loose Distributor CAP = Perfect idle

[ira]

Hey folks, and tnx in advance.

1982 Datsun 5 speed Trubo.  at cold start the car idles steady at aprox 1100 RPM.  10min later after it warms up i start to hear little miss here and there.  the longer it runs, the more miss and rough idle starts to happen to a point that needle is all over the place.  I place a vacuum gauge on the Fuel Reg hose and i see that at first its steady in the green but again once car is warm, gauge is all over the place form RED to Green.

 

here is the interesting part, when i loosen the 2 screws on the distributor cap, and raise it up just a tiny bit, the idle becomes perfect (about 800 or so), the engine sounds prefect and the vacuum gauge is solid steady in the green area on 21 or 22.

 

 

[Zed Head]

Have you had it apart?  It's 38 years old.  Check that the rotor head is firmly seated.  Look at the top of the inside of the cap for wear marks.  Make sure the shaft is tight and the disc spins smoothly.  Maybe the cover is causing misalignment of the shaft.

[Mark Maras]

 Interesting that you can raise the cap slightly and it runs better. I agree with ZH's advice about being sure the rotor is seated. I also wonder about gear wear that would allow the distributor shaft to raise the rotor slightly. Try grabbing the installed rotor and try raising it and the distributor shaft. While lifting it allow it to twist one way or another to get maximum height. There should be very little vertical motion in the distributor shaft.

[Dave WM]

an old trick I used to use on dirt bikes that were run on two stroke oil (and tended to slightly foul plugs from the burned oil). Sometimes I would have a rough running engine, or even an engine that would not start. I would pull the plug cap off and replace but not seat on the plug, but rather space it off (estimate about 1/8 inch). With that it would start and run fine, at least long enough to get back to the pit area. Not sure of the science of why this worked, but work it did.

Pull the plugs and take a look at them, see if partial fouled. better yet just replace them with some new ones see if the issue goes away.

Edited November 2, 2020 by Dave WM

[Zed Head]

Might be carbon tracking.  With the cap up it breaks the path to ground.  

[ira]

Zed, Mark and Dave,

Thank you all for the prompt replies

yeah, i forgot to mention that i did take it apart.  i cleaned up the little corrosion under the seal plate.

New NGK sparks plugs. don't know what spacing they need to have so i compared to old ones.

Rotor is firm, hardly has any up & down movement.  i did use a very fine sand  paper and gently scrubbed the tip to get rid of any accumulations

here are some pics 

[Dave WM]

that cap looks a bit beat up from the rotor, maybe that is normal.

may as well put some spacers in there and run it with the cap up since that solves the problem. Looks like some alum dust sprinkled around, maybe enough to provide a ground path same as a carbon track?

Have you looked at the new plugs since you put them in?

Next time this happens you could try pulling the spark plug wire off a little bit and seen if any one plug is effected. I am sticking with the partial fouled plug until you look at them. New plugs can foul if there is a problem.

Edited November 2, 2020 by Dave WM

[Zed Head]

Make sure you check the other odd things like the main wire from the coil to the distributor cap.  Things that move when you raise the cap.   

Seems like there's a lot of crusty stuff floating around in there.  The spark can jump anywhere along the path on top of the rotor.  When you raise the cap you move the rotor away from places to jump to.

I might try a piece of electrical tape on top of the metal as an insulator, just for an easy test.  Overall though, a simple tune-up with new parts might be the way to go.  The old pocket knife electrode scraping might not be good enough.

[ira]

Dave,

i see ur point w foul plug.  i will test for sure, but the fact that raising the cap gives me great results i figured no need to pursue the plugs &/or cables at the moment.

so far i've been able to reproduce/fix for a day.  im gonna try that for couple more days.

any one knows the spacing for the spark plugs?

 

[ira]

got it...

is there a spray i can use to clean up connector ends?  i see couple with some greenish build up in there

[Dave WM]

warming up rough idle, things that come to mind is a leaner air fuel mix, which is harder to ignite. I presume you only have this roughness at idle?  A leaner mix is more taxing on the ignition system, IIRC fsm calls for .035, but not sure of that. Any weakness in the ign will show up in the leaner idle mix. I have the same issue, idle cold is great get it fully warmed up and the idle is not as smooth. I don't worry about it since its fine at any thing other than idle, guess I could/should look into it. I did try new plug wires and cap recently, no change. Have some new plugs coming.

It maybe something as simple as adjusting the idle bypass on the AFM for a slightly richer mix (for my car that is). IIRC I have noted this on a swap I did with a spare AFM (better idle vacuum).

You can also see if you have a poor injector, maybe one is flowing a bit less, again a relative power test can isolate this, unplugging one injector at a time and listen for rpm or drop in vacuum is a easy way to find the loafing cylinder.  Check it for compression, put a new plug and wire, if still loafing you can try a color tune plug and do a comparative flame color to a good cylinder. if its lean swap an injector and see If the laziness follows that injector. The color tune plug is expensive but a nice tool to add to your diagnostics tricks.

Edited November 2, 2020 by Dave WM

[Dave WM]

1 minute ago, ira said:

got it...

is there a spray i can use to clean up connector ends?  i see couple with some greenish build up in there

a small brush like used for cleaning paint guns and some contact cleaner from the auto store, followed up with some deoxit will help.