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MG MGB Technical - 71 B rough at startup

My 71 B is equipped with a Crane electronic ignition, otherwise stock. When I start it with the choke pulled, it runs on 2 cylinders, then 3, finally 4. It takes maybe 30 seconds to start running smoothly. From then on, no problem. It starts instantly and runs smoothly when warmed up.

I have examined the choke linkage and both carbs are activated when the lever is pulled. I don't know if I have an electrical or carb issue. Any thoughts?
G Nicholas

If it is only happening when starting from cold, it might be due to oil seeping down the valve guides when standing and fouling the plugs.

Do you know which two cylinders it is/isn't running on?
Dave O'Neill 2

Thanks Dave. I am not sure which cylinders are not firing initially. I bought 4 new plugs today, and will install them tomorrow. No smoke when firing up though, which I would expect with leaky guides.
G Nicholas

Check the grade of the old plugs. Your signs tend to suggest as the engine warms, so do the plugs hence fouling is burning off and a good spark results. It may be just that the plugs on offending cylinders are just past it. I had a Triumph Bonneville ( lovely bike, wish I had never sold it!) That missed somewhat, I put two new plugs it was worse! I hummed, and tried everything to find the problem, in desperation I thought, alright two more new plugs. It started beautifully, no misses! There you go, you can even get 'duff' new plugs at times! Mike
J.M. Doust

You may also have a sticking carburetor piston. Pull both of them off and spray them with some carb cleaner. It's good preventive maintenance. RAY
rjm RAY

"fouling is burning off and a good spark results"

Can't really see this, if the plugs were fouling during normal running I can't see this burning off in the first 30 seconds of a cold start, it would take ages for a fouled and hence non-firing plug to get even warm from the heat of adjacent cylinders, let alone hot enough to burn off fouling.

Also they don't seem to be fouling during normal running as a warm start is fine.

Can't see it being oil seals either if it doesn't smoke with the throttle blipped after a long idle, or after a long downhill on a trailing throttle.

Nor a carb problem, as that is more likely to affect a pair of cylinders and not progressively less cylinders as it warms up.

Nevertheless I'd take the plugs out after a good run and compare them, and do the same after a cold start while it is still firing on 2 or 3 cylinders.

It sounds more like spark to me, and the first stage of that would be to replace the plugs unless they are new, in which case I'd swap them between cylinders.
PaulH Solihull

This thread was discussed between 10/07/2011 and 12/07/2011

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