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Old 11-07-2018, 07:26 AM
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Join Date: Nov 2018
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M116 rough when warm.

Hi.
I know that this thread is now old but someone may be able to help.
My 1973 350SLC was running fine then started to run rough when warmed up.
When I stopped, there was a strong smell of fuel.
I pulled the plugs and some were clean and biscuit coloured, others were black, wet and smelly.
The distributor internals and cap were changed last year. I have fitted a new Bosch coil and resistors and new Bosch W7DC plugs but no difference. I have a set of leads on order so that will be all the ignition bits replaced.
Does anyone have any clues about what may have suddenly changed to cause this.
I am learning about the D Jetronic but can't see what would cause these particular symptoms.
Any help would be appreciated.
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Old 11-08-2018, 08:17 AM
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Join Date: Aug 2002
Location: near Scranton, PA
Posts: 5,164
Hey Chrismin!

I moved your post to a new thread because your issue sounds a bit unrelated to the previous one.

Because you have mismatched plugs, what you truly need to do is find out which ones are mismatched. This will tell you what's going on with your D-Jet.

For example, if plugs 1 and 5 are black, 4 and 8 are fine, 6 and 3 are black, and 7 and 2 are fine, this can point to an issue with either the D-Jet ECU, or the trigger points in the distributor. I don't remember if the D-Jet system only uses the idle setting when the car is warm or not, but I THINK it does, so it may be ECU-related.

If plugs 1 and 8 are black, but the rest are fine, that's more likely to be injectors stuck open. They are pretty old at this point and may need a good cleaning or replacement.

The D-Jet ECU fires injectors in pairs. It has only 4 driver circuits, so it's shared with the ones I listed above.
1+5, 4+8, 6+3, 7+2

So if your plugs are not consistent across the same driver circuit, injectors; if they are, ECU or trigger points.

Now, that being said, you may also have distributor shaft wobble which is also firing-order dependent, as the shaft wears it can wobble around causing misfires because the breaker points put enough pressure on the shaft to misalign it at idle (but not at higher speeds). That can be examined with a timing light, checking to see if cylinder #1 and #6 fire at the same time, and consistently, as they are 360 apart.
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