Almost a misfire

[100SRV]

Went green laning on Sunday, my Bowler developed an odd problem - part throttle at lowish rpm it would stumble, not exactly a misfire but it wouldn't pull cleanly - as though the HT system was damp.

Cure 1. I took the king lead off (as I figured the problem affected all eight) - the socket for it in the dizzy cap was dirty and had dark oily in it so I gave it a good clean out and also cleaned up the coil end. I even ran a bit of emery cloth over the HT terminals to expose a bright surface. Rubbed a WD40 cloth over it, back together and it seemed to cure it. None of the HT leads were touching the engine and the coil wasn't overly hot.

Later on it was doing it again so this time I took the dizzy cap off. All OK in there, no wet or dirt. Rotor arm was difficult to remove but I took it off, cleaned it up and did the same inside the cap. Seemed to cure it for a while.

You've guessed it!

It started doing it again. Engine starts and runs OK, pulls nicely even revs well but doesn't like part throttle, low rpm running.

I wondered on the way home whether it was fouling the plugs so I pulled 1 and 8 out but if anything they were lean rather than rich.

I will try enriching the mixture but other than that I have run out of ideas - any suggestions (besides fitting EFi :-) )?

100SRV

[sheepy]

Try a new dizzy cap and rotor arm. My MGB does this from time to time, especially as the center pin in the dizzy cap wears (the carbon pin connects the king lead to the rotor arm)

[100SRV]

Hi,
replaced the rotor arm with a healthier looking second hand one I found in my parts box - a bit better but still not perfect. I might shell out on a new distributor cap, rotor arm and coil (all three are about nine years old). Any recommendations on whether a "performance" coil is worth fitting? The engine has Optronic ignition triggering - what performance coils would PH'ers recommend?

Cheers
100SRV

[PJR]

Vacuum leak someplace? It almost sounds like the dreaded fuel problem, but not being an EFI engine, your motor is likely immune from that.

P,

Edited by PJR on Thursday 1st March 21:03

[mave]

100SRV said:

Hi,
replaced the rotor arm with a healthier looking second hand one I found in my parts box - a bit better but still not perfect. I might shell out on a new distributor cap, rotor arm and coil (all three are about nine years old). Any recommendations on whether a "performance" coil is worth fitting? The engine has Optronic ignition triggering - what performance coils would PH'ers recommend?

Cheers
100SRV

If it was OK beforehand, cleaning the rotor arm improved things temporarily, and putting a secondhand one on made it a bit better, then I would concentrate on a brand new rotor arm etc. before uprating anything else..... Do you need it to be any better than it was before your recent outing?

[sheepy]

Have you checked the center pin in the dizzy cap? If a secondhand rotor worked better, fit a new one and a new dizzy cap. I doubt that its the coil.

[100SRV]

Hi Sheepy,
the centre contact in the distributor cap is fine, the spring is springy and there is plenty of length left on the contact. I will buy and fit a new distributor cap and rotor arm today.

Hi Mave,
the car is fine as it is but I did wonder whether, whilst the wallet was open, if it was worth fitting a better coil, on reflection I doubt if I'd notice the difference and changing too many things at once usually hides the culprit.


many thanks...

100SRV

[MR2Mike]

100SRV said:

Hi,
The engine has Optronic ignition triggering - what performance coils would PH'ers recommend?

Is it the old fixed dwell angle system (silver coloured ignition amplifier) or the newer constant energy system (black box)? If the former then you need to buy a coil suitable for points use, a modern electronic coil will be cooked. The gold coloured Lucas Sport coil (DLB105) is suitable for the older Lumention system, and can be had new for reasonable money on Ebay.

[100SRV]

V8 update:

Fitted a new genuine Lucas distributor cap and rotor arm - better but I did notice that there was a bit of a stumble under the same conditions. I noticed that the once-good spare rotor arm I had fitted now has a circular depression where the centre contact runs.

Next step is to remove and clean the HT leads I guess...

MR2Mile - the lumenition is the old silver type - I'll have a look on ebay - thankyou.

100SRV

[watson1]

Are you sure it is only an ignition / electrical fault?
Like the manuals say: if the engine misfires, check fuel system and ignition system.
With a petrol it can only ever be fuel or ignition - which covers every damn part of making the thing go!
Seriously; you say you still stumble at low rpm - I've had similar things with my old rangey on SU's, and after replacing every part of the ignition, including swapping the points for an electronic ignition system, found that the problem was, infact, caused by the carbs going out of adjustment (presumably due to vibration so the adjusting screws move) and it running too rich.
After having them set up, the same thing happened about three months later - carbs again out of adjustment - this time set them up and thread locked the mixture screws in!
A year later I've haven't needed to touch the carbs...
Do check for leaks from the inlet and exhaust manifolds too, but as these are uncommon stick with the ignition check and carb check.

[100SRV]

Hi Watson1,
have been working away and not using the car much. I will make the carburetters my next point of inspection. I have recently adjusted the "lost motion" bit where the two butterfly levers are linked. I noticed afterwards that the idle speed was a little fast - that will be reduced. I think I'll do as suggested and check the mixture etc as well as making sure there is enough oil in the dashpots - haven't checked that for a while. I'd be surprised if the inlet manifold has a leak, that was removed about a year ago and only seems to need re-torquing once after it has heat cycled for a couple of hundred miles. An hour with the spanners should cure it.

100SRV