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Thread: SU Questions

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    Newbie ninjazombiemaster's Avatar
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    Default SU Questions

    Hey folks. I've always though myself to be pretty familiar with our carbs by now, but recently my car has been running pretty crummy and I think they may be at fault. I mess with them way more than I ought to trying to get it to run its best, but I can never get it. It seems like right now it idles rough at low RPMs, and my MPG seems to be really poor. (I think it is leaking from the float bowl from the poor condition of my gaskets. I have to run the engine with quite a few turns past the standard 2.5 to get it to run off a single carb as the tuning process demands. And although I've set my floats to the 23 mm mark, it seems to run out of gas as if there isn't nearly enough in the float bowl, or as if its using way too much. Also, one of the pistons is harder to lift than another. They both have identical SAE 20 fork oil in them. I inspected the dampening rods, and they both appear to function and look identical.

    What do you guys think? Obviously new gaskets would be a bright idea - but would no gasket at all be better than one that's not working right? , but should I mess with the floats to allow more volume of fuel in the bowl, and compensate for the difference with the mixture nuts?

    Suggestions for the dampener? I guess I could put a heavier oil in the other and see if it evens out. But I'll try removing the dome and cleaning the pistons first.

    I'm just at my wits end with them. Almost to the point of driving it to a carbmaster and letting them deal with it.
    1973 240z w/ ztherapy round top conversion.

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    Registered User madkaw's Avatar
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    First of all, are you sure all other aspects of your tune are right. I'm talking ignition, points or whatever, timing, valve adjustment, fuel filters. You will be chasing your tail if you don't have those things right.
    You might need to start from ground zero on the carbs. What are your plugs telling you. You say its bad on gas, but you are wanting to richen it up?
    How is the power?
    "It seems to run out of gas"-statements like this will get you no help on this site. That means nothing with out other info. Try and do better on your posts when asking for help, be specific, logical and scientific
    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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    Newbie ninjazombiemaster's Avatar
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    Quote Originally Posted by madkaw View Post
    First of all, are you sure all other aspects of your tune are right. I'm talking ignition, points or whatever, timing, valve adjustment, fuel filters. You will be chasing your tail if you don't have those things right.
    You might need to start from ground zero on the carbs. What are your plugs telling you. You say its bad on gas, but you are wanting to richen it up?
    How is the power?
    "It seems to run out of gas"-statements like this will get you no help on this site. That means nothing with out other info. Try and do better on your posts when asking for help, be specific, logical and scientific
    Power is pretty decent until it stalls from fuel starvation. If I let off the throttle it comes back in a few seconds. Spark plugs are only a few months old, and every time I look, they're good. Fuel filter is a year old, and clear. Valves adjusted in the spring. Points/condenser are 6 months old, and we're set along with the timing perfectly at that time. Its possible the ignition could be part of the problem, but I haven't touched that.

    I don't want to enrichen it, quite the opposite, but enriching it is the only way I can get it to perform on any level.

    So I guess, restart my tune. If I can't get it to run well I should direct my attention elsewhere.
    It could very well be a problem with another system. But correct me if I'm wrong fuel starvation can only be the pump or the bowls if the filters are good. The pistons moving at different speeds can only be engine vacuum or the dashpot oil.

    Rough idle can be a lot of things.. so I'll give you that, but I have been rather specific in most aspects.
    1973 240z w/ ztherapy round top conversion.

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    Registered User madkaw's Avatar
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    Do you have color on the plugs?
    How are you sure this is a starvation issue-by sound??
    As far as the dampers , you could experiment with different weight oils and amounts to get them the same. Do they both fall freely and clunk?

    Don't be afraid to richen up the floats a little. Take the lids off and measure and then tweek them enough to know you made a considerable change. This should make a change at top end if you are running out of fuel.

    Sorry if I came off gruff, you are doing better with your posts.
    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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    Registered User Stanley's Avatar
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    The little cylinder filters, one inside of each banjo bolt, get them with longnose pliers and get them a good blast of carb cleaner. Also if your car has an electric fuel pump on the frame between the gas tank and the right rear wheel well, get a new filter for it from black dragon, wear some safety goggles, use a channelocks to unscrew the bottom of the pump (when the tank is about empty) and pop the new one in there. Doubt if it's really the problem but good to do anyway. Mine looked like no one ever changed it.
    Did ZTherapy do a complete rebuild including float valves, needles and nozzles, or did they just rebuild the carb bodies?

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    Newbie ninjazombiemaster's Avatar
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    Quote Originally Posted by madkaw View Post
    Do you have color on the plugs?
    How are you sure this is a starvation issue-by sound??
    As far as the dampers , you could experiment with different weight oils and amounts to get them the same. Do they both fall freely and clunk?

    Don't be afraid to richen up the floats a little. Take the lids off and measure and then tweek them enough to know you made a considerable change. This should make a change at top end if you are running out of fuel.

    Sorry if I came off gruff, you are doing better with your posts.
    No worries, I know you're just trying to get the relevant info to help.
    Both the send and feel when it happens. It feels exactly like actually running out of gas in the tank, except it comes back if you let off for a sec.

    They both fall freely, and seem to do so at the correct speed. I'll try enriching the floats and leaning the nuts if the mix goes to rich.

    Quote Originally Posted by Stanley View Post
    The little cylinder filters, one inside of each banjo bolt, get them with longnose pliers and get them a good blast of carb cleaner. Also if your car has an electric fuel pump on the frame between the gas tank and the right rear wheel well, get a new filter for it from black dragon, wear some safety goggles, use a channelocks to unscrew the bottom of the pump (when the tank is about empty) and pop the new one in there. Doubt if it's really the problem but good to do anyway. Mine looked like no one ever changed it.
    Did ZTherapy do a complete rebuild including float valves, needles and nozzles, or did they just rebuild the carb bodies?
    Banjo bolt filters were checked maybe a month ago, but I'll do it again. They always come out clean..
    No electric fuel pump, but I'm sure I'd benefit from one. They were done before I purchased the car, but as far as I can see, it was a complete rebuild. Honestly, it ran spectacular until winter. I replaced the plugs at that time, as well as the starter and recharged the battery.

    While I waited for the starter, I got too curious and pulled off the carbs to clean them, and learn hands on about them. It hasn't ran as good ever since. Its had its highs and lows as I've tweaked it, but its never been the same.

    Maybe its time I pulled them off again and made sure I didn't goof something up all that time ago.
    1973 240z w/ ztherapy round top conversion.

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    Registered User madkaw's Avatar
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    Sure you got everything back together right when you reinstalled the carbs. Possible air leak would cause lean mixture and idle problems.
    They were running great and you pulled them off to clean them? Now see, this is more info relating to the issue that is slowly leaking out.
    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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    Quote Originally Posted by madkaw View Post
    Sure you got everything back together right when you reinstalled the carbs. Possible air leak would cause lean mixture and idle problems.
    They were running great and you pulled them off to clean them? Now see, this is more info relating to the issue that is slowly leaking out.
    Well, as I remember it was running great. But that was in the dead of a 0 degree winter, so that's relative. However, it really did run incredible the fall before said winter. I would expect it to be running about the same now, but its not. The linkage and carbs themselves should be right. If I move things around, they seem to go where they're supposed to. I did not separate the needle from the piston, so I doubt that could be it. I suppose it could somehow have an air leak. I could only imagine them being in the spacers between the carbs and the engine though, but I got them pretty tight, and the gaskets were still good.
    1973 240z w/ ztherapy round top conversion.

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    Registered User Gary in NJ's Avatar
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    The original problem could have simply been bad/old fuel. Now it could be anything.

    In my experience, when all else fails, start from scratch. I would suggest a rebuild kit from ZTherapy and the Just SU DVD. Using the DVD as a guide you should be able to get the carbs back to good working condition. If you don't want to buy the DVD, Blue has provided some very good information on rebuilding the SU's. Also, there are hundreds of posts on this forum that discuss rebuilding and correcting problems.
    Gary
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    Previous Owner of a 10/70 240Z ('83-'85)

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    Quote Originally Posted by Gary in NJ View Post
    The original problem could have simply been bad/old fuel. Now it could be anything.

    In my experience, when all else fails, start from scratch. I would suggest a rebuild kit from ZTherapy and the Just SU DVD. Using the DVD as a guide you should be able to get the carbs back to good working condition. If you don't want to buy the DVD, Blue has provided some very good information on rebuilding the SU's. Also, there are hundreds of posts on this forum that discuss rebuilding and correcting problems.
    I'm guessing these were our carbs from the get go so a Tune Up kit will be like throwing new parts at new parts.

    We have had situations like this before where things have been fiddled with (see float level adjustment etc) to the point the carbs are totally out of sync/adjustment. Steve has been very generous about taking carbs back under his wing and taking them back to square one and returning them...... Probably shouldn't have mentioned this but can't have our carbs out there causing difficulties through no fault of theirs....
    Bruce Palmer
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    Newbie ninjazombiemaster's Avatar
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    Quote Originally Posted by Gary in NJ View Post
    The original problem could have simply been bad/old fuel. Now it could be anything.

    In my experience, when all else fails, start from scratch. I would suggest a rebuild kit from ZTherapy and the Just SU DVD. Using the DVD as a guide you should be able to get the carbs back to good working condition. If you don't want to buy the DVD, Blue has provided some very good information on rebuilding the SU's. Also, there are hundreds of posts on this forum that discuss rebuilding and correcting problems.
    I have the DVD, have watched it several times, and have read and reread blues posts.
    Quote Originally Posted by Bruce Palmer View Post
    I'm guessing these were our carbs from the get go so a Tune Up kit will be like throwing new parts at new parts.

    We have had situations like this before where things have been fiddled with (see float level adjustment etc) to the point the carbs are totally out of sync/adjustment. Steve has been very generous about taking carbs back under his wing and taking them back to square one and returning them...... Probably shouldn't have mentioned this but can't have our carbs out there causing difficulties through no fault of theirs....
    Thanks Bruce. I'll keep working with them longer though.
    I made some interesting progress last night. From what I can tell, the rear carb is doing alright, but the front carb is running quite lean.
    -I took off the float, and it did seem like it was wrong. Adjusted it, tested by blowing into it while lifting the float, put it back on, and got fuel pumping out of the air hose at the top of the bowl.
    -Took it off again, made a *very* minor adjustment, put it back on and had to turn the mixture screw all the way out to get any action from that carb.

    This went back and forth for some time. I cant find the middle ground with this. So here are my theories:
    -Maybe its the fuel shutoff nozzle not closing fully enough under typical range of motion, requiring the float to be over adjusted. This would cause too little fuel in the bowl for WOT, and because of the lower fuel level, the mixture would run lean.
    -Maybe the float is fine, and there is an air leak somewhere effecting the front carburetor. This would create a lean condition that would make me to compensate via the mixture. Dumping more fuel in the engine trying to enrich the engine would cause worse MPG and might explain why I have to run the nut so many turns down.

    Also, part of the MPG problem appeared to be the gaskets letting fuel leak from the bowl under some conditions. I managed to work around that, and get a good seal. But I'll need to order new gaskets from you later, Bruce.

    I'm thinking of removing the fuel shutoff nozzle from each bowl lid and switching them. If it is responsible, this would cause the rear carb to begin malfunctioning and the front to begin working. That being said, it seems to work properly when blowing into it and raising the float.

    I'll also keep adjusting the float and see if I can make any ground there. But after 3 hours of that last night (mostly fiddling with gasket and dropping screws under the car), I'm really thinking thats not the problem. At least not the entire problem.

    Should all else fail, what would be the cost to have Steve give them a look over? If at all possible I'd like to avoid this, 'cause she's my DD. But I live close enough to work I could make do.
    1973 240z w/ ztherapy round top conversion.

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    I don't know what did it, but its done. Put in a few more hours effort and, although it could still use some fine tuning its running the best it has all summer.
    Here's what I did.
    -Removed the air cleaner assembly, re-re-re-synchronized the air flow on each carb. Lowered the idle to 600. It was real rough.
    -Checked the venturi of each carb with a clear hose - again. Interestingly, the front carb, which was struggling the most, had a higher level slightly. But they were both acceptable.
    -Removed the domes of each carb. Cleaned the mouth, piston, needle, etc. The pistons still moved at a different speed. I switched the dampening rods of each carb, and miraculously, they are now fine.
    -Blew air through the nozzle of each carb, in case maybe their was an obstruction.
    -Reassembled the carbs.
    -re-re-re-re-checked my air flow. It was idling at 600 still. Still rough.
    -Piston-lift tuned each carb until I was reasonably satisfied with the idle and action.
    -Put everything back together, and took a spirited drive to reward myself with a hamburger.
    -Rewarded the car with an interior detail.

    Again, still not running perfect, but much better than any recent point. Some fine tuning the mixture, and I think it'll be perfect.
    1973 240z w/ ztherapy round top conversion.

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    Once you get air balanced, set the idle to 1000rpm then lift the back carb's piston with your finger to run on thr front 3 cylinders. The car should idle rough but chug along ~ 400 to 600 rpm. If not, adjust the fuel jet height of the front carb.

    Repeat but lift the front piston so that you chug along on the rear carb and the back 3 cylinders. Adjust the back carb's jet height to run "equally as sh!tty" (comment for Brian).

    Recheck the air, recheck the finger lift then enjoy. Check the plugs and adjust the jet levels both as needed to get the plug colour's where you like.
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