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'73 240z starts, runs 10min, dies, then won't start for a while


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First off, thank you for all the answered questions I've gotten so far.

 

I'm trying to get a '73 240z running solid enough to be my daily driver, but I've run into an issue I can't seem to solve.

 

It's able to start, with a little help from the choke, and will idle for a seemingly infinite period of time, but as soon as it's driven around for about 10 minutes or so, it slowly loses rpm and dies, and then is unable to start for quite a while.

 

I charged the battery, since the issue seemed to start after the battery was left to drain on accident over night, as well as tested the alternator (14.4v), checked fuel flow (seems to be consistent - stock mechanical fuel pump), replaced the battery with a brand new one, changed out the spark plugs and run over everything I can think of.

 

It doesn't seem to get all that hot, but at this point I figure it has to be something heat related, but I have no idea where to go from this point. The car is pretty much completely stock, with '72 SU carbs, and the only thing that changed between it driving 100 miles flawlessly the day before, and being unable to run for more than a few minutes, was running some seafoam aerosol cleaner through to try and hunt down an exhaust leak (crappy non-stock exhaust from previous owner), and the battery being left running (due to somebody breaking into the car, stealing my seats, and turning the ignition to on and leaving it). 

 

Once it dies, I can try to start it all I want, and the engine turns over, but it doesn't seem to catch at all. Coming back a few hours later, it seems to start fine again. I've checked the output of the fuel pump while it was in its "can't start" mode, and it seems to be squirting out as usual, so I don't think it's related to fuel flow, and if it is, I'm not sure how it could manifest in this way. When it dies after being driven for a bit, it's usually as I bring the car to a stop, at which point the rpm lowers, can't recover, and the engine dies, unable to restart. 

 

If you have any light to shine on the issue, or any questions I can answer to help clarify, I'd hugely appreciate the input. I'm at my wit's end trying to get this car running again. Thank you in advance!

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compression, ignition, fuel.

Give an engine all three, at the right time, in the proper amount, it will run.  Millions of engines prove that every day.

If the engine ran yesterday, and nothing catastrophic happened, you can assume the compression is good, for now.  Especially if the engine has a even rhythmic sound when cranking.

 

So if the engine cranks, and no start, go and smell the exhaust,  Smell gas, that points to an ignition problem.  Do not smell gas, possibly a fuel related problem.

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datzenmike - It does have a manual choke, and putting it all the way on seems to extend how long it's able to stay alive for a bit.

 

DanielC - Pretty sure I'm smelling gas, so, points towards ignition then?

 

wayno - Sometimes it gets fairly hot, but it will still refuse to start long after it's cooled down. It's also night time near Seattle, so, I think it's fairly unlikely it's vapor lock, unless it's amassing way more heat then I think would be possible in the current climate, and for as short of a time as I'm running it. It drove about a hundred miles the day before fine, so I'm inclined to think that the issue would have presented itself during that trip if it was present

 

thisismatt - Seem to be getting a spark from what I can see, but I've got a spare coil sitting around somewhere I can try to throw in it. It's a "Flamethrower" coil if I remember correctly, and came paired with an electronic points system that I believe it's supposed to be installed with. It worked fine on my previous '72 240z.

 

A lot of the symptoms seem to point at times to the alternator, but doing a quick test on it while running showed 14.4v... any chance it could be that still? I've got an extra alternator I could try throwing in if there's any chance that could be the issue. 

 

Thanks for all the replies so far!

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Just went out about an hour and a half after driving it for 10 minutes then having it die while pulling back into my driveway. Starts again, but won't stay alive above a weak idle. Pumping gas seems to brighten dash lights and headlights, pointing towards power issue possibly?

 

I did the super basic test last night of removing the positive lead from the battery once the car was started, and the car kept idling fine, until the heat, lights, and wipers were turned on, at which point it would die. Hope that provides a little more useful information!

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When it doesn't start or barely starts, have you pulled a spark plug wire and stuck a screwdriver in it to see if it's arcing nice and healthy?  Or remove the high tension lead from the coil and attempt to start.  Spark should jump out the coil readily.  If it doesn't, you have a weak coil.  If it does, chances are fuel/air.

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When it doesn't start or barely starts, have you pulled a spark plug wire and stuck a screwdriver in it to see if it's arcing nice and healthy?  Or remove the high tension lead from the coil and attempt to start.  Spark should jump out the coil readily.  If it doesn't, you have a weak coil.  If it does, chances are fuel/air.

Checked spark - didn't seem super strong, so I replaced the coil with one I had lying around and had working fine before. Spark looks about the same, and nothing seemed to change. Pointing towards fuel/air then I suppose?

 

Fuel system plugging up.

Went ahead and replaced the fuel filter (no change), and fuel pump (made it worse... couldn't start at all, so I changed it back), so tomorrow I'm going to try an electric holley pump I had setup for a previous Z I was working on, to see if that effects the problem.

 

Also replaced the alternator with a spare I had, still reading 14v on multimeter when running, so guess that wasn't the issue. Also, my carbs are definitely due for tuning, but that didn't change between the car running and not running. One is hissing (good, from what I understand?), and the other is not (which is bad, I'm pretty sure). I have had a terrible time trying to fully understand the dual carb tuning process up to this point, but if that somehow could have suddenly changed enough to cause this issue, I'd love suggestions for somebody with little to no carb experience (still very new to working on cars in general).

 

Thank you again for all the information so far, I'm really hoping I can get this car acting fine soon

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I was thinking about heat and 10 min doesn't sound like a lot, even less so in Washington this time of year.

Yep.In that car,the only thing that that can cause a shut-down in 10 minutes of run time is a plugged fuel system or a coil(real long shot-but possible)and this assumes points distributor.Put a fuel pressure gauge just after the pump-this will rule out fuel as an issue.

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If the tank has rust flakes in it, it can cause an issue like your having.

When you start the engine up the fuel pump starts sucking fuel up, rust flakes will eventually plug the end of the pick up tube in the tank, when the bowls run out of fuel, the engine dies, when it dies the fuel pump quits sucking and the fuel in the pick up tube can actually flow backwards clearing the rust flakes from the end of the pick up tube, but it is not a rush of fuel, just a little bit of fuel, gravity at work.

But when you start it up again, the fuel in the main line fills the bowl quickly, and rust flakes get picked up again and clog the end of the tube.

There was an old timers trick that was used when someone got mad at someone else, they would dump a bunch of moth balls in the tank and the same thing would happen, the moth balls plugged the pick up tube/screen, and when the fuel pump quit working because the engine died, they fell off the tube/screen, but 10 minutes later it would be plugged again.

It sounds like a fuel issue, next time it dies, squirt just a small amount of fuel in one or both of the carbs, if it starts/hits, it's not electrical.

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Yep.In that car,the only thing that that can cause a shut-down in 10 minutes of run time is a plugged fuel system or a coil(real long shot-but possible)and this assumes points distributor.Put a fuel pressure gauge just after the pump-this will rule out fuel as an issue.

It does have points, so you're probably right. I'm going to swap in the electric fuel pump I have from a previous Z, and throw in the little inline pressure gauge I have with it, so I should know by tomorrow how the fuel pressure is looking right before it hits the carbs.

 

Did you say it has a matchbox dizzy?  If so, change the matchbox.  Mechanical fuel pump is also suspect.  They can have bad diaphragms that take a bit to stop generating pressure.

I'm not super familiar with what a matchbox is, but I think I just have the stock distributor on there at the moment, sorry if that's not the right response >.<

 

I'm hoping all it is is the fuel pump, so experimenting with it tomorrow should tell me whether that's what's causing the issue or not. Fingers crossed!

 

If the tank has rust flakes in it, it can cause an issue like your having.

When you start the engine up the fuel pump starts sucking fuel up, rust flakes will eventually plug the end of the pick up tube in the tank, when the bowls run out of fuel, the engine dies, when it dies the fuel pump quits sucking and the fuel in the pick up tube can actually flow backwards clearing the rust flakes from the end of the pick up tube, but it is not a rush of fuel, just a little bit of fuel, gravity at work.

But when you start it up again, the fuel in the main line fills the bowl quickly, and rust flakes get picked up again and clog the end of the tube.

There was an old timers trick that was used when someone got mad at someone else, they would dump a bunch of moth balls in the tank and the same thing would happen, the moth balls plugged the pick up tube/screen, and when the fuel pump quit working because the engine died, they fell off the tube/screen, but 10 minutes later it would be plugged again.

It sounds like a fuel issue, next time it dies, squirt just a small amount of fuel in one or both of the carbs, if it starts/hits, it's not electrical.

This could definitely be the case, but as of yet I haven't seen any rust accumulating in the fuel filter, which with my '72 Z was a huge indicator of the issue when it arose. It might be clogging up further back then the fuel filter, so I'm going to try sticking a gauge in there and seeing what it reads tomorrow with both the stock fuel pump, and a holly electric one I have sitting around (blue one, so it shouldn't overflow the carbs). I'm hoping that that will flush out whether it's an issue with the fuel getting all the way to the carbs, or some problem that's arisen with the carbs themselves.

When I dealt with rust clogging the fuel line before, I solved it by washing out the gas tank with some cleaners meant for the job, and recoating the inside, so I'm hoping if that's what's causing all my issues, I can just do that again and have my car running happily =]

 

Thank you for all the suggestions! I'll update this as soon as I've tried everything suggested so far - wish me luck!

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 Also, my carbs are definitely due for tuning, but that didn't change between the car running and not running. One is hissing (good, from what I understand?), and the other is not (which is bad, I'm pretty sure). I have had a terrible time trying to fully understand the dual carb tuning process up to this point, but if that somehow could have suddenly changed enough to cause this issue, I'd love suggestions for somebody with little to no carb experience (still very new to working on cars in general).

 

 

 

Missed this before. Hissing may be the one carb is more open at idle and doing more work than the other. On multi carbs they should be 'synchronized' or tuned to be working equally. You'll have to ask or look up the procedure. Also the proper fuel level in the carbs is critical and has to be equal as well.

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It does have points, so you're probably right. I'm going to swap in the electric fuel pump I have from a previous Z, and throw in the little inline pressure gauge I have with it, so I should know by tomorrow how the fuel pressure is looking right before it hits the carbs.

 

I'm not super familiar with what a matchbox is, but I think I just have the stock distributor on there at the moment, sorry if that's not the right response >.<

 

I'm hoping all it is is the fuel pump, so experimenting with it tomorrow should tell me whether that's what's causing the issue or not. Fingers crossed!

 

This could definitely be the case, but as of yet I haven't seen any rust accumulating in the fuel filter, which with my '72 Z was a huge indicator of the issue when it arose. It might be clogging up further back then the fuel filter, so I'm going to try sticking a gauge in there and seeing what it reads tomorrow with both the stock fuel pump, and a holly electric one I have sitting around (blue one, so it shouldn't overflow the carbs). I'm hoping that that will flush out whether it's an issue with the fuel getting all the way to the carbs, or some problem that's arisen with the carbs themselves.

When I dealt with rust clogging the fuel line before, I solved it by washing out the gas tank with some cleaners meant for the job, and recoating the inside, so I'm hoping if that's what's causing all my issues, I can just do that again and have my car running happily =]

 

Thank you for all the suggestions! I'll update this as soon as I've tried everything suggested so far - wish me luck!

 

Sometimes it isn't simply rust in the tank but something else that got in there. To see if the carbs have fuel in them when it won't start simply remove one of the float bowl draining screws. No fuel comes out you either have a faulty pump or a floater in the tank. You may also want to check the screens on the fuel inlets of each carb within the banjo fittings.

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I didn't have as much time as usual tonight to keep tinkering with it, but I'll respond with what I know so far, and do the rest of the suggested troubleshooting tomorrow - 

 

right when this goes bad ck for spark, then pull the fuel pump hose and see if it shoots gas. If both yes then those are OK.

I would do a valve lash ck while your at this , Not being that its bad.

How about ck the oil water level? Gas cap vented ??? Pull the gas cap and drive it.

 

Looks like I still get spark when it's dead, and the fuel pressure going to the carbs is a steady 4psi with the electric fuel pump I threw in today. I'll do these other suggested tests tomorrow and get back to you - thank you for the suggestions!

 

Missed this before. Hissing may be the one carb is more open at idle and doing more work than the other. On multi carbs they should be 'synchronized' or tuned to be working equally. You'll have to ask or look up the procedure. Also the proper fuel level in the carbs is critical and has to be equal as well.

 

Definitely out of sync like you're saying - I'm starting to think more and more that the issue is focused around this. I'm just not sure how it changed on me....

Either way, I either need to hunt down somebody who can get them tuned, or try my best to get the process down myself (I've had very little luck at that so far though).

 

Are there any good resources for learning the tuning process of dual carbs for people with effectively zero experience in tuning carbs at all? I've tried reading through some step-by-steps, and they're overwhelmingly dense for somebody not aware enough of the terminology involved to picture the instructions at all. I'm still trying to figure out with screw does what on them >.<

 

Sometimes it isn't simply rust in the tank but something else that got in there. To see if the carbs have fuel in them when it won't start simply remove one of the float bowl draining screws. No fuel comes out you either have a faulty pump or a floater in the tank. You may also want to check the screens on the fuel inlets of each carb within the banjo fittings.

 

I'll do these tests tomorrow and let you know what I find! I'm pretty confident in the pump I just installed today, and up to the carbs it's reading a steady 4psi, so I'm pretty sure if it's a fuel delivery issue, it's gotta be one or both carbs having an issue (or simply being hopelessly out of sync). I'll get back to you with what I find tomorrow!

 

 

Thanks again for all of the information so far - it's definitely acting a little better now that it's got a fresh fuel pump and regulator in there, but it's still not driveable. I'm hoping with the tests you've all suggested, I'll be able to hunt down the issue once and for all, but in the mean time, my primary suspect is the carbs. If anybody has any suggestions for mechanics in the Seattle area who could tune them for me, I'd love to get it done as soon as possible to either solve the issue, or rest assured that the carbs aren't causing it.

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You have points- have you changed the condenser?

 

Whenever I've had a fuel delivery issue that would let it run for a while, then shut off due to starvation, it would start up again within minutes (then die again) and it was rarely predictable.  This was from a plugged up fuel system- rust everywhere.  I still have problems to this day with that car, since rust flakes got everywhere.  But this doesn't sound like your problem.

 

Having the ignition left on with a points system can easily overheat the coil and make it weak.  Had a truck do that, and symptoms were similar- would start up fine, but after a few minutes it'd die and take around an hour to be able to run again.  Coil was really hot to the touch.  Replaced coil, problem gone. 

 

But you said you replaced the coil, so I would then look at the condenser.  It's usually under the dist cap, normally changed when the points get changed since it's normally part of the points kit.

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It runs!

 

Thank you everybody for all the help! After changing the plugs, coil, alternator, fuel pump, fuel filter, several fuel lines, and battery, it ended up being due to something in the distributor. I'm guessing it was due to a bad condenser as datsunaholic suggested, so thank you again for that!

 

It's running quite rough at the moment, definitely needs timing and tuning, but it doesn't die any more, and I successfully journeyed down the freeway a ways and back with no threat to the RPM bottoming out upon stopping, and only one or two backfires. I pulled out the distributor from a previous 240z project, and brought over the pertronix electronic ignition kit I had it setup with as well, and it immediately resolved the issue I was battling.

 

I couldn't have gotten this solved in such a short time frame without all of your suggestions, so you've definitely made a permanent member out of me, and I hope I can repay the assistance someday. Thank you!

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if a wrong coil is installed it can saturate the condensor as tto much current running thru the points. A point coil is needed if running points. NO other coil no matter how COOL is looks.

 

But you gave indications you were using a Pertronix set up so thats why it was harder to trouble shoot.

 

Put a Pertronixkit in there with the correct stock coil and ballast and it will last a long time!!!!!!no more fucking with a old point dizzy. (also busshing wear)

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It runs!

 

Thank you everybody for all the help! After changing the plugs, coil, alternator, fuel pump, fuel filter, several fuel lines, and battery, it ended up being due to something in the distributor. I'm guessing it was due to a bad condenser as datsunaholic suggested, so thank you again for that!

 

 

Well some of those things probably needed it like the fuel filter but it's expensive replacing everything to fix a problem. Often the 'new' part is much poorer quality than the part it replaces and adds to the problem or soon will. 

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