Fuel or Ignition? heat related problem

[ta240]

As soon as the temperature gets into the mid 70s my Z will start acting up. 

If I sit at a light for a couple minutes or park for 5-10 minutes it will start and idle just fine but as soon as I try to go it will cut out and buck and jerk along.  Once I get moving at 30 mph or so for around 10 seconds it usually smooths back out.

I have replaced the mechanical fuel pump with an electric one, added a fan shroud to the radiator.  The temperature gauge sits right at the middle, except when going 70+ on the freeway when it moves just a bit to the right of middle.  When it is hot and off if I spin the fan by hand it will go about a quarter turn (about the same as it does cold).  It has Petronix ignition with MSD 6A (mounted inside the cabin) and blaster ss coil.

It just baffles me that it starts and idles fine and only under any power does it have the problem.   I realize it uses more gas at that point but if it was

After it did it yesterday at the next light I kept the revs around 1500 and it didn't do it when I pulled away from that light.  But since it doesn't do it every time I idle for a bit it may have just been coincidence as it also does it worse if I have been driving at highway speeds for awhile right before I stop and the first light was closer to that.  It runs fine on the highway for the hour long commute.

When it is hotter in the summer even stopping for gas at lunch after driving for 10 minutes or so it will often do the same thing.

would an ignition issue behave like that or would it be strictly a fuel problem?

Edited February 10, 2016 by ta240

[Mark Maras]

 This sounds like a temporary fuel starvation problem usually caused by heat. Many Zs, of all years, have had this problem. I would try to duplicate the conditions that cause it with one exception. Leave the hood cracked open allowing heat to escape. The results should tell you if excess heat is the culprit. Another easy thing to try anytime you suspect a lean mixture is pull the choke on a bit and see if it runs better. The choke trick only works if there is a sufficient fuel supply in the floats bowls.  

[ta240]

I'll give that a shot.   I keep hoping it will happen some time when I've got a good area to pull over so I can see how hot the fuel lines and fuel bowls feel.  From when I've felt around on different parts when it was hot the fuel rail feels hotter than the carbs.  The gas return line is working, when I first turn the ignition on I can hear it pouring back into the tank, but the rails get uncomfortably hot. 

So it does sound fuel related rather than the heat affecting an ignition part?

Edited February 10, 2016 by ta240

[Mark Maras]

 That depends on the year. You don't mention the year of your Z. The first few years had little to no electronics. Little to go wrong with points in the dist. The later Zs have had problems as you describe. I believe it's the ECU that's the usual culprit. (I'm not very knowledgeable about engine management systems.) RedBird is about to replace hers to see if she can eliminate her intermittent problems. 

[ta240]

woops, it is a '73 but I've changed out the ignition to Petronix with MSD 6A (mounted inside the cabin) and blaster ss coil.   It also has '72 carbs on it.

Edited February 10, 2016 by ta240

[Mark Maras]

Fuel percolation and vapor lock were discussed a few months ago at length. There wasn't a consensus of opinion on either the cause or fix but unwrapped headers seemed to aggravate the problem. Summer and winter gas formulations seemed one of the more likely possibilities. Heated carbs were also not ruled out.

 As I recall some of the fixes were; The factory heat shield below the carbs seems to be a necessity. Wrapping headers, Some have rerouted the fuel lines away from the valve cover. Some have added an elec. fan from the later Zs. some have installed a vented 280 hood. The list goes on. 

[Jeff G 78]

I was part of the previous discussion.  My racecar was a nightmare.  It would run great for 30 minutes and then start acting up.  At first, it would break up at high RPM and then after a bit more time, the engine would misfire at lower and lower RPM until it wouldn't pull at all on the straights.  We changed everything including fuel pump(s), ignition, exhaust, heat shielding, carbs, etc.  The only thing that worked was ditching the steel fuel rail and replacing it with rubber hose.  The heat was transferring from the head into the fuel via the rail mounts.  Once we made the switch, we have never had a minute of trouble with vapor lock.

[Chickenman]

The heat transferring via the rail mounts is a very good observation. Would explain why wrapping the rails in insulation does not help all that much and why on FI cars, switching to an Aluminium rail usually eliminates vapor lock. The aluminum rail will dissipate heat a lot faster.

I always buy lots of ice cold bottled water when travelling in the summer. Stop for fuel.  One bottle of water for me... two to pour over the fuel rail  LOL.

 

 

[Chickenman]

I seem to remember that some of the 240Z models had plastic insulators where the fuel rail attached to the mount. Like a plastic sleeve. Can someone have a close look?

We always ran our 240Z's with the back of the hood open about 1/2" to 3/4" when racing. Seemed to help.

Edited February 11, 2016 by Chickenman

[Jeff G 78]

29 minutes ago, Chickenman said:

We always ran our 240Z's with the back of the hood open about 1/2" to 3/4" when racing. Seemed to help.

Yep, that's what we do.

 

[ta240]

The fuel rail does have little plastic pieces at the top between the mount and the fuel line but the metal from the mount wraps all the way around the line so I think a lot of heat still makes it through there.   Whenever I've checked when hot the fuel rail has been one of the hottest parts in the system.  Ironically, my adding heat wrap to it at one point probably helped keep more of the heat in than out.

Yesterday I connected the mechanical fuel pump back in along with the electric one and it helped some so the heat in the fuel system is definitely the problem.  I'm going to go with Jeff's idea and run rubber fuel line and see how that does.
I had thought about trying to cut the mounts and add something in-between that wouldn't pass as much heat but going with rubber lines sounds easier.

thanks for all the input!

[siteunseen]

I've never thought about what Jeff's saying but it sounds good.  I made an aluminum fuel rail for my EFI 280 and it is held in place only by the 6 injectors rubber hoses.  I've never had a problem with vapor lock.

Now I will try and figure a way to get rubber heater hose in the place of the plastic on my 240, just in case.  But some nice stainless braided hoses would look good too.