Idle problem with heat

[240ZMan]

I have a strange problem with my 73 240z that has got me stumped. My setup is as follows:

- Original 240 bottom end

- E31 head - stock

- 72 3 screw SU carbs

- early 70's aircleaner

- Pertronix electronic ignition with coil

- stock exhaust

- all smog removed, except EGR which I've put a gasket under to make sure it stays closed

Whenever the engine compartment gets hot, the idle begins to slow down and the engine stumbles. This will happen almost everytime I park and let the car sit (engine off) with the hood closed for about 5 or more minutes. Even now when the highs for the day are only about 50. Yet a few weeks ago when we had highs in the 30's the problem never showed up. Sometimes just letting the car sit idling will build up enough heat to slow the idle down. Pulling the choke back a bit will bring the idle back up, but not as smooth. Giving a little throttle will do the same thing. Driving around for a few moments on a cool day makes the problem go away.

I put a fuel pressure gauge in and the pressure stays between 2.5 and 4 psi always. I don't see any difference when the problem occurs.

One thing I can't tell yet is why the idle is slowing. By that I mean that I suspect the mixture is going rich because I can tell the smoothness of the idle is not right. Yet when the engine compartment is not hot, say when the hood is open for a few moments, the idle is perfect, nice and steady and smooth so I think the tuneup is ok. I've checked the timing and dwell while the problem is present and both are spot on.

I've rebuild both carbs completely following the ZTherapy tape. Both have new needle/seats in the float chambers.

One thing that I can't tell if it is normal or not is that sometimes when I open the fuel cap there will be a small amount of pressure escaping. I would have expected the tank to build a small vacuum, so this has me wondering if it's part of the problem. But when I open the cap it makes no difference on the idling.

I tried taking the hose off the 3 way purge valve and can blow back into it. Leaving it off doesn't make a difference so I figure the valve is ok.

At this point I am out of ideas. I still believe that the problem is with something in the engine compartment that changes the fuel delivery when too hot.

Any ideas? Thanks,

Daniel

[2ManyZs]

Are you running the stock 73 intake manifold with the water line still connected? It could be the water control valve at the rear of the manifold that is causing the problem, overheating the manifold.

[240ZMan]

The intake manifold has an "N33" cast on it and I believe is stock. The water lines are hooked up but I can't find the valve you mentioned. The water line at the rear of the manifold goes through a rubber hose then to a metal assembly. The other end goes behind the engine around to the passenger side and connects to the coolant hose there. If there is a valve in it it must be inside the block of metal in the middle as I can't see any signs of anything that moves from the outside. Also, the fitting off the bottom of the assembly was cut and crimped/welded (poor man's cap?). I can take a picture and post if that would help.

[2ManyZs]

The 2 inch or so long metal cylinder that is on the back of the manifold that the rubber hose connects to from the hard line around the back of the head is the water control valve.

It's actually a small thermostat that is supposed to cut off the water supply when the water temp reaches around 80 degrees.

If you can block off the water supply at the Y junction on the passengers side of the engine, you can find out for sure if that is the problem.

You can still get the water control valves last time I knew. Motorsport has them listed in their catalog, and Chloe at MidwestZ can probably source you one as well. Or, you can just block off the water supply which will make it a bit colder blooded on those cool mornings, but you will never have to worry about it vapor locking.

[240ZMan]

So if I understand your thinking on this, if the thermostat isn't cutting off at 80 degrees, I may be getting too much heat in the manifold/carbs and seeing some of the symptoms of vapor lock?

I'll try plugging it this weekend. I'm not too worried about losing the heat in the manifold during warm up as drivability has been excellent, even within the first minute or two when it's 30 outside.

Thanks,

Daniel

[240ZMan]

I tried 2manyZs suggestion tonight and blocked off the water intake to the back of the manifold. It did help a bit, but not all the way. But I think I figured out the real issue: my throttle bushings on the rear carb are leaking like a sponge.

I had tested them earlier after rebuilding them by spraying carb cleaner, and the front had slowed the idle a little, but it wasn't too bad. The rear had been fine. But I had done this with the hood up and not much latent heat in the engine compartment. Tonight I let it idle in the garage with the hood closed for 10 minutes and then sprayed carb cleaner. The rear almost killed the engine!

I put some grease on the ends of the shafts for tonight, but I suspect I'll be calling ZTherapy tomorrow. $$$ Ouch.

[Sean240Z]

I know this is an old thread, but I had a similar problem with my 1973 240Z which had the same set-up as yours. My problem ended up being fuel vapourization because my exhaust manifold was leeking at the #1 and 6 cylinder (both manifold bolts had sheared off in the head) and the underhood temperature got very high. It was my fault, fiddling with SU mixture, ran way too rich for a while and warped my exhaust manifold.