What year MGB? The early (non-synchro tranny) starter pinions had a habit
of getting jammed in the flywheel gear teeth. The rocking you described
is precisely the method to free it up. In my case it got progressively
worse as the flywheel teeth got chewed up. This in turn would chew up the
pinion teeth until I had to replace the starter. After three starters in
three months I bit the bullet and pulled the flywheel to get a new ring
gear fitted. (As it turned out I never refitted that flywheel since I
stumbled across a 4-synch OD tranny which I installed instead. I must say
that the later starter is much more confidence-inspiring in its
configuration, and in fact is more reliable in operation).
Anyway, if you suspect a similar problem, get under the car with the
tranny in neutral and the spark plugs removed, and turning the engine
over a bit at a time, examine the flywheel gear teeth through the starter
opening. Two bad teeth in succession will be sufficient to cause a jam,
in my experience. The jamming is somewhat intermittent due to the
"random" initial position of the flywheel at startup. But the position
does not seem to be entirely random -- the engine has a tendency to stop
with the crank in certain positions of greatest friction (or least
energy, or whatever). This means that a total of 3 or 4 bad teeth can
force you to resort to the rocking procedure on almost every start.
You can also look at the starter pinion teeth, since they are right out
there exposed where you can see them. Are the corners of the teeth
chipped and broken?
Of course, if you have a '68-or-later car, this is irrelevant, and I
apologize for wasting bandwidth. When you say, "no response", did you
hear a faint "clunk" sound? I should have said that the situation
described above assumes so.
If nothing happens at all, it is just as likely to be poor electrical
connections, usually at the battery post clamps, and not the starter.
This can be intermittent as well, for inscrutable reasons of temperature,
humidity, and charge.
Good luck, hope it's the latter and not the former situation.
Martin C. Jones had this to say:
>After paying at a petrol station at the weekend, I returned
>to my MGB, turned the key - no response. Indications seemed
>to point to the starter motor. I rocked her to and fro in
>fourth gear, tried again and hey presto she fired up.
>My question is:
>Can this happen as a one off or should I check out the
>starter motor? I seem to be OK with it this week so far.
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.