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Intermittent Misfire

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DM6 or 25D?

If 25D, suspect the primary (low tension) pigtail lead from the nylon block to the points...as sugested in post 2. Try the dark test, as in post 13, but remove the cap and crank or turn the engine with the ignition on and look for any spark that's not between the points contacts.

Danny
 
Well guys, I found the problem and I’m embarrassed to say that it was something as simple as the timing. Should always start with the very basics!

Thanks for the suggestions anyway.
 
I'm glad your found and fixed your problem, but I'm perplexed by the cause. Retarded timing would cause consistent poor running and performance, and probably worse mileage (timing advance at cruise is mostly for more efficient combustion, hence more power from a given amount of fuel, hence less fuel required), advanced timing could cause detonation ('pinging'), whether you hear it or not. Did you make any other change besides changing timing? It's conceivable just dis/reconnecting some wires removed some corrosion or caused a better connection to be made. Sounds unlikely, but I'd swear just washing my car makes it run better.
 
Bob, I did pull the distributor cap off to check for any arc tracks, and while I had it off I reset the point gap. But then I test drove it and the misfire was still there. It wasn’t til I reset the timing that it went away. If the car had been pinging I wouldn’t have been able to hear it as I usually drive without my hearing aids in :smile:
 
Check back of ignition switch for heat.
 
Bob_Spidell said:
I'm glad your found and fixed your problem, but I'm perplexed by the cause. Retarded timing would cause consistent poor running and performance, and probably worse mileage (timing advance at cruise is mostly for more efficient combustion, hence more power from a given amount of fuel, hence less fuel required), advanced timing could cause detonation ('pinging'), whether you hear it or not. Did you make any other change besides changing timing? It's conceivable just dis/reconnecting some wires removed some corrosion or caused a better connection to be made. Sounds unlikely, but I'd swear just washing my car makes it run better.
Click to expand...

Bob,

I can agree with you on the timing, too.

With my past experiences, when I would be cruising at higher speeds and maybe slightly accelerating at times, would I experience a misfire, but not often. Still, after such an episode, I decided to do a thorough tune up and left my timing alone. Hard to say exactly in my case. I do believe it was a matter of replacing the plugs, points, rotor, condenser, wires, cap...all that sort of stuff and never had any misfiring after that.

In addition to my above experience and what I tend to wonder is, because I have a high voltage ignition coil in place, if that in some way may cause excessive wear on the related components. Specifically, I find that if I trim off about a 1/2 inch of the ends of each ignition wire where they connect to the spark plugs and at the distributor cap entry points, (due to what appears to be 'burnt' endings after time), well...let me say that after performing such intermittent trimmings of the wires, (and of course, periodic points adjustment, cleaning of rotor, etc.) any subtle misfiring's I may have are eliminated once again.
 
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I got to thinking about my own last comment, and remembered Healey engines' advance is driven by port vacuum, which is created by airflow through the carb throat (downwind or upwind side of the throttle plate; I don't recall). Most older American cars used manifold vacuum, which has a port on, needless to say, the intake manifold. Some cars, even some Brit cars use a vacuum retard system, possibly in combination with manifold and/or port vacuum. Are there any advance/retard gurus on this forum? I confess to not completely understanding how the two+ possibly systems work (I do understand the need for advance based on increased engine/piston speed).
 
Vacuum read at manifold with advanced timing will yield instant throttle reponse. Vacuum read at carb and stock or near stock timing yields smooth running and mpg. If you want more throttle response you would advance weights in distributor and lock down and set timing at max. Stock could be 0- 12* advanced would be stuck at 32-34*. Mostly used on hot rods.
 
When you open the throttles wide, there is very little manifold vacuum--1" is usually cited, caused by manifold restrictions--so mechanical advance would be predominant; conversely, at part throttle there is more vacuum so vacuum advance is employed for efficiency. Port vacuum, AFAIK, would produce more vacuum, due to Bernoulli/venturi effect at WOT, less at part throttle. Still not able to wrap my head around this; and why vacuum retard (I think some Brit cars use it)?

Time for some googlin'.

Edit: Found this, for a start: http://www.georgiajag.com/Documents/Vacuum%20Advance%20versus%20Vacuum%20Retard.htm


Best explanation I found: https://en.wikipedia.org/wiki/Ignition_timing
 
I'm having trouble grasping why the timing would cause an intermittent misfire. Was the timing only occasionally wrong?

Danny
 
Danny, I can’t explain the whys and wherefores, but I can confirm that the timing was off to the tune of about 20-25* btdc. How it got so far out I don’t know unless I had my eyes crossed when I first set it. Once I set it correctly the misfire disappeared. By the way the car idles much better now. :smile:

Griz
 
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