Rough Idle - HELP!!

[CG901]

This has been going on for a year. Here are the symptoms:

1) At idle, #4 is not firing, but getting spark (plug is good). Pulling plug #4 while idling does not change anything.
2) At 2000 rpm and above, running fine on all four and can pull plug lead #4 and engine starts to miss as you would expect
3) Inspecting plug after idle shows wet fuel on electrode, otherwise plug is clean.

Assumed left Weber was a problem yet undiagnosed, so replaced with known good Weber.
Result: Same rough idle, and no combustion on #4 at idle.

Any ideas from the experts?
Thank you.

[Chrispy]

Poor compression? Done a test?

[CG901]

Leakdown and compression test are excellent (newly rebuilt engine).

[types26/36]

High resistance in ht cable, distributor cap or spark plug suppressors or caps.
Wear in distributor shaft or cam lobe, if points, dwell angle too large (small points gap) ?

[mark030358]

Have to g with Brian here...

[Mick6186]

Sounds like a balance problem, with the throttle closed the butterfly on #4 could be absolutely shut while the rest are very slightly open.

[Grizzly]

When is the last time you set the idle mixture up? you can literally stop a cylinder by winding in the idle mixture screw and it will run ok once the throttle passes the progression holes. If it was a balance issue you would expect no3 to be doing the same thing (assuming the carb hasn't been played with / the throttle plates not fitted correctly)

That said 90% of issues like this are electrical so make sure you have a good look at leads, cap, plugs etc.

This is where a colourtune comes in handy, with one fitted you can see the spark while it's running and see if it's running too rich / lean or not getting enough fuel to fire.

[Ray656]

I have had a similar problem on my Elan S4, it seemed to be a combination of an over rich mixture on no 4 and the ignition timing. Part of the issue seems to be that no 4 has a tendancy to run a little richer than the rest of the cylinders with the standard air box. If you find the car will idle better with the air box off, leaning the idle mixture by fitting 45F8 idle jets rather than 50F8 may be worth trying, it did with my car. The other issue was the the ignition timing varied by several degrees between cylinders, even though the dizzy was a remanufactured unit from a reputable dealer. Fitting a new old stock dizzy improved the timing, the car now idles ok and progression is acceptable on all 4 cylinders. It is worth checking the ignition timing directly on no 4 rather than setting on on 1 and assuming that the other cylinders will all be the same, as I had always done, they may not be.

[bitsobrits]

Don't neglect to inspect the idle jet on #4 for an obstruction. All it takes is a tiny flake of rust from the fuel tank to block a jet.

[MarkDa]

Same problem with two different carbs tends to support Brian's suggestion of an electrical issue.

[Hawksfield]

HI CG901
Is your car fitted with a servo if so check running with the vacuum intake blocked.
Good luck

[CG901]

Thank you for all the great suggestions. I think I have found the problem, although, have yet to confirm
I removed the cap to check the rotor and measure the points gap. The #4 contact post in the cap had a section missing that didn't appear on the other three. Is this the culprit? New cap ordered. See photo.

[types26/36]

Looks like the rotor has been catching #4 segment, check if the cam lobe shaft can be rocked side to side, it would mean the bush and/or shaft is worn.
It could also mean that the distributor cap was not on correctly.

[CG901]

Rotor shaft doesn't feel wobbly at all. In fact, very tight. The cap was correctly fitted.

[MarkDa]

With a bit of luck the new cap will do the trick.
Have you checked that there is no eccentricity to the rotor arm movement?