Q-Jet part throttle lean condition

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

  1. buick64203

    buick64203 Just plum crazy Staff Member

    Im fairly sure I have a part throttle lean condition on my beater GS. At highway/ cruise rpm of around 2500-3000, Im getting a very subtle random misfire. Feels like a intermittent hesitation. Plugs had about 800 miles on them and came out super clean with almost no color on them. The porcelain had a pinkish hue but I chalked that up to the additives in the fuel (?). I threw a new set of plugs in it just for kicks.

    Original 7040240 carb was gone through by a very reputable shop. Shafts were re-bushed. Carb runs well otherwise. Secondary rods are brass with no stamp and look to be similar in shape to an AX or AU rod. Im going to finally pick up a portable wideband O2 kit this week and a tailpipe clamp to verify my suspicions

    So lets assume Im correct and the carb is running lean at cruise, what the best way to go about richening it up? Would installing an APT screw be the path of least resistance?
  2. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    mess with the power piston spring. stretch it out a small amount.
  3. Taulbee2277

    Taulbee2277 Silver Level contributor

    Brett Slater likes this.
  4. BadBrad

    BadBrad Got 4-speed?

    Jason, how's that timing curve? At part throttle, high speed, cruise too much vacuum advance will cause surging that can feel very much like a misfire.
  5. buick64203

    buick64203 Just plum crazy Staff Member

    That is a very good point. It does feel a little like a surge. Its so slight its hard to put your finger on it. And that's what I initially suspected as well. That it might be over advancing itself. But I disconnected the vacuum advance and the issue was still present.
    BadBrad likes this.
  6. techg8

    techg8 The BS GS

    IF ...and that's a big if.. you can get that baseplate apt to loosen up then that's an easy way to lift the rods for more fuel at cruise.

    I don't mess with the baseplate APT screws anymore because for me they rarely come free.

    I'd rather go directly to installing the next size down primary rods.
  7. buick64203

    buick64203 Just plum crazy Staff Member

    Seems that taking that original screw out is a bit of an issue. So assuming I went with swapping out the primary rods, you think just going to the next size down would give me enough of a change? According to the service manual, I should have an .041" primary r0d and a .067 jet. So an 039" rod with the same jet?
    Last edited: Sep 24, 2017
  8. techg8

    techg8 The BS GS

    From the description it doesn't sound way off so small changes might work.

    Nothing wrong with trying an .039 rod.

    Don't need to change the jet since you're only trying to richen the steady state cruise mix.

    You're increasing metering area under high vacuum with power piston down. Lean cruise mode.

    (Worth mentioning is that a .030 spread between rods and jets is generally a good neighborhood to be in for performance and economy.)
    Last edited: Sep 24, 2017
  9. buick64203

    buick64203 Just plum crazy Staff Member

    One thing I didn't mention that Im trying to reconcile is the fact that I drove this car for a year with the automatic and 3.08s all over the place and it didn't seem to have this issue at all. Ir at least I didn't notice it. The carb sat for a year while I converted it to a stick and 3.42's. Once I got the car back together with the new combo, it has this issue. The only thing that's changed is the fact that my cruise RPM is a lot higher- 2200 before vs 2800/3000 now. So the engine is spinning a lot faster. .......did I just answer my own question??:D
    techg8 likes this.
  10. telriv

    telriv Well-Known Member


    I had the same type of problem with two Pontiacs. Of course these had AFB carbs. on them, but I believe since the symptoms appear to be the same this may also apply to your situation. Had a '64 Gran Prix, 389 3 spd. from the factory. Very light throttle pressure & you could feel the car was kinda "bucking". Almost as if you were turning the ignition on & off. Had rebuilt the distributor, re-curved it & installed electronic ignition. Numerous times I played with timing, etc. No change. Put it back to stock the way it was. Finally put it back to the modified way I had done. Ran MUCH better & had more power this way. Had gone COMPLETELY through the fuel system including rebuilding the AFB. Went through the carb. numerous times checking for problems, etc. Called & asked MANY people questions, even Dennis Manner. I hooked up my timing light & mounted it to the dash so I could watch/see if I was losing light. After driving myself nutso for awhile I finally went to my sons house & used his Innovative 2 & hooked each O2 sensor to each tailpipe. The exhaust system was new with no leaks so the readings were correct. When it was bucking the AF ratio was extremely rich. This was with ALL STOCK, original parts. I couldn't find the metering rods I was looking for so made/modified my own to lean it up some. Solved the problem.
    This was ALSO happening on that '64 Black Bonne wagon I had with the 421. Leaned it some & the same problem went away.
    Both of these cars had been sitting for quite awhile before they came to my shop. MANY YEARS.

    Just a little more FYI.

    Tom T.
  11. buick64203

    buick64203 Just plum crazy Staff Member

    Tom, your spot on with your description of the issue and it looks like Im following right in your footsteps!
  12. telriv

    telriv Well-Known Member

    Slightly leaner metering rods are an EASY, non time consuming fix if indeed it fixes/re-mediates the problem.
  13. buick64203

    buick64203 Just plum crazy Staff Member

    Well, the Innovate wideband O2 kit is on its way! Lets see if the wideband confirms my seat of the pants diagnosis
    techg8 likes this.
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Jason, make a small vacuum leak and take it for a ride. See if that leans it out and it runs better that way. Are you sure the power piston has full travel? Try pushing it down through the vent, maybe it is gummed up and isn't hitting bottom?
    techg8 likes this.
  15. telriv

    telriv Well-Known Member

    That's also a good & easy idea. I did it & made NO diff.
    Carry on with the power piston idea. Maybe even make a stopper to hold it down some???
  16. buick64203

    buick64203 Just plum crazy Staff Member

    Yep, tested that. I put a small screwdriver in there and the piston is definitely moving up and down. I got my new toy on its way so lets see what that tells me. You know how much I love buying tools :D
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yup, you'll probably need a sedative when it arrives.:D
  18. Cliff R

    Cliff R Well-Known Member

    The power piston used is NOT a player at steady cruise at light throttle, and NEVER clip one or stretch it out, get the right part instead for what you are doing.

    The carb has an APT system and would be the BEST way to cure lean running at cruise if you can get it out of the baseplate. I'm able to get about 95 percent of them out w/o issues, there will be one here or there that just isn't coming out no matter what you do, but it is always worth the effort and makes part throttle tuning seconds and fully adjustable w/o taking the carb apart.

    We make an external APT screw for those with a spring on it so it will hold it's adjustments.

    Before doing any jet or metering rod changes make sure that the jet size is sufficient by doing some heavy primary side only pulls and lug the engine enough to know that the jet size is adequate. With this new fuel sometimes just going up one jet size will cure both problems if you aren't using APT for fine control of the part throttle A/F.

    It was mentioned above but also verify that the vacuum advance isn't adding too much timing at the new higher cruising RPM. If the distributor has lighter than stock springs (or a spring/weight kit installed) it may simply have too much timing at part throttle and can be cured by reducing the amount of timing from the vacuum unit.

    No mention of engine modifications but changes to improve engine efficiency such as camshafts with better cylinder filling abilities, cylinder head port work, tighter quench, higher compression, or combinations of all of these will have the engine wanting LESS timing at every rpm.

    One also has to factor in that this new fuel isn't as good as requires more of it, so it may simply be a matter of needing more fuel at cruise to compensate for recent changes in the fuel quality.........Cliff
    BYoung likes this.
  19. wkillgs

    wkillgs Gold Level Contributor

    Or to make it a bit richer, close the choke a little and take it for a test ride.
    Will changing the rod hanger to one with a higher position richen it at cruise?

    You'll like the Innovative A/F gauge! Which one did you get?
  20. buick64203

    buick64203 Just plum crazy Staff Member

    I got the"LM-2" portable one with the tailpipe clamp. I'm curious to see how well the aquamist car is running as well. That one has an adjustable APT thanks to Cliff that I adjusted by the seat of my pants at the time.

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