I'm stumped.  I own a '72 240Z, L24 series engine with twin SU's.  When at idle, either when the engine is hot or cold, the idle will drop in RPM with a loss of engine power.  If I rev the engine past 5000 RPM, this might or might not clear the problem (sometimes with a backfire).  The same thing can happen while on the road.  The 240Z runs like a hot rod, then at any given moment the RPM drops along with power.  With the clutch in, a rev to above 5000 RPM might or might not clear out the problem.

 

Things that I have done recently to remedy the issue, but with no success:

* Service the fuel tank with a new liner.

* Frequently change out the fuel filter.

* Replaced the vacuum lines.

* Check SU's damper oil.

* Installed new fuel pump.

 

With an backfire when rev'ing the engine, I am now suspecting an ignition issue.  Could there be an issue with my distributor.

 

Again, when there is no drop in RPM and power loss, the car runs like a rocket sled.  Any suggestion you might have will be helpful.

 

Thanks, Bryan

 

Does the car still have the stock points distributor or is it upgraded to electronic ignition?

Maybe if electronic, an intermittent or failing module.

If so, does the problem mainly appear when motor is warmed up?

No, the Z still has the stock points distributor.  The problem can happen when the car is either hot or cold.  I was out and about in the Z today and it didn't miss a lick, so the problem is intermittent. 

Check your HT leads (spark plug wires). 

 

If you pull off one lead at a time while the engine is at idle and just hold it near the spark plug you will hear if the RPM will drop or stay the same. If you get a jolt when doing this then your HT lead has a break in the shielding and is intermittently grounding to the engine block causing your engine stumble.

Is the fuel pump mechanical or electric?  Could be a voltage issue at the pump, at idle.  Low fuel supply.

 

But your description is a little unclear.  You said "at idle" first, then "at any given moment".  Two different situations.

 

 

 When at idle

 

 then at any given moment the RPM drops along with power.

Could be rich on the carbs. Burns off the plugs and runs better. Take the plugs out and see if they're dark. Might need to lean the mixture knob a hair. Weather change could cause this.

The Z has a mechanical fuel pump, and I actually replaced it with a new one last night.

 

For clarification, the drop in RPM/power can happen either during idle or on the road at speed.  During idle, it can happen when the engine is either hot or cold. 

 

I'm liking the HT lead idea.  I have reatively new plug wires from a recent tune-up, but that doesn't mean anything.  I hope that a break HT lead shielding doesn't mean a jolt to me!

Don't overlook the coil or coil wire as a possible culprit.  A coil usually fails completely but you might be getting intermittent HT performance at the source.

 My dad used a spray bottle with water in it to check plug wires. Start the engine, turn out the lights,& mist the wires. If they are bad, kick back & enjoy the light show. Much better than getting shocked & banging your head on the hood,

Mark

Sounds like an ignition problem to me also.  Check the wires on the coil, resister, wires to distributor, etc. 

Great tips.  I'll check into the coil wires, etc. this weekend.  Thanks!

I'm curious.   Any update?