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240z dies when choke is fully open


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#1 5 Star Rising

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Posted 21 August 2016 - 09:38 AM

I have a 12/70 71 240z that i bought recently that runs like crap. I do not know the past history of the car or the past owners so I'm starting with a running car that runs, but not great. I have been replacing old parts these last few weeks and in the middle of a tune up.

Removed funky electric fuel pump and switched back to a mechanical pump, new fuel filters,new spark plugs and wires, ign cap and rotor, I have not yet done a compression test on the motor. I have fuel going to the carbs and can start the car by pulling the manual choke when cold like normal, but after the car warms up and I open the choke fully the car starts to idle rough and dies out almost like it's getting too much fuel.

When I have the car running best is when the choke is slightly pulled back about a quarter of the way. When you Rev up the motor there is a slight spudder or miss through out the rpm ban. Just feels like the carbs are out of tune or not adjusted right. The car lacks that snappy throttle response, and still has a slight miss. I was getting ready to pull the carbs and get a rebuild kit for them but thought I would post this first, I have not rebuild SU's before and may be over looking a few things, like a proper adjustment while on the car.

I found a few rebuild threads on here that I will follow, but I'm wondering why the fully open choke kills the motor and what may cause that.

#2 Miles

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Posted 21 August 2016 - 10:40 AM

POST DELETED


Edited by Miles, 18 September 2016 - 11:56 AM.

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#3 Tony D

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Posted 25 August 2016 - 02:14 AM

First, it's not a 'choke' it's a 'starter system'...

 

It does not 'choke' anything, but rather admits more fuel than normal by altering the jet position.

 

If you have pulled it all the way down to get it to start, my bet is that one or both of the nozzles are stuck in the down position and that is why you chug chug chug after it's disengaged.

 

Go stick you hands under the carbs and tracing the cables, find the jets and, after pushing the lever in the centre console all the way forward (off) go out push the jets up against the bottom of the carbs where they should be... see what happens.

 

Likely they are sticky. Now that it's solved, watch the SU Therapy Video and find out how to fix it.


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#4 5 Star Rising

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Posted 18 September 2016 - 11:42 AM

To be honest I'm not interested in how the SU'S work or to become and expert of knowledge about them. I would like a video or book on how to rebuild them step by step and set them to factory specs. Do they cover this info in the "Just Su's" video?

#5 Miles

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Posted 18 September 2016 - 11:58 AM

This is just sad.


Edited by Miles, 18 September 2016 - 11:13 PM.

“Life is like arriving late for a movie, having to figure out what was going on without bothering everybody with a lot of questions, and then being unexpectedly called away before you find out how it ends.”    -Joseph Campbell


#6 seattlejester

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Posted 19 September 2016 - 10:11 AM

If they are out of sync or misadjusted then they could dip into a too rich or too lean condition. If they are sticking like tony says then you have a massive fire hazard on your hands. 

 

I don't know how you plan on rebuilding something without knowing how it works. The adjustments would also require you to become somewhat familiar with them. If you don't plan on learning how to do it yourself then send it off to be rebuilt, just fair warning it won't be cheap, but given that the carbs sit right on top of the exhaust it is better then catching your car on fire.

 

Tune up should also check compression, valve clearance, spark strength, spark timing etc etc etc, just so you know if your car is out of time or you put the spark plugs in an incorrect order or the compression is piss poor due to valves being out of spec or bent because of the timing messing with the carbs won't do anything to help.



#7 5 Star Rising

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Posted 19 September 2016 - 07:34 PM

So does the video show you how to rebuild the carbs?

This is my 9th 240z in 10 years.. most all of them had not run in years, installed and tuned many Su's, BUT have never rebuilt the internals of the carbs, this is the info I'm looking for.. I didn't mean that the info in the video was not important as far as tuning and explaining what they are and how they work but my question was, does this video provide step by step of the rebuilding process of the Su's? Gaskets,seals,specs? Or is a Chilton my best bet?

#8 HuD 91gt

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Posted 22 September 2016 - 07:33 AM

Just SU's is the rebuild bible.  Get it.  Thank Tony later.

I agree with him. I bet one nozzle is stuck down out of position.  It's easier to find the nozzles when someone it operating the lever, and you can feel around for what is moving.


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#9 5 Star Rising

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Posted 22 September 2016 - 10:27 AM

Before I pulled the carbs I noticed that the choke levers (where the cable attaches on the carbs) were bent and binding.

Good to know that the video covers rebuilding. Thanks Hud

#10 5 Star Rising

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Posted 24 November 2016 - 02:26 PM

First I was a little iffy about being told to buy a 16 year vhs/DVD about SUs. But I ended buying it from Z Therapy. Gotta say it was indeed worth it, Wish the video had chapters to jump to but I watched it a few times,rebuilt my SU's and tuned them correctly. Car runs great now. Thank you. Think I have been a member here for like 10 years or so. Should probably come to the forum more often.. Cheers

#11 5 Star Rising

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Posted 27 November 2016 - 03:29 PM

OK I lied car doesn't run that great. I now have a back firing issue after plugging a major vacuum leak. I was able to tune the carbs with this major vacuum leak before. The large port on the air rail that I believe had a large hose connected to it gong to the air cleaner box was wide open. When I bought this car they had put a hose on large air rail nipple and it hung down under the carbs .it had a rubber plug on the end that had cracked open. I kept hearing a hissing sound from vacuum but thought it was the carbs pulling air. I plugged this port on the air rail and now the car idles super smooth, but after about 60 seconds both carbs give a small backfire followed by a small backfire out of the tail pipe. I tried to tune the carbs to get rid of the back fire but so far unsuccessful. It all happened as soon as I plugged the port. Do any of these ports need to be open to suck air? I have seen pics on the net of other L24s with these ports plugged. Thanks

#12 5 Star Rising

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Posted 27 November 2016 - 03:31 PM

Here is a pic of the port I plugged..

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#13 Miles

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Posted 27 November 2016 - 04:16 PM

Back fire through carb  =  too lean

 

Too lean = lean mixture adjustment or air leak or both.

 

Resolve all air leaks and reset mixture per the video


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#14 5 Star Rising

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Posted 29 November 2016 - 05:12 PM

Yeah I heard that in the video too. I set both carbs as rich as they would go and still got the back fire at idle and when when you hit the gas. The idle is smooth but all the sudden you heAR a "click" and the rpm drops and both carbs spray out puff or mist of fuel and then you hear the tail pipe pop continously. As soon as I plugged that major vacuum leak on the port this started happening. The clicking from the carbs almost sounds like a AC compressor kicking on and the RPMS drop and then the backfiring happens. (I don't have AC in my car, just sounds like the click of a compressor). It there another valve of some sort opening in the engine like some sort of EGR or something?

#15 74_5.0L_Z

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Posted 30 November 2016 - 03:19 AM

What oil are you using in the dampers on the carbs?

Did you verify that the floats are set correctly? Float level is critical for these carbs.

 

You do have the springs installed that hold the piston in the down position right?

 

The clicking sound you describe sounds like uncontrolled movement of the piston inside the SU carburetor.  This would occur if the springs were missing.  Also, the oil inside the SU acts to damper the movement of the piston.

 

At idle, the butterfly is nearly closed and airflow through the carburetor is low.  Thus the pressure differential across the piston is low.  This allows the mass of the piston and the force of the spring to keep the piston low.  When the piston is low the needle restricts fuel flow to control the air fuel mixture.

 

When you open the throttle, the air flow through the carb increases and the pressure at the venturi decreases as the velocity increases.  This pressure decrease causes the piston try and rise.  The piston will rise until the force provided by the spring and the mass of the piston is in balance with the force on the piston cause by the pressure differential.

 

The piston should rise and fall smoothly as the throttle is opened and closed.  If the spring is missing, then the piston will slam upward and downward making your clicking sound.  If the oil is missing, then the piston will move too quickly when the throttle position changes.


Edited by 74_5.0L_Z, 30 November 2016 - 08:07 AM.


#16 5 Star Rising

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Posted 03 December 2016 - 11:28 AM

These are 4 bolt SUs. Both springs are installed. I'm using a cap full of Merc/Dex Transmission fluid in each carb. I had no back fire what so ever and was able to tune and Rev the car with the vacuum leak, as soon as I plugged the vacuum port the idle went smooth but the back firing started. It's almost like the engine can't breath and it gasps for air, thus causing a small back fire at the carbs which I think slams the Pistons upward causing the "click sound".

The back fire at the carbs is not a loud pop it's more of a cough in which both carbs puff out a mist of raw fuel, almost like the pop is happening in the intake and then back out both carbs simultaneously. When the car is running and you shine a light inside the face of the carbs it is not wet with fuel or does not have fuel pouring out the nozzles.

When setting the float levels I do remember being unsure if the needle valve was supposed to be partially closed by the weight of the float or if the new valve was supposed to be stiff enough to hold the weight of the float with out depressing the needle. I installed a new set of valves but they seemed cheap compared to the ones that came out. As I was setting the float level I noticed that the weight of the dry float pushing down the tab was actually closing the valve. it sure if this is normal.

But I set my float levels with where the float rested naturally.

#17 Miles

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Posted 03 December 2016 - 01:23 PM

"But I set my float levels with where the float rested naturally"  The  video gives the procedure for setting the float. They also sell a good quality rebuild kit.

 

There is a member here who invented and sells a sight device for setting the correct full level, but the Z Thereapy procedure works.

 

Google "how to check for intake air leaks"


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#18 5 Star Rising

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Posted 03 December 2016 - 07:39 PM

Right but the video doesn't specify whether you may encounter a problem with a weak spring tension on a cheap needle valve. My floats are set to specs but it's with the natural weight of the float collapsing the needle partially that I was concerned about.

On my old needle valves that I removed I noticed that the needle spring tension held the needle all the way up so that the valves were fully open even with the weight of the float and tab pushing down on the valve. Again the back firing only happended after plugging the large vacuum leak. Guess I could try to install the old needle valve and set float levels again to see if that makes a difference.

#19 Miles

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Posted 03 December 2016 - 08:30 PM

Here is picture of what you are trying to achieve when adjusting the float level. A float set too high will run rich etc. See the red line in the picture just under the carb? What you are trying to achieve is a puddle of fuel right at that red line. High velocity air passing over that puddle is how fuel mixes with the air in the correct ratio and gets sucked into the intake.

 

The other thing that can happen is that the needle can get stuck and not open. I seem to recall removing the top of the float bowel and flipping the top upside down and blowing in the inlet. No air should leak past the needle valve.

Flip the top of the float bowel over right side up and blow through the inlet.  Air should pass freely.

 

 

As per the video air leaking around the throttle shafts can cause a lean condition that prevents ever getting the mixture set right.

 

The SU carb system is simple, but you have to be methodical and pay attention to the details.

 

Have you confirmed that there are no air leaks?

 

Did you check the ignition system?

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Edited by Miles, 03 December 2016 - 08:38 PM.

“Life is like arriving late for a movie, having to figure out what was going on without bothering everybody with a lot of questions, and then being unexpectedly called away before you find out how it ends.”    -Joseph Campbell


#20 5 Star Rising

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Posted 05 December 2016 - 03:48 PM

After I set my float levels I will blow through the valves to check for leaks at first these cheap needle valves were not sealing great I had to push them up and down a few times and get them wet to where they would fully seal air. These were the only things I questioned in the rebuild the cheap after market parts. I did a gravity test on the Pistons and they both fall at the same speed and you hear the metallic sound when the hit bottom at the same time. The jet needless are set like the video states aND I am aware of howhat the fuEl operation works in these after watching the video 3 times. "The gardenext hose affect". I will check for any more vacuum leaks using carb cleaner around the intake carfully. But I doubt finding more leaks and fixing them would make the back fire go away because the whole reason the back firing started was because I plugged a HUGE vacuum leak. If anything fixing more vacuum leaks would make it worse haha. But yeah I definitely don't want any leaks.. But got to get to the bottom of the back firing. I will check float levels again and timing as well, I have yet to check timing at all since I got the car.




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