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Thread: Help! SU carbs...(of course) idle is at 1800 can't bring it down....

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    Registered User GCSS's Avatar
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    Default Help! SU carbs...(of course) idle is at 1800 can't bring it down....

    well to start with I took the carbs apart to clean. The car just wasn't right.so after about ten fuel filters,I figured the gas tank had way too much trash in it.... I found a quart of rocks and a pine cone and pine needles and an 1156 light bulb in the gas tank!!! Like i always said...WATCH THAT CHILD!!!
    so...I decided to clean the carbs. been wrenching for 40 years and doing carbs is no biggie. my first with the SUs.
    I just took them apart, found some sediment in the bowls and blew out the passageways.
    I did drop one piston and needle assy.I looked at it very cosely and I just can't see anything wrong with it. The tip is nice and sharp. well I put it all together and the car was liking to idle about 3000 rpms. I did get it down to 1800 by turning the thumbscrews under the carbs. The carb with the dropped assy is sucking alot of air, the rear carb, is not.
    I suspect there is something wrong with the front carb, but don't know enough about these to be sure.
    I did back off all the idle screws and the motor is racing too high.
    what should my next step be??????
    Thanks...
    (no wise guys please)

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    Registered User LeonV's Avatar
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    No wiseguys, eh?



    nyuk, nyuk, nyuk!


    But seriously, take a look at the SU carb section of the forum and search this site and google as well. SU functionality, syncing procedure, vacuum leak testing, etc. is all available online.
    2/74 260Z

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    While you're at it, make sure you put the carburetor back together properly. Did you use a new gasket set, too?
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    Nova Scotia,Canada,Earth Blue's Avatar
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    Quote Originally Posted by GCSS View Post
    I did get it down to 1800 by turning the thumbscrews under the carbs.
    (no wise guys please)
    The two things to balance are:

    1. Air into engine
    2. Fuel mix with air


    For number 1 you have to adjust the throttle plate and linkage
    For number 2 you adjust jet depth


    Here are some links to help. If you want to be walked through it, just send me a personal message with your phone number and a time to call and I'll ring you.

    http://www.classiczcars.com/forums/s...irty-SU-Tuning

    http://www.classiczcars.com/forums/s...ing-fall-rates

    http://www.classiczcars.com/forums/s...ploded-Diagram

    http://www.classiczcars.com/forums/s...281#post376281
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    Registered User GCSS's Avatar
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    Quote Originally Posted by LeonV View Post
    No wiseguys, eh?



    nyuk, nyuk, nyuk!


    But seriously, take a look at the SU carb section of the forum and search this site and google as well. SU functionality, syncing procedure, vacuum leak testing, etc. is all available online.
    I went through all that. I put it together back to the way it was . the gaskets were all OK, the only thing that I changed was covering up the coolant passage, and in the carb in question, I did bend the needle,but got it very straight. I don't know if that could be the problem still or not. The forward carb is sucking air more than the rear one is. I'm gonna say that 99% of the high "idle" is from that carb. I even applied rtv to the base gasket to insure a good seal. I'm going to remove the carb again tomorrow and redo the gasket to be sure. The FSM is vague where it states the locating points to measure and what the actual specs are for the float settings.If anyone knows what these are, I'd like to know so I can double check that. From what I can see there are no check balls,so I don't think I could have lost one of them, question.... if ,supposedly there is a vacum leak between the carb and intake, will this cause the motor to race? I don't know much about these sidedraft carbs. They really are simple in design and I really am suprised there are books written on them.

  6. #6
    Registered User GCSS's Avatar
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    I don't think I should have to mess with the throttle plate and linkages as the car was idling fine to begin with. all I was doing was cleaning the carb due to the trash in the fuel tank.
    I blew out all the passageways hoping to dislodge any dirt particles that may be trapped.
    Quote Originally Posted by Blue View Post
    The two things to balance are:

    1. Air into engine
    2. Fuel mix with air


    For number 1 you have to adjust the throttle plate and linkage
    For number 2 you adjust jet depth


    Here are some links to help. If you want to be walked through it, just send me a personal message with your phone number and a time to call and I'll ring you.

    http://www.classiczcars.com/forums/s...irty-SU-Tuning

    http://www.classiczcars.com/forums/s...ing-fall-rates

    http://www.classiczcars.com/forums/s...ploded-Diagram

    http://www.classiczcars.com/forums/s...281#post376281

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    Did the idle work OK before you started this process (you said the car did not run great but that is very limited info) - Did you work any on your choke cables - a common culprit of high RPM idle is having your chokes too tight - you may try loosening them - relax the choke cable - and then reset without any pressure on the choke cable.................Another common mistake is folks working on these carbs before they have read up on them and actually doing more damage than good. There are 100's of threads on this topic with exact float setting (see the FSM). When I was running Hitachi's (and I'm sure most of us that are) will make a float guide to assist in setting the floats. This should be a set once and forget function (just so you are careful on your float bowl annual maintence and inspection). If I remember correctly front and rear carbs are a different setting.

    I would doubt float setting would make the RPM's run high.
    Last edited by Travel'n Man; 10-09-2011 at 05:08 AM.
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    Registered User Gary in NJ's Avatar
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    Regarding the carb with the "dropped piston", have you confirmed that the piston/needle move freely without binding?

    Have you conducted a vacuum leak test with a can of carb cleaner?

    Have you checked the linkage for binding or high over-throw in certain locations?
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    Registered User GCSS's Avatar
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    I did disconnect and reconnect the choke cables. I did pull them tight...in such that what I did was I pushed down on the choke lever (In the Open position)and pulled the choke cable wire up and then tightened the screw. that way there would be tension holding the choke in the OPEN position. you're saying a common mistake is making them too tight...OK, I will go and loosen them, but I don't think it will help because the front carb (the one that has the dropped piston) is the one giving me grief and I di the choke cable the same way on BOTH carbs.
    The car ran good at idle and drove ok but had poor performance. so I adjusted the bottom thumbscrew (actually is the needle's seat) unscrewed them about a turn. and then the car would have good performance, but bad on the slow speed example...driving through town at 35 mph steady it would buck. and regardless of what settings I had the carb at it woukld sometimes buck at even high rpms. The dwell angle is correct, new points, new plugs, new wires,new cap,same coil.at this point I am going to try a can of carb cleaner and check for leaks question...what is the float specs? I want to double check it to be sure even though it may not cause the high "idle"
    Thanks.....
    Quote Originally Posted by Travel'n Man View Post
    Did the idle work OK before you started this process (you said the car did not run great but that is very limited info) - Did you work any on your choke cables - a common culprit of high RPM idle is having your chokes too tight - you may try loosening them - relax the choke cable - and then reset without any pressure on the choke cable.................Another common mistake is folks working on these carbs before they have read up on them and actually doing more damage than good. There are 100's of threads on this topic with exact float setting (see the FSM). When I was running Hitachi's (and I'm sure most of us that are) will make a float guide to assist in setting the floats. This should be a set once and forget function (just so you are careful on your float bowl annual maintence and inspection). If I remember correctly front and rear carbs are a different setting.

    I would doubt float setting would make the RPM's run high.

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    Nova Scotia,Canada,Earth Blue's Avatar
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    You have to bite the bullet and learn how these carbs function if you are going to tune them, haphazardly turning a screw to a jet will not help if your problem is elsewhere.
    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


  11. #11
    Registered User GCSS's Avatar
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    Quote Originally Posted by Gary in NJ View Post
    Regarding the carb with the "dropped piston", have you confirmed that the piston/needle move freely without binding?

    Have you conducted a vacuum leak test with a can of carb cleaner?

    Have you checked the linkage for binding or high over-throw in certain locations?
    Hi Gary,
    I don't know what you mean by a high overthrow,but the linkages seem to be just fine. the piston needle assy that was dropped goes up and down freely. I checked it against the other one and they both are the same.they go up and down real smooth and steady. I am using 30W oil in the dampers.
    I wonder if somehow I accidently lost a small o-ring or something...a check ball???
    I looked at the carbs real good and I just don't see anything wrong.
    One thing of note and I should have mentioned it earlier is that I initially placed a small piece of gasket material over the coolant holes and put it all together and it didn't hold under a radiator pressure test, so I removed it and cut out a whole new gasket for both sides. when I went to start the car I had hydrolocked the motor with antifreeze. I pulled all the plugs and blew out the holes and after a new set of plugs were installed, I finally got the car running again.
    I don't think this deal is related to the high idle. I even check the carb baseplates for warpage with a Machinist's Straight-edge and the bases were not bent or warped.

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    Registered User GCSS's Avatar
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    Quote Originally Posted by Blue View Post
    You have to bite the bullet and learn how these carbs function if you are going to tune them, haphazardly turning a screw to a jet will not help if your problem is elsewhere.
    there is no haphazardly turning of the screws going on here. I read the FSM, and even went to a few websites and reap up on tuning. It is after the tuning didn't work that I decided to check elsewhere....read my postings to see what I did and what the processes were. The problem is within the first carb.

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    No more body roll! SteveJ's Avatar
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    Are you using a tool like the one pictured to check your flow?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Registered User GCSS's Avatar
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    I had one on there earlier when the car was running better than it is now. At this point I don't need one of them until I get the engine settled down some. It is just way too wild right now. I either have a leak or possibly unbalanced piston/needle assemblies or both. I am going to Church right now and will be back around 1:00 CST. I went to blue's videos and they are good. I plan to utilize them when I get home.
    thanks
    Dave

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    Registered User GCSS's Avatar
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    well...we are rolling again!
    I believe it was the needle up too far into the piston.
    I got it to idle now . I adjusted it good enough to go to town and back. good...very good performance. low to no load is crappy though. I'm not going to worry about that until I adjust the valves and get the airflow meter back, and rebuild them and check the floats. at least the idle isn't gone wild anymore. THANKYOU!!!!!
    you're a real champ!
    Thanks again! Blue!!
    Dave

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    Nova Scotia,Canada,Earth Blue's Avatar
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    Very happy to help! It was great talking... your enthusiasm and energy to beat the problem is inspiring.

    If you need anything else I'll be glad to help.
    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


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