We have been working on rebuilding a 1973 240z which we recently purchased and came across another puzzle that I was hoping someone else had experience with. We recently got the car running and took it for a test drive. After driving about five miles we stopped for anoint half an hour and then took off again. Almost immediately the car would buck and shudder any time we tried to accelerate.

For a short time it would idle but then shudder and die under acceleration. We ended up towing it home. My initial thought was that the fuel filter was bad. We added some gas to the tank (just in case) and put some Heet in the tank. We then pulled the fuel filter. It did not look dirty nor could I see a water layer. I cut the old fuel filter open just to be sure and it still appeared clean.

While this was off I blew air through the gas line back to the tank to make sure it was not plugged and we could hear the air blowing through easily. We then installed the new fuel filter and then disconnected the fuel line on the carburetor side of the fuel pump. The fuel pump is a mechanical kind and it appears to be a new/newer pump. We turned the car over and lots of fuel came through. I dont have a pressure tester to do a real pressure check.

We then reconnected everything and fired it up. It started up and ran pretty well. We then tuned the carburetors and found that they were not balanced hardly at all. We used an air flow meter and got them balanced and the idle set and the car was now sounding and idling much better than it had.

So we took another test drive. The initial drive was around the neighborhood, low speeds and gently acceleration. The engine ran smoothly so we took it out to the main road. As soon as we began heavier acceleration the engine shuddered and bucked and promptly died. We let it sit for just a moment and it fired back up but would start to die under heavier load. We were able to drive it home carefully.

We then did some checking with the engine running while parked and I could not get it to stumble or load up and die. I noticed that the fuel filter barely had any fuel in it. We shut the car off and the filter slowly filled up with fuel. We restarted the car and the fuel filter again ran to where there was barely any fuel in it but I still could not get the car to die.

My guess is that either the fuel pump is not pumping at a high enough pressure which starves the engine under load or that something is plugging the fuel line from the tank, again starving the engine for fuel under load. I rechecked the new fuel filter and it is still clean. There is not sludge or grit in it at all.

Any ideas would be appreciated.

Check your fuel vent line. If it is kinked, blocking off air, you could be getting vacuum fuel starvation.

Drain the tank and see what comes out.

Also,don't rule out the distributor as the culprit.It can cause problems under load that seem like fuel.

P.S. Here is no-load fuel flow at 3000rpm and 800rpm:

I drove the car for 600km no problem.

btw that fuel line ran along the outside of the car to the fuel door then down into the fuel tank as the hard lines were plugged.

It had black duct tape over it so it looked like a racing stripe.

Well I started leaning towards there being a vacuum building up in the tank which created the low fuel flow. We removed the gas cap and checked the vacuum valve on it and there was absolutely no movement. It did not even feel like it was made to move. So initially we ran the car without a gas cap and we immediately noticed significantly more fuel staying in the fuel filter. We then took it for a nice long drive under various amounts of load and speeds and had no problems at all. We soaked the filler cap in WD-40 and quickly got the vacuum valve to function properly. We then replaced the cap and did another test drive and still had no problems!

Thanks for all the help. It is amazing how daunting a problem seems at first only to discover that there is a very simple fix.