spark plugs uneven burn

[timberlite]

recently replaced 4 new plugs on my carb engine - correct plugs specs based on the service manual. after running for about 50kms saw the plugs' color are not the same:

plugs no. 1 & 3 are very dark, carbonized
plugs no. 2 & 4 light to dark brown. - looks normal

we checked the compression pressure but cylinders are nearly equal above 155psi.

what could possibly be wrong? what should i check next?
is valve clearance related?

thanks in advance for your ideas or suggestions

[Ungas]

It could be possible that the valves don't fully close with respect to its clearances. Or valves in 1 & 3 chambers don't fully open to let the air or fuel in.

Have you checked on air and fuel delivery?

[jackaroo]

Also check hi tension cable resistance, your spark may be weak on 1&3.

[timberlite]

Originally posted by Ungas
It could be possible that the valves don't fully close with respect to its clearances. Or valves in 1 & 3 chambers don't fully open to let the air or fuel in.

Have you checked on air and fuel delivery?

thanks Ungas,
so i'll have the valve tappets corrected? could this be related: sometimes when i pop the hood, i hear a slight "tst-tsk-tsk" sound under the engine cover.

did a check on the air & fuel delivery. saw the stock paper filter looks clean enough; used carb cleaner spray on the carburetor's mouth (some sludge/dust removed from the choke)

Originally posted by Jackaroo

Also check hi tension cable resistance, your spark may be weak on 1&3.

thanks Jackaroo,
i'll also look into this. high resistance, weak spark? anyway, i'll compare the wires 1&3 to 2&4, they should be equal right?

i'll get back to this thread once check-ups are made over the weekend. thanks again!