Dellorto DRLA 40's - Mixture Screw Setting Procedure - Best Lean Idle

Discussion in 'Mech Tech' started by andyo, May 21, 2014.

  1. I have the Dellorto book, but it's not mentioned until far later when trouble shooting (which I find strange)

    To be fair, I'm going to balance via a manometer (I'll document, for fun) which seems to be a rare approach, but the best!

    So I want to understand the best approach to set mixture adjustment at idle, I have conflicting approaches (I'm favouring the latter). I'm guessing the aim of the game is to get the best vacuum / high idle, but, honestly, so much searching has not clarified matters for me.

    Approach 1

    Adjust the Idle Mixture screws to produce the strongest manifold vacuum for each individual barrel (strongest vacuum, or loudest hiss, or peak rpm). Don't worry about balance, just make each one as strong as it can be. On properly jetted Dellortos, the idle mixture screws usually end up being out 3 to 3 1/2 turns from fully seated. Anything more than 4 turns gets beyond the effective taper on the needle and will make no additional change. Anything beyond 3 3/4 turns probably means larger idle jets would be better. Anything less than 2 turns probably means a smaller idle jet would be better. Some older Webers used a pretty coarse Idle Mixture screw, and the final setting could be as low as ¾ to 1 ½ turn.

    The peak vacuum/ rpm will probably hold over a range of screw movement, so adjust the screw each way… in and out… until the vacuum/ rpm drops noticeably from the peak. Note the screw positions where the drop-off occurs relative to the peak. After the range is established, the final setting is made by backing the screw out a bit, then turning it in and stopping half way between the high and low limits you just found.

    Approach 2

    The idle mixture adjustment screws are located at the base of the carburetor, and usually on the outside (Weber or Dellorto). Back each of them out 3-4 turns. With the engine fully warmed up and idling, slowly turn them in (do one at a time), and CAREFULLY listen to the idle quality and speed. As you turn the screw in, you will hear the cylinder misfire (it’s the one you are turning in, and it’s running out of fuel). Once it misses, turn the screw back out until the cylinder fires properly again. Play with it, and adjust to achieve maximum idle speed. The biggest mistake in this part of the adjustment process is not waiting 10-15 seconds after an adjustment of 1/4 turn at a time, for things to “stabilize”. Don’t rush it! At idle it takes a bit for fuel to get flowing, or stop/evaporate, don’t be in a hurry.

    When you find maximum idle speed for that cylinder, I go 1/2 turn richer with Dellortos, and 1/4 turn richer with Webers. This will result in a more stable idle speed as temps and conditions change.
  2. Manometer (for balancing / synchronising)


    Need to put some oil in, but ready for a trial on Sat.
    MarcT512 and paradox like this.
  3. either method will achieve very similar results: each cylinder will have minor variations and it's own particular requirements. As you're effectively running one carb per cylinder each can be dialled in's all seat of the pants stuff unless you have a means of measuring individual and combined F/A ratios....this all goes to pot when you change altitude/temperatures etc.
    andyo likes this.
  4. Cheers, so the aim is to get the best / highest idle / vac on each barrel. Then balance them up.
  5. Let me know how you get on with your manometer, as I've just built one to set up my twin weber ict's
    paradox likes this.
  6. I took the easy route. Set throttle plates with feeler gauges as per the book and set idle screws out about 2 turns. Needed to up the throttle slightly (1/4 turn) Then had it rolling roaded. I was rich on idle but that got sorted.
  7. Cost me about 80 quid for the session and goes way beyond what you can do at home!
  8. Thanks for the advice. Now I need to find a rolling road near Halstead Essex
  9. I will do squire. I am based in Southend btw, and will be looking for a rolling road soon. I was recommended MoWatts in Benfleet, i understand the Father is the man with carbs, his son(s) do the newer stuff.
  10. Yes, I was looking at those guys.
  11. Booked the bus in with T and M auto in Colchester. Got some good reviews with setting up vw's
  12. When u book it in for? Let us know how you get on. Didn't get the manometer on until today, issue with no 1 not pulling enough vacuum, compared with the rest, no matter what I do with the other mixture screws - so need to get to the bottom if that.
  13. So removed carbs to check for blockages etc, looked ok, blasted the hell out of them with carb cleaner, and some dirt could be found in a clean bucket. Also ordered a re-build kit which I have put on (mostly) what are the washers for in this pic?: [​IMG]

    Also thinking now that the reason cylinder 1 was showing low vac, was because that is the barrel that has the vac port for the dissy. Ill see when I bolt them back on (and block it this time)

Share This Page