240Z breaking up after 5 minutes of running

Hey there folks, I have an issue with my Z i'm looking to diagnose and I'd like to see if you folks can help.

After about 5 minutes of running and revving great, the car starts to break up, rev slowly or not at all, and fall on it's face. After letting it sit for about 10 minutes I can drive it another 5 miles or so before it starts to do it again.

I replaced the spark plug wires and it still did it. Then I replaced the coil and it still did it.

The carbs need to be tuned and that will happen in a week or so but it is not what I was expecting. I was thinking that i'd be loading up the plugs from running so rich and thats why it would be breaking up. But after pulling the plugs, they actually look on the lean side.

I don't know, the symptoms to me seemed electrical like a coil that was bad but it still does it after that was replaced.

Any ideas?

Thanks

Jay

Fuel filter? or floats are stuck...?

RossD wrote: Fuel filter? or floats are stuck...?

There is a little sediment in the fuel filter by the tank but the front one is clear. Looks like its still flowing fine.

Not sure if the floats are stuck, maybe a problem with the returns or it over filling the bowls?

I bet RossD's onto something.

I went the same place you did and was bummed when you'd already tried the coil. Speaking of electrical items that might behave badly when heated, does this car have a points-and-condenser ignition? Probably a reach on my part there...

Is it points or transistor ignition? If its transistor try freezing the module with a can of air duster next time it dies/acts up then see if it runs.

Whats the choke system look like on one of these?

Fuel Pump

Sounds like a fuel tank venting issue. Or a collapsed fuel line.

Do you have a heat shield between the header/manifold and carbs? The L-powered Z's I have dealt with have all seemed to have drive-ability issues if there was no heat shield and the carbs got hot. It's worse with a coated header. What carbs do you have?

Kenny_McCormic wrote: Is it points or transistor ignition? If its transistor try freezing the module with a can of air duster next time it dies/acts up then see if it runs. Whats the choke system look like on one of these?

I have a points and condenser ignition, both are new as of 7/6.

The choke cables are not hooked up.

Elsmere wrote: Do you have a heat shield between the header/manifold and carbs? The L-powered Z's I have dealt with have all seemed to have drive-ability issues if there was no heat shield and the carbs got hot. It's worse with a coated header. What carbs do you have?

This or something like it. Something is emptying the bowls. The float could be junk, too.

PHeller wrote: Fuel Pump

Brand new electric unit. Are you saying it's overflowing? Its 3 psi so I don't think it's that.

It starts revving slowly and actiing like somebody just unplugged 3 wires as I was driving. ?????

The carbs are round top early model SU's with a factory exhaust manifold. Heat shield is gone I believe

Elsmere wrote: Do you have a heat shield between the header/manifold and carbs? The L-powered Z's I have dealt with have all seemed to have drive-ability issues if there was no heat shield and the carbs got hot. It's worse with a coated header. What carbs do you have?

It could be vapor lock. Have you tired really beating the tar out of it? If it runs better then, I'd look at that.

Check the fuel tank for rust It is fairly common in the z's

zeeguy wrote: Check the fuel tank for rust It is fairly common in the z's

The fuel tank was cleaned and put back in. Again the rear fuel filter has a little amount of sediment at the bottom but not the front filter.

A clogged fuel filter wouldn't cool down and run again after 10 minutes though?

Kenny_McCormic wrote:

Elsmere wrote: Do you have a heat shield between the header/manifold and carbs? The L-powered Z's I have dealt with have all seemed to have drive-ability issues if there was no heat shield and the carbs got hot. It's worse with a coated header. What carbs do you have?

It could be vapor lock. Have you tired really beating the tar out of it? If it runs better then, I'd look at that.

It does it whenever. Hard driving or light, I've tried both ways.

My `71 had the exact same problem, turned out to be an issue with the fuel tank and the evap system at the back of the car. It wasn't venting. There is a small tube on top of the tank where it vents into a smaller tank inside the car on the right rear. Once I repaired this the problem was solved. It took me FOREVER to find it though. Same symptoms.

Points suck, ballasted coil sucks, I'd upgrade to an electronic distributor and 12 volt coil. SU's are moody, open up the float bowls and make sure you don't have any crud down there plugging the output.3psi is about as much as the stock needle valves can hold before you start spurting fuel out the top of the float bowls. Count the turns on the jets, should be 2 or 3 turns out if it's substantially more you're rich. Check the SU fluid, should be fluid in the top atf is a good substitute if you're out.

Same symptoms, same car; I'd be checking out racerdave's tip...

That said:

crankwalk wrote: A clogged fuel filter wouldn't cool down and run again after 10 minutes though?

Depending on the crud, sometimes they can pick stuff up, smash it against the filter material with pressure, and after shut off , enough of it falls away and settles back down to allow flow again. I want to say this is more common with the in-tank sock than the inline filters, but it's something to consider...

The stock tank isn't even baffled, there is a single rib maybe 1.5" tall that runs front o back inside the tank and a single pickup tube that hangs near said rib, so yes rust and corrosion could also be issues.

racerdave600 wrote: My `71 had the exact same problem, turned out to be an issue with the fuel tank and the evap system at the back of the car. It wasn't venting. There is a small tube on top of the tank where it vents into a smaller tank inside the car on the right rear. Once I repaired this the problem was solved. It took me FOREVER to find it though. Same symptoms.

Yeah I may not have ALL of the vents hooked back up at the tank. If I recall there we like 4 vents and I capped like 1 or 2 except for the ones on top.

So test that, could I drive around with the fuel cap off and see if it still does it? Would that be enough venting?

crankwalk wrote: Yeah I may not have ALL of the vents hooked back up at the tank. If I recall there we like 4 vents and I capped like 1 or 2 except for the ones on top. So test that, could I drive around with the fuel cap off and see if it still does it? Would that be enough venting?

Yes. Problem sounds just like when I mistakenly replace the gas cap on my Alfa with an unvented one when I lost the original. One quick test is when It acts up is to remove the gas cap. If it is hard to get off or you hear air rushing in, you have a vent problem

Z is fixed as of this morning. The rear plastic fuel filter nipple was crimped down not clogged.(kudos to the fuel people) New filter and she is good. Drove her 10 miles and I think ill take her to work. Wish me luck.

Thanks for all the help!

excellent.. I live good news.. especially good news that was cheap to fix

crankwalk wrote: Z is fixed as of this morning. The rear plastic fuel filter nipple was crimped down not clogged.(kudos to the fuel people) New filter and she is good. Drove her 10 miles and I think ill take her to work. Wish me luck. Thanks for all the help!

Hurrah for cheapo fuel starvation issue!

$4.74 fix and I got to drive it to work for the first time!