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72 240z Twin Hitachi round top su carb HELP HELP HELP

Old 08-05-2012, 02:11 PM
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72 240z Twin Hitachi round top su carb HELP HELP HELP

I recently traded a VERY tired 64 thunderbird for a 72 240z and haven't had any regrets ever. That being said, I really need some help though. I am pretty new to the world of carburetors and I now wish i would've done more research of the SU carbs before tackling the issues at hand. The first issue I had was that a bad float needle assembly. I did some work and got all of that fixed. It had been getting stuck and was flooding out. I got that fixed and got the floats in both carbs to be doing pretty well. I even drove the car a little bit. However, it was idling pretty high. It had been idling at around 1500rpm and I got it down to about 900rpm. Now this is where it gets sticky and where I should've done more research. The screws I adjusted to get the idle speed down were unfortunately not the idle adjustment screws, they were the balance and the fast idle screws. So after the car seeming to run fine for a little while, it all of a sudden would have very sporadic idling speeds. I grabbed a haynes manual and followed how to properly set the idle speed and when I turned the idle adjustment screws (i got it right this time) to what they said to do after slackening completely the balance and fast idle screws my car was then idling at well over 3,000-3,500 rpm. I traced that to probably a leaking vacuum line. So i replaced all of the vacuum lines. After doing that and getting to the same step for adjusting the idle speed it was now down to about 1,200-1,500 rpm. Any time I try and adjust it down with the idle adjustment screws, once I get to about 800rpm it runs very rough and wil then drop to about 500-600 rpm and then eventually die. I haven't checked the ignition timing or the valve clearances yet. My father seems to think that it isn't the valves or timing that is off but rather that it is still a carb problem. I was just wondering what everyone on here thought and exactly what I should do. I am really hoping to get this car on the road soon because my wife and I will both be needing a car now and she needs the one that is reliable. Thanks.

Ben

P.S. Sorry if this is posted in the wrong place.
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Old 08-07-2012, 09:42 AM
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On the bottom of the carbs is a thumb wheel. That regulates the idle mixture. Turn them closed and back out 1-2 turns. Start the car and adjust each one to get the best idle. You will have to do this several times along with the linkgage adjustments. Let us know how you make out.
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Old 08-08-2012, 08:15 PM
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Well, I went to try and start the car shortly after I posted this and it won't fire up now. It sounds like every so often while trying to fire it up that it wants to ignite and go, but it doesn't. I haven't tried starter fluid yet to see if that will get it to fire up and allow enough to get the car to run on its own. But for now I can't get the car to get to normal operating temperature to properly adjust the carbs. I haven't been able to get ahold of a timing light but it appears to be pretty good from what I can tell (#1 piston is at least close to TDC when the rotor is at #1). I checked the jet needles on both carbs yesterday and found a little bit of debris on the front carb. I cleaned that off a little and blew out anything else that might've been in there but it still won't fire up (I believe the debris was from parts of the float that had deteriorated, but i have that fixed now). I did notice that whenever I'd lift the carb pistons up and down a little bit and then try and start the car that it sounded like it was closer to firing up. I have the balance and fast-idle screws completely slackened, should i change that to try and get it to start now? Thanks
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Old 08-08-2012, 09:01 PM
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Make sure you're getting fuel to fill the bowls. With the air cleaner off set the idle stop screws to where you can see equal opening of the butterflies. Now set the linkage to equal openings at about half throttle. Check your fuel filter and that you are getting sufficient fuel. When you push up on the pistons they should move freely and drop smoothly. Check the oil in the domes. Turn the fuel thumb screws closed and back them out 3/4 to 1 turn.
Make sure you're getting spark to the plugs, this is my favorite test, get your wife to hold the plug wire while you crank it over.
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Old 08-09-2012, 07:50 AM
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hopefully this doesn't sound like a stupid question. But when you mean to adjust the linkage till it is about half throttle, do you mean just adjust the throttle screw at the top or should i use the balance and fast-idle adjustment screw also? I'm pretty new to carbureted engines so a lot of this is pretty new to me. I've also used gumout carb cleaner and cleaned up the carbs inside and out a bit. Thanks for all your help so far. I'm using 20w fork oil for the dashpots, is that fine to use or is that too heavy? Any other tips are welcome as well.
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Old 08-09-2012, 11:46 AM
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Another update. I cleaned it out with carb cleaner and put everything back together and it fired up...for a few seconds. I think it was running on the carb cleaner. I'm getting fuel into the float bowls but I wonder if I'm getting fuel to come out of the jet tube. I have taken a hose and blown through the jet tube into the float bowl and it seems clear of debris. Would a small leak at that jet tube cause the carbs to not get gas into them?
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Old 08-09-2012, 01:09 PM
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Fork oil is hydraulic fluid. I don't think that is good. If you want the carbs to open faster you can use 10w.
What you need to accomplish is to have the butterflies equal, that is easier when you can visably see them at half open. Your idle stop sets both carbs at idle speed. a common mistake is to compensate the idle error by adjusting the butterfly pening, wrong. The idle mixture screws regulate the amount of fuel going in at idle only.
You must get it to start in this state or you will never get it right.
Make sure your pcv valve is not stuck, it will make it imppossible to get idle correct.
Spark, timing, vacuum advance and fuel come first.
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Old 08-09-2012, 06:23 PM
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i honestly believe that everything is set up right, I can even spray starter fluid into the carbs and it will start and run, but for some reason gas isn't getting from the float bowls into the carb body. I've made sure that the jet and the tube connecting the two are free of any debris. I'm not sure what could be going on here. Any ideas are definitely appreciated. I just dont think there is anything else wrong because I had it running and driveable just a day before all of this happened. And because of adjusting some screws on the carbs it eventually started acting like this.
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Old 08-10-2012, 07:06 PM
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another update. I got it running...sorta. It will run for about anywhere from 10-30 seconds and for a couple of times it ran longer than that even but in order to adjust a few things and because the idle was extremely high I had to turn the engine off. It appeared that the carbs were maybe pulling too much air in because it took my buddy holding his hands over the carb intakes to get it to run, but then it would stay running for a little while after he moved his hands away. Pulling the choke cable seemed to help some. But at times i would need to close the choke in order to get it to run. Could the gaskets where the carbs hook up to the intake be bad and causing too much air? And I believe that the main reason why I couldn't get it running was because some of the vacuum lines I replaced I forgot to tighten down the hose clamps. Figures it'd be something that simple. Thanks for your help so far.
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Old 08-19-2012, 10:49 AM
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The oil I have always used ( 27 yrs) in my carbs is ATF fluid. Never ever had a carb problem using this fluid. To correctly adjust your carbs shop around a get your self a device called the uni sync. This device measures air flow into the carbs. The factory repair book if you can find one will be your best guide outside of this site as to the proper adjustment of these carbs. Good luck with this car!!
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