977 Pontiac) which differs the most in having a large front vent tube in the airhorn.  It is set up very close in specs to yours and uses .073" main jets and out tapered .044" primary metering rods. 

It has powered 4 different engines to date with the same basic arrangement.  The current engine is well over 550hp/600tq and it brings in plenty of fuel for "normal" driving and increases fuel delivery with any increase in throttle angle/engine load.

Maybe changing the airhorn on your carb is part of the problem, or something else not quite up to par someplace?  When you use good parts, the correct settings, and don't get acceptable results it's time to look a little deeper at things to find the problem(s).........Cliff

[Seff]

I had the same line of thought. Will return with test results.

[Seff]

A "stock" idle calibration woln't work well for that engine combo. 

In case my 170 series carb doesn't work out, will a third recipe idle and/or main system mods be appropriate for the 70 series carb?

Thanks.

[Cliff Ruggles]

Provide the part number of the carburetor you plan to use, then we can discuss setting it up exactly for what you are doing......Cliff

[Seff]

Part number 7041250.

[Seff]

The carb has 70 jets, doesn't that mean that 0.070" air bleeds would be too large?

[Cliff Ruggles]

7041250 is an early Oldsmobile unit.  They used small MAB's with those, so leave them as-is.

I'm not overly fond of the Olds carbs thru 1974 for high performance work.  They still used the early float/hinge pin arrangement and don't like high fuel pressure, or larger N/S assemblies.

They can be difficult to keep full on hard runs.

The later carburetor (when working correctly) is superior in just about every area......Cliff