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A local buddy of mine had a similar situation and his theory is crud in the gas tank. The crud gets sucked all to one spot as the car runs and eventually plugs the tube feeding the fuel pump. Shut the car off, vacuum stops, and crud settles back out. Car starts and runs again for a while. Another (simpler) suggestion is the valve on the gas cap isn't working correctly and you're drawing a vacuum on the tank. Maybe pop the cap next time the problem happens and see if there is a big woosh of air into the tank? That feeling of not being able to trust the car really sucks. All you want to do is enjoy it, and you can't stop worrying. Hope you get to the bottom of it!

I'm thinking that if you had more carbs, your engine might run longer? I mean, if you started out with ten or something.....

The clue that it happens on an incline confirms fuel starvation. Along with a fuel pressure test a fuel volume test could isolate the problem to the carbs. Have you checked the last chance fuel filter screens in the banjo bolts on top of the float covers? Post #13 you mention the front carb piston being a little sticky and a minor problem with the rear carb choke. Are those working OK now?

It doesn't hurt to check your fuel pressure. Also, are you sure your float levels are set properly?

Even if it just drops the air temp in the cabin to the mid 80s, that would be a BIG improvement for me.

I did not pour over the 75 wiring diagram, but for other years, a temperature switch controlled things like ignition timing and EGR engagement. I understand that you're not running the original ignition module, but maybe somehow you're getting EGR pumped into the intake manifold when you shouldn't be? Just tossing out ideas.

Among many cool workshop tools, the shop I worked in at the local electric utility has one of those. I used it frequently to tune the distributor for my race Z. If I recall correctly, 34 to 36 degrees was about it. Even running 115 octane leaded racing gasoline. I also used the machine to prove that my younger brother changed the advance curve after I specifically asked him not to. I had set the distributor to around 35 degrees. Put it in the engine, put the engine in the car, tuned it, did a bit of a break in run around the neighborhood. Changed the oil, and got everything ready for the upcoming race meet. The day before we loaded up to hear to the track I caught little brother fiddling with the distributor. Asked what he was up to and he said we could get more power with a bit more advance. I told him I set it on the machine, and asked him to leave it be. At the track the next Saturday I went out for morning practice. Yes, the car was pulling hard. I couldn't hear it detonating over the exhaust. A freshly built engine destroyed in the first 15 minute practice session at Pacific Raceways. Bent rods. Burnt and broken pistons and rings. Scored cylinder walls. Loaded up and headed home. Unloaded, pulled the distributor and went to the shop at work. I found it set to 47 degrees. I confronted little brother about it, he didn't say a word. Got in his car and drove off. Hasn't talked to me since. That was 1991. The boy has never been able to take ownership of his actions. Sorry, I drifted off. But yeah, those distributor machines are awesome.

I bought an inline fuel pressure gauge, but I haven't used it yet. Will get it hooked up. The carbs were rebuilt by ZTherapy about 10 years ago and the car's probably only been driven 20 miles total since then, so I don't have any reason to believe the floats have moved. I'll check though. Yes, the carb pistons both move freely, and the carb chokes are both working fine. I'll check the fuel filter screens in the banjo bolts. Another potential clue is that the problem seems to worsen the less fuel I have in the tank. The gauge shows about 1/3 full at the moment. Got that sinking feeling like I may need to drop the tank and have it reconditioned after all. Maybe it just needs more protein instead...? 😉

I've done a lot of playing around with timing since I have a programmable ecu. You should have most of your advance in by 3000rpm, but sometimes you might have to drag out the last few degrees beyond there. You can run quite a bit of vacuum advance especially when you can control it with this 123 ignition. I run 45 degrees of advance on my stroker in high vacuum/low KPA areas .

WOW, mine made a terrible noise that sounded like the chain tensioner let go! I would hate to be driving down the FWY and throw a rocker. Thanks for the input.

Hello I know it has been a while since my last post, but sometimes other things get in the way of playing with our cars. I did want to close the loop on my post. After many trials and tests we think we have the problem resolved…and IMHO it was not vapor lock, as I originally suspected. Shortly after my last post, I ran the car on a relatively cool day (mid 60s) and sure enough it died after 40 minutes and would not restart for a while. Given the mild temperature, I began to suspect some other cause, as some of you suggested. I drove the car to my mechanic’s garage. The drive is about 20 minutes. I got there without problem and left the car running. Within 15 minutes the car died. At that point we disconnected the fuel line at the carb to check the flow. Upon cranking the engine, no fuel was reaching the carb. We checked and confirmed the fuel line between the pump and carb was clear. So, we installed a new replacement (GMB brand) fuel pump. The car started and ran for 30 minutes or so and died. Again. we disconnected the fuel line at the carb…no gas. At this point we suspected a possible problem between the tank and the pump, so we ran the fuel pump inlet hose directly into a five gallon can of gas. Cranked the engine but no fuel at carb. I bought a different replacement (US Motor Works brand) fuel pump. Installed it…would not work right out of box. Ran the test with fuel pump inlet hose in can of gas. Cranked engine…no fuel at carb. We installed a used Kyosan Denki pump…the car ran flawlessly for three hours. So, I acquired a new old stock (NOS) Nikki pump. The engine has been running fine for hours at a time. Totally baffled…could these modern replacement pumps be that bad? During all these tests where we had the car running for hours, we also discovered a couple intermittent ignition issues…which may, or may not, have contributed to the original problem. Twice while running the car. it suddenly died (no stumbling…just died). A quick check revealed no spart at plugs. The first time this occurred, we had power all the way to distributor. A check of the original cap revealed the center contact was damaged and perhaps causing an intermittent problem. The second time we had no power from the coil. The resistance on the coil tested within specs but since it is a NOS coil, I suspect it might be compromised and failing at operational temperature. All seems to be working fine with new distributor cap and coil. Thanks for listening and all your help. Jim

no worries, ditching the idea of trying it with the stock EFI setup. I I was doing a thought experiment to answer my own question about why its not spooling up and making boost on the test stand. I assume it takes a LOT of power to spool a compressor wheel up enough to create boost at the volume needed by an engine. With that in mind the amount of drag I am seeing is minimal, assuming that is the drag does not go up with RPM. I just wanted to see if it would spool up, I have seen a lot of turbos where the guy just spins it with fingers and round she goes, but the more I read the more I find out about the differences in journals vs ball bearings and even more about the drag of a carbon seal vs what is called a "dynamic" seal. I seems the carbon seal is used for very good sealing on the intake side (I have also read that carbon seals are used on ac compressors, that would need a very good seal indeed to not lose refrigerant). Anyway back to my setup. I do have the stock turbo manifold/harness/ECU and sundry sensors, got them with the engine. So the plan now is to stop futzing around and start by sorting all out to a diagram to make sure I am not missing any parts for the EFI, then I will just build another test stand dedicated to the turbo. If the stock setup eludes me I will consider an aftermarket setup, but the data on those is fast moving, I can see some ads for complete plug and play but those seem like a lot to me (2k+) maybe that's just the way it is but I would rather try the stock setup which I have 1st. I am not in a hurry since my stock NA engine is pretty solid and I have a better NA engine as a spare. Its the one with the MN47 head that has much higher compression. Anyway its the journey that motivates me at this point so no real needs for short cuts like the plug and play at least for now. I would however like to tackle pikes peak again and not run out of power above 12.5k feet....

Here's a good conversion tool for converting mm hg (millimeters of mercury) to pressure. Manifold absolute pressure (MAP) is the pressure inside the manifold. So it is kind of the inverse of vacuum readings. A high pressure number will need low ignition timing advance, whereas a high vacuum number will need a high ignition advance. Anyway, for the purpose of creating a MAP map you can just take the points on the Nissan curves and convert them to pressure. Convert the mm-Hg to whatever unit your 123 is set for. I think that there are two settings, metric and imperial maybe. Not sure. Pick a curve, convert the numbers to make a new table to match what 123 software needs, plug the numbers in. https://www.onlineconversion.com/pressure.htm p.s. I had to wake up some sleeping (deeply) brain cells to get in to this. No guarantees that it's correct but I think it's close. Feel free to correct anything.