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to carb or not to carb

Discussion in '40- & 55-Series Tech' started by talkinggoat, May 26, 2003.

  1. talkinggoat

    talkinggoat

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    09/75 fj-40
    I-6cyl
    edelbrock headers (SOR mfg)
    mallory distributor
    weber 38mm DGES carburetor
    stock intake
    stock heads
    stock short block
    no emmissions systems except for the pcv valve etc.

    i have a 75 cruiser that i've been fighting with for about six months now and just can't seem to get the thing to run right. i just put a new mallory distributor in it, but that didn't seem to help much. i brought it into correct time and checked it with a timing light. the engine is in time. the mark jumps arround slightly, but it's barely enough to notice.

    one of the main things that i have found was the 5th and 6th cylinders were not firing inside the engine while at idle. once the engine would rev up, they would start to hit. i thought it may have been something with the dwell on the distributor, so i went to my local auto zone and bought a dwell meter. the book says, for that distributor, to bring the combined (dual points) dwell to ~40 degrees. it's at that... and the engine is in time, so i began looking at the carb and found out that the idler screw was in too far. once i began bringing it out, those two cylinders evened out and started running.
    that solved problem #1

    the second (and most serious) problem is that the engine hesitates upon acceleration, and occasionally "pops"/backfires (it doesn't make a loud noise) both during acceleration and while the engine is at a higher, but steady rpm. i have kinda fixed this problem by bringing the choke in a little bit. if the choke is fully open, it will hesitate, but when partially closed, it runs fine, as long as the second barrel of the carb does not open. if that happens, the engine runs rough, misses, etc. with a sharp pop on the fuel pedal, it will rev up like it is supposed to, and the choke opens fully to allow for the flow of more air. at this point, there is no hesitation or missing, but, like i said, if the second barrel opens, it will miss.

    i am thinking there is something wrong with the fuel mixture. with the choke closed it is causing more of a vacuum on the upper part of the throttle body, thus causing more fuel to flow from the fuel boal. (increased richness) i think that the engine is not geting enough gas in the mixture, and when the second barrel opens, it throws the whole thing out of whack, increases the air in the ratio, and causes the engine to run lean. do any of you have any suggestions on what to do?
    does the carb need to be/can be tuned? anything at this point will help.

    further, i have checked the compression on all of the cylinders. they all hold as normal. i have checked and double checked the backlash of the valves. i know the distributor is installed in phase.
     
  2. CruisinGA

    CruisinGA

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    If that was an Aisin carb i would say accelerator pump, but I don't know if a Weber has one?
     
  3. Pin_Head

    Pin_Head

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    Problem 1 is most likely a vacuum leak at the 5-6 intake runner. This is often due to a leaking vacuum brake booster because this connects at the 5-6 runner. Fix this first.

    Weber 38s are not known to run lean, so there is something very wrong with its operation. &nbsp:Did it just start doing this or has it always run like this? The options are to rebuild/ repair the Weber or replace it with an OEM Aisin.
     
  4. CruisinGA

    CruisinGA

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    Replace the manifold gasket first, unless it is new, I thought I had a bad brake booster, turned out it was a bad manifold gasket that made sounds like a bad brake booster. A gasket is much cheaper than a $400 rebuilt booster.
     
  5. talkinggoat

    talkinggoat

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    where is the line going to the brake booster supposed to go? to the 5th and 6th cylinder manifold intake, or the carb?
    i went and looked at it today, and it was attached to the bottom of the carb. i moved it and replaced it with the hose that runs to the pcv valve on the back passanger side of the engine. was this right?

    the engine has been doing what it is doing all allong. i am still not 100% sure that it is the carb. every thing points to it, but there is some doubt. i have checked just about everything else on the engine, and can't seem to find anything wrong with it. everything that has to do with the operation checks out ok. unless it is something deep inside, (camshaft pushrods) it must be the carb. i checked the accelerator pumps to see if any gas comes out. when i depress the accelerator, fuel shoots out just above the venturi on the #1 throttle body. i could not realy tell on the second; if i'm not careful it will backfire up through the carb.

    i just put a new gasket on it, but i will try putting two.
     
  6. CruisinGA

    CruisinGA

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    On my 10/73 (74 year model) and I believe on most if not all Cruisers with power brakes, the line from the brake booster goes to a fitting sticking out of the side of the intake manifold.
     
  7. 73lndcrsr

    73lndcrsr SILVER Star

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    Problem #2:
    It sounds like you have jetting issues with the Weber. Let the truck idle for a few minutes after it was warmed up and the choke is all the way off. Check you plugs. They should not be dark. If they are you have the idle screws too far in. I usually start with the idle screws about 1.5 turns out. &nbsp:Double check where yours are before making any adjustments though.

    After you determine the idle mixture is good. Put the plugs back in and drive the truck around the block.
    Check the plugs again. Again they should not be too dark. I think light tan is good. If they are bright white you have too small primary and secondary jets.

    I would recommend getting the haynes book for webers, they are at most parts stores around here (atlanta).

    You may need a plethera of jets to get it right unless you have a buddy with a gas sniffer.

    Hope this helps.

    Getting the proper jetting can be tricky.