no power over 4000 rpm

[bnxbombers]

Hi all - new member here. Need help with a long time issue. I have owned my '73 for 10 years and have had a perceived problem since I have had it. The car has SU's, a 6 into 2 header, 2.5" MSA exhaust. All smog equipment removed and air injector holes capped. It starts and runs great - sounds fantastic. But - has a rough idle and doesn't want to run over 4500 rpm - at 4000 it flattens out and there seems to be no additional pull over that. No sputtering or stalling, just loud and rough with no additional power. Almost like the tach was wrong - like 4000 is really 6000. But with that big exhaust it should pull much stronger. I also have a '70 bone stock, and although I have not run it to over 50 (it needs restoration) it seems much smoother and stronger.

The carbs were tuned by a reputable mechanic in NJ and the timing set and points/plugs/filters replaced. Voltage is good at over 14.5. Prior to that the car ran very rich and dirty. It seems to be much cleaner.

Here's the kicker - along with the car came a box of parts - a cylinder head was included. I suspect the head/cam were replaced - but with what? How do I figure this out and who in OC/LA can work on it? Thanks and sorry for the long story.

[Curtis240Z]

Sounds like you are close to Motorsport Auto, the biggest Z retailer around. I'm sure they would have recommendations. Good luck.

[Zs-ondabrain]

Most heads have the model no printed on the right, front side near the fuel pump area.

Look for something like E-31 or E-88 or P-90 or N-42 or something like that. it should also be in the same location on both heads (in car and in the box)

The ( E ) indicates a head from a 240z. you'll find that a lot of 240Z items have an (E) in the part number.

And I think (don't quote me on it) than an ( N ) indicates a 280Z type head

hope that helps a little and I'm sure givin enough time, some one will correct me and fill in the blanks.

Dave.

[=Enigma=]

I had a similar problem and it turned out to be bad plug wires. Once the new wires were on, it was good to go, revving all the way to 6000 and a lot smoother. If you haven't already, check and/or replace your dizzy cap & rotor as well.

You'll find the head type on the right side of the head between the #1 and #2 spark plugs, maybe #2 and #3. If it's stock, it will be an E88. Apparently there was more than one version of the E88. Go to the link below to learn more about the various heads, although I'm not sure it details the difference of the various E88's. In a nutshell the differences are due to increasing smog restrictions and include larger combustion chambers and lower compression.

http://www.geocities.com/zgarage2001/

Cheers,

[Zs-ondabrain]

See there, what did I say? what was that, 8 minutes? Thanks Enigma!

[e_racer1999]

i think the prob is mos def ignition. had a similar prob, replaced the dist. with a ZX one and all better

[bnxbombers]

Most heads have the model no printed on the right, front side near the fuel pump area.

Look for something like E-31 or E-88 or P-90 or N-42 or something like that. it should also be in the same location on both heads (in car and in the box)

The ( E ) indicates a head from a 240z. you'll find that a lot of 240Z items have an (E) in the part number.

And I think (don't quote me on it) than an ( N ) indicates a 280Z type head

hope that helps a little and I'm sure givin enough time, some one will correct me and fill in the blanks.

Dave.

e88 - which I believe is stock

[bnxbombers]

I had a similar problem and it turned out to be bad plug wires. Once the new wires were on, it was good to go, revving all the way to 6000 and a lot smoother. If you haven't already, check and/or replace your dizzy cap & rotor as well.

You'll find the head type on the right side of the head between the #1 and #2 spark plugs, maybe #2 and #3. If it's stock, it will be an E88. Apparently there was more than one version of the E88. Go to the link below to learn more about the various heads, although I'm not sure it details the difference of the various E88's. In a nutshell the differences are due to increasing smog restrictions and include larger combustion chambers and lower compression.

http://www.geocities.com/zgarage2001/

Cheers,

Plugs wires are blue NGK - replaced them recently - no change

[Arne]

No, I'm thinking it's a fuel issue. Rough idle makes me think of carb synchronization, worn throttle shafts, or (probably more likely) a combination of both.

In fact, now that I think about it, I wonder if perhaps only one of the two carbs is really performing properly. If you are running mostly off the front carb for example, the rear cylinders will fire weak at idle, since they are only getting their mixture through the balance tube. Again at higher revs, the single carb doing the bulk of the work can't supply enough for all six cylinders, so it runs out of power up top. Something to consider?

[kenz240z]

Check the fuel filters. There is one in the electric pump back by the tank that can get clogged with sediment from the tank.

A clogged fuel filter is a potential cause of the problems you are having, especially no power at higher revs/speed.

As Arne mentioned, it could also be due to the carbs being out of balance, but if the carbs were tuned recently, then hopefully they are ok.

[Arne]

Ooh, good thought about the fuel filters. The fuel pump itself could be falling behind as well. That could definitely cause the higher RPM issue. And the rough idle could be worn throttle shafts, which can cause that no matter how well they are adjusted otherwise.

[mlc240z]

try packing some heavy grease onto the ends of the throttle shafts and see if the idle improves. this will somewhat slow/stop the air leakage and help in diagnosis.