Montanazx

I recently rebuild the engine in my 1982zx non-turbo. I'm having a problem I can't seem to trace. The engine start fine and runs fine when cold but after warming up it has a very noticeable stumble exactly between 1800 rpm and about 2100 rpm. Doesn't matter weather I have it in neutral and rev it to 1800 rpm or drive it under load, it consistently has problems in the 1800-2100 rpm range. Anybody have any ideas what would cause this?

NismoPick

Welcome to ZDriver!

Did you replace the spark plugs? Have you read the tips / checked the gap?
What's the timing at?
Have you checked the AFM and CHTS & connectors?

Zdon

Maybe bad compression rings after warm up.

pnoyslimm

It could be going in "limp" mode, did you replace your knock sensor? When i got my car the knock sensor was disconnected (i didnt know it at the time) and it would run the same way like yours....or your distributor may not be timed right?

NismoPick

On a fresh rebuild?

FYI... the non-turbo doesn't have a knock sensor.
Yes... check timing.

Montanazx

Thanks for the suggestions. I've been at this for some time so I do have new plugs, distributor (vacuum advance was bad and electric advance didn't seem to work right on the old one) and new air regulator (wasn't closing). I've taken the AFM meter off twice to made sure it worked smoothly (after sitting for over a year it had some corrosion on the walls that interfered with the flap). Each of these repair helped but there has been so many gremlins I swear they are breeding.

Timing issues seem to make sense. The timing is set at at 8 btdc. Hooking up the timing light and leaving the vacuum and elect adv connected it appears to me to do some strange things. If i open the throttle a small amount the timing drop back 3 or 4 degrees and you can feel it lose power. If I open it more it advances as you would expect. Does this seem right.

I plan to warm it up tomorrow and test the timing again with the light.

Montanazx

Just to clarify the timing is set at 8 d advance with both the vacuum and elec adv disconnected. Hooking them back up it reads about 15-16 degrees when the engine is idling and running cold. If you open the throttle a small amount it drop back to 4 degrees and then as you open it more it advances up to 25 or so as I would expect. I noticed driving it that there is a lack of power when I first opening the throttle but once you get above 2000 rpm it kicks in and pulls strong all the way to 5000 rpm. Since its still being broke in I haven't run it much over 5,000 rpm. I've checked for vacuum leaks and everything looked good and my fuel pressures checks out good.

pnoyslimm

yeah, scratch what i said. i got confused looking at the FSM. EFI and ECCS little different. so what i really meant was CHTS....theirs not much power past 5000, just wasting gas.

Montanazx

Thanks. I'm now looking at the CHTS as the possible problem. Appears this may be a more difficult part to find. Is it best to get an OEM sensor?

Montanazx

Checked the CHTS in a pan of water on the stove with a temperature range from 68* to 170*. Each reading was well within specs. I also tried another CHTS and still had the same problem. Eliminated this as a possible problem so I'm looking at the distributor more closely. Its a new rebuild distributor but the way my lucks been running it could still be the problem. Its my understanding that the distributor is not controlled by the ECU. Is that correct? May be time to have it hooked up to a scope.

Evaluating the problem more closely I've noted that when the throttle is first applied it seems to loss power momentarily and then pick back up. Checking the timing the distributor appears to retard the timing significantly when the throttle is initially applied and then advances normally. Right at 1800 rpm there's a sharp change as the engine really struggles until it gets past 2000 rpm and then clears up an revs up fine. Any other thoughts?