Dieseling since 3rd engine pull - what causes it?

[rbastedo]

I pulled my motor a few times this past summer and since the last reinstall it's been dieseling whenever I turn the ignition off. It never did this before.

 

I didn't make any adjustments, just pull the motor & tranny then put it all back in and bolt it down.

 

Have I inadvertently broken my anti-dieseling solenoid? How would I test that?

What else could cause it? I guess it could be one of the wires to the thing, how does it work? Is it constantly energized then voltage removed with ign off, or does turning ign off put power to the solenoid?

[datsunaholic]

If the Anti-dieseling solenoid wiring was broken, it wouldn't ide at all. It's only powered when the ignition is on. Otherwise it'd quickly drain the battery.

 

I'd suspect you've inadvertently knocked the idle or timing out of spec. If the idle setting leaves the throttle cracked open, it still can run-on off the primary system.

 

Or, the idle-cut solenoid is damaged and physically stuck open. You'd have to take it off and see if the plunger operates normally, that the plunger isn't broke off, and that it doesn't have something blocking it open.

[ariascarlos1990]

Sounds like timing to me.

[rbastedo]

Well that all sounds easy enough, I'll locate my timing light as that sounds like the easiest thing to check first. Thanks guys!

[rbastedo]

I got out there to take a look today and at the very least my timing was definitely off. Not saying that's the only problem, but it was a problem. It was right on the "0" (zero). I set it to about 15 BTDC and then adjusted the idle back down to 950 rpm (it climbed up to about 1400 rpm due to the timing change).

 

I'll see how it goes tomorrow, hopefully this will improve performance & mpg too.

[MikeRL411]

If your head is heavily carboned up, that plus the timing issue will promote Dieseling. The carbon hot spots provide an ignition source.

[ggzilla]

The more advanced the timing, the more likely to diseil. Stock timing is usually 5 to 10 degrees BTDC.

 

> the idle back down to 950 rpm

 

That's the major cause right there. Put it down to 600 rpm and I'll bet it stops.

[rbastedo]

It wont idle down that far, likely my carb is way to old & tired.

[ggzilla]

Unfortunately then you will have to live with the run-on.

[rbastedo]

Unfortunately then you will have to live with the run-on.

 

I don't really understand that.

 

Until the most recent engine pull and reinstall it idled at about 1000 rpm all the time and shut off with no dieseling. It's just not as simple as "get the idle down to 600 rpm or live with run on". Not in my experience with this engine.

[ggzilla]

High idle airflow is #1 cause of dieseling. When Datsun raised the idle speed is when they started fitting the idle cut-off solenoid to compensate.

 

What changed? When you pulled and re-installed the engine what did you do? Different engine? Rebuilt engine? Just took it out to clean the outside and put it back in?

[rbastedo]

What changed? When you pulled and re-installed the engine what did you do? Different engine? Rebuilt engine? Just took it out to clean the outside and put it back in?

 

Thanks, that was good for a morning laugh!!

 

No - I'm usually not the one that gets the rap for cleaning much of anything and I wouldn't (afaik) go to all the trouble to pull an engine just to clean it.

 

Originally I pulled it to put a 5 speed transmission on it, then I found that the tranny wasn't good so I pulled it again and put my old 4 speed back in.

I guess that makes two times, don't know where I got three - maybe it just seemed like three.

 

That was it, pull it out & r&r the tranny and there you go.

All was good the first time, then it started with the run-on the second time.

 

Is checking the cut off solenoid as simple as applying power to see if it kicks?

 

BTW this morning it ran great. It's been a bit harder to start until today, after setting the timing. It really just started right up like it used to. When I turned it off it quit, no dieseling.

[ggzilla]

Good to hear. If it's not broke, don't fix it further... see how it goes for a few days. Maybe it has cleared up.

[datsunfish]

Almost every high idle situation for me has been a vacuum leak. If it was good before you pulled it its probably something hooked up wrong but hard to say. Many times for me,I have done one thing and something else completely unrelated happened. Are you running points or electronic ignition? Carbon buildup,as stated, and overtemp will cause deiseling too.

[rbastedo]

Almost every high idle situation for me has been a vacuum leak.

 

Maybe you misunderstood, it started idling higher when I advanced the timing from 0 to 15 btdc. 950 - 1000 rpm idle is normal (I believe) for a 1980 A15 with the economy head and the hitachi downdraft carb. This has an electronic distributor.

 

I've got it idling at about 850 now, the dieseling is much better (less) but still there a bit. Next I'll pull the cut off solenoid and verify that it works.

[rbastedo]

Just changed the plugs, wires, cap & rotor.

 

Plugs were at about .040 - new ones at .030 and I had never put in new cap, rotor or wires since I bought it.

 

It blew a big cloud of smoke on startup, weird! :blink:

[rbastedo]

Well my gas mileage has improved and it doesn't 'run on' as bad but still a little.

I'd say that problem is 90% gone.

 

The carb is really old & worn out, I'm thinking of replacing with a Weber 32/36 as that seems a pretty simple bolt on change.

I also wonder if it might be just a little shorter profile than the stock Hitachi carb.

[banzai510(hainz)]

maybe pulling the motor out caused soemthing with the float to put a little more gas at idle.

 

I know that you should set the idle and then the mixture scew the same time and balance between them to get idle down and timming correct. cheap gas will seomtime make it idle also when motor is HOT. try a cooler stat?

[rbastedo]

Oh, that's an interesting thought - thanks I'll take a look at that.