Umm... no one seems to know how to fix this...

[240zDailyDriver]

So, I bought my car about a year and a half ago... a 1972 240z. A month later, the cylinder head went, so I replaced it with a 1975 280z n42 head. I have had some problems here and there with the car but overall it runs fine. There is just one little problem that no one seems to know the answer to... that is, when it reaches ~4500rpms the engine will die and then catch at 4000rpms. If you keep flooring it, it will just repeat this action over and over and over again... never going higher than 4500. Now, if you give it 75% throttle, it will pass the 4500 mark without any problems. I am totally confused... could it be the n42 head? If you need any more information on the car and/or have any ideas please feel free to respond! Thanks! 240zDailyDriver

[JimmyZ]

Sounds like a fuel related issue assuming that it runs smoothly. IF it's not running smoothly at 4Krpm + then check the ignition first. Do a search on this site regarding checking play in your distributor shaft. It's possible that it's causing your woes. Grab a timing light and hook it up to any plug wire and do your little rev experiment while watching the flashes. If the timing light starts flashing erratically above 4.5K then you know it's an ignition problem. Start by checking the condition of the plugs then replace the points and condenser and see if that helps. If replacing the plugs points and condenser doesnt get it then the replace the cap/rotor and finally the coil last. Plug wires should not be touching anything metal. It might also be possible that you have some loose or bad plug wires.

If the timing light's flashing shows no problems then start by changing the fuel filter and checking that the float bowls are clean and the needles are clean and working properly. Also check that the SU pistons are clean and able to move freely. Check the cleanliness/condition of your plugs.

I've had little experience with this but sometimes an engine that's been overheated will have it's pistons swell. In some cases you can't run it for more than a minute or two before it shuts down. I knew one guy who "fixed" his swollen pistons by revving the crap out of it the moment it started.(6K rpm+) Amazing he didnt snap something but it worked.

2c

jim

Edited November 7, 2008 by JimmyZ

[240zDailyDriver]

Thanks! Every little bit of information helps!

[John Coffey]

Check the float level in the carbs and the fuel pump.

[Stickerman]

If you are running SU carbs make sure the needles are not bent.

[blue 72]

I had that exact same problem in my '72, and mine turned out to be the distributor.

[Zs-ondabrain]

I was just thinking and had the same issue years ago..... Turned out to be the Coil. It could build up slowly with a little peddle but if I floored it, it would drop on its face around 4K.

I swapped the Coil and never had the problem again. Try it.

Dave.

[240zDailyDriver]

Hello again! I have some updates on this problem. BTW, thank you guys for your feedback! One has actually led me to a question. Okay, so I took apart the distributor to see if the advance was working right. It was kind of rusty in some places, so I cleaned it up and greased the joints. Now when I floor the car, it just stops at 4500rpms... no fluctuation. When I looked at the advance though, I noticed that one spring had a lot of play... and the other one was really tight. Are they suppose to be like that? I am going to try and find a picture of the advance to show you what I am talking about. If this is a problem, how do I fix it? Thanks again!

Advance.doc

[EScanlon]

I know that's the case with the Sports (aka Roadster). One spring is not only a different length but also a different tension. The concept being that it takes higher rpm's to extend the longer and heavier spring and changes the advance curve accordingly.

Don't know if that is the case for the 240, but it may be so. You might also check the vacuum advance mechanism, as those little ball bearings have a tendency to pop out of the plastic holder.

FWIW

E

[240zDailyDriver]

Thank you EScanlon for your comment! I did find that the springs are the way they are suppose to be. I did check the ball bearings and they are all in tacked. *sigh* It seems the distributor is running correctly.. so probably not the problem. I am stumped. I put on a rebuilt cylinder head not to long after I bought it. It is a N42 from a 1975 280z. I keep wanting to blame it on that. I put it on myself, and I never trust my own work... The car runs fine at those lower rpms... and when you gradually bring up the rpms. Right now it is running with almost no vibration... aka it is running well. But when there is a problem I like to fix it. But I just can't seem to figure this one out. Are the valve clearances the same on an n42 and the e31? I have the intake at .010(.25 mm) inches and the exhaust at .012(.30mm) inches. Those are hot clearances. The question is, what would fail when the "petal is to the metal" and yet work correctly when at 3/4 throttle? I know the vacuum advance works and so does the centrifugal advance... If anyone might have any new ideas please share! I will take any little bit of information to ease my worried/ curious mind.

[Arne]

Is the cam timing perhaps a tooth off?

[240zDailyDriver]

if the timing was off, would it effect it at all rpms? just asking, not sure myself.. what is an easy way to put the #1 cylinder to tdc?