Misfiring on tickover

[Dominic]

Hello folks

I have been having great fun setting up the timing and mixture today, after the 500 mile tightening up of the cylinder head bolts, and resetting of the valve clearances. (I had changed engines, for a reconditioned one, using my almost new lead free cylinder head). Initially, I had to refit the distributor drive, as rather mysteriously, the timing was 180 degrees out, this was after replacing the timing chain. I was bemused, to say the least, as I took enormous care not to disturb the crank or camshafts! Today however, I solved that mystery, as I discovered that the drive dog on the dizzy shaft has been fitted 180 degrees out! This is a new Lucas recon dizzy, from a reputable Morris parts supplier, who will of course be hearing from me tomorrow. All the other ignition items have been replaced over the last few months also, and apart from the dizz, were used on the previous engine with no apparent problem.

The problem I am having now is that it is impossible to get a smooth tickover. I used a colortune plug to set the timing exactly to 5 deg BTDC. The dwell is at 60 deg on the nail. However, on running the engine at tickover speeds, anything from the slowest possible to about 1000 rpm, it can be seen that the spark (I was using no. 1 plug position) misses every second or so. This happens whatever the mixture setting. Has anyone ANY idea as to what could be the cause? I'd be so grateful, as I have very little hair left to pull out!

Happy tinkering!

Dom

[Willie]

I think you mean that you used the Colourtune to set the mixture,not the
timing? If you are observing the spark and it is missing intermittently it
has got to be an electrical problem, nothing to do with too much advance or
retardation just an electrical problem. if you check ALL four cylinders with
the Colourtune that will tell you if the missing sparks apply to all of them or
one in particular. If it applies to all of them then you have to go through the
ignition circuit methodically i.e. do not change points/plugs/capacitor/and
rotor arm all at the same time because you will not know what,if anythiing,
cures the fault.
On reflection, I suppose you did use the Colourtune to check the static timing though it is easier to use a bulb with a couple of crocodile clips since you
can observe the bulb and the timing pointer at the same time.

[Dominic]

Hi Willie
I used the colourtune to view the spark while slowly rotating the distributor. I turned it back first, then slowly brought the top towards front until the spark, then clamped tight. I'll try all 4 plug positions this week. Thanks

[bmcecosse]

I suppose it is a possible use for a colourtune - it's a waste of time for setting the mixture! Just do that with the lift pin on the carb. It may be your mixture is so weak it can't fire properly - but most likely it is ignition related - as above but consider also the little flexible 'low tension' wire inside the dizzy, at the base plates. Also check the vacuum advance is working (suck it and see), and is connected to the manifold ok. As for the dizzy drive - could be the old dizzy was wrong, and the new one correct! Don't jump in till you are sure of the depth!

[Willie]

BMC... how can the colourtune be a waste of time? like all good ideas it is a very simple and effective idea which enables you to compare mixtures in
each cylinder under firing conditions and,if you know what you are doing, will
enable you to check if the accelerator damper is working and even pin point
an incorrect needle profile if tuning has taken place. Your remark is about as
false as your comments about it being dodgy to replace the diff pinion oil seal which is one of the most straightforward jobs on the car.

[Dominic]

BMC, the dizzy is a new Lucas one, and when setting the drive spindle by the book, (both Autobooks & Haynes) the same thing happens, 180 deg out! At least I now know how to set them up without dropping the aforementioned spindle into the sump!
I'll replace the rubber connector to the vacuum advance however, as that is a little loose, having hardened with age. I'll try all your suggestions during the week though.

[Dominic]

I've just come in from testing all 4 plugs with the colourtune, and I have to admit that I was wrong regarding the sparking. Seeing it in the dark, I can see it is actually pretty damn steady at all speeds. It is the ignition of the fuel/air mix that is intermittent at low RPM. Any ideas? The carb has new gaskets between it and the manifold, the heat-proof block is in place, new gaskets at every joint in fact. I was also very careful to ensure clean surfaces at the joints. Squirting WD40 around the joints made no difference to engine speeds, so I guess there are no air leaks. Any ideas will be gratefully received!

[bmcecosse]

Well if it's not carb/mixture it must be ignition components!
Willie - The colourtune is not needed on a car with SU carb - the IDLE mixture is set by checking with the little lift pin Mr Skinner installed on the carb for that purpose. And it is absolutely no use whatsoever at any other revs, because unless the engine is under load the readings are completely meaningless. There is no way it can check the 'accelerator damper' whatever that is !, or the carb needle profile. Colourtune may well have a use on cars that sadly don't have an SU carb, but no sensible use on an SU engine.
I could take offence at the way you constantly jibe at me Willie - but I realise you are well-meaning , so let's just leave it at that!

[chrisd87]

The accelerator damper is the bit which stops the piston in the carb from shooting straight to the top when you put your foot down - i.e. the spring and oil damper.

[Willie]

Yes, and if it is working properly the colourtune will immediately show
if the enriched mixture is being supplied as intended. Yes BMC we will have
to leave it at that.

[Dominic]

Hey chaps, sorry to interrupt the argument,

I've just come in from testing all 4 plugs with the colourtune, and I have to admit that I was wrong regarding the sparking. Seeing it in the dark, I can see it is actually pretty damn steady at all speeds. It is the ignition of the fuel/air mix that is intermittent at low RPM. Any ideas?

but I really am at a loss as to what to try now!

[chrisd87]

Well, I'm only guessing but I assume as it's recon the engine has good compression? It must be ignition-related then. Maybe have a look at the wiring in the dizzy to make sure there are no shorts or poor connections, and also check that the condenser and coil are both in good nick, and that the points are properly gapped. It sounds like you may be getting a weak spark.

[Dominic]

The engine gave readings of about 155lbs when warm, it was about (25 mins after a 5 mile run home from work), and the readings were all with 3lbs of each other. I'll try swapping condenser & coil for old working ones, and will clean & regap 3000 mile sparkplugs. Determined to get it set up properly!
Cheers!

[Dominic]

Rather a red face here........... I have just fitted new spark plugs, for some bizzarre reason I thought that they were set to correct gap..... They (& the old ones) were set to about 45 thou instead of the specified 25 thou.... and guess what? It now ticks over like a sewing machine, rock steady!!!!

[Gareth]

Fab! Nice to hear everything is resolved and running nicely again, chaps.

[bmcecosse]

Well done - and well done too with the confession!

[brixtonmorris]

i like the colour tune plug.

ive owned one as long as i have owned morris.

dont use it much now though

(its in the blood.)

i bet a running engine with 4 of those instead of plugs would look great after a rave.

[Cam]

ive owned one as long as i have owned morris.

WOW! I didn't know you owned Morris!

i bet a running engine with 4 of those instead of plugs would look great after a rave.

I see.

[brixtonmorris]

wow
what a difference a "A" makes