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Thread: Front carb running rich

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    Default Front carb running rich

    My past experience has been with British car SUs so that may be confusing me. I acquired my 1972 Z a few months ago and have been trying to fix several things the PO did not fix. One is the front carb is causing the first three plugs to become black and gummed up. When I clean the plugs they are black again after about 100 miles. The back plugs are a fine light grey color. In the past my British cars were leaned out by tightening the jet up to the needle. In all the three repair manuals I have for the Datsun, on carbs they focus on the air adjustments. With a Unisync I see both carbs are drawing the same amount of air so I think that adjustment is okay. When I reach under the front carb there is a wheel with indents that may be adjusted by rotating the wheel. I think that is the jet adjustment. If I rotate that wheel clockwise will that lean out the jet and fix my problem? Do I need to go back and start the air adjustment from scratch again? Is the air control more important than the jet on my car?
    Thanks in advance,
    Charlie

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    Registered User madkaw's Avatar
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    Charlie,
    It could be several things-and one of the most important is float adjustment.
    Yes, the wheel is an adjustment and clockwise is lean.
    Make sure your needle and seat assembly is tightly screwed down and the float moves freely.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Rust Free'ish zKars's Avatar
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    Charlie,

    Float levels are key, then make sure the front choke is not still "on" compared to the rear. The cable choke mechanism pulls the jet tube down, effectively moving the needle farther up richening the mix.

    If the front jet tube that gets pulled down is gummed up on the outside and dirty, or the cable mechanism is not adjusted right, then the front tube may be partially left down even though the chokes are "off".

    Use the chokes and look and see if the tubes are both drawn down equally and return to be fully up when the choke level is all the way to "off". Take off the air cleaner and backing plate if you haven't already to clearly see what's going on.

    If the jet tubes are both all the way up, then re-set your mixture on both carbs to be the same at least as a starting point. Turn them all the way in (CW looking from the bottom) then out 2 1/2 turns.

    ztherapy.com sells a wonderful and much respected DVD on the topic of tuning and re-building SU's. There are also many many posts on this forum on the topic.
    Last edited by zKars; 07-15-2013 at 12:36 PM.
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    Make sure nothing is sticking. As mentioned, float levels are key to adjusting mixture.

    There is a recent thread concerning the sight glass method. The OP started the thread gaging interest in selling but went a bit further.


    http://www.classiczcars.com/forums/c...wl-levels.html
    things will only bother you if you let them.

    82 280zxt 4 spd auto
    73 240z--lsd, cv axles
    short throw info

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    I'll check the jets and float levels this week. I have to get out in the early morning as we are having a heat wave right now.
    Thanks for the replies!
    Charlie

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    Just a question here guys...... Why if carbs have been running adequately and all of a sudden start doing something odd like one carb running fat, do you all suggest that some one or some thing has gotten in there and changed the float level?

    In this case smart money is on a stuck choke. Reach under there and bump the nozzle and see if it doesn't snap back up into place. If it does, time for at least a good cleaning....
    Bruce Palmer
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    Quote Originally Posted by Coastalman View Post
    My past experience has been with British car SUs so that may be confusing me. I acquired my 1972 Z a few months ago and have been trying to fix several things the PO did not fix. One is the front carb is causing the first three plugs to become black and gummed up. When I clean the plugs they are black again after about 100 miles. The back plugs are a fine light grey color. In the past my British cars were leaned out by tightening the jet up to the needle. In all the three repair manuals I have for the Datsun, on carbs they focus on the air adjustments. With a Unisync I see both carbs are drawing the same amount of air so I think that adjustment is okay. When I reach under the front carb there is a wheel with indents that may be adjusted by rotating the wheel. I think that is the jet adjustment. If I rotate that wheel clockwise will that lean out the jet and fix my problem? Do I need to go back and start the air adjustment from scratch again? Is the air control more important than the jet on my car?
    Thanks in advance,
    Charlie
    They weren't running adequately. New car for him and he's trying to run down the problem. I would start from the beginning. Is there that big of a difference in the choke mechanism between the british su's and ours? Unknowns, did he check the chokes? Did he adjust the knobs and was there change in mixture?

    A new to me car, i would want to start from scratch from the beginning.
    things will only bother you if you let them.

    82 280zxt 4 spd auto
    73 240z--lsd, cv axles
    short throw info

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    My car was doing the same thing a year ago. From looking at the plugs, the front 3 were way rich, all black and fouled. Finally noticed the choke was sticking. Took off one screw and pulled the nozzle down, hit it and all the linkage with carb cleaner, followed up with the spray grease on the nozzle and linkage, and that fixed it. Not to say that's your problem.
    Yeah, you're supposed to re-balance the carbs again after adjusting the mix - according to the FSM anyway. I don't always do it, it's sorta fun though, just a hassle taking off the air cleaners every time.

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    Thanks for all the suggestions. What I found was this. One, the wheel needed adjusting 1/2 turn clockwise to lean the jet and the suggestion to check the choke cable turned out to be spot on. The cable on the front carb needed to be adjusted because it was not pushing the linkage all the way down. I adjusted these two items and now, after several hundred miles, the plugs are showing a light grey color and all the black soot and sludge is gone. Thanks so much for your help. I am enjoying the car very much and now need to get some grey film on the back hatch to keep the heat at bay.
    Charlie

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    Enjoy the zzzzzz!!!!!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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