Colour of the spark plug - setting up the carburettor


I'm running a 1967 Vespa 50N with a Dell'orto SHB 16.10 and a 52 main jet. Trying to find an ideal carburettor setup, I did what I read already on many places:

I set up the carburettor to the basic setup (3 turns out) and used a new spark plug to be able to identify what it looks like after driving (spark plug Bosch WR5AC).

After driving about 10 km, I checked the plug and it was of some clear brown-grey colour. I thought, everything is as it should...

But, after another 10 km, the plug had already a darker brown colour and started to turn black on some spots The washer on the plug isn't altered except the it looks a bit oily.

Is everything as it should be or should I change something?

Thank you very much!

I've been driving a 1:45 ratio, but switch now to a 1:50. Can a 1:45 ratio make it look so oily?

I recently replaced the oil seal behind the brake plate, but this doesn't cause oil to get into the cylinder...?

Do you think, there's a problem with another seal?


After another 20 km, the colour remained almost the same. The spark plug is still oily and one part still remained black. Everything as it should be?


1:45 is fine. in the races we drive 1:33 without a problem. i suggest, you check the other seals...

yes. looks pretty oily. what's your mixing ratio? are the oil seals intact?

drive more km; and check again. Make sure the engine gets to operating temperature

If your seal are broke you're oil will smell of gasoline. Broken seals can also causes to disrupt the vacuum in de crankshaft camber; it can cause situations where the engine revs without throttle


Here are some pictures after 40 km of driving. In reality, they look a bit less clear.

So what do you think?

What about the oil?