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exhaust systems and carbs

Discussion in 'Technical Q&A' started by pcelenta, Oct 7, 2004.

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  1. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    #1 pcelenta, Oct 7, 2004
    my euro ansa system just keeps getting louder...has no rust holes in it but definately breaking down inside...since I have noticed the increase in noise (music?) I have also noticed that I have developed the traditional popping of the carbs in the transition phase (3000-4000 rpm) from Idle's to mains which I can't seem to iron out with the idle mixture screws...carbs are balanced, I changed all the exhaust donuts...went through all the vacuum lines for leaks, running xr700s so the timing is a little advanced....car makes real nice power with no flat spots and it sounds good and sets off other peoples car alarms. Currently running 55's on the idle jets...considering bumping up to 60's..question is what is the relationship of the exhaust flow to the carb popping...what actual causes the lean condition?
     
  2. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,934
    Outside Detroit
    Full Name:
    Don the 16th
    #2 don_xvi, Oct 7, 2004
    You halfway answered your own question in the last sentence, if you "remove" the baffles from the muffler, reducing backpressure, the engine can pump the endgas out more readily, allowing more intake air (less internal EGR). Since carbs respond to velocity rather than massflow, you need to rejet for increased airflow.

    Now, the question I've been wondering about is why do you get a backfire through the carbs when lean? I can't imagine it's related to valve face temp, so what causes it to come through the carbs?
     
  3. Sean F.

    Sean F. F1 Rookie

    Feb 4, 2003
    3,067
    Kansas
    Full Name:
    Sean F
    #3 Sean F., Oct 7, 2004
    I asked this question on Ftalk and someone mentioned the points might be off. But generally, the caugh/spit condition comes from the carbs being to lean, but could also happen if it's too rich.

    I got the problem solved, but then moved to a lower elevation and warmer climate and it's come back a little bit.

    What condition are your carbs in? I know mine have never been rebuilt and need to be done. Then, a major tune up in in order.
     
  4. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    #4 pcelenta, Oct 7, 2004
    carbs are rebuilt...look all nice a spiffy...just have the damn popping...I'm running the crane xr700 ignition system..so no points...the only other thing I was thinking is that I am getting an air leak from the throttle shaft bearing area, as I did not open this up and re-grease...I understand that the grease acts as a seal...but I suspect that if had a leak there I would experience problems through-out the rpm range and not just the 3000-4000rpm transition range.
     
  5. Spasso

    Spasso F1 World Champ

    Feb 16, 2003
    14,656
    The fabulous PNW
    Full Name:
    Han Solo
    #5 Spasso, Oct 7, 2004
    Is it actually 'popping' or 'stumbling' through the transition? Mine stumbles briefly then delivers smooth power. It's a multi-carb car, a 25 year old one at that. I don't worry about it. My 240Z has 3 Weber twin's on it and it too stumbles a little at 3k RPM. Not as bad as my 308 though.

    I wouldn't rely on grease to stop a vacuum leak around a throttle shaft. If the vacuum leak is bad enough to cause abnormal running the leak is strong enough to suck the grease through.

    Leakage past the throttle shafts would be very noticable at the lower RPM ranges and virtually gone at higher speeds. Nearly impossible to syncronize carbs with this problem.

    The best and cleanest method I have found for finding vacuum leaks is to make up a small bottle of propane, similar to what plumbers use, with a 1/8" diameter rigid tube about 12" long. With the engine idling, turn the propane on and stick the tube against each throttle shaft end and run the tube around the base of each carb at the manifold gasket level. If there is a vacuum leak of any magnitude in any of these areas the engine RPM's will increase.

    Don't worry about propane explosions. There isn't enough propane in volume or accumilation to cause one. The worst that can happen is you may end up with a quart size cigarette lighter. It beats spraying flammable liquids like WD40 or carb cleaner on the top of your engine to find the leaks.
     

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