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Thread: Rear carb running super rich. HELP

  1. #1
    Registered User metalmonkey47's Avatar
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    Default Rear carb running super rich. HELP

    Hey guys, just want to start this off by saying that I am a total side draft noob, and the only thing I know is what I've read over the last few months on places like this.

    Anyways,

    I've been fighting an issue with my SU's. I thought I was just doing a crappy job at tuning them, but it turns out the rear carb is running super rich a full up. It's so rich the exhaust smell burns your eyes. It makes it very dangerous to drive, as the smell fills the car.

    What I've done so far:

    -I've reset both needles in the pistons, and filled them with fresh fluid.
    -Check the chokes to make sure the jet tube was moving up and down freely on BOTH carbs.
    -Set float height @ 10mm (specs that came with my rebuild kits)
    -Replaced Needle & Seat (both carbs)

    I'm sure I missed something I'll remember in a minute...

    I've determined it's the rear carb by running the screws full up, (you can hear it running very rough on what seems like half of the engine) where it should have stalled. Cupping hand over the front carb causes the idle to increase slightly before dropping back down, telling me it's lean as it should be. Cupping hand over the rear carb, the idle drops immediately and the motor starts to stall. Obviously that tells me the rear carb is rich.

    I've done a lot of research, I feel I've been very thorough but obviously I'm missing an important step somewhere. I've spent the last 6 hours (I'm not even exaggerating) trying to track this down. If someone could help point me in the right direction I'd appreciate it, this is my DD and gets me back and forth to work everyday. It sucks I can't enjoy it right now!

  2. #2
    Registered User madkaw's Avatar
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    You need to verify by checking plugs too. Are the rear three sooty or fouled?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
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  3. #3
    Registered User metalmonkey47's Avatar
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    Quote Originally Posted by madkaw View Post
    You need to verify by checking plugs too. Are the rear three sooty or fouled?
    They are all sooty and black. In trying to balance the carbs, I ended up running the front carb a little rich as well. This was before I realized I had one just one carb richer then the other. The rear plugs are obviously much nastier then the front.

  4. #4
    Registered User Walter Moore's Avatar
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    Check that the choke is releasing.
    '71 240Z, Because any fool can drive fast in a straight line.

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    Nova Scotia,Canada,Earth Blue's Avatar
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    Set fuel level using a clear hose connected to the bowl. It should be ~20mm down from the top lip of the fuel bowl.

    Measuring the tab and hanging amount of the float from the roof of the bowl is not as accurate.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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  6. #6
    Registered User metalmonkey47's Avatar
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    The choke is releasing fune, i greased it up for good measure.

    Funny thing, using the clear hose I found the float height was dead at 20mm and even on both front and rear carbs.


    This thing just runs rich one minute, and fine the next. It's so inconsistent it's IMPOSSIBLE to find out why. I can't break 65mph either, because it starts to run like absolute ****, and after that I'm completely down on power and run poorly. I've been pretty thorough on this car and just can not seem to get the damn thing running right, it all seems to circle around the super rich carb issues.

    The car takes hills very poorly, in fact spirited driving is impossible in the car as a result. It has an issue revving high and breaks up at about 4000 RPM, I can only assume because of how rich it is.


    Maybe this will provide some info into the 'revving' problem.

    http://youtu.be/cQXYH_2TyP0
    Last edited by metalmonkey47; 06-21-2014 at 08:14 PM.

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    No more body roll! SteveJ's Avatar
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    Matt, if you're not working tomorrow, bring the car over to Stephen Rogers' place. Tim & I can look at it.
    73 240Z
    74 260Z

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    Registered User metalmonkey47's Avatar
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    Unfortunately, I'm working all day tomorrow. The cars not really up for a 'long' drive. It has a tendency to start running poorly. Probably a result of the fouled plugs.

    I really wish I could make it out.

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    Nova Scotia,Canada,Earth Blue's Avatar
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    How far down are the jet knobs turned down from their top most position?


    Don't rule out timing however the sooty plugs seem indicative of too much fuel.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


  10. #10
    Registered User metalmonkey47's Avatar
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    Well they aren't even, I had to drop the needle on the rear carb about 1mm further to help lessen the rich running issues... didn't do much, so I have a feeling it's getting fuel another way. They're about 2 turns down, give or take about 1 turn.

    Timing is dead on, about 30 degrees @3000 rpm and 10 degrees at idle. I'm going to replace the plugs today. They are pretty shot.

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    To really understand these carbs, ZTherapy,com has a DVD that has all the info that you will need. I considered myself pretty good with SUs, having run them for decades with little to no trouble but I was surprised at the tips & tricks that I picked up from the video.
    I don't understand why you changed the needle height to lean the mixture, Needles should be the same height on both carbs Shoulder of the needle even with the bottom of the piston not even with the groove. The mixture adj. is done with the the nozzle nuts after ensuring the float levels are at the right height.

  12. #12
    Nova Scotia,Canada,Earth Blue's Avatar
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    Maybe the hole in the jet is ovaled or the needle worn
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


  13. #13
    Registered User metalmonkey47's Avatar
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    Sorry about the delay guys, I figured it out last week. The needle in the rear carb was bent so slightly, the piston wasn't dropping all the way. It sat about 1/4'' above the venturi. Now it runs a lot better after straightening the needle. i'm going to order new needles and nozzles from Z therapy soon.

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