News

308 popping?

Discussion in 'Technical Q&A' started by joeyy, Feb 12, 2004.

  1. joeyy

    joeyy Karting

    Nov 11, 2003
    190
    long island
    Full Name:
    joe
    78 308 carb car with single point dual dist setup. when held at 2000 revs a slight popping can be heard and sometimes has a backfire, almost sounds like a lean condition but it is running rich as heck,whats the deal?
     
  2. To remove this ad click here.

  3. thecarreaper

    thecarreaper F1 World Champ
    Silver Subscribed

    Sep 30, 2003
    16,015
    Savannah
    Full Name:
    I NEED A JOB
    could be a vaccume leak at carbs or intake, may be a timing difference between the 2 banks. seems to me a vaccume leak would be evident at idle. spray around the carbs with carb cleaner after engine is warmed up and see if there is a change in the sound of the motor or a definative rpm change. this will help find leaking intake or carb gaskets but will not show a leaking vaccume hose or fitting unless you happen to spray it. the timing issues with the stock set up i will leave for the owner/experts. i hate points and will change them out as soon as i find a car! there are some great folks here, i am sure you will get better replies , but i wanted to at least give you a direction to go in. michael
    ( edit if your plug wires or extenders are bad this will show up as described, the higher rpm allows for more current flow which wants to find a " easy " path to ground, if there is a burned extender or bad wire it will show more at the specified rpm than idle speed. the miss would tend to be on one bank unless you have many bad wires or extenders.)
     
  4. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Seems like there are a zillion fuel paths in the DCNF, but it simplifies to idle, progression, and main. You can be rich in one place, and lean in another. At 2000rpm and no load, you are in the progression circuit, which is entirely controlled by the idle jets. At idle, mixture is controlled principly by the idle adjustment screws. (Fuel flow is small enough that the idle jets don't present much restriction.) (Also, there are some 'idle air bushings' that bleed some air into the idle fuel mixture, but these aren't normally replaced or altered.) One possible scenario is that you have small idle jets causing a lean condition when the progression holes are uncovered, but mixture screws turned out too much causing a rich idle.

    Note that many of the late 70's cars have idle jets that are probably smaller than optimum for emission reasons. Dyno testing and A/F measurement is the only way to know for sure, but it probably wouldn't hurt to go up a size in idle jets. I did, and it got rid of some low rpm popping.
     
  5. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    What are the idle jets in there now?
     
  6. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    I'd lay odds that they're 55s. It'll be interesting to see what he pulls out.

    BTW, it seems - at least to me - that the idle jets are more sensitive than the other fuel paths to changes in exhaust and air filtering. Also perhaps altitude. My car idled and drove at low rpm like crap at Crater Lake (7000ft), but seemed fine when I put the pedal down.
     
  7. To remove this ad click here.

  8. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    Hmm, I think 78 might be 50s with 125 mains.
     
  9. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    My 78' has 55 idle jets stock. 125 mains.
     
  10. Slim

    Slim Formula 3

    Oct 11, 2001
    1,735
    Pacifica, CA, USA
    Full Name:
    richard
    My vote is distributor timing. At least check that out and make sure it's perfect before you dive into the carbs. I had the dual point single distributor on the dino and getting the two sets perfectly 180 off each other cured all the popping: I suspect a similar problem might arise if the two single point distributors aren't working in perfect unison.
     
  11. joeyy

    joeyy Karting

    Nov 11, 2003
    190
    long island
    Full Name:
    joe
    you guys are good!! the mains are 55's up from 45's wich i took out . it wouldn't idle with the 45's it came with. the prograssion circuit is what i was pinpointing but even major adjustments seemed no help. maybe the dist. i will chech them out and get back with you all later.
     
  12. To remove this ad click here.

  13. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    573
    NW Rural Nevada
    Full Name:
    Mike Florio
    I had a similar problem - checked the timing, and both distributors were spot-on at idle. At 3000 RPM they differed by 15 degrees! The advance mechanisms needed major work. Its a good idea to check your timing at 3000 or 4000 RPM as well as at idle.
     
  14. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Joeyy: You can't really 'adjust' the progression circuit. The idle mix screws will have some effect, but minimal. Only changing the idle jets will give a meaningful difference.

    Oh, and PS: Mike is correct, in that dist curves need to match. To be safe, I set my timing using the 34deg mark at 5000rpm. Serious timing errors at high rpm/load can cause damage, while minor differences at idle or low rpm won't matter much - within reason!
     
  15. joeyy

    joeyy Karting

    Nov 11, 2003
    190
    long island
    Full Name:
    joe
    i have checked the timing and is ok even at high rpms. i have isolated the problem to the 5-8 bank. i think maybe the msd has something to do with it (crossfire of something) or possibly air or vacuum leak?
     

Share This Page