Weak spark . . . One cylinder more than others - PeachParts Mercedes-Benz Forum

Topher39 · #1 10-14-2013, 09:31 PM

My 69 w115 has started to have a rough idle, sometimes missing at low rpm/idle. This has gotten worse over a 2 week period. I pulled each plug wire to see which was missing and sure enough, the most rearward cylinder had no spark at idle! Swapped plug wires and same thing. With the other cylinders I could see the spark jump and hear the lack of fire. When I idle up, the cylinder kicks in, stops missing and I see the spark jump. So what's the deal?

I replaced the coil with new points/condensor 8 months ago, along with new set of wires and plugs. Been running the car as a daily driver since.

What's the scoop? Where is my issue?
Why one cylinder over another? Worn distributor causing one out of 4 to be weaker?

Chris

Carpenterman · #2 10-15-2013, 10:40 AM

Gotta be something going on with the points. Remove the cap, turn the car over and watch the action. Your distributor may be worn. Is there excess play in the cam? Check the dwell for grins.

Or you could just spend the $90 for a Pertronix ignition. It will compensate somewhat for a worn distributor. You will need the Bosch distributor model number to get the correct unit.

Topher39 · #3 10-15-2013, 10:45 AM

I feel a little play in the distributor yes, been wondering about it. Perhaps a distributor rebuild is in order, and yes replacement of points with electronic would be nice. For the same, single cylinder to be weak at idle would seem to indicate such. Wondering if I adjust my gap/dwell, what direction should I go? Wider gap?

Tomguy · #4 10-15-2013, 10:53 AM

It does sound like you may have distributor shaft wobble, which at idle is enough resistance from the points to cause a cylinder to miss. As the engine speeds up the shaft no longer can wobble as much, and the points no longer push the shaft, which at idle are causing the rotor and cap to move out of firing range.

You can try a junkyard or other used distributor to test this, or see if you can get yours rebuilt.

Carpenterman · #5 10-15-2013, 12:26 PM

If you wanted to play with dwell, I would try decreasing it from 37 degrees (IIRC) to increase the gap. Of course, that will retard the timing some so it still may not run right.

Tomguy · #6 10-15-2013, 02:29 PM

The issue being that increasing the gap will actually put more stress on the shaft... you'd want to decrease the gap as much as possible to see if that could resolve the issue. Of course, weak spark may be the result.

Topher39 · #7 10-15-2013, 02:49 PM

Who on this thread rebuilds distributors? And/Or - where can I get the parts to rebuild it myself?

Chris

Carpenterman · #8 10-15-2013, 08:13 PM

Good point, Tomguy.

Topher39 · #9 10-15-2013, 08:53 PM

I just want a strong spark! LOL I am less worried about stressing the shaft.
So even if I have a little wobble, an electronic ignition would not really be phased by that - sounds like a good idea

Palolo · #10 10-16-2013, 11:56 AM

Quote:

Originally Posted by Topher39

Who on this thread rebuilds distributors?

Screen name Dr. Benz

Tomguy · #11 10-16-2013, 01:12 PM

Benz Dr, actually.
http://www.peachparts.com/shopforum/members/benz-dr-.html

Topher39 · #12 10-16-2013, 01:25 PM

OK - got back to basics - checked the Dwell and it was at 70! almost no gap in the points. I reset the gap, and tweaked the Dwell to 45 - strong spark is back, no issue.
Pretty steady on the dwell meter. I do have play in the distributor, timing bounces a bit but not terrible. Will get my dizzy rebuilt.

The lobe was dry, I am assuming the point's contact on the lobe was wearing down - hence the issue. Now I remember why I swapped my points for E-Ignition on my MGB, no fussing around every 3000 miles.

So much to do about nothing. Thanks Guys.