



MG MGB Technical - Cold start issues

I have a 78 B that is very difficult to start. It will crank with no hint of firing for the first multiple attempts to start it. It will then give a hint of turning over for several more attempts at starting. Finally it will begin to run and I have to continue to give it gas to keep running until it is warmed up and it finally will run. Once warm there seems to be no issues. I have put a manual choke in place of the automatic. New Plugs, coil, plug wires, cap, rotor, points, condenser and battery. I did remove the pollution equipment and plugged the holes per the "how to improve your MGB" article by John Twist. Any thoughts would be appreciated.
B.D. Harland

The wrong type of sparkplug can cause this problem .Barrie E
B Egerton

Would your car have had a balast resistor and 6V coil when new? If so do you still have that set up?

A cold spark plug finds it more difficult to spark that it does once it has heated up. So the ballast resistor system gives greater HT power while cranking to compensate.

If you have increased the compression ratio through rebore, head skimming or pistons with a smaller bowl that will have made the spark plugs job more difficult as well.

I am running 10.5:1 compression and find the engine does not start straight away from cold. Changing to the later ballast resistor and coil is something I have considered but not done yet as the longer cranking does start the oil moving at least.
David Witham

All rubber bumper cars had the 6v coil and external ballast that is bypassed when cranking. It *can* make the difference between starting and not starting under adverse conditions, but judging by the number of people who seem to replace that with a 12v coil when fitting after-market ignition systems, either by choice or because the manufacturer tells them to, is certainly not essential.

I don't really see how the plug type can affect a cold engine if the gap is correct - a hot engine yes.

This sounds to me like inadequate enrichment and fast idle on starting. With the manual choke, when the engine is hot and running, pulling the choke should have a very obvious effect on running - making it hunt rather than run smoothly.

Were all those new parts fitted in an attempt to fix the problem i.e. did the problem happen before?
PaulH Solihull

Sounds like a "choke" (cold start) failure.
The most common choke conversion simply replaces the thermostatic operation with manual operation, so most of the characteristics and problems of the autochoke remain.
See here:

Also, the Moss MGB catalogue has a good writeup from John Twist.

A better but rare conversion uses a true manual choke from a TR4/6 or Volvo.

FR Millmore

Thanks for the posts. I will check which coil I put in the car but I do not think it has the external resistor. I replaced all of the parts because the car has not run in 11 years. I thought about luminition ignition but have not done that. I did check the timing and it was about 2 degrees after TDC. I moved it back to 10 degrees BTDC and regapped the points but the car was warm at that point so I will try it this weekend. When I do pull the choke cable I do not really notice a difference in the cars idle so I suspect it may not be working either.
B.D. Harland

As an update. I removed the bosh platinum premium spark plugs and put in cheap champion plugs and it started right up.
B.D. Harland

Would I lie to you ? Barrie E
B Egerton

I would like for somebody to explain why the Bosch platinum are such a POS. I put some in my Dodge truck and
it started eating distributor caps. Turned out the thin hot electrode had eroded down inside the insulator, which you couldn't see except under magnification. I found it when they turned green, because the platinum is welded to copper down in the hole. I've since come across some more that did that.

FR Millmore

I pur some NGK Rhodium tipped plugs in the everyday Fiat at ir's last service, just for fun really. They have the much thinner central electrode. It's designed to run hotter and have a higher energy spark. I had always thought a bang was a bang before I opened the throttle and was amazed to find they make a difference. They are designed to run hotter and generate a fatter flame kernel when they discharge. This effectivley gives you bit of advance and more complete combustion. So far they have worked like any NGK I have ever fitted, once you take the torque wrench off thats it until they are next due for a change. I was thinking of a set of NGK upgrades next time the B is due, or some Denso ones which have a shaped earth electrode as well to "anchor" the spark landing position. Going OT I dont buy any Bosch domestic appliances either after experinces with a washing machine and a freezer both of which were about as good as their spark plugs seem to be from this thread. We replaced them with Miele which are built by a privatley owned German outfit with no shareholders/anylysts on their back and normal Teutonic service was resumed.
Stan Best

This thread was discussed between 25/04/2012 and 20/05/2012

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