Another spark plug diagnisis...what's going on with #4?

[The Geologist]

In the process of tuning my Weber 38 DGES (described here), yesterday I pulled the plugs to inspect for lean vs rich running.  Should've done this before I started, but this is what I have now, after getting it close to dialed in and running it for ~ 10 miles.  #1 through #4, left to right.

 

 

I suspected one cylinder was misfiring because as I was testing the car under load conditions (a hill with +200' elevation over 3/4 mile) I could hear one cylinder misfiring slightly, and the car seemed down on power.  This was accelerating uphill at ~2000 rpm.  Something's going on at #4.  Both idle jets are set at 1.75 turns from closed, so I don't think there should be a significant imbalance in mixture between #1-2 and #3-4.  Bad ignition source or wire?  Bad oil ring maybe?  

 

 

The car has a stock M10 except for the Weber 38 and 123 distributor.  Plugs and wires look OK.

 

I'd appreciate suggestions on next diagnostic steps.  Thanks.

 

 

[Zak]

You should check compression across all four cylinders. The most important thing is that the readings for all four are no more than about 10% of each other. To determine ring wear squirt a little engine iul into each combustion chamber after taking the initial reading. Take a 2nd reading, and if the it is significantly higher, your rings are worn. But, from the look of your plugs I wouldn't say that your rings are really worn.

 

#4 cylinder usually wears a little sooner. But, if not the plug, you may have a bad spark plug plug wire. Check your distributor cap as well.

 

I wouldn't run that carb on any 02 motor, especially a stock motor. While it is touted by some here, it wasn't designed for our cars and is better suited for a 6 cylinder motor. The 38/38 simply lacks the flexibility of the much better progressive carb which is much better suited for our cars. It's pretty much all or nothing with the 38/38. And, fuel mileage is awful. 

 

Edited 6 hours ago by Zak

[TobyB]

Given the way that plug looks, I'd check plug wire and plug itself, then the inside of the distributor cap...

you're not getting much, if any, combustion there.

 

Also,  if they're resistor plugs, you have a resistor rotor and resistance wires, that can stack up to give a misfire 

under load.  Usually across all 4, but if something's already a bit weak on #4, then that may be where it's showing up.

Or if #4's getting extra air...

 

...cap off the brake booster vent, drive carefully, and see if symptoms change.  Usually if it's leaking constantly,

the #4 plug goes even leaner, but in your case, your a/f is already in the right range, so extra air into #4 might take it

so lean it'd misfire.  Not super likely, but easy to rule out, since #4's your offender.

 

But it really does look like it's not firing much at all.  I stick with, confirm plugs and wires first,

and confirm that the 1231345245345 is really popping a spark on #4.  

 

I ran a 38/38 for a few years- it was lots of fun, with a bit of extra cam.  Got low 20's mpg, compared to mid 20's

with the 32/36.  Lap times didn't change much, but the butt dyno loved it.

 

t

 

[The Geologist]

Thanks gents.  I inspected and cleaned all contacts from the plugs to the rotor and I think the problem is the loose fit of the 123 distributor cap to its body.  I could easily wiggle it by hand, and when I tried the same on my stock '76, the fit was as tight as it could be.  As you can see from the second photo, the hinge of the buckle is attached to a mounting bracket connected to the body of the 123 dizzy.  The hole in the bracket is oblonged but there's no further room to lower it and tighten up  the buckle.  Rather than dissassemble, drill out and reassemble, I think I'd rather try a different cap with the buckle ears a little higher up.  I guess I could start by swapping out the cap from the '76, to see if it fits better.

 

Any other suggestions?  Part numbers that solve the problem are always welcome!

 

 

 

 

[popovm]

Can you bend the clips so that they put more pressure on the cap?