Almost Done (need advice)

[NC280z]

Good news everyone!

 

     Swapping out the Schneider valvetrain for a stock setup brought my engine's vacuum at idle up to a rock-steady 16". It seems to run much cooler, and sounds better as well but I've still got a very noticeable stumble/hesitation just off idle (1000/1200rpms to around 1500rpm), if I rev the engine past that point the hesitation goes away. I've set the timing to 7' BTDC, idle rpm is at 800 and I've set the valve lash per the FSM to .008" and .010". When the engine stumbles just off idle it acts like it wants to cut out. I've  adjusted the throttle position switch so that it breaks continuity just off idle, per the FSM so I'm lost as to what the issue is.

 

What could be causing this stumble? I'm willing to bet that the other 4" of vacuum I'm short of is due to the fact that the engine still hasn't had any street miles put on it since rebuilding it, but perhaps the vacuum I'm missing and the stumble are results of the same issue? I've read on other forums that adjusting the AFM for a richer mixture helped smooth out the stumble other people had, and brought their vacuum up from around 15" up to 20". The AFM I've got in the car is a brand-new unit from Black Dragon, so I'm hesitant to mess with any adjustments on it. The dizzy and the TPS are new as well.

 

As always, thank you for any help you guys can provide.

 

**Side Note: could a messed up BCDD cause any of the above issues? 

 

 

[NC280z]

****Update*****

 

So I pulled the plugs today, and found them to be sooty-black, which means the engine is running rich. I still have good vacuum at idle and no starting issue whatsoever, but now the engine wont rev past 23-2500rpms. If I try to rev the engine past that point it dies off, and if I hold the throttle open at 2000rpms it misses a good bit. Messed with the TPS, and driving it around the block showed that it's real boggy in the lower rpms.

 

Any thoughts?

[EastBay521]

ignition timing? cam timing? my L18 would fall on its face at high rpm if ign timing is too advanced

[DrRocks0]

Cam timing, 16 inches on a new motor is really low.  Vacuum on my built motor started out right at 20 after a few minutes of running. I'd be willing to bet its a link off retarded

[620Turbo4X4]

Black sooty plugs could be because you haven't put much of a load on the engine yet? Idling and revving up in your shop will not produce enough heat for them to stay clean. Try replacing or cleaning with a propane torch and see if that helps the misfire.

 

What condition is your harmonic balancer in? Curious if it might have slipped on the rubber dampener, and is throwing your timing marks off.

 

Only other things I can think of is fuel pressure regulator not working correctly, cold start injector stuck open, or exhaust plugged or restricted.

[Sealik]

I believe his cam/head timing is correct....as 'stated' in his other thread.

If the rings haven't seated yet, vacuum numbers could be a little low....but...that would not explain the stumbling/cutting out.

[NC280z]

Wow, thanks for all the replies!

 

I think I found at least one probable cause. I had Motorsport Auto send my AFM off to be rebuilt, and the numbnuts that rebuilt it did an extremely poor job with the pin soldering (Fuel Injection Corp). Pin 8 was pressed hard against the carbon trace pad that connects to pin 9, causing a short between pins 8 and 9. FSM states that these pins are supposed to have 100 Ohms resistance between each other, I had zero up until I fixed pin 8's solder joint. AFM now tests out per the fuel injection bible. I don't know exactly what these pins are used for in the ECU's measurements, but I'm certain this was an issue. I'm going to contact Fuel Injection Corp. and MSA and let them know that they should consider reviewing their quality control procedures, as this was very poorly done.

 

As far as cam timing goes, I'm certain that it is where it needs to be, since after rotating it by hand I can still get the bright link on the #1 cam tooth with the damper at 0' TDC, with the V notch just right of the cam tower notch. Hopefully tomorrow will show that the AFM was the root cause of my current issue.

 

**Update**

 

According to AtlanticZ.ca, the 100 Ohm resistor is used by the ECU as a reference to monitor changes in resistance of the primary carbon trace pad due to environmental temperatures affecting the AFM. Since the ECU was seeing 0 Ohms it's easy to see why it was acting the way it was.

[NC280z]

So it was in fact the AFM that was causing all of my current running issues. I corrected the pin inside the AFM, fired it up and took it for a spin. Runs outstanding!! However, now the drivers-side rear wheel has a chat-chat-chat noise that increases with speed. I put the rear of the car up on jack stands and found that the drivers-side rear wheel is hesitant to spin, especially when running in first gear.

 

Differential issue, maybe? The wheel also spins at a much slower rate than the passenger-side when in gear.

[dgi]

Have you checked the brake adjustment?  Brake dragging?  Emergency brake releasing properly?

 

Doesn't sound like a differential issue to me.

[NC280z]

Looks like the brakes are the issue, dgi. The rear driver's side caliper has two different style guide pins! one is a bit longer than the other, and sits taller in the caliper. I checked the passenger side to make sure I wasn't seeing things, and yep two different guide pins. I ordered a replacement set through Autozone, should get them put in next weekend. Thanks for the advice!

[dgi]

???  1975 280Z has rear drum brakes standard.  

 

You wrote "caliper" which makes me think you are talking about disk brakes. 

 

If your car has rear disk brakes, something has been changed.  If so, what are they from so you can get correct parts?

 

Check your PMs.

[NC280z]

I converted my Z to the Maxima/280ZX rear discs earlier in the year, I hadn't had a chance to see how they worked until recently when I got the engine running right. The calipers are from the '85 model Maxima, and I was able to get new pins for $10 from Autozone. Since the driver's side is the only side making noise (and the only side with different guide pins) it leads me to believe that the pins are the issue.