More DRLA 40 nonsense

Discussion in 'Mech Tech' started by Zed, Aug 13, 2020.

  1. mikedjames

    mikedjames Supporter

    I think one of the things they keep doing is see the venturi as an obstruction to flow, which it is, rather than a means of extracting a vacuum derived from mass air flow, which gives smoother behaviour as you arent having to put a "steep" fuel feed characteristic (big jets) against a "weak" flow rate vacuum signal.
    Smaller vents mean a stronger flow rate signal.

    And the vacuum signal should be due to fast flow in a smooth tube not purely obstruction(where some carbs have large structures smack in the middle) . Past the venturi the pressure increases again as the airflow slows in the larger diameter inlet manifolds.
     
    Zed likes this.
  2. Got 30's at the mo.. venturi and jet changing is as far as I'll go ...not statring to drill anything out just yet ... wait for Zedders to try first....

    Sent from my SM-G930F using Tapatalk
     
  3. I have no idea what you lot are on about, but hope you don't mind me asking some hopefully simple dellorto questions here?


    How often (mileage wise) do dells need 'servicing' & what should be 'changed'?

    Can servicing be done with the engine still in - no engine access hatch & type 1 engine.

    I've heard talk about preventing heat soak with a riser / gasket. Are these a good idea? How do I know if I already have them. I can post some photos.

    Most I've ever done to my dells is check the fuel filters and change the double fuel banjos to single ones.

    Happy dellorto ing.

    Hope this hasn't hijacked the thread too much
     
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  4. @paulcalf I rebuilt mine if I can do it anyone can - bought the dellorto book and service kit.
    They run great they have been on for a couple of years. I would recommend rolling road after if you do it. I have the heat sock just looks like a spacer below the carb.
     
    paulcalf likes this.
  5. Thanks.
    I've had mine rolling roaded a couple of times, but quite a few years ago. They seem to run OK, but noticed they felt a bit juddery on low revs the other day, then they seemed OK the next day, possibly just a bit of dirt in the idle jet, but made me think of servicing them
     
  6. Just take the jets out given them a blow put them back in
     
    paulcalf likes this.
  7. I'm not brave enough for that quite yet.

    Which dellorto book did you get, I may attempt it if I'd read how to do it or seen some pictures
     
  8. Zed

    Zed Gradually getting grumpier

    You would know if an idle jet was blocked it would be popping and backfiring.
    Once they're set up you shouldn't have do any servicing apart being sure the linkage moving parts are greased.
     
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  9. Zed

    Zed Gradually getting grumpier

    I set out with my Christmas cracker jet drills intending to enlarge the top two from 1.40 to 1.60 this morning. My fatal mistake was assuming the drills would actually drill. Doh!
     
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  10. Thanks.

    I do get a bit of popping until it's warmed up & some back firing occasionally if I rapidly take my foot of the gas.
     
  11. Zed

    Zed Gradually getting grumpier

    Popping only while it's cold is good - an indication that someone fitted the right idle jets. :thumbsup:
     
    paulcalf likes this.
  12. Barry Haynes

    Barry Haynes I dance in leopard skin mankini’s

    You've made my day Steve, the carp bus pops and back fires when cold then once warmed up everything is fine so this is good:thumbsup:
     
  13. Let me know if you want a pdf of the Delloto book (PM me an email.) and I might know someone who can email one over...

    Sent from my SM-T580 using Tapatalk
     
    Lasty, S1mon, paradox and 1 other person like this.
  14. Zed

    Zed Gradually getting grumpier

    Due to my port drilling failure I've decided to embrace my lean transition as it only happens at constant low load speeds. After I considered this I went back from 200 to 180 airs to deliberately get it back, then down to 135 mains to make it even more pronounced and get WOT safe but not too rich. 30 vents.
    So now I'm idle cruising below 30mph at low 14s
    Transition from 30 to 40mph anywhere up to 17-18 AFR
    Then mid to high 13s to 50mph
    Heavyish acceleration mid 13s
    WOT high 12's
    Goes nice, getting better.
    @mikedjames how do those AFRs compare to yours?
     
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  15. Zed

    Zed Gradually getting grumpier

    I've now managed to man up and drill the top two ports so I have 100 - 140 - 140 -160 -160.
    That doesn't sound like much, but the cross sectional area increase from 1.4mm to 1.6mm is from 1.5 sq mm to 2.0 sq mm.
    While it was easy and knowing that the ports add air to the idle emulsion while above the throttle plate I upped idles from 47 to 50.

    The idle mixture is tuning like somewhere between a 45 and a 47. By which I mean before drilling 50 would be a sharper "correct" point, after drilling it's fuzzier and more like the smaller jets were.

    That's as far as I got. I might fit 52 idles before I drive - just a hunch.

    To recap, the basic idea is to change the progression curve which is rich to lean by adding more air to the idle emulsion via the upper ports as the throttle plate sweeps the lower ports which is compensated for by larger idle jets. As the throttle plate passes the upper ports and they change direction and supply emulsion, that emulsion will be richer through larger ports which will extend the progression and fill the lean hole.

    There may be more drilling...
     
    77 Westy, Deefer66 and docjohn like this.
  16. :eek:
     
  17. Zed

    Zed Gradually getting grumpier

    Did you know that at WOT air bleeds in reverse through the idle circuit into the main jet fuel well, right at the bottom? Thusly the idle jet size has a small effect on the WOT AFR. You could lean your top end with a bigger idle jet. :)
     
    snotty likes this.
  18. Did you know that randomly drilling holes in your Dells can completely shag them ;)?
     
  19. Zed

    Zed Gradually getting grumpier

    Have you seen the price of dells?
    If anyone else drills their carbs - that up to them, but if the situation was reversed I wouldn't be following in my footsteps. ***This thread is not advice.***

    Yanks were supplied with exactly the same carbs (yes I know their fuel is poop) but instead of
    100 - 140 - 140 - 140 - 140 they got
    100 - 160 - 160 -180 - 180. My change to
    100 - 140 -140 - 160 - 160 seems tame to me but I'm wary enough to stop there.

    I was thinking that a good lot of driving will show what I've done and I can adjust if I think it'll help. Or cry. :)
     

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