Carburretor Refurbishment. A step into the dark... | Page 3 | FerrariChat

Carburretor Refurbishment. A step into the dark...

Discussion in '308/328' started by Andy 308GTB, Nov 3, 2018.

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  1. Andy 308GTB

    Andy 308GTB F1 Rookie
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    Hi Martin, the spitting reduces as I wind out the idle mixture screws. The exhaust noise is occurring when the idle mixture screws are 2 or 2 1/2 turns out.

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  2. Martin308GTB

    Martin308GTB F1 Rookie

    Jan 22, 2003
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    What is your idle speed?

    Best
    Martin
     
  3. Andy 308GTB

    Andy 308GTB F1 Rookie
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    The idle is set to 1000rpm.
    The cracks and bangs are very prominent if I blip the throttle.

    I am starting to think that all is not well and I may need to take them back off the car. Put the sealed bearings in. Check the bases more thoroughly that they are true. Basically eliminate the possibility of a vacuum leak. It shouldn't take more than a few hours.
     
  4. derekw

    derekw Formula 3
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    #54 derekw, May 4, 2019
    Last edited: May 4, 2019
    If you take them off, re-stake all the plugs as the US may have loosened them (a little epoxy into the holes too can't hurt.) Did you check and adjust float levels?
     
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  5. Martin308GTB

    Martin308GTB F1 Rookie

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    Talking about sealing lead plugs. What also works great is Loctite 290. Actually it's a thread sealer. But given its extremely low viscosity it works with capillarity and seals threads without dismantling the joint and even seals casting porosities. After restaking my plugs each one received a drop.

    https://www.henkel-adhesives.com/us/en/product/threadlockers/loctite_290.html

    I assume, that Andy adjusted float levels, since his rebuild sets came with new needle valves and washers. After replacement adjusting float level is actually mandatory.

    I will look into my records later, how far out my idle mixture screws are.

    Best from Germany
    Martin
     
  6. Andy 308GTB

    Andy 308GTB F1 Rookie
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    Thanks chaps. I did check all the float levels and they are bang on.
    Before I put the carbs back on I did inspect all the core plugs and prodded them with a screwdriver. They all seemed solid.
    When you say 'restake' them, you mean simply hitting them with a suitable drift? You aren't suggesting that I pull them out and put new ones in?

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  7. Martin308GTB

    Martin308GTB F1 Rookie

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    Hi Andy,

    regarding re-staking. I did it the way I described in my thread many years ago. It's absolutely sufficient to freshly expand the lead, that it seals again. My procedure also has the effect, that the possibility of losing a plug completely is eliminated.

    https://www.ferrarichat.com/forum/threads/308-carburettor-rebuild-some-specialities.35524/#post-134477920

    Pulling and replacing them can open a can-o-worms I wanted to avoid.

    BTW. According to my records my idle mix screws are 3 - 3 1/2 full turns out.

    Best
    Martin
     
  8. Andy 308GTB

    Andy 308GTB F1 Rookie
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    That's perfect, great thread and information.
    I totally agree about removing the lead plugs. I can see, why, in a perfect world you would remove them and replace them. But that's not the world I live in!
     
  9. jmaienza

    jmaienza Formula Junior
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    What are the values for the various jets? Modern gas is very different from the gas of 40 years ago. I'm in the US so my state has 10% ethanol in the gas which substantially changes things. I had to change several of my jets to accommodate the modern fuel.

    Also really check the air corrector screws. Loosen the lock nut and back out slightly then turn them in. These screws really allow you to fine tune each half of the carb so it is balanced.
     
  10. Martin308GTB

    Martin308GTB F1 Rookie

    Jan 22, 2003
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    Hi Jim,

    Andy has the situation, that the carbs worked o.k. prior to his rebuild and now no more after his reinstallation.
    So it's quite sure, that it has nothing to do with UK-fuel or jet sizes.
    Since he reported, that he didn't touch the bypass-screws, I think they aren't the culprit as well.

    BTW. In Europe we still get fuel with max, 5% ethanol or less. But I once accidentally filled up with 10% and noticed nothing.

    I actually think, these carbs are, once set up correctly, bullet-proof. I touched mine in 2008 the last time and the car still runs perfect. This past winter I checked synchronization/airflow and there was nothing to do. Corrected idle mixture a tiny bit and regreased linkage balls. That was it.

    Best from Germany
    Martin
     
  11. 308ROB

    308ROB Formula Junior
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    Andy, i read somewear that you set the airflow on 4- 5 kilo/h but i remember that i always put them on the 3 -4 kilo/h only saying.

    Rob,
     
  12. Andy 308GTB

    Andy 308GTB F1 Rookie
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    My airflow meter is secondhand (eBay) and is showing more like 12 kilo/h. Yes, at idle! I am thinking that this reading is wrong. So, the value is being ignored and the meter is being used simply to make sure the air flow is the same across each carb.
    From what I have seen, anything around 4 kilo/h is normal at idle.



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  13. jmaienza

    jmaienza Formula Junior
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    +1

    Mine flow at 3.5-4.0 kilo/hr at 950-1000 RPM.
     
  14. Andy 308GTB

    Andy 308GTB F1 Rookie
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    The tedium continues...

    I removed the carburettors from the car. I flattened the bases of the carbs using some very fine wet & dry sandpaper taped to a sheet of glass, happy that they are now perfect.
    I did not install the sealed bearings as I didn't want to get involved in manufacturing mini pullers at this point! (Especially in light of what I found, see below) I did check that the ball races were fully packed with grease (they were) but I packed them out more for luck!
    What I did find, was that when I held the chokes up to the light, it was possible to see light at the edges of the butterflies - primarily on the unit that I had messed with the spindles. I re-positioned the butterflies and re-installed the carburettors, convinced the problem would be resolved...

    Nope, the engine is still not dropping back to idle as I would wish.
    I removed the Front to Back linkages. So that the front pair and the rear pair were only connected via the side to side linkage. But this made no difference - although it did confirm that the Front to Back linkage isn't binding.

    My theory is now that the setting the airflow across all 8 chokes to be the same, is all well and good provided they are all airtight. But if one has a vacuum leak you are simply adjusting the other 7 to match the behaviour of the one with the vacuum leak?
    So it looks like they are coming back off the engine again. And the butterflies on all 4 units will come under very, very close scrutiny...
     
  15. derekw

    derekw Formula 3
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    I would try to get more info before pulling them off again. Try pulling the plugs one at a time to see the differences per cylinder and per carb, what colour and smell each plug, disconnect linkages and just use the idle screws to throttle back the carbs to the target air flow, then connect linkages, the air box and filter may have been keeping things in balance so without them it may be flowing too much air. As suggested, be very sure ignition is all good as that may have been affected by the dismantling. If you have or can borrow a compression tester, check that while plugs are out. How long since you checked valve clearances?
     
  16. Andy 308GTB

    Andy 308GTB F1 Rookie
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    Hi Derek,
    I've got to say that I was hoping that this job could be done in isolation!

    Overall the car was in good shape before I started. The ignition was rebuilt only a few years ago and very few miles have been covered since.
    I've never checked the valve clearances. Perhaps I do need to take a more holistic view of the problem!
     
  17. Martin308GTB

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    Hi Andy,

    don't panic. It's pre-war technology :) And like you said, your carbs worked o.k. before the rebuild, so a deeper problem is unlikely.

    have you meanwhile tried to set the idle mixture more rich? Do you have a CO-gas analyzer? Your symptoms still match "idle mixture too lean". What happens, if you turn the screws three full turns out -like mine- and correct speed with the idle stop screws?
    Be aware, that the idle circuit plays its role till higher revs. So it is not surprising, that exhaust banging worsens when you blip the throttle.
    BTW. I remember, that I experienced the same -slow return to idle speed- when I did my carbs myself for the first time. IIRC it was the mixture setting. I'm no more quite sure, but I'm sure at the end they worked and I didn't have to pull them once more.

    Best Regards and good luck
    Martin
     
  18. Andy 308GTB

    Andy 308GTB F1 Rookie
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    Thanks Martin for the encouragement, as long as I have some ideas to work with, I won't give up!

    I was still getting the issue yesterday with the screws 3 1/2 turns out.
    BUT bear in mind that the car had been running with the screws between 5 and 6 turns out, so that's my next trial. I should have tried this first but I wanted to set the car up per the instructions. The car was running rich but several mechanics told me that often these cars preferred it that way.

    I will try this tonight (i.e. setting the idle mixture rich) but I don't understand how a lean set up would induce the slow the return to idle.
     
  19. Martin308GTB

    Martin308GTB F1 Rookie

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    Hi Andy,

    if the idle speed rises while screwing out the idle mixture screws more and you have to compensate idle speed then with the stop screws, you will approach the final solution.
    This -closing the butterflies more- would also decrease the amount of air which seems high to me according to your original post. No matter if the used device you used contributes to an indication error.
    Regarding understanding why a lean mixtures induces slow return to idle speed I am currently discussing with a 'carburettor-pope' :)

    Best Regards
    Martin
     
  20. derekw

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    Ask the question another way and it might make more sense-- If my air passages are open sufficiently for 2000rpm and I lean out the mixture a lot, why do the rpms slowly drop? I agree with Martin that you need to get the air flow at idle right and then adjust the mixture with the idle circuit screws/jets. It would be helpful to check the A/F ratio with an exhaust O2 sensor (I got two from Ebay for £35 for this purpose.)
     
  21. Martin308GTB

    Martin308GTB F1 Rookie

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    Link? I would be interested in one. I just have a cheap CO-analyzer.

    Best Regards and thank you
    Martin
     
  22. Martin308GTB

    Martin308GTB F1 Rookie

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    If the mixture is too lean and you compensate idle speed with opening the throttles more, the transition/progression circuit comes into play. The bigger airflow draws fuel through the progression circuit and this affects idle speed coming down. It may not be enough fuel flow through the progession circuit to keep idle speed permanently high, but enough to slow things down. This is the answer I just got from my Weber-specialist.
    Next step. Close the butterflies more and screw out the mixture screws. I'm curious what happens.

    Best from Germany
    Martin
     
  23. Andy 308GTB

    Andy 308GTB F1 Rookie
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    That's my evening taken care of!

    Thanks very much indeed - that does make sense
     
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  24. Skippr1999

    Skippr1999 F1 Rookie
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    I haven’t read the entire thread and I’m no expert on this, but I encountered similar issues while dialing in the carbs on my Dino.
    What I found was opposite of what’s being reported here.

    I had the high idle not returning to 900 when hitting the accelerator. I had the adjustment screws too many turns OUT. They were about four to five turns out at the time. I was also getting the pops and bangs.

    Returning the adjustment screws to TWO turns OUT remedied the situation. Idle was readjusted AFTER returning the the adjustment screws to TWO turns out.

    It seems to me that you’re running too rich if your getting bangs and high idle not returning. Too much fuel.

    Airflow 3.5 per meter
     
  25. Skippr1999

    Skippr1999 F1 Rookie
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    The pops and bangs I describe above were in the exhaust, ie. unburned fuel, too rich in my case. I read more of the thread and see you already tried having screws at 1 turn out.
    I’d try adjustment screws 2 turns out with idle screws totally backed off and then set idle screws appropriately after you do the adjustment screws at 2 turns out. And use air meter to set idle around 900 to 1000 at 3.5 air flow



    Just trying to help.
     

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