OK... here's the Puzzler:
The Car: '73 MGB, twin HIF4 carbs, crankcase breather connected directly
to front carb.
The Chronology: Engine rebuilt in February, no oil consumption after
break-in period, switched to synthetic oil 750 miles after rebuild. 3500
miles on first batch of synthetic, due for oil change. Oil is dark but
not opaque.
Friday afternoon: Thinking about my friend with the '72, complaining
about having blow-by already 2 years after his rebuild. Pull breather
hose to check blow-by... WOW! lots. Plug it all back up and meet family
for dinner. As I'm driving my mother around I notice an oil smell... her
husband (following in the diesel) notices puffs of oil smoke out the
tailpipe when I switch gears.
Saturday morning: Car is working fine around town. When I go onto
interstate on-ramp, billowing clouds of oil smoke stream out the back,
and I can smell oil burning. This lasts about 30 seconds and quits. 5
minutes go by. More oil smoke for a few seconds, then the smoke quits.
This happens a couple more times on the way down... each time the smoke
is billowing I also hear a clatter or pinging from up front.
I suspect oil being burned external to the engine since I can smell it at
60 mph. I check plugs at my destination: The first two show a moderate
lean condition with granularity (evidence of pinging??) with the rear
plugs slightly sooty but smooth. Compression check shows 160, 165, 170,
and 90 for cylinders 4-3-2-1! This was with the engine cooled down.
I richen the front carb 1/2 turn, and check the valve adjusters on #1
cylinder. They are slightly loose but no big deal. I drive back home with a
few more smoke incidents.
Next morning: I check the oil and buy a couple quarts. I'm low on oil!
I drive the 65 miles again, with more smoke / clatter incidences. It's
happening more often but is still intermittant.
I race the car in an autocross. The oil smoke is evident only in the
first run in once corner (but it's an enormous cloud). I check the plugs
after the run, and the rear plugs are the same as before. The front two
show an EXTREME lean condition and granularity. I also fracture the
ceramic on #1 plug when removing it. Compression check shows (engine hot
from race) 150-155-160-35 !!
Trip home: Continuous oil smoke now. I have to stop every few miles and
add oil. I finally wise up and 1) Disconnect breather from carb, plug
carb inlet 2) disconnect carb 1 from throttle linkage, and 3) leave oil
filler cap off (to vent pressure). Oil consumption drops significantly,
and in fact the oil level seems to rise a half-quart or so over 20
miles. Naturally since I have to drive slower at this point I am taking
a round-about route on back country roads.
I'll start tearing things down tonight, but would appreciate any
guidelines for what to look for:
Summary:
1) blow-by increased dramatically in a few days
2) Compression in #1 cylinder has dropped quite a bit.
3) #1 and #2 have been running lean, perhaps because of blow-by. Clattering
while smoking could have been pinging due to excessively lean mixture?
4) Problem was intermittant for a 1 1/2 days and then continuous.
5) Most of the oil consumption seems to have been via the breather system,
forced back into #1 and #2 cylinders. When pressure was relieved, oil
level seemed to climb slightly (1/4 to 1/2 quart).
6) Radiator fluid level is good, pressure is held, and no oil is seen in
coolant.
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire E Stock | '88 RX-7 C Stock
Home Page: http://www.widomaker.com/~trindle
"Anthony's Law of Force
Don't force it - get a bigger hammer."
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