Just by way of a change I've recently changed the head gasket on a friends
Midget 1500 (see the full tale at http://www.mgb-stuff.org.uk/midgebits.htm)
A couple of days later and on the way to an MOT (annual test) it starts
running really badly, and Keith limps it home. When I get there and start
it up it runs fine for a few moments, then obviously starts running on 3
cylinders. Clipped the timing
light onto the coil lead and each plug lead but everything is flashing as it
should be. Quite by chance I pulled No.3
HT off the distributor cap and it made no difference - i.e. that was the
faulty cylinder. Not believing the chances of
that I replaced it and pulled the others off one-by-one, but no it was
definitely No.3. Held near the cap No.3 lead
sparked just as all the others did, so that and the timing light flashing
told me the rotor and cap were OK. Removed
the plug and it was fouled. Replaced it (with a used plug) and it fired up
OK, but after a few seconds started firing
on 3 again. Tried another used plug and that was the same. By now I'm
beginning to fear the worst and suspect a
sticking valve. There is no popping in the intake so it must be an exhaust,
but for *that* to be affecting
running it has to be sticking a fair way through the induction stroke, at
least. But looking at the valve spring it seems to be doing exactly the
same as all the others, and the rocker is showing no signs of excessive
looseness. It isn't tight either, for although the gap has closed up
somewhat because the engine is hot (they were set
to 10 thou cold) there still *is* a gap, and is much the same as the other
valves.
Then I have a brain-wave and realise I can use the adjustable timing light
to 'freeze' the valve (low ambient light levels), by connecting the light to
a suitable plug lead and varying the timing control. Indeed this is exactly
what happens, and by varying the control I can freeze the valve anywhere
between fully up and fully down. No.3 is regular and consistent, and the
timing the same as the other exhaust valves, so I'm beginning to doubt my
diagnosis. As a further test I press down on the top of the spring cap with
a screwdriver, and lever up under the upper turns of the spring, thinking
that if it *is* a
sticking valve one or other of these tests should have some effect, but
neither did. Then all of a sudden it started
firing on 4 cylinders again. Thinking that possibly this was because the
valve cover had been off a while, and the top
of the valves might be a bit cooler and it had stopped sticking, we replaced
the cover. The engine continued to run
perfectly for several minutes even being revved hard, on a short test-drive,
and a drive of about 20 miles or so next
day. So at the moment it remains a mystery.
One thing that did occur to me subsequently was that with the plug being
fouled, wouldn't that mean it wasn't firing? I.e. no HT reaching the plug
(we'd tried three plugs remember) despite it leaving the cap and going into
the lead. If it *is* firing with a sticking valve wouldn't that keep the
plug clean even though the effect of combustion is lost? Also another test
I could have done while it was running on 3 was listen to the misfire in the
exhaust while Keith alternately removed and replaced the plug lead for that
cylinder. If it *was* firing, but the exhaust valve was sticking open, the
misfire would surely make a very different sound with the plug connected to
when it wasn't. Whereas if the problem was simply that the plug wasn't
firing, there would be no change in the sound between connected and
disconnected.
Comments? Suggestions?
PaulH.
_______________________________________________
Edit your replies
Mgs@autox.team.net
http://autox.team.net/mailman/listinfo/mgs
|