What are the symptoms of running lean? I have the following problems with
my '74 MGB ('69 Engine & Weber 45 DCOE 9 carb). It misses/backfires under
hard acceleration and backfires loudly under engine braking. I first had
the problem about a month and a half ago when I autocrossed my B for the
first time. It ran great on the 130 mile round trip between here and
Columbus (OH), but it missed badly under hard acceleration. On the trip out,
I bottomed out and broke my exhaust pipe in front of the back muffler, so
this could have been the cause. Since then I have replaced the exhaust
pipe and muffler with a Cherry Bomb Glasspack. I have never rejetted the
Weber though.
I also had to replace a blown headgasket, I was losing coolant out the side
of the head, though compression was still good. After I did this, the
problem got worse, now I was running on only two cylinders most of the time.
I think this may have been because in removing the head I had to remove
the Weber and in doing so, I shifted some crud around in there.
So, this past weekend I rebuilt the Weber (basically just cleaned it out,
replaced the floats, needle and seat, and put new gaskets all around.) It
now runs okay, except that I get misses/backfires under hard acceleration
and backfires when I lift off the throttle. When I was working on the carb,
I discovered that the idle mixture screws are pretty rotted out (I have
new ones coming) and the only way to get it to run was to tighten one all
the way shut.
Here is my theory, the rotted screws caused the Weber to be rich at idle,
causing me to think that I needed leaner jets (I got new jets earlier
this spring one size larger air correction & one size smaller mains.) This
worked fine until I freer flowing exhaust which caused it to run leaner.
So, I think I need to put the old jets back in. Unfortunately, I can't
find them (see, I am a wanker :-). So, before I go and order more jets,
I thought I'd see if what I am thinking sounds reasonable from what
I have described.
Thanks,
Bob
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