Misfiring on startup (when cold)

[warrenperkinson]

I would appreciate some advice on where to start looking for the cause of this issue...

When I start up my 1971 series IIa SWB (2.25l petrol), it runs rough (on 3 cylinders) and blows a bit of smoke (white) until it's warm. Once it's warm it runs really, really well (purrs even) and starts again perfectly. It has only started running rough when cold recently...

In the last 14 months I have:
- replaced the dizzy with a new one and put electronic ignition in it (Powerspark).
- had the (Zenith) carb reconditioned and tuned.
- taken the head off and replaced all valve seals and seated the valves (and replaced where necessary).
- replaced the HT leads and coil.
- replaced the spark plugs.
- set the timing.

Until recently it has been running perfectly (well, for a 2.25l LR engine!), so I'm not sure why it's begun to play up when I start it cold. (It starts easily and doesn't need any coaxing). I'm in New Zealand, so it's not the cold weather that's causing this...

Does anyone have any suggestions where I should start looking into the cause?

Cheers,
Warren.

[Les Parker]

Have you tried replacing the electronic ignition with the original points/condenser/rotor arm? I would try that to eliminate the ignition side of the equation.

[slowmo]

Hi Warren,
Have you checked the compression? You've done almost everything on the top end and I'm wondering if you have a problem with rings. The white smoke is a hint. Perhaps once it warms up you get a better seal? How does the oil look?
David

[warrenperkinson]

Thanks for your suggestions. I have bought a compression tester and will give that a try. I also bought some new spark plugs and replaced those - in doing so I may have found the problem... the old spark plugs were fouled with carbon - black but dry.

If I understand the spark plug diagnosis in my Haynes manual correctly, this means that the mixture is too rich. The carbon build up will eventually cause misfiring - which seems to be my case.

I replaced the sparks with NGK B6ES which I understand is an appropriate replacement for Champion N8s.

I have a Zenith carb, so I may have succumbed to the dreaded zenith warp which. I've checked my air filter, which is good, so it's unlikely to be that.

I will still check the compression, as I suspect that one of my cylinders is a little low, so this may still be the problem.

Cheers,
Warren.

[warrenperkinson]

Here's an update...

It turns out that my choke cable was sticking and the choke was not going opening all the way - hence the over rich mixture and carbonised spark plugs.

The choke cable, it seems, was getting stuck because the little plastic half round that locks the choke knob out had disintegrated and fallen into the cable housing.

So now that I have removed and refurbished the choke cable (and added a new plastic half round to the cable) the engine is running nicely again. Yay!

If anyone is interested in how to repair the choke cable lock, I found this really useful post on one of the MG forums:
http://www.mgexp.com/phorum/read.php?1,2071172

[Les Parker]

Thanks for the feedback and glad to know you are running well again.