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Erratic Idle-308 carbs

Discussion in 'Technical Q&A' started by docweed, Sep 22, 2005.

  1. docweed

    docweed Formula Junior

    Dec 8, 2004
    452
    Morgantown,WV
    Full Name:
    Chuck Stewart
    I think I've read every thread on erratic idle problems so I know there are many of you out that are having difficulty "tuning" your Weber setup. I started by adjusted the ignition timing, set the dwell, and even rebuilt the DCNF's. Proceded to equlize the carbs...air bypass closed, mixture 3 turns out, and idle at 1000 rpm's. Using the Synchrometer checked all mated throats in each carb. All matched except one. Loosened the nut and turned the air bypasss screw out just enough to balance the two. Completed the whole adjustment/balancing procedure and then spent the next several weeks trying to get the engine to idle correctly. Used both ways to set the idle...idle stop screws and then use only the linkage. It would shoot up to 2000rpm and hang there or start to drop and stall. Kinda all over the place. I finally isolated the offending bank of carbs by disconnecting the linkage and revving each bank separately. The front bank returned to idle immediately but revved the rear bank and the idle would hang up. Spent hours trying to figure out what was causing the problem. Finally looked at the throat that was out of balance from the beginning, you know the one that I opened the air bypass screw to adjust. When I would turn the air/fuel mixture on that throat it would not effect the engine rpm's. I pulled the spark plug wire and no change in the engine speed but there was spark at that wire. I thought that by opening the air bypass screw I was creating a vacuum leak in affect and I could not feed in enought fuel thru the mixture screw to get the cylinder to fire. I deceided the heck with what the meter said and closed the air bypass screw, adjusted the mixture and low and behold the cylinder joined in and the idle stablized and held at 1000. The car is now running great (except for a flat spot which I hope to fix). The point of this threads to alert you on the possibility of the air bypass adjustment may be causing your idle problems. I know that this was not mentioned in previous threads so here it is for your info. Hope it may help someone.
     
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  3. Dave

    Dave F1 Rookie

    Apr 15, 2001
    2,722
    Little Rock
    Full Name:
    David Jones
    Chuck, I don't know if it will help or not,
    but I have found that when rebuilding Weber carbs,
    many times the base plates are warped and don't provide a proper seal.
    I like to sand the base plates on a flat surface, in a figure-8 pattern to remove irregularities and give an airtight seal.

    Good Luck!
     
  4. docweed

    docweed Formula Junior

    Dec 8, 2004
    452
    Morgantown,WV
    Full Name:
    Chuck Stewart

    NEXT MORNING. After I had the night to sleep on it I thought I'd better clarify something. I didn't mean to imply that you should ignore the Sychrometer's readings, on the contrary, I used it to do all my adjustments (carb to carb and bank to bank). I wanted to suggest to everyone that you might want to double check your settings introduced with the air bypass screw. When I closed the screw the idle problem went away and went I opened it again the problem came back. Maybe something wrong with my carb.
     
  5. marks308GTB

    marks308GTB Karting

    Jun 6, 2004
    114
    Los Altos Hills, Ca.
    Full Name:
    Mark S.
    I hope I dont confuse things, here. But I am having a bit of the same problem on mine. I have it working reasonably well, but not perfect.

    The service tech who did some work on it has been working on these since about 1975. He was a race tech and really knows his stuff. He seemed to remember that there was an air passage inside the carb that occasionally got plugged with carbon (or something) and caused instability of the type we are seeing.

    He said they had to drill out the passage from the outside, clean it out, and then reseal it. No other way to blow it clear.

    I have not done this yet. Sounds nasty to me. So will go down that path only if I have to.

    Again, not sure if this has anything to do with your exact problem. Just threw this in as, sometimes, every little hint can help.

    Mark
     
  6. docweed

    docweed Formula Junior

    Dec 8, 2004
    452
    Morgantown,WV
    Full Name:
    Chuck Stewart

    Mark
    Thanks for the info. I don't think that removing the lead plug would be that difficult if you knew which one it is....there are some 20 + plugs in one DCNF. It shows how in the Haynes "Weber" repair manual and there was a good thread on this chat line not to long ago. After you remove the plug you reseal it with a new lead plug and tap in back in with a pin punch. I don't know where you get the lead plugs but they looked like bird shot which would probably work. Pierce Manifold may have them. The thing is which one is the offending passage.
     
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