280z stalling issue (I've tried the search feature!)

[HybridzNJ]

 

So I've tried searching several z forums for this exact issue as well as asked a few mechanics I know to help and have had no success solving my problem... 

My car is a 1975 280z, it has a 1972 240z head and a 1982 280zx block. Dual su carburetors and electronic ignition that I beilive was sourced from another 280zx by the previous owner. The car ran fine for the first year I owned it.

My issue is that the car will start and idle with no issues but after driving for half a minute or so the car wil stop accelerating and wont respond to the throttle. It then will bogg down, my tach will drop and the car will stall out. Sometimes I can keep it going other times I can't and sometimes it will turn back over, other times I have to push it home. When it does this there is a distinct change in the sound of the engine where the tone of the car drops to a low droning sound before it cuts out. 

 

So far I've replaced the coil, alternator, starter, ballast resistor, fuel filter, fuel lines, cleaned the tank, rebuilt the cabuerators, changed the cap and rotor as well as the wires plugs and the battery. 

If there was a good z shop or mechanic near me I would happily pay to have it fixed but I've been unable to find a shop anywhere near toms river nj that is willing to take on the car (and if anyone knows of one please let me know)

 

Anything you guys can tell me is greatly appreciated, I really want her back on the road! Also if there's anymore information about the car that could help let me know and I will post it. Thanks in advance!

[Zed Head]

46 minutes ago, HybridzNJ said:

My issue is that the car will start and idle with no issues but after driving for half a minute or so the car wil stop accelerating and wont respond to the throttle. It then will bogg down, my tach will drop and the car will stall out. Sometimes I can keep it going other times I can't and sometimes it will turn back over, other times I have to push it home. When it does this there is a distinct change in the sound of the engine where the tone of the car drops to a low droning sound before it cuts out. 

So far I've replaced the coil, alternator, starter, ballast resistor, fuel filter, fuel lines, cleaned the tank, rebuilt the cabuerators, changed the cap and rotor as well as the wires plugs and the battery. 

Does it do the "start and idle, then bog after a half-minute" thing even when the engine is warm?  Or are you describing a cold engine?  One could be fuel supply, the other could be a warm engine problem.

And did you do all of the replacing after the problem showed up?  Or are you just listing all of the work that you've done and the problem happened  in the middle somewhere?

S/U's are an odd type of carburetor and can have their own odd behaviors.  

[HybridzNJ]

It only boggs when I drive it, it'll idle just fine. It does this wether i let the car warm up or not. And all of that work was done after the problem started to try and resolve it. The SU carbs are tricky and i do plan to replace them but I was told repeatedly that they probably aren't causing this.

[Zed Head]

Who told you?  If they don't understand SU's then they don't know what they're talking about.  Like me.

I do know though that if the pistons don't move, then air won't flow and the engine won't make power.  But it will idle because the pistons are at their starting point (Edit - actually I don't know if this is true).  What kind of oil are you using in the carburetors?

Somebody had a very similar problem recently.

Edited July 22, 2017 by Zed Head

[HybridzNJ]

I bought piston oil of off z therapy, description said it was specifically for the su carbs. Both pistons seem to move freely. And the mechanics I talked to we're not too familiar with SUs so maybe that's part of it. 

[Mark Maras]

 When the problem starts, does the engine rev fine if it's not under a load? As in neutral or clutch in? Does operating the choke (plus or minus) make any difference?

[HybridzNJ]

Yes it does, in neutral it will still respond to the throttle, the choke doesn't seem to have any affect on it.

[HybridzNJ]

The car still won't stay running when I try putting it in neutral though, it will rev up but again it will drop off and then stall.

[Mark Maras]

 I'm thinking that there's a couple of problems. 1st, if the engine won't pull a load but rev's freely when not under a load the problem is usually lack of fuel. There can be many causes. A fuel pressure and volume test is first. That should rule out everything up to the carbs. Check the filters in the banjo fittings above the float lids if you haven't already. Even with a poor fuel flow the engine should idle smoothly. Recheck the float levels, be sure the nozzles slide up and down easily when operating the choke and  run thru the carb balancing-tuning procedure again. What color are the plugs?

[HybridzNJ]

Plugs look normal no scoring, but they are usually wet when this happens.

[Dave WM]

Scoring? not sure what that means. Are they a light tan in color on the insulator? wet is not good, if that is the case I would think there is no combustion happening, if wet from fuel. If wet from oil, then you have a competently different issue, as in rings or valves.

How about installing NEW plugs, drive it for a while, then take them out and post up a pic.

While the plugs are out you could also do a compression check just to know the stats.

 

Edited July 22, 2017 by Dave WM

[Mark Maras]

 Wet plugs, Hmmm. OK, let's go that direction. Have you determined if the plug wetness is oil or gas? Sniff test might do it, a burn test would work too. Have you checked to see if there is a strong spark at the plugs before and during the problem? If all is well with the spark (new plugs is a good idea)  then it's on to too much fuel. SU's will run well with 3 - 3 1/2 psi. Anything over that can over power the needle and seat and overfill the float chambers. Did you re-set the floats when you rebuilt the carbs and what method did you use? That's enough for now. Keep the info coming. We'll get to the bottom of this.