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This Problem is Eating me Inside Please Help.. Stuttering, Jerking. loss of power after 20-30 min


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I am new to carburation but i am very mechanically inclined , this prob has been haunting me for a while and slowly i am eliminating the culprits. and i think i have narrowed it down but i would love some input and similar experiences to once and for all fix it... 

my z starts fine i let it warm up for about 5- 10 min and it does well,,.. it drives fine around town and in the freeway no problems. but after about like 30 40 min of  driving it starts.. acceleration suffers and driving around is torture because it totally bogs down like its not getting fuel and i fear it will leave me stranded so im driving at that point with a handicapped engine sounds like its running on 2 cylinders and i just cant get it to go .. it does not shut down but instead loses power.. and want to die. next day rolls around runs fine and it starts again after i been driving it for a while wtf!! also my car  feels like its skipping a beat when im driving down the street like it misses slight engine jerking at speeds u can feel it and you can also hear it when you open the hood and rev the engine..  


my 73z  has 3 screw round tops su carbs


this is what i have done: float bowls ave been checked and adjusted to correct fuel level.. with new float bowl gaskests


fuel filter under the hood has been changed , spark plugs and wires have been changed, synced carbs, timing is correct


checked the fuel needles they are ok. removed those tiny fuel screen filters. 


checked the dampening oi fluid all good .


my 73 240 z does not have the rear electric pump already checked ,, not there the wiring is there but not the pump


what i think is the prob..


it still has what it looks like the original mechanical fuel pump could that be the prob?


my z does leave a lot of crud in the fuel filter, i heard of ppl saying all that crud clogs the fuel line maybe thats it?


and could explain fuel starvation?? i have never checked the fuel tank i know its the original..


any idea or input or similar problems would greatly be appreciated .









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Guest Rick-rat

Pull the tank, clean it out ,blow back the fuel lines with some alcohol,sounds like it is picking junk up from the tank that has settled while it sat overnight, junk gets stirred up after driving it for a while.


Could be the pump, check it out first, then try above

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I would think that a fuel issue would be present all the time. It sounds like something in your ignition system. It could be that your coil is going bad. If it is heating up after some use and not delivering the proper amount of spark energy on the secondary side, you would experience these issues.

Just a thought.

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Had a problem like this before. The fuel tank vent was plugged and starving the pump from fuel. After driving enough to drop the fuel in the tank some it was creating a vacuum in the tank. Was more noticeable when tank was full.


Next time you drive it for awhile and it starts acting up pull the tank cap off and see if it has vacuum.

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If it's a hindered fuel line, it would refill the bowl at a stoplight, and you would have good acceleration until the bowl emptied. This sounds related to heat to me. Are you losing water? Oil? This can happen when the head gasket leaks. Gets hot and let's water /steam into a cylinder. Pull your plugs and post pics. It'll tell weather the ignition is lean, rich, steamed, or oiled.

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Vapor lock is a possibility. Make sure that your fuel lines are far from heat or insulated. Heat shield installed behind the carbs? I experienced the exact symptoms you mention and solved it by insulating the metal fuel lines that crossed over the engine.

If you have crud in your filter you should clear out the tank too but I don't think that's your main problem.

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So where does the crud go every night? Back into the tank? If crud was plugging the filter, lines or carbs it would be there the next time you drove it.


If you think tank not vented drive with the cap loose and see.


Can't be hot enough in the Bay area in February for vapor lock on SUs even without the heat shield..



MSD? throw it away and put the points dizzy back in or better yet a late L28 EI dizzy and coil. Have the correct coil. This is the problem.

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So where does the crud go every night? Back into the tank? If crud was plugging the filter, lines or carbs it would be there the next time you drove it.


If you think tank not vented drive with the cap loose and see.


Can't be hot enough in the Bay area in February for vapor lock on SUs even without the heat shield..



MSD? throw it away and put the points dizzy back in or better yet a late L28 EI dizzy and coil. Have the correct coil. This is the problem.

Ive had issues with crud falling back in the line or loosening up while sitting overnight and letting the car run for a minute or two... but that is a good point as he would be building the pressure up substantially over the 30-40min drive and line blockage would not be an issue

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UPDATE... drove my car to work today same route i always do 30min drive anything from 70 to 10 mph.

i looked under the hood and the ballast doesnt seem to be used only one wire is hooked up to it. if u were standing right above the ballast there is only one wire tied down to it on the left.


i decided to unscrew the gas cap and loosely let it sit and see what happens on my way to work, i have the unvented cap btw.


so my car didnt act up as much it kinda started jerking and acting funny at the end but not as abd as before, im going to keep driving woth the cap unscrewed and see what happens.


yes i have stock points ignition, and no name brand red coil.


and yeah the stock heat shield is there .


a while back i changed the fuel filter cuz it will,accumulate crud in the bottom so i changed it and again the new one will have visible crud after a few months.


any yeah i will post pics tonight of my z , the car you guys are helping me trouble shoot, as soon as i get off work .

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OK Next time it's acting up grab the coil. If you burn yourself it's the wrong one for that system. Probably a later EI coil designed to run on full 12 volts is needed.


I suppose you could re connect the ballast to take the load off of it but if it's been overheated, best to replace. Any '78 and up coil will be the ticket.

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so i drove back home from work with the  gas cap totally off .. the problem did not arise.. i will keep driving with the gas cap off and continue to monitor.. if it is the venting issue how would i get around that.. buy a vented gas cap? or do i really gotta take the whole fuel tubing out and restart ?


and king rat : i did touch the coil... it was pretty warm.. not hot .. used my heat gun and read 119f.. i could wrap my hand over it for like 5 min .. oh and its a bosch coil 12v

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No, It needs wires connected to both sides the ballast for it to work. That is for the old points system. looks like it's setup for the MSD box to do the switching on/off of the coil now. The original distributor points are just telling the MSD box when to fire the coil. This setup reduces the electrical current that the points would normally have to deal with, making them last MUCH longer. The wire that is connected to the ballast is most likely to old switched 12V., that comes from the starter while cranking. Don't need that anymore, or the ballast.


That's a heck of a wire mess on your coil there...


MSD Ignitions use 12v coils so your fine as far as coil voltage goes there. Don't know about the cheap no name coil though. Coils can cause problems when they get warm, and be fine next day too. I would suggest updating to the factory electronic ignition and canning the MSD. You really do not need multiple sparks. One good hot spark is all thats needed. The multiples that come after do nothing but wear out your ignition system faster.


I've found when trying to diagnose intermittent problems it is best to wait for the problem to arise, then look for the problem. Not make a bunch of changes and see if it comes back. Who knows if it's fixed, if you don't know what you did to fix it? 


My suggestion if you have original cap and want to keep it is to wait for it to run shitty, pull over and check for vacuum in the tank (heck leave it running too), you will know right away and without question if this is what's causing your problems when you hop in and drive away. If fuel cap is already aftermarket. just go buy a vented one.


Just my $0.02. Thats what I would do…. With that wire mess there. Who knows? A crapy connection could be heating up and causing voltage drops??? If you have a volt meter, Bring that too.. If it turns out not to be vacuum in the tank, you could do some quick voltage checks while it is actually having problems.. Key on intermittent problems… Regardless,  I would get a diagram, clean up some of the wiring and be sure all the grounds are cleaned up good too...


Nice Car  :thumbup:

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Feel the coil when it starts to cut out. If its a Hot MOFO then you have the stock point coil still in there and your running 12-14 volts thru it all the time. which is a NO NO.

You need like a MSD Balster 2 coil.


a stock point ignition only get full coltage at START as there is a bypass wire that give the full battery voltage at START then when key goes to ON the batter voltage goes thru the ballast resistor to cput the volage down to help preserve the points and the coil.



coils will be marked needs ballast  or not.  yours is painted so if could be a stock coil or a aftermarket one already and its not the proplem. Key is feel if HOT!!!!!!!!  and maybe pull the center coil wire place near ground and start and see if the spark is weak or not even there


This might not even be the proplem. if this rig was running before fine.


I dont like MSD as the relaibility isnt all that good with them in my opinion.  plus after 3 k rpms they go to 1 spark anyways.



you got a nice set up there this is a simple fix. your just missing something

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After looking at how your MSD is hooked up a little more, I see a few problems...


I'm not seeing the heavy red wire that connects directly to 12v power all the time. I see a yellow running across. Is that it? Looks a bit small. Whenever extending wires it's a good idea to go one size larger and go the the battery side of the starter solenoid to avoid corrosion. I prefer to use a fuse. But, if you don't be sure it runs in a spot where it won't get the insulation damaged and away from vibration.. like the firewall. not across the valve cover. Not seeing a good ground for it either.


The MSD is capacitor discharge. Which means "The MSD Ignition features a capacitor which is quickly charged with 520 - 535 volts and stores it until the ignition is triggered." 


The only two wires that should be on the coil are orange and black from the MSD unit. You don't want to be hooking your tach up to 500+ volts!!!


White wire from MSD should go directly to points.


All those connectors are showing their age. Each one could be easily adding .5V voltage drop across, or more. The less connections the better. Soldiered and sealed with adhesive heat shrink tubing is the way to go. Those cheapy crimp connectors are good for temporary stuff, but thats about it...


Here's a link to the MSD 6A manual. This should help...



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