'81 Datsun 720 Idle Problem

[shirley]

My '81 720 recently started stalling on me when I stop.  This happens when the truck is both cold and warm.  No smoke and have not done anything lately to the truck.  It has been colder(ish) here in sunny San Diego, so maybe it may have something to do with it.

Any ideas?

[datzenmike]

Yes

 

1/ Listen near the carburetor while someone turns the ignition switch on off on off on off, but don't start. What you should hear is a soft clicking sound. This is the idle cut solenoid that allows fuel down into the idle circuit. If not working the engine will run but won't idle.

 

2/ Separately pull one intake side and one exhaust side plug wires off and put a spark plug in the end. Hold firmly against the intake or exhaust manifold so it is well grounded and have someone turn the engine over with the starter. Is there spark on both intake and exhaust side plugs??? Both sides must be working for smooth idle.

 

See if this helps narrow things down. There are more things to check so don't worry.

 

 

 

 

 

 

[shirley]

Thanks!  

I can definitively do #1 but am having a problem understanding #2.  Specifically the intake side and exhaust side wires and putting a spark plug in the end.... hmmmm....not sure how to do this.

[DwayneOxford]

2 hours ago, shirley said:

Thanks!  

I can definitively do #1 but am having a problem understanding #2.  Specifically the intake side and exhaust side wires and putting a spark plug in the end.... hmmmm....not sure how to do this.

You not have 8 plugs?

[shirley]

Yes, 8 plugs.

[datzenmike]

All 720 Z series engines were dual plug per cylinder with 4 on the intake side and 4 on the exhaust side. See below...

 

 

Just unplug any wire on the left and the right side as seen from the front and check for spark.

 

[shirley]

Got it!

Thank you.  Will give it a go.

[datzenmike]

The dual plugs allows a shorter burn time so the timing can be reduced to like 5⁰ BTDC. If one side is not working the timing would like to be about 10 to 12⁰ so in effect the engine would be acting retarded and run very poorly.

[shirley]

Makes sense.  It's running pretty well, but the idle seems unstable.  I was going to try to adjust it, but thought otherwise.

I will keep you posted.

Thanks!

[datzenmike]

Well there is the idle speed screw and the idle mixture adjustment screw. Idle mixture only adjusts the idle mixture.

 

On a thoroughly warmed engine, turn idle speed down as low as possible. This makes any changes more noticeable.

 

Turn mixture screw in (lean) or out, (richer) till the idle drops off. Turn the opposite direction till it improves as much as possible. There may be half a turn where nothing seems to change so set roughly in the middle. Idle has probably gone up in response to the better mixture so turn it back down. Repeat the in and out adjustment of the mixture screw to find the best strongest and fastest idle speed. Again turn the idle speed down and repeat as many times as needed till you can't improve the idle quality any more and the idle speed is 700+- RPMs

 

 

[bottomwatcher]

An unstable idle is usually a vacuum leak. Check your vacuum hoses.

[shirley]

Yes, and some black smoke is also key to this, which I am having.

[Landen_cesalYT]

On 2/14/2023 at 1:20 PM, datzenmike said:

All 720 Z series engines were dual plug per cylinder with 4 on the intake side and 4 on the exhaust side. See below...

 

 

Just unplug any wire on the left and the right side as seen from the front and check for spark.

 

Aren't the intake plugs for emissions?

[datzenmike]

Dual plugs are excellent for reducing emissions. But not just the intake or the exhaust sides. Both sets of plugs fire together all the time.

 

Dual plugs shorten the burn time so the timing can be reduced. This shortens the time peak cylinder pressures are experienced and this reduces the production of oxides of nitrogen.