Sounds identical to the problem with my 77 MGB. I took it apart last
week and found a burned piston and shattered ring on #3. #3 compression
was 90 cold and about 50 hot with 100% leakdown. I found a set of
pistons with rings and pins for $148. The rebuild should cost a mere
$250 with new mains and rod bearings. I chose to leave the engine in the
car and just hone the cylinder walls (they were OK even after the burned
piston and honed nicely, taper was within specs).
On Mon, 21 Aug 1995, John M. Trindle wrote:
> 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."
>
>
|