Q-Jet part throttle lean condition

Discussion in 'The Venerable Q-Jet' started by buick64203, Sep 24, 2017.

  1. Cliff R

    Cliff R Well-Known Member

    Nothing at all wrong with a meter to measure A/F, but what numbers are you shooting for? Depending on what engine you have and how it is set up will have them being happy with considerably different A/F numbers at different engine speeds/load. I've seen some engines do fine with A/F ratios clear into the high 15's/low 16's where others started bucking in protest much past 14 to 1. So for any particular application what do you strive for?

    I tune instead for best performance in all areas, which includes fuel consumption when tuning the APT or making jet/rod changes. If I'm using an A/F meter I'll put a piece of tape over it until I've nailed things down the best that I can, even if it took several tankfuls of fuel to find out what the engine likes best. Then I'll remove the tape and look at the numbers and use them as a baseline. From this point on I'll make very minor changes, do a lot of driving, and see if the changes helped anyplace?

    These engines vary greatly in the A/F they will tolerate and still perform well w/o any negatives anyplace, such as running hot, overheating, detonation, surging, "flat" on acceleration, down on power, requiring to much throttle angle to maintain vehicle speeds, etc, etc.

    I'd also add here that some folks do not use a vacuum advance for street driven cars either. Not sure why this happens, just got into a discussion today with an engine builder who prefers mechanical advance only, and "all in" by or before 2500rpms. Said he's been tuning street engines this way since Moby Dick was a minnow with perfect success.

    Well folks, timing "all in" on the mechanical curve at any rpm still doesn't eliminate the want or need for more timing at light engine load. The vacuum unit is a load sensing device, and not using one on most street driven vehicles is costing you money and engine efficiency. We need to be a lot smarter these days, or at least one would think folks would be with all this great tuning information at your fingertips.

    Anyhow, I can't save the World and no intensions of trying anytime soon. However, if you are using a Q-jet employ the APT system if it has one, and if your distributor has a vacuum unit use it, and include tuning with different amounts of timing, as it is just as important as fuel delivery from the carburetor. With a little effort one can achieve great performance from these old engines, and have them sipping fuel lightly at the same time.......Cliff
  2. buick64203

    buick64203 Just plum crazy Staff Member

    Well Cliff, while its true you cant save the world, you can save my baseplate! Any secret tips on freeing up the APT adjustment or should I just send it to you! ? I took the plug out and its not budging. Not sure if I should soak it in PB Blaster or apply some heat to it.

    The main impetus for buying the A/F meter is to determine why I have this part throttle bucking/ random misfire issue. Normally if it were at idle I would richen it up with some propane Im really not shooting for a particular A/F ratio but more so to baseline where I am and which direction I need to go in. The car is a bone stock 70 GS 455 "driver". Right now the APT adjustment is a no go because of the frozen screw.

    Ibe gone though everything. Timing, carb, etc. The vacuum advance is connected and working.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I have had an air fuel gauge in my car for several years now. It is this one,


    My engine uses 34* locked out timing. My vacuum advance is limited to 10*, so my car cruises down the road at 44*. The air fuel ratio at a steady 60-70 MPH is anywhere from 14.6-15.2. No surging or bucking, instant throttle response, no audible detonation when I punch it. At WOT, the gauge reads 13.0. It likes to idle at around 13.5 or so with nearly 15" of vacuum at an out of gear 900 RPM. I usually get between 14 and 16 MPG with it if I am careful.

    Just thought I would give you something to compare your results to. Jim Weise did this Q-jet for me when he built my current engine. Pretty sure he used one of Cliff's recipes, but I have no idea what the jetting is. It's a 7042240 carburetor that I have had for years. It was previously done by John Osborne.
  4. MrSony

    MrSony Well-Known Member

    Not meaning to hijack, but how would this work on a car with true duals?
  5. buick64203

    buick64203 Just plum crazy Staff Member

    You normally run one 02. Ideally, the carb should run the same on bot sides. The air fuel meter has the capability to run two O2's but I don't think thats really needed. In my case, I bought the tailpipe clamp so its a non permanent install and I can move the sensor to the other pipe to test the other bank
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Also, with a dual plane intake, one side of the carburetor feeds cylinders on both sides of the engine.

    Last edited: Sep 28, 2017
  7. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    Jason you also could have gotten a 4MV rochester that has the APT accessible through the airhorn .
  8. buick64203

    buick64203 Just plum crazy Staff Member

    This one has a screw in the baseplate. Im going to try heating it up and dripping some candle wax on it.

    I got the Innovate wideband O2 kit this week and Im planning on getting some baseline measurements tomorrow. I can hardly contain myself!!! Im anxious to see what the reading are and if my butt is correctly calibrated

    I noticed another issue last night un-related to this issue- All the lights are pulsating. I have disco interior lights. I think its due to the combination of running points and a mechanical voltage regulator. And the alternator has a bunch of crickets in it. Its chirping pretty good. I took the belt off and the noise went away so its definitely the "new" auto parts store alternator.
  9. BadBrad

    BadBrad Got 4-speed?

    It's that regulator. you might be able to hear it clicking on and off, right through the firewall, to the pulsation. I know mine did. I rewired everything to a 3-wire 12SI 1987 GM G-body alternator that was a near direct bolt-in after pulley swap and grinding a small nub off the case. It's about an hour worth of work for 94 amps and parts store availability, with lifetime warranty.

    Death to disco!
  10. BadBrad

    BadBrad Got 4-speed?

  11. buick64203

    buick64203 Just plum crazy Staff Member

    So today I was able to play with my new O2 meter and I have some results...

    I bought the portable style wideband kit. I also bought the optional tailpipe clamp as I didn't want a permanent install. The thing looks like an aluminum crack pipe. First issue- the tailpipe of my GS is bent at a 45 degree angle at the end so the tailpipe clamp didn't fit in too well. I didn't feel it was going to get a "straight shot". So I got a 2" 45 degree elbow off the shelf at Auto Zone and attached it to tailpipe then installed the crack pipe and O2. Fit better

    Plugged it in with the car off and after its warm up routine, it read 20.9 which if I remember my gas analyzer training from years ago is ambient air. So far so good. Start the car up and take it on a ride to get it fully warmed up. Get the car home and the water temp is reading 190, so up to temp. So lets see what its idling at. I look at the gauge at the meter is reading 5.8:1. Hmm, isnt that pig rich? At that AF ratio, the car should be belching black smoke or at the very least making my eyes tear. But no. And the old plugs came out clean. Anyway, moving forward.... I start adjusting mixture screws. I got it a little leaner but not by much. You would think giving it a massive vacuum leak would get it to go real lean but I didnt really experience much of a change aside from the vacuum cleaner noise the open port made and the higher idle. Not sure what I was doing wrong if anything.

    So being a little confused by all this, I decide to take the carb off the Riviera and see how that one tested. The Riviera runs really well with zero issues. And it just happens to have a 71 Stage 1 "242" carb on it. So I swap carbs and right off the bat there is a noticeable improvement in the idle. Definitely running smoother.

    Now I didn't touch a thing on the Riv carb aside from the idle speed screw. I take it for a ride and the bucking and hesitation at part throttle are all gone. Its a different car. Thing runs like a top just like the Riv does. As an aside, this carb is old. If its been rebuilt, it was a long time ago. So now lets see how this carb reads. At idle this one is reading 9:1, 9.2:1. Possible inaccuracies aside, and all things being equal the Stage 1 carb is reading leaner not richer that the "240" carb.

    I still need to get it on the road and see what the part throttle reading are. I kind of concentrated on the idle measurements today and hashing things out with my new toy. For giggles, I want to see what the idle measurements are on my 2013 Escape. That thing should be damn near perfect.
  12. Aaron65

    Aaron65 Well-Known Member

    Widebands are supposedly inaccurate at idle, but 5:1 seems excessive. There's no way it's running that rich because it wouldn't be running. If you already calibrated the gauge, chances are you'll be sending it back. :(
  13. buick64203

    buick64203 Just plum crazy Staff Member

    I read the instructions afterwards and you need to perform an initial calibration...ooops! I guess I glossed over that part!

    Back to square one....
    techg8 likes this.
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Well, get crackin there Slim:D
  15. techg8

    techg8 The BS GS

  16. buick64203

    buick64203 Just plum crazy Staff Member

    Your post this afternoon not only reminded me but gave me some inspiration to go out after work and play with it. So that's what I did!

    As you know, I took off the offending carb and put a 71 "242" carb which was on the Riv. As I said it ran well.....or so I thought! Running AFR meter on the car revealed an 11.7-12.0 AFR at idle and cruise. I got the car home and adjusted the mixture screws. One full turn in on the right side and a half turn on the left side brought the AFR up to about 13.2 (I swapped it from side to side). Quite a difference just by turning a screw. I wasn't able to take it out on the road to see if the 13.2 idle translated to a 13.2 at cruise. Im taking it to the LIBC meeting tomorrow so I'll hook it up again and let you know.

    From what Larry posted, 13.2 seems to still be a little rich for cruise correct?
    techg8 likes this.
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yeah, that is definitely rich. That looks more like a Holley DP. Mine leans right out on the highway. It will idle in the low 13's.
    techg8 likes this.
  18. buick64203

    buick64203 Just plum crazy Staff Member

    As a side note, I swapped out the alternator. The cricket noises were driving me crazy. I put an old alternator in it and no more noises! Funny, this car seems to like used, old parts.
  19. buick64203

    buick64203 Just plum crazy Staff Member

    So might as well mess with this one first since its now taking up residence on the car....

    So, the 71 service manual says this carb should have a 75 jet and a 45 rod. Lets make some assumptions here for the sake of the conversation. Assuming the rods and jets are factory and assuming the carb is running the same 13.2 AFR under part throttle cruise as it is at idle, what would I do? Install a 47 rod and see where Im at? What sort of number should I see from the change?

    Once this carb gets dialed in, we're gonna move back to the 70 "240" carb and fix that. :D
  20. Aaron65

    Aaron65 Well-Known Member

    I don't think you can assume it'll run at the same AFR at cruise that it does at idle. Nothing I own works that way; it's usually leaner at cruise than it is at idle.
    Last edited: Oct 11, 2017

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