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Idles with a miss & stutters between 2k-2.5k..

312 Views 1 Reply 1 Participant Last post by  James 72 240Z
...and not as much power as I'd expect after that.

New or rebuilt everything, and I mean everything, in the engine bay. Been tuning and adjusting for a week now and while things are improving I'm still not there yet. I'm kinda confused at this point so thought I'd share my progress to date in case there is anything I've missed...

Freshly rebuilt forged L24 motor with 232 dur. 450 lift cam and 280 valves, rebuilt 72 3 screw SUs, rebuilt 79 ZX distributor, MSD coil. Nitrous plumbed in but inactive.

First I determined TDC. Rotor pointing at 1, #1 intake valve pointing straight up, first timing notch is right on the mark. Made sure it was on compression stroke by taping a piece of thin plastic over #1 spark plug hole and watching it puff up as #1 intake valve comes around.

Set initial timing to 7-8 degrees. As I only have the Haynes manual (waiting for a FSM to appear on Ebay...) it is not exactly clear what each timing mark is on the crank. I am assuming 5 degrees.

Set carb floats to 9/16" right on the money. Balanced carbs, adjusted according to ZTherapy video. Checked if either carb is running lean using Norm's technique. Did the high speed balance. Pistons, choke action all perfect.

Adjusted valves cold (.008 int and .010 exh) then again hot (.010 int and .012 exh). Doing it hot I can get it done in about 4-6 minutes. I keep the garage door closed and the hood down to retain as much engine temperature as possible.

Went around the intake and exhaust and retorqued every thing that could be causing a vacuum leak.

For the last two nights I have reiterated the above in the following manner: adjust valves cold, start car and set initial timing, adjust valves warm, retime and then adjust carbs.

The best lowest idle I can get is around 850-900 with a definite periodic miss. What I mean by that is that the idle will be really nice for like 2-3 secs then the miss will kick in for like a second then it will go back to the nice idle again. No backfiring, just a miss. Valves are quiet until the miss happens then they click a bit.

With no load it revs nicely but there is a definite, although muted exhaust backfire and slight stuttering through 2k-2.5k. Above 2.5k it jumps to life and races on upwards. With a load, it stutters hard through 2k-2.5k, can't floor it, but if I feather the throttle from there I can then go full throttle above 2.5k. Doesn't seem to have the get up and go that I am expecting though. Spark plugs look good (brownish) , although every once in awhile, one of them (random) will appear to be rich.

Oil is still golden.

My vacuum advance appears to be working. If I drive the car without the vacuum advance it's sluggish in low rpms. I'm not sure, however, how much mechanical advance I should have and at what rpm should it all kick in. I know that my total advance should be less than 35-38 degrees when it's all in, wherever that point is.

I'm going to pull one plug wire at a time while it's running and see how the engine behaves. Other than that, and checking for vacuum leaks, I'm not sure what to do at this point. Any ideas anyone? (and sorry for the long post!)
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Think I figured something out

Lost spark today. Coil and module are good. Am suspecting distributor internals now...magnet possibly? ('cause the air gap is to spec)

It's supposed to be a rebuilt distributor but looks shady inside. Wires to the module are old and dirty, the rubber boot surrounding the module fell apart in my hands, and the screws holding down the stator, magnet and reluctor are chewed up.

I guess I should test everything but I'm tempted just to send it back to MSA.
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