Carb tuning + hot idle

Discussion in 'Mech Tech' started by nooster, Nov 22, 2014.

  1. Hello good folks :)

    as my engine's recently been back in the van and running (sorry no video as yet!)

    I've been messing with the carb in a vain attempt to tune it.

    Seems like the engine's idling too hot now, dippys a bit too hot to the touch etc. Could this be caused by incorrect carb settings?

    The valves are spot on, timing is pretty good, electronic ignition.
    Tinware is all in place, belt is tight, wind blowing from under van etc

    I must confess despite all of my readings I don't exactly understand the function of the carb screws 34pict3, other than their basic function and default position. And hence relatively indiscriminate fiddling follows :)
    Has anyone got a really solid explanation?

    Happy weekends peoples!
     
  2. Baysearcher

    Baysearcher [secret moderator]

  3. Thanks :) its a really good article and have gone through it several times over the last few days! Although there's not much by way of what the screws actually do - other than turn it in 1/2 a turn till the revs drop, and so on. Would be nice to read a really comprehensive account to understand exactly what's going on.
     
  4. I'm still not sure for instance how to make the mixture more rich - which screw to turn which way.

    Though being daft I may well have missed it somewhere!
     
  5. Sure timing is ok ? gets hot if to advanced
     
    paradox likes this.
  6. I'm sure there is an slight error in this otherwise useful guide.... No13 - turning the smaller screw in slightly at the end makes it run a touch richer, not leaner.... ?
     
    Cawstan1 likes this.
  7. This is a slightly simpler way carb adjustment vw t1 t2.jpg
     
  8. cool cheers all, very helpful. To get a decent idle at the mo (without spending a day tinkering!) Seems the fast idle screw needs to be most of the way in - ie giving some throttle. I know this isn't right obviously though.

    So the conclusion is that turning the little screw in increases the amount of fuel in the mix?
    And turning the big screw in increases the idle speed?

    cheers again peeps!
    Fairly sure the timing is good - hard to tell exactly when the idle is a bit all over the place. But its certainly not a mile advanced.
    I shall be checking for air leaks next - guessing mostly the carb/gasket/manifold boots/manifold to head.
     
  9. Check for air leaks, set your timing statically with a light bulb (engine off) - then you know it's right, set the fast idle cam clearance and idle screw/idle mixture as per the instructions and your engine should run. Avoid fiddling wildly with the two screws.
     
  10. Zed

    Zed Gradually getting grumpier

    Set the timing properly first, then the fast idle screw as above. After that fiddle like mad and you'll get there. If it won't start with the mixture screw (small one) 2 1/2 turns out try a few more. If it still won't idle, unscrew the idle speed screw (big one) until it will. Then gradually adjust both turning the speed screw in to get a reasonable idle speed, then adjusting the mix as per book, then speed again etc.

    You do not want to let the idle speed get over 1000-1100rpm while adjusting as the timing advance may be kicking in.

    If the idle speed is flctuating you have an air leak.

    At idle, the vac advance should NOT be working - if it is, take the carb off and look up the barrel to where the plate rests. It should not touch the side, that's the purpose of the 1/4 turn on the speed screw. If it has been allowed to rest on the side it can wear into the barrel and forever expose the vac advance signal hole which will cause constant vac advance and big trouble. If so the carb is scrap.
     
    paradox and snotty like this.
  11. Thanks ever so much for you help chaps :)

    @zed not sure I completely understand the last paragraph but will definitely follow up on what you say (if I can!)
     
  12. Zed

    Zed Gradually getting grumpier

    Hopefully you won't need the last bit.
     
  13. Update...

    Engine is running better. Have a steady 850 ish idle with timing set to 7.5. Bypass is 2.5 turns out and mix is 4/5ish. It running much cooler too. I think I may well have had a leak - fully tightened the boots and manifolds.

    So just a couple of things:
    @zed In your last post you mentioned about the throttle plate wearing and continously exposing the vac advance. I don't know whether that's the case could you take a look at the piccy? Is the vac hole the one on the left? It seems the vac is active at idle - I can time the engine with the vac line on to 7.5btdc but when it's plugged the idle drops until near stall and the timing falls to just after TDC. Could there be any other cause?
    carb2.JPG

    Finally there's a small (about a mil) amount of axial play in the throttle arm. There doesn't seem to be a leak there after spraying with gas but just wanted to check that was normal.

    Cheers to everyone once again

    nick
     
  14. Zed

    Zed Gradually getting grumpier

    Yes i think that is the hole, it should be covered by the throttle plate at idle so that it makes no difference whether the vac pipe is plugged in or not. Your carb body looks worn.
     
    nooster likes this.
  15. ta for the reply zed, it does look a bit knackered on a big close up yes. Will have a fiddle to see if I can alter anything, otherwise might be a bin job I'm guessing. Just out of interest it says on the side 34peak3 instead of pict. Wonder if its a cheapy....?
     
  16. Zed

    Zed Gradually getting grumpier

    While you do have it fitted, set the timing to 7.5 degs with tube OFF. Then rev up and check it advances to approx 30 degs also without tube attached.
    all that being good set the carb again. Now you can either block the advance tube at the carb and run your distributor as if it was mech advance only type, or plug it back in and put up with high revs over-advanced tickover.
     
  17. any guesses how far advanced the tickover would be to cause some troubles?
     
  18. Zed

    Zed Gradually getting grumpier

    You won't cause any damage at tickover. If you set as I suggest, the moment you apply any pedal at all it will behave as normal. Just tickover will be over advanced and the engine may race a bit.
     
  19. Zed

    Zed Gradually getting grumpier

    A new carb is the only cure of course, I'm just suggesting how you can get by safely.
     
    paradox and nooster like this.
  20. Ta zed, so very helpful as ever. =)
     

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