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MG MGB Technical - Engine runs best with rich fuel mixture II

This is a follow up on a thread I started a few weeks ago about my rebuilt engine
running best with a rich fuel mixture. The last time I reported I tested with some
altered AAL needles and ATF damping oil in carbs and reported that the car was
running ok. I reworked the mechanical advance in the original 41155 dizzy so it
would reach full advance around 3000 to 3200 rpm. I ordered a Edelbrock A/F
monitor and installed it to see what I could learn. I had the sensor plug welded in
the front exhaust down pipe about 6 inches below the exhaust manifold. Idle speed
showed A/F around 13:1 and cruise in the same general range. On very slight
acceleration the monitor lights would all go out and come back when when normal
cruise was resumed. Average reading was in the 13 to 13.5 range. The engine ran
well during this run. On completion of the test run the spark plugs looked rather
white/lean

Changed to AAA needles, 20-50 damping oil and NGK BPR5ES plugs (higher heat
range). Another road test showed the same idle and cruise readings of around 13 to 13.5.
On very hard acceleration the monitor lights would all go out and quickly come on
and still settle back in the 13 to 13.5 area. I could never get a reading in the 14 range
even though I tried different speeds, overdrive in/out, etc. All my tests were on a
divided controlled access highway will some slight grade changes. The engine ran
well on this test and plugs showed mostly brown. I tried to adjust the idle A/F readings
with the jet adjusting nuts. When I turned them in the A/F readings would not change
but would begin to flicker and then go out. Turning them back would bring the A/F
lights back and settle around 13.

The engine is running great but if I believe the A/F monitor I need needles leaner than
the emission needles at idle and cruise and some that go much richer than AAA's
for acceleration. The A/F monitor no longer resides on my car. I'm beginning to think
SU carbs may be, to some degree, self compensating.

Thanks to all those who made suggestions on the previous thread. The hotter plugs,
careful carb readjustment and reworking the dizzy advance curve seems to be what
I needed. Regards, Clifton

Clifton Gordon

The chemically correct (stoichometric - sorry I cant spell) AFR is 14:1, so you are a little rich for no load. I would expect this to drop to about 12:1 at high rpm and even 10:1 at max power. It should also richen under transient. At light cruise you might get away with leaner, but the combustion will probably be unstable on an older design like B series at 18:1.
Paul Hollingworth

Clifton, I ran into the exact same situation last fall when I installed the same Edelbrock meter on my header, on my freshly rebuilt engine. I asked the machinist who rebuilt the engine his opinion, and his experience was that until the engine is broken in and the rings are seated, the gauge will not read properly. I'm not sure how many miles you have on your engine at this point, but I only have about a hundred on mine, and besides a little oil blow-by, I'm still playing with distributor advance, engine timing, testing ignition coils...
Another option is to play with ignition timing. A properly timed spark will require more fuel for the perfect burn, and of course vice versa.
Jeff.

Jeff Schlemmer

Jeff. I have around 500 miles on the engine. I'm also still trying to find what the
engine likes best for distributor advance curve and timing. No rolling roads in
our area. Clifton
Clifton Gordon

Cliff,

I agree with Jeff, it may be too early to get a reliable reading. It was not until my rebuilt engine passed the 5000 mark (and a couple of oil changes) that it seemed to "settle down." While it was breaking in the Champion N6Y plugs (provided by the enigne builer) looked normal and it consumed about one pint of oil per 1000 miles.

With around 8000 miles on it it consumes one pint of Mobil 1 15W-50 per 1300 miles, the plugs are normal (i'm using NGK BPR6ES) and it seems to have lots of reserve power while crusing at 65 mph.

Make sure that it's not running too rich, vary your speeds, don't push it too hard, run mineral oil until you have at least 2500 miles, torque the head at 1000 get a magnetic oil plug, and change the oil and filter at 1000 and then at 1500 intervals until your satisfied its broken in then switch to synthetic oil and K&N oil filters.

Just relax and sit back and take the time to smell the roadsters along the way.

rn
RN Lipow

This thread was discussed between 14/02/2002 and 17/02/2002

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