D-Jet Light Stumble, Partial throttle only... Options

[GregAmy]

Here's an interesting one.

1974 2L D-Jet. Tangerine-rebuilt and -adjustable MPS (could prob use professional tuning). TPS replaced 3 or so years ago and properly adjusted (to the best I can with a couple alligator clips and an audible ohmeter). Pertronix electronic points; stock untouched D-jet plate. Injectors are all untouched though I've checked thoroughly as I can for vac leaks, nothing found.

I'm getting a light stumble at cruise, but only at partial throttle. It never DIES, it's just a light misfire for a cylinder or two, seemingly at random, and not constantly. It seems to go away when I floor the throttle (and release that somewhat-less-than 90 poines of fury) and it's under a hard load.

It's initially imperceptible to my passenger (wife) but I can feel it. The longer the cruise goes and the warmer the engines gets, the more prominent the stumble gets - never "bad" - to the point where my passenger (wife) will say "yeah I felt that one" but probably only because she was listening for it.

The car never dies. Starts, runs, idles fine (after getting past the CHT donut hole).

Possibly related: poor fuel economy - low to mid 20s - but I've put that down to not driving it a lot (evaporation) and as noted I've not properly tuned the MPS (giving thought to adding an AFR to the car).

Thoughts? Easy things to check?

As soon as Chris gets the racecar engine done and that project is completed, I'm planning to remove the street car drivetrain and go through it, including replacing intake hoses, lines, seals. Might even get the injectors serviced by Marren. Until then I'm still driving it around...

GA

[JeffBowlsby]

Lube the dizzy weights once a year per the FWSM if you have not done so already. 1 drop of 30 wt oil is recommended per the manual.

[GregAmy]

Nope, I wasn't aware of that requirement. I don't have a factory manual; anyone have a photo of where to lube?

[JeffBowlsby]

Center of dizzy shaft...its hollow and direct conduit to the weights below.

[GregAmy]

...that was easy...

[GregAmy]

Ok, so today was the first time I had a chance to drive the car since dropping a couple drops of oil into the distributor shaft...and I am amazed at the difference this seems to have made on the way to work. Thinking it was just in my head, I drove it the long long way home after work with a lot of steady state throttle on the highway, and did not even hear one stumble.

What does this do? What was happening without that lube?

I've never been inside the guts of the D-Jet disty other than to install the Pertronix so I am genuinely curious.

[Bleyseng]

The weights stick so your timing advance is sticky causing the off part load issue

[JeffBowlsby]

Ah the power embodied in mere drop of oil in the right place at the right time to unleash the genius of mechanical ignition advance.

The weights were frozen yo. The lube freed em up.

[rhodyguy]

Willing to bet back in the day that could have been an expensive service order.

[914_teener]

There are pivot points inside the dizzy for the advance plate and the weights. If the car sits for a while water can consensate inside.

Not only is this an electrical contact point but relies on the free movement for part load as well as full advance.

Most of the stock dizzies are well beyond their lifespan.

[914_teener]

On second thought....after reading you post...can you pull vaccum on the advance diaphram and have it hold vaccum?

[GregAmy]

Of course! Distributor advance weights...I was thinking it was a reference to a function of the D-Jet pickup...I actually am really old enough to know better...

Reminder to myself to recheck full timing again this weekend...3000 RPM and what advance for 93 octane?

Disty vac diaphragm holds a vacuum and is properly connected to the throttle body.

Thanks, all ('specially you, Jeff (IMG:style_emoticons/default/wink.gif))!

[jagalyn]

Lube the dizzy weights once a year per the FWSM if you have not done so already. 1 drop of 30 wt oil is recommended per the manual.

What is ‘FWSM’?

[GregAmy]

Factory Work Shop Manual. Usually "FSM" (Factory Shop Manual). Of course, almost no one really has one of those...

BTW, drove it to/from work again today. Engine is butter smooth.

Co-workers are like, "what is it?" and "how old is it?" and "is it a manual?"

Now to figure out how to get rid of the harmonic wheel shaking at certain highway speeds. I think it's because of the non-hubcentric Panasport wheels. And the speedo needle bouncing between 5-40mph is annoying...