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Question about poor acceleration


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Another timing/tuning question for you guys/gals:

I have a 72 240z with a completely stock engine setup, SUs, points ignition, etc.

I rebuilt the SUs with the guidance of the Ztherapy video, and tuned it with the FSM I have. I also checked the gap of the ignition, installed new plugs and points.

The car runs fantastic at idle, it is smooth and level. I've got it set at 800-900 rpm there. The car is good if you punch it and accelerate up to 3000 rpm. I can easily get it to squeal the tires if I wanted. It drives great between 0-60 mph. However, once I get it to 3-3200 rpm in neutral or under load the car will no longer accelerate and start to stumble and pop. Once I decelerate it will pop and backfire again. I can't get the car to go past 80 mph either. After reading other threads I figured the timing was out of place and got a timing gun for the car. However, I can advance the car completely or pull it back a bit and the problem still remains.

I am at a dead end with this acceleration issue. I think I have covered most of the details on the car and what position it is in. What's confusing is it will start right up without the choke and idle great, so I have half the acceleration it can deliver. I would greatly appreciate any addition insight on this or suggestions. I think the only thing I have not replaced that is apart of the ignition system are the plug wires. Could they be the culprit to my acceleration stumbling under load? The wires are at least 27 years old and NGKs. I wasn't sure and didn't want to start throwing money at the car if I didn't need to.

Thanks in advance!

Chase

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Also after all the tuning I pulled each of the spark plugs and they were lightly brown and similar to what the FSM describes them to look like. The coil is also at least 20 years old.

Chase

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Throwing money at it?

It's called a tune up. You cannot expect to troubleshoot a problem when the basics( plug wires ) are 27 years old. Most of the time troubleshooting is the process of elimination- so eliminate the basics, give it a tune up.

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After reading other threads I figured the timing was out of place and got a timing gun for the car. However, I can advance the car completely or pull it back a bit and the problem still remains.

I hate to be "that guy" but..... did you try setting the time to the specified correct value? More timing doesn't always mean more power.

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Throwing money at it?

It's called a tune up. You cannot expect to troubleshoot a problem when the basics( plug wires ) are 27 years old. Most of the time troubleshooting is the process of elimination- so eliminate the basics, give it a tune up.

Woah easy now, madkaw. If you had read properly for more than a few seconds I stated that I have given the car a tune up. I am already past the point of a basic tune up and my problem still remains. I like most people don't have an unlimited budget and would like to spend money on what's necessary rather than replace it all and hope for the best. After some more investigation the plug wires seem to be fine with the check of an ohm meter.

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Throwing money at it?

It's called a tune up. You cannot expect to troubleshoot a problem when the basics( plug wires ) are 27 years old. Most of the time troubleshooting is the process of elimination- so eliminate the basics, give it a tune up.

Woah easy now, madkaw. If you had read properly for more than a few seconds I stated that I have given the car a tune up. I am already past the point of a basic tune up and my problem still remains. I like most people don't have an unlimited budget and would like to spend money on what's necessary rather than replace it all and hope for the best. After some more investigation the plug wires seem to be fine with the check of an ohm meter.

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As far as the mechanical advance goes I can put suction on the vacuum tube and see it advance the distributor with the cap removed. However, being an old vacuum advance it might not be enough for what the engine needs at 3000 rpm. I also tuned back the timing a bit, finding the best spot where the motor revs without the "pinging sound"

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The mechanical advance is not affected by the vacuum servo. The mechanical advance is a fly-weight govenor that lives below the breaker plate. To test it simply rotate the rotor lightly by hand. It should move freely a few degrees and then return on its own when you let go.

Your symptoms suggest inadequate fuel flow. A dirty fuel filter could be the problem. There are 3. One in the right fender and a small screen in the inlet of each carb. The inlet screens are easily overlooked and also easily cleaned and reusable. If the filters are good perform a fuel pressure and flow rate test in accordance with the factory service manual instructions.

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beermanpete,

Looking and testing the mechanical advanced, it seems to move freely like you described it should. I have also replaced the filter on the right fender and cleaned out the two in the inlets of the carbs during the rebuild.

Chase

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Chase,

Ok, so those items are off the list. Check the fuel delivery. There could be a flow restriction for other reasons.

As has been pointed out, your symptoms could also be due to weak spark. You may not need all new wires, cap, rotor, and so on but you need to make sure they are in good condiiton. Make sure everything is clean and dry. Look for cracks, carbon tracks or other evidence of arcing to ground from the high voltage. As the engine speed and load increase the voltage requirement increases. If the insulation on any of the high voltage parts is marginal it will arc to ground before the voltage is high enough to fire the plugs. Make sure the carbon brush in the cap is long enough and moves freely. An old cap and rotor might look good and not have any high voltage leakage but as they wear from use and get cleaned during service the air gap increases which adds to the voltage requriement.

Make sure the primary connections are clean and tight as well. Don't forget the primaray circuit goes through the tach on the early Z so chech the connections there as well.

Set the timing to the factory setting for your car. If you do not have a timimg light dead time it. That is the one upshot to still having points is being able to set the timing without running the engine. Also, if you have a dwell meter check the dwell. If the points gap and the dwell do not agree to a reasonable degree it indicates a problem.

Trouble shooting is a process of elemination. Start with the most likely suspects and work toward the unlikely along a logical path.

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Chase,

Ok, so those items are off the list. Check the fuel delivery. There could be a flow restriction for other reasons.

As has been pointed out, your symptoms could also be due to weak spark. You may not need all new wires, cap, rotor, and so on but you need to make sure they are in good condiiton. Make sure everything is clean and dry. Look for cracks, carbon tracks or other evidence of arcing to ground from the high voltage. As the engine speed and load increase the voltage requirement increases. If the insulation on any of the high voltage parts is marginal it will arc to ground before the voltage is high enough to fire the plugs. Make sure the carbon brush in the cap is long enough and moves freely. An old cap and rotor might look good and not have any high voltage leakage but as they wear from use and get cleaned during service the air gap increases which adds to the voltage requriement.

Make sure the primary connections are clean and tight as well. Don't forget the primaray circuit goes through the tach on the early Z so chech the connections there as well.

Set the timing to the factory setting for your car. If you do not have a timimg light dead time it. That is the one upshot to still having points is being able to set the timing without running the engine. Also, if you have a dwell meter check the dwell. If the points gap and the dwell do not agree to a reasonable degree it indicates a problem.

Trouble shooting is a process of elemination. Start with the most likely suspects and work toward the unlikely along a logical path.

Very good BeermanPete,

Also check for vacuum leaks in the engine compartment. You tested the distributer for correct operation, make sure the vacuum hoses are in good working order.

--Justin

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beermanpete and buysell,

Thank you for your input and suggestions on what to do. I went through the process of checking the ignition components like suggested and could not see anything that was obviously at fault. While doing this I managed to find another coil in my extra parts box for the car I've accumulated. Surprisingly the other coil I had seemed to fix a good portion of the problems. I will fine tune the timing according to the FSM and see where that takes me.

Chase

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Didn't mean to sound like an a$$, but I still stand by my point that to ask us to help diagnose an issue that could diectly related to a simple tune up(Wires), which are 30$, just doesn't make sense. You are NOT "past the point of a basic tune up".

As pointed out-during heavy loading the spark has to be strong and good wires definetly could be the culprit, so I would replace your 27 year old stuff. The coil swap was a good idea, and you might be on to something. I have had coils going bad on newer cars and the car would buck during heavy loading like the trans was going bad, but it would fine everywhere else. Searching , you might find the ohm reading for a stock coil and check that against yours.

Lean condition also fits the your description.

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Didn't mean to sound like an a$$, but I still stand by my point that to ask us to help diagnose an issue that could diectly related to a simple tune up(Wires), which are 30$, just doesn't make sense. You are NOT "past the point of a basic tune up".

As pointed out-during heavy loading the spark has to be strong and good wires definetly could be the culprit, so I would replace your 27 year old stuff. The coil swap was a good idea, and you might be on to something. I have had coils going bad on newer cars and the car would buck during heavy loading like the trans was going bad, but it would fine everywhere else. Searching , you might find the ohm reading for a stock coil and check that against yours.

Lean condition also fits the your description.

He measured wire resistance and found it in spec, as far as he said. Only other thing I'd do is run the car in the dark and see if there is any arcing wire-to-wire.

Checking the coil per the FSM is a good idea...

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The wires do not have any breaks or damage in the casing, and meet the required resistance spec in the FSM. LevonV I did the test of checking them in the dark while running and no arcing happened.

madkaw what is YOUR definition of a basic "tune-up" New cap, coil, points, fuel/oil filters, fluids flush, SUs rebuilt, new fuel lines, spark plugs, everything gaped and adjusted according to the FSM. That;s just in the engine bay. I don't think I left anything out with the exception of the plug wires but they have already been tested and in good condition.

The new coil seemed to do the trick. No backfiring under deceleration, the car will easily hit 5000 rpm and has the power through the whole power band. Thanks guys for the tips and suggestions.

Chase

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Its a fuel problem.

John,

The fuel pressure and filters were checked and verified to be in good shape. My new coil solved the issues and the car will accelerate easily under load up to 5500 rpm with no problems

Chase

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