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[Ben_nz]

One or two of the spark plugs on one side of my engine were often black and the motor's a bit rattly, so I bought a cheap compression tester and did a test.
Assuming this is how the cylinders are numbered:

Front of the car

2 - 1
4 - 3

Rear of the car

the initial compression test results I got were
1: 165 psi
2: 162 psi
3: 145 psi
4: 120 psi

According to Haynes and the compression tester instructions, those readings aren't encouraging. I tried cylinder 4 again just incase, then sprayed a little WD40 in. After adding WD40 I measured 140psi in cyl 4, then 153 psi in cyl 3, then 133 psi when I rechecked cyl 4.

When I open the oil filler cap while the engine's idling, the patented Alfa boxer hurricane howls out of the tube, but this reduces significantly as engine revs increase.
The sooty spark plugs were in cylinders 1 and 3 (i.e. the same carburettor) so seem unrelated to the compression results.

Do I worry about the apparently uneven compression? Manual suggests it's piston/ring related because adding oil improved it, yet the engine uses no oil.

Why would the spark plugs on one side be blacker? Could the plugs blacken due to idle mixture in the 20~30 seconds of idling I do as I coast home before turning the engine off?

[paulhide]

If it were carbs it might be a matter of tuning. Is the black smooth and shiny like burnt oil or more sooty?
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http://www.alfa-pages.co.uk/viewtopic.php?t=5188

[Ben_nz]

I believe the black is sooty kinda black.
I have fooled with the idle mixture screws a bit (without much method to my madness) and swapped the plugs all around. I'll recheck them next weekend.