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Carb sync update - problems: air leak, high air flow rate

Discussion in 'Technical Q&A' started by Mike328, Jan 30, 2004.

  1. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
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    So I've got the carbed sync'd pretty well. I understand the process fairly well, was able to sync each barrell, then each carb between banks, and then each bank.

    Everything is flowing, according to my syncrometer (STE SK1), at 7 kg/hr +/- 1/2 kg/hr. I know that this seem high. Idle is fairly smooth and steady at 1000rpm. I've adjusted the idle mixture screws and am fairly confident that each is in the "range" of contributing to highest RPM, although where in that range I don't know (probably need to get a colortune).

    So, the symptoms that I know are not good:

    ** 1. High airflow rate: 7 kg/hr for each barrell

    ** 2. When I "goose" the throttle, and then release it, the RPMs continue to climb even after I've released the throttle, and are very slow to come back down (Maybe takes five seconds to return to normal)

    ** 3. When I spray starter fluid at the carb bases (right on the insulator gaskets), the RPMs rise significantly (maybe 500rpm). Each carb appeared to exhibit this behavior.

    NO WAY! I just had these rebuilt by Pierce Manifolds. There is NO WAY that I can have an air leak?? They straightened the carbs so that they were true, and every gasket is new in the thing.

    For each carb, I used ONE 3mm thick black plastic base gasket piece, with very thin (1/3mm) light brown colored, pattern-printed factory gasket on either side of it. There were four additional regular 2/3mm thick gaskets, part of the rebuild kit which I did not use; they weren't on before and don't show up in the parts manual.

    So, either I put them on the intake manifold wrong, over- or under-torqued the carb nuts, didn't use the proper intake manifold <--> carb gaskets, or Pierce Manifolds rebuilt me some leaky carburettors.

    Any ideas?
     
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  3. bill308

    bill308 Formula 3
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    Mike,

    Starter fluid is pretty volatile stuff. I wonder if it is evaporating, like a local cloud, and being sucked into the carb through the throat? This could explain the increase in rpm.
     
  4. Steve Magnusson

    Steve Magnusson F1 World Champ
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    7 kg/hr per barrel at 1000 RPM? -- sounds like you're running on 4 cylinders. If you remove each spark plug wire one at a time at idle does the RPM drop the same amount for each?
     
  5. Mike328

    Mike328 F1 Rookie
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    I'm in the process of replacing the plugs now. They look a little on the lean side.

    I'm pretty sure I'm running on both banks; I've been down a bank before and know what it feels like. That said, I'll do the spark plug pull-test to make sure. I was able to affect the RPM by adjusting the mixture at each carb barrell.

    Now, I'm not sure this has anything to do with it, but the needle on my SK1 does fluctuate a little, I'd say by 0.75 kg/hr to 1.25 kg/hr. I've heard other comments that the SK1 is "dead on," so I'm wondering if this could be indicative of some problem.

    Steve, how could running on one bank double the airflow?

    I agree that the stater fluid could be evaporating and being sucked in... What concerns me more is the slow RPM drop after the throttle plates have closed.
     
  6. Mike328

    Mike328 F1 Rookie
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    Just talked to Mike at Pierce Manifolds.

    1. I'm trying to set the carbs with the linkage on. Don't do this. Linkage is coming off.

    2. Spraying starter fluid at the base gaskets is not the way to test for air leaks. Exactly as Bill pointed out, it is very volatile and its very vaporization is going to make the engine race a little bit. If you have to test this way, Mike recommended motor oil, something with viscoscity, to help seal the leak.

    3. Mike notes the odds of having an air leak on all eight barrells is obviously next to nothing.

    4. Mike confirms I'm using the right 3mm thick insulator base gasket, with factory "vegetable" gasket on either side, with NO other gaskets even though four extra small ones were supplied with the rebuild kit.

    5. Mike confirmed that the carbs were put on a granite block, and were flat to well within tolerances. He also noted, of course, that carbs rebuilt by them just don't have air leaks (within the carb, anyway).

    6. Regarding torque specs, he mentioned it didn't take a lot to mount a carb but at the same time, you'd really have to overtorque things to warp the carb base. I think they're torqued to a happy medium.

    7. I mentioned the slow idle returning to "base idle" after revving, indicative of an air link. Mike said, disconnect all the linkages, and rev each carb ONE at a time to isolate the problem carb.

    8. I mentioned I'm getting 7 kg/hr flow rate on the SK1. He said a number of things, paraphrasing though, he seemed to suggest that the absolute number on the SK1 meant very little (and can be manually adjusted if you open up the SK1). What matters is, of course, the relative flow readings across all 8 barrels, correct mixture, and the tachomoter reading.

    So, that's what I got out of our conversation. I'm going to go back out there, get these new plugs back in, and have another go.
     
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  8. bill308

    bill308 Formula 3
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    Mike,

    Are you sure our distributor advance mechanisim is operating freely? A sticky mechanism could cause the slow to idle symptoms you are experiencing.
     
  9. Mike328

    Mike328 F1 Rookie
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    Hmm... I see what you're saying. For a split second earlier in the day, I did consider that. I'm not sure how I would test this.

    Hell, I've got an Electromotive HPX in my garage, waiting to be installed!

    This may become clearer if the same engine symptom happens for each carb, which I'll check here shortly...
     
  10. bill308

    bill308 Formula 3
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    Mike,

    Too much initial advance can also cause this problem.
     
  11. 4re gt4

    4re gt4 Formula 3

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    Retarded advance will cause a slow idle, which in turn you'd compensate for by cranking up the flow rate.

    Assuming that your syncrometer is calibrated somewhat similar to the rest of ours, a flow rate higher than others are reporting could happen if you are using both sets of points. Many of us (me included) don't have the 'retarded idle' set of points. (R2?? I can't remember which is labeled which.)

    If you don't have both sets of points, err, to paraphrase Gilda Radner on SNL: "Never mind!"
     
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  13. Mike328

    Mike328 F1 Rookie
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    Just running one set of points now.

    Timing was "fine" before the carbs came off!
     
  14. 4re gt4

    4re gt4 Formula 3

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    Timing may be 'fine', but what I'm referring to is that, at idle, timing is 7deg retarted with both points, and 3deg advanced with only one. *note*: I'm pulling these numbers out of memory, as I don't have the books here, but the 10deg difference will still be there even if I did get the numbers wrong.

    If you only have the one set, I'd say that the advise given by others above is definitely a good place to start. In particular, that given by the guys at Pierce.
     
  15. bill308

    bill308 Formula 3
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    Factory timing with both sets of points on a 1978 308 is 3 degrees ATDC.
     
  16. rudy

    rudy Formula Junior

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    I didn't read all the posts, but if the RPMs rise when you spray at the base, there's a leak. If the timing was right before, and nothing was disturbed, then the timing is right now. The worst thing you can do is start to change EVERYTHING. You need to diagnose.
     
  17. Peter

    Peter F1 Veteran
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    I'd hate to suggest this, but you can use a very minuscule amount of sealant on the baseplate, between carb and intake manifold.

    To keep the engine from ingesting goops of sealant, I highly stress "minuscule" amount. Apply a thin smear of it and wipe some more off. Almost to the point of wiping it all off. Should help with your leaking issues.

    Then go about rechecking timing, flow, etc...
     
  18. rivee

    rivee F1 Rookie

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    A light colating of "Gask-a-Cinch" is what was recommended to me on the carbs.
     
  19. joeyy

    joeyy Karting

    Nov 11, 2003
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    i think you should go back to the timing. if you are running the single set on the R-1 point you will automatically be running 10 deg advance. check to see that you didn't go too far. once the advance plate moves it will come down slowly. from what you wrote about the carbs it seems to me that they are going to check out ok.
     
  20. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
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    I think that before you "change" the timing think back to how the car ran before you had the carbs done. If it ran well and you were on 1 set of points then continue to work the carbs. Also I have never used starter fluid to check for vac leaks with the carbs. Unfortunately you need to use oil (I use ATF) to check for leaks which is a bit messy but it should fix the problem. As for the sealant on the base gasket you can go either way with that. If everything is clean and flat then you don't need the stuff but a little dab won't hurt. I time my 77 with pertronix's pointless set up at 7 ATDC and it runs like a raped rabbit up to 7500rpm.So I usually start with things that I touch before I explore other areas. My 2c
     
  21. Steve Magnusson

    Steve Magnusson F1 World Champ
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    "...how could running on one bank double the airflow?"

    The internal friction to run the engine at 1000 RPM is about the same regardless of how many cylinders are actually running correctly. If you've only got 1/2 the cylinders contributing power at idle, then they need 2X the airflow to make up the power difference.

    However, your comments about the slow RPM return after fully closing the throttle plates says there's more/something different wrong than 4 cylinders always working well and 4 cylinders always dead.

    Always possible that your particular STE gauge is mis-calibrated, but again the slow return to idle says there's more going on then that.

    My experience, too, is that retarding the idle timing increases the needed airflow rate to maintain 1000 RPM, but not 2X the airflow:

    1000 RPM at 7 deg BTDC = 3.5 Kg/hr (the R1 only set-up)

    1000 RPM at 3 deg ATDC = 4 Kg/hr (the stock R2/R1 set-up)

    and in both cases, I'd say the short-term airflow measurement fluctuation is not more than +/- 1/3 Kg/hr (i.e., about +/- the width of the indicating mark).

    Assuming you're very sure that both distributors are at ~7 deg BTDC at 1000 RPM and you're measuring 7 Kg/hr per barrel, I'd still ask the same question:
    If you remove/disable each spark plug wire one at a time at idle does the RPM drop the same amount for each? (But please take the necessary precautions to avoid getting shocked.)
    If it does, I guess the next question would be: Are your exhaust manifolds glowing?;)
     

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