Varying idle rpm and lack of power at take off

[lpraun]

Hi

I’m a new Datsun 240z enthusiast from Denmark who has imported a 240z from USA. I’m now nearing the end of the restoration of the car and just fired up the engine this weekend. As the headline suggest I encountered some idle problems, but first I will just describe my car.

1973 Datsun model 204z - produced Dec 1972.

Manual 4 forward, 1 reverse gearbox.

As far that I can tell it’ a stock L24 engine with emission control

Round top 3-screw SU carburettors, must at one point have been changed from the original Hitachi flat top carps (?)

I removed the air condition system.

New ignition coil

Compression test shows between 135 and 150 psi.

The car seemed to run fine before I started the restoration and I haven’t done anything to the engine other than messing around with the air/fuel mixture screws as described below.

First I closed the mixture screws (the ones positioned at the bottom of the carburettors) and turned them 1.5 turns out. After starting the engine it backfired out the carburettors. I leaned using this forum that it most likely was due to a lean mixture – I unscrewed the mixture setting screws until the problem stopped.

As the engine warmed up and I released the choke it settled into a idle cycle where the rpm goes down to about 600 and then up to about 1300, then back down to 600. This goes on and on and on – each cycle tacking about 5-10 seconds. The rear carp seems to draw a little more air than the front one. And if I spray a little engine starter on the shaft going through the carps the engine rpm goes up a little indicating an air leak (this happens on both carps individually), but I don’t know if it’s serious enough to cause the engine rpm cycle described.

Now, when I try to drive of I put it in gear release the clutch with a little throttle (as you would normally do) there is a complete lack op power – I have to engage the clutch in order for the engine not to stall. If I rew the engine to about 1500 rpm there is no problem other that the car leaps forward as if I only have had my driving license for a week. If I just rew the engine in neutral it will go higher than I dare.

Is there anyone out there who might know what I can do to solve these problems?

Regards

Lars Peter

[d240zx2]

This sounds like a classic (and serious) vacuum leak. You've identified one source, at the throttle shafts, and there may be others. Have you synch'd the carbs recently?

Z Therapy can be of immense benefit to you at this point.

Frank

[Bruce Palmer]

The JUST SUs DVD will walk you through the trouble shooting..... front to back top to bottom.

[lpraun]

Sorry I haven’t been updating the progress with the car, but I have been working on my master thesis in naval architecture and just handed it in. Therefore there haven’t been much progress!

It seems that after the engine have been warmed up a couple of times, the problem disappeared. Now it idles fine and the power delivery is strong. Some carp-adjustments may still be needed to get better fuel economy!

I will try to set up some before and after pictures elsewhere in this forum – and some pictures from in-between.

Lars Peter

[Weasel73240Z]

Lars,

Even if you think its running well, you should check out the ZTherapy video. It really makes balancing and tuning the SU's easy. Better yet, when the budgets there, get a new set of ZTherapy SU's, you"ll be glad you did every time you turn the key.

[lpraun]

Yeah – a set refurbished SU’s from ZTherapy would be very nice. They are at the top of my “must have list”!