carb tuning DNCVA/DCNF

Discussion in 'Workshop Forum' started by AKimball92, Aug 10, 2019.

  1. AKimball92

    AKimball92 True Classic

    I recently installed an AEM wide band sensor to help tune my carb. The car runs and drives and we just went for a quickie drive around the neighborhood. Some notes about its current condition.

    Engine spec is in my signature. Carb is a 36 DCNVA, very similar to a DCNF but with a manual choke.

    The engine idles high. While driving the wide band was reading in the 10-11 air/fuel ratio, very rich. Now that I have the sensor, I adjusted the idle jet, to raise the ratio and reduce the fuel. Idle jet is all the way in and its idling really high. I did not plug in a timing light or more accurate RPM reading. throttle screw is all the way out (not touching). It's idle at about 3000 RPM but still only at about 12.5 A/F. The exhaust is also a bit on the black side, which if rich, would be unburt carbon deposits?

    Do I need a set of larger or smaller number idle jets? Anything else possibly wrong?
     
  2. carl

    carl True Classic

    Location:
    Virginia
    At true idle, your idle mixture screw makes whatever idle jet you have an infinitely variable jet so the real idle jet size you need is one that gives a proper reading just off idle or at cruising speed with barely any push on the gas pedal. At 3000 rpm you are either on the main circuit or you have a fairly large vacuum leak. Disconnect the throttle linkage at the carb to verify the linkage is not too tight and is pulling your throttle open. You will not be able to determine jet sizes or mixture screw setting until you get the idle speed down to normal.
     
  3. EricH

    EricH Eric Hamilton Moderator

    Location:
    Durham NC, USA
    You’ll already have checked for a vacuum leak, and you have the throttle stop all the way out, but it’s getting air from somewhere if it’s idling at 3000.

    If it’s like a DCNF it’ll have two air bypass screws. How do you have them set?

    Disconnect the throttle linkage in case it’s tight and holding the throttle open despite the stop screw being all the way out.
     
  4. What size idle jets are you currently running? You will need to run something like a 47 or 50 from my experience.

    Cheers,
    Dom.
     
  5. AKimball92

    AKimball92 True Classic

    I confirmed today that the idle jets are 47s. I started with 40s as it idled too high when I first got the engine running. It currently is idling at about 2000 cold and increasing to 2300 after heating up. The idle screws are turned all the way in. A/F ratio shows about 12.3 at idle and 2300 rpm. The throttle stop screw is all the way screwed, not in effect. Should I purchase 50s idle jets?

    I need more time tuning to fully figure out the other settings. That 2300 RPM is with the choke fully open.

    While driving, the engine stays in the 10-13 A/F range. What jets should go bigger, or smaller? I don't fully know if this is just a jet thing or how to determine the effects of a wrong emulsion tube, air correction jet, float valve, etc. What causes and effects should I be looking for specifically?

    Fast accel, Blips, constant RPMs through all ranges, etc.
     
  6. Are the throttle plates closing enough to block the transition orifices? If not, you need to figure out what is keeping that from happening before messing with the jets.
     
    EricH likes this.
  7. EricH

    EricH Eric Hamilton Moderator

    Location:
    Durham NC, USA
    When you say "the idle screws" do you mean the idle mixture screws, or the bypass screws? If the engine runs with the idle mixture screws turned all the way in, then it is running on the transition or main circuits, not the idle circuit.

    But in any case, the first question to answer is where the air is coming frrom. Your idle is too high because the engine is getting too much air (the carb just delivers fuel to match the airflow) and there are only a few ways that can happen: throttle plates not fully closed, bypass passages open, vacuum leak.
     
  8. Ulix

    Ulix True Classic

    Location:
    Stuttgart, Germany
    To say it again, more clearly: you are not going to fix your high idle with a jet change.
    So do look for a source of air as described above.

    For the mixture at cruise, you should step down by a size or two on your main jets.

    The idle jets should be optimzed after the main jets are set.
    Make the idle jets richer until you don't have a stumble (lean spot) anymore when the main jets are coming on.
    I can't say more since I have never had a single DCNF, only duals.
     
    autox19 likes this.
  9. carl

    carl True Classic

    Location:
    Virginia
    I'll reinforce, again, what Ulix and others have said, do NOT mess with jets until you get the idle down to normal.
     
    autox19 likes this.
  10. AKimball92

    AKimball92 True Classic

    Thank you all. I was approaching this the wrong way. I was assuming since it is <14.7 it is too rich. In that case it don't have enough air, thus no vacuum. The way EricH worded it, I will look for a vacuum leak. I will play with some starting fluid and see what I can find. Any other way to spot a vacuum leak?

    By your statement below:
    This leak would have to be internal to the carb, as you hint at, correct? A warped manifold or carb base would only allow too much air make the O2 read lean correct? Something is needed to pull more vacuum to through the venturi, requiring the leak to be internal to the carburetor.

    Also, while driving, I never read a lean condition. There was some stumbling under load when accelerating. I'm hoping it's just too rich.
     
  11. Ulix

    Ulix True Classic

    Location:
    Stuttgart, Germany
    The vacuum leak could be anywhere between throttle plates and intake valves.
     
  12. AKimball92

    AKimball92 True Classic

    How would a vacuum leak from the manifold create a rich fast idle? Honest question, not questioning your reasoning.
     
  13. carl

    carl True Classic

    Location:
    Virginia
    Don't get target fixation, have you determined that the cam box mounted linkage is not pulling on the carb linkage and thus openings the throttle?
    I would be curios to see a picture of this carb.
     
  14. EricH

    EricH Eric Hamilton Moderator

    Location:
    Durham NC, USA
    Not necessarily, and that’s one of things that makes a vacuum leak so insidious. Say there’s a leak downstream of the carb....

    Yes, that makes the mixture too lean. In fact, it makes the mixture so lean that the motor won’t run with a properly jetted carb. So you go with a bigger idle jet and/or open the mixture screws so that the air coming through the carb draws in more fuel, enough to compensate for the extra air being introduced downstream. That makes the engine run, and the poor atomization of the fuel means that it runs even better as you further richen the carb - but in fact you are pushing the carb further and further away from proper tune.

    So whenever you find yourself dealing with a carb that obstinately refuses to idle properly, you want to find where the air is coming from. It’s air, not fuel, that determines the idle speed.
    You may also want to take the plugs out and wirebrush them because they will foul quickly under these conditions, and once fouled the engine will run badly even when you’re getting the carb right.
     
  15. Ulix

    Ulix True Classic

    Location:
    Stuttgart, Germany
    Exactly.
    Having a vacuum leak downstream of the carb is just like opening the throttle plate a bit more, more air is getting into the engine.
    I would treat idle mixture separately.
    Idle mixture does not affect idle speed by much at all.
    So speed first, mixture second.
     
  16. AKimball92

    AKimball92 True Classic

    A bit of progress was made on the car (and myself) last night.

    1. I realized I was only playing with the idle jet screw and not the mixture screw. I realized it was mentioned up above. I knew i was only messing with the idle jet screw but for some reason ignored the connection... wheres the head banging on wall emoji when you need it?

    2. I had a bit of tension in the throttle cable. The holy bolt and nut were adjusted on the cam cover. My idle is now around 2000 according to the tach. I knew it was there and before, I would intentionally pull the throttle cable to close the plates when attempting idle in the garage. I was waiting to see if that settled a bit. The idle sounds good, a bit high but I really don't think it sounds like 2000 RPM. I was unable to get my Dwell meter to read RPM. I am still working on that.

    3. I played with some starting fluid and was unable to get an engine response external to the carburetor inlet. I still need to clean the plugs for a clean spark.

    4. I pulled the air jets, emulsion tubes, and main jets. Thanks for the pipe cleaner tip I read somewhere on this forum! My carb currently has:
    main jets: 130
    Emulsion Tubes: F36
    Air jets: ??? I couldnt find any markings on these
    Idle jets: 47

    From my readings and suggestions I've seen here, I believe my mains are small. I will play with the idle mixture screw and do more seat of the pants analysis in the meantime. Should I purchase 140s + or - after I try to optimize the idle mixture screws and idle jets?
     
  17. EricH

    EricH Eric Hamilton Moderator

    Location:
    Durham NC, USA
    That’s a plausible source of extra air. It’s worth completely disconnecting the throttle linkage, even when you think you have it right and it can’t possibly be holding the butterfly open.
     
  18. carl

    carl True Classic

    Location:
    Virginia
    I'm with Eric, it's amazing how the linkage on the cam box can mess up thing. Remove the ball connection between the carb and cam box and see what that does. As I recall, you have an A/F meter? If so, what is the reading at just enough throttle to get off light cruise but before the secondary open? That reading will tell you if you need larger main jets.
     
  19. rx1900

    rx1900 1981 X1/9

    If you want to set - or check - your idle speed accurately, the stock tach in the dash is pretty useless.

    If you ever venture across the border, stop in at a Princess Auto ( the Canadian equivalent of Harbor Freight ) and pick up one of these:

    https://www.princessauto.com/en/detail/actron-heavy-duty-digital-multimeter/A-p8707598e

    Measures RPM quite accurately. Also reads dwell ( if you still have points ). And measures duty cycle on injectors and such. Also checks diodes. And ohms and volts of course. Basically it is an automotive multimeter.

    Goes on sale occasionally for $19.99 Canadian. That is like $4 US or something......
     
    AKimball92 likes this.
  20. AKimball92

    AKimball92 True Classic

    I did install a A/F meter. It's really fasinating to know by a quick look why the engine is bogging down. When you're new to carb tuning its hard to know exactly what your changes are doing or why the RPMs drop. In my case the RPMs tend to really drop at around 10.5 A/F.

    This time I played with the idle mixture screw and within like 15 min felt like the idle is considerably better than anything I've had before. Its currently idling in the mid 13s. I will continue to see if I can get that up into the 14s. No point in wasting fuel at idle. Idle is nice and low. I even raised it to a more comfortable level with the idle speed screw. I would say its about 75-80% dialed in.

    Under a slow accel (approaching steady state at each rpm) the gauge stays in the 12 to mid 13 range. That still is a bit low and rich. Any suggestions?

    Under fast accel it varies considerably. I need to look closely for the trend. I think it goes a bit rich, low numbers. In the future perhaps, I will dial in the pump system to see if this improves off the line feel.

    I continued to check the cabling and believe nothing is applying an extra load to the throttle. I would attempt to turn the throttle lever on the carb to no effect.
     

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