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

[Arne]

Never mind. After re-reading your initial post, I now see that cam timing isn't your problem.

[doradox]

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

When you hit 4500 rpm @ full throttle does the engine shut down like you turned off the key and then return to life again @ 4000 like you turned the key back on?

You can try turning the key off and on at 4000 / 3500 to get a feel.

Does temperature make a difference?

Do you have the stock air cleaner?

Age/condition of ignition components?

Steve

[biddljj]

As I learned from the list... The best imperical way to start with the #1 cylinder at TDC:

1. Remove the Valve Cover

2. Remove Spark plug #1

3. Find a long piece of wire to feel piston travel

4. Rotate the motor with a wrench on the crankshaft

5. When #1 is TDC the Cam lobes for exhaust and intake should be unloaded

6. The distributor rotor should be point at the radiator/left headlight

What color are the plugs ?

Not just a Z - zdisease

[Zs-ondabrain]

One easier way I found is just remove the #1 plug

rotate the motor with your thumb over the plug hole, if you're on the "Compression stroke, you should feel air pushing out past your thumb)

do this till the air stops, then use a flash light to see if the piston is up all the way or slide a screwdriver in the hole and watch it move up and down till it's at it's highest point.

No need to pull the valve cover and replace it. That's just more work, (no offence Biddljj)

Your rotor should be pointing somewhat forward. place the cap on the dizzy to compare where the rotor is pointing and where your #1 plug wire is.

Loosen the dizzy and adjust accordingly. Basically, try to make the rotor point at TDC and move the plate on the dizzy to 0 degree's.

Then adjust the dizzy to 5 degrees??? and put the plug back in and lock the cap back on and start her up.

Once again, Did you try the coil yet??

[jmortensen]

I had a similar problem on my first Z that I bought from the original owner. It was an automatic and I actually changed the breaker plate in the dual point distributor to a single point distributor in an attempt to fix (among other things). Turned out after much frustration to be bad spark plugs. They looked NEW, ran fine with no load, for whatever reason if you got the rpms above 4500, they'd cut out. I don't remember that problem happening in lower gears, just in 3rd, so I think it was a load issue. I chased that problem round and round for months with the help of a Nissan master tech, and it was the stupid plugs...

It definitely sounds secondary ignition to me. Rotor, cap, plug wires, plugs, points and condensor if you're still running them.

[240zDailyDriver]

Once again, Did you try the coil yet??

yeah.. I tried the coil... but no luck, thanks again for the idea!

[240zDailyDriver]

When you hit 4500 rpm @ full throttle does the engine shut down like you turned off the key and then return to life again @ 4000 like you turned the key back on?

You can try turning the key off and on at 4000 / 3500 to get a feel.

Does temperature make a difference?

Do you have the stock air cleaner?

Age/condition of ignition components?

Steve

Hmm, good question. It does feel similar to turning the key at 4500, but it is a rough deceleration until it gets to 4k. I have noticed it gets worse when it is hot. It will sometimes just stop at 4 and just kind of sit there running really rough. I do have a stock air cleaner and the condenser, vacuum advance, cap, rotor, point, spark plugs and now coil, are all new.(new being less than 3 months old) hmm, the only thing I haven't replaced are the spark plug wires... I know they are about three years old... and they aren't touching anything metal (I just recently bought the plastic spark plug holders)... anywho, I hope this helps! thanks! anymore questions, feel free to ask!

[240zDailyDriver]

hey guys really quick... I read up on how to check and see if the timing chain is in the right position a while ago, just to check and see if that was the problem... it said if "the notch was to the left of the oblong groove than the chain is stretched" (on the camshaft sprocket). Well, do they mean left if you were standing in front of the engine or left if you are standing "behind" the engine? (picture they show is confusing... reading chiltons repair manual) anywho, if the chain is stretched, wouldn't I hear it rattle around a bit? My first thought on this problem was that it might be a timing chain problem, like at 4500 it would just stretch a slight amount, throwing off the timing... I don't know, just throwing it out there...

[doradox]

hey guys really quick... I read up on how to check and see if the timing chain is in the right position a while ago, just to check and see if that was the problem... it said if "the notch was to the left of the oblong groove than the chain is stretched" (on the camshaft sprocket). Well, do they mean left if you were standing in front of the engine or left if you are standing "behind" the engine? (picture they show is confusing... reading chiltons repair manual) anywho, if the chain is stretched, wouldn't I hear it rattle around a bit? My first thought on this problem was that it might be a timing chain problem, like at 4500 it would just stretch a slight amount, throwing off the timing... I don't know, just throwing it out there...

Look at the motor from the front.. then think of the cam sprocket as the hand on a clock. (engine rotates CW when viewed from the front) If the mark on the cam hasn't made it to the mark on engine yet the chain is stretched.

A small change in cam timing won't cause your problem.

Steve

[JimmyZ]

Looking into the mechanical aspect ie valve timing issues is barking up the wrong tree. (IMHO) Slop in the distributor may be happening though.

After having carefully read all of the posts it dawns on me that there may be some things you've possibly overlooked or discounted. When you "grease" the points I hope you mean a apply minor amount on the cam or cam wiper and NOT the contact faces.

It looks like you replaced all of these a few posts ago but if you haven't..

Replace these items. Don't even assume the parts are good just try it. It won't be that expensive and it can only help.

Replace the

Plugs

Points

Condenser

Cap

Rotor

Fuel filter

After the parts are installed do a proper tune up with an eye for diagnostics.

Use a dwell meter to fine tune and check your points while it's running @600RPM. You may then find that you have play in your distributor shaft in which case a new/reman distributor is on your list. (Wavering dwell on meter)

Use a timing light to check the timing. Even if the advance doesn't work it should still rev smoothly.

Check the plug wires and make sure that they are NOT laying on the bare valve cover. If they are breaking down with age then they will arc to each other or the valve cover. If you can swing it replace the wires.

Balance the carbs and set the mixture. Compare the clean new plugs to pictures to determine if the mixture is rich/lean or if you have oil fouling. You may want to look at getting a colortune so that you can "see" any changes in fuel mixture when doing your rev experiments. It's great for tuning these cars. Well worth the $80.

I've owned my '71 240 for almost twenty years. It has been my daily driver for most of those years. I've found that problems like these are very easy to diagnose especially when you have stock components.

Jim

Edited December 7, 2008 by JimmyZ

[JimmyZ]

Just a second thought. Years ago my tank was shedding rust into the fuel system. It behaved very much like an ignition issue. Things got so bad I went to a glass filter and had to clean it every mile or two! Using the colortune mentioned above would display a lean mixture at higher RPM if you had a fuel starvation issue.

http://www.eastwoodco.com/shopping/product/detailmain.jsp?itemType=PRODUCT&RS=1&itemID=2299&keyword=colortune

Take the float covers off your carbs and check to see what debris has gotten past and settled into them. It's possible that your fuel rail has some crud in it also. Try reverse blowing the rail for a bit. Verify that it is flowing fuel. Don't open up the restricted opening on the return end of the rail. It's there to help maintain fuel pressure.

It's got to be somethin' simple.

Jim

[beandip]

On SUs there are vary fine screens located in the fuel line where it connects to the carb at the float. The fitting has been called a ''banjo'' fitting because of it's shape. Check and see if it is clogged. I had a car that acted like your describing and it wasn't RPM related as much as load. On the flat the car was fine but pulling any grade it would start missing and loosing power. The fuel pump was failing and could not keep up with the demand. Also check the fuel level in the float chambers, if the setting is off it can cause what you are experiencing . Gary