72 240z Running poorly when hot, great when cool.

[mec011]

So I'm having some issues with my 1972 240z. When I bought the car, it was running but really down on power. I went ahead and did the basic maintenance work on it, I installed a Crane XR700 optical points system, a Crane coil, new plugs, spark plug wires, rotor, and distributor cap. Checked the timing, and I have good compression, 145-155 across the board.

Initially it still felt down on power after all of that, better, but not great, then I found some bad connections at the ballast thanks to jiggling the right wire at the wrong time. Cleaned up the electrical connections around the ballast and coil, and I had power!

The car then would run great if you started it cold, and drove it from that point. If I stopped, like to go into a store, 5-10 minutes, it would have trouble starting, but would with a bit of gas as it started to catch. However at that point, the car would not run right. It still had power at low RPMs but it stumbled after about 2500 rpm when you gave it any throttle. It seemed to smooth out some with the choke on, so that led me to believe that maybe it was having some sort of fuel issue. Interestingly, it seemed to still run fine if it got cold again, or if it sat for an hour or so before being restarted.

At this point I may have messed up, but I watched the Ztherapy SU video, and went ahead and cleaned the domes and piston, used ATF fluid for the pistons, reset the mixture and needles as was described, started the mixture at 2.5 turns down, and then attempted to tune the car. I let it get warm, temp gauge up to operating temp, and adjusted the mixture, 2 turns was the right spot it seemed (I'm running an SM needle that the previous owner said runs rich). I had just a slight flow imbalance that I fixed with a tiny turn of the rear carb idle screw. The car was running good at around 1000 RPM Idle. I went to do the High Speed balance adjustment, and the car started to get worse and worse. I didn't get the balance done because I couldn't get my flow meter to read at the high RPM, but when I backed it off of 3000 RPM it was running terrible again, worse than ever before really. It was sputtering badly, and seemed to be running very lean, (Hand over the carb opening would make it run smooth briefly) and I'm almost thinking I heard a vacuum leak, but it could just be the carbs sucking a ton of air. All my vacuum tubes look good, and I couldn't pinpoint the sound, but it was not there when I started tuning it, or before that I remember, which makes me think it may just be the poor running condition.

One thing that seems odd to me, is that the fuel filter on the fender, seems not to stay full. While running it seems about a quarter full at best. The other day it was full up for a while. Based on that, maybe I'm not getting fuel, but I'm not sure how the filter is supposed to look.

I'm a bit out of ideas at this point, any help would be greatly appreciated!

Also, the mods to this car are the Crane XR700 which I put in just recently (I never had the car running right before that, I put it in as part of the tune up) and it has a Stage 1 Crane Cam per the previous owner, if that makes a lot of tuning difference.

Edited June 8, 2010 by mec011

[Gary in NJ]

Could be poor flow or pressure from the fuel pump. Easy to check with a flow gauge. I also would take the time to make sure the return line to the tank isn't clogged. That too would cause poor running only when warm (vapor lock).

[Zs-ondabrain]

Is the Heat sheild on and doing it's job?

Are your chokes slightly on even after dissengaging the choke handle?

Are your fuel lines insulated to keep the heat off of them?

[mec011]

The heat shield is on for sure, and I imagine doing it's job because of that?

The thing about the choke, the light will not go off, but I've checked that it's got full range of motion on the carbs, it just seems that there is always a bit of tension on the handle, but it's not moving the actual linkage, I've gone as far as to remove and reattach the choke cables to kill the light, but no luck on that end so I'm not really sure what to do about that. But I'm sure it's not keeping it choked, I checked the operation while I had the domes and pistons off, it returns to normal just right.

The fuel lines are the standard metal ones, they run along the valve cover, with a rubber tubes to each carb, then back along the cover to rubber lines between the fuel pump and fender then metal tubes along the passenger frame rail. Where would you suggest insulating them (along the valve cover I'm guessing?) and what would you use to do so?

I'll check the return fuel flow. Is there any sort of filter or servicing that the mechanical fuel pump needs?

Thanks for the advice!

[mec011]

One thing I thought of, what should I torque the PCV to? It's in good, but not really torqued down hard, maybe 10 ft/lbs?

[mec011]

Well, I don't have a fuel flow meter, the auto parts store said they didn't have one either, can you point me towards what I'm needing on the net or something?

Also, no sort of insulating tape either so I'll have to look another place.

I do have a engine vac/fuel pump pressure gauge. I used a T fitting and put it in the fuel line in several spots.

Between the filter and the pump in 5/16 line, it seems to show no positive or negative reading, but the needle jumps and pulses around.

At the rear carb inlet in 1/4 line, it's showing a solid 3 PSI.

At the end of the 3/16 return line next to the pump it's showing .5-.9 PSI with a bit of pulse

Don't know how these numbers sound to you guys?

But again, the car held a nice tune until it started getting really warm, then it seemed to be running very lean, even with 3 turns of the idle mix nob, based on the fact that the flow gauge or my hand would make it quickly run right as I covered the mouths of the carbs. I would also get some back firing as I revved the motor up. If the fuel pressure sounds good, maybe it's a float issue? Some choke still seems to be smoothing the car out a bit, which still makes me think it's very lean.

I'm a bit befuddle! It seems really odd that it will run good for 5 minutes (after warming up with choke for 5 or so) then start running like crap.

Thanks again for any help!

[Gary in NJ]

, you used the right gauge - the fuel pressure gauge. Did the fuel pressure change as the engine warmed up? Have you checked for vacuum leaks by spraying carb cleaner around the base of the carbs? The reason I ask was you noted that the engine ran better when you restricted the airflow at the carb inlet.