Hi, all can anyone tell me the correct main jet size for 1977 mk3, twin amal mk1 carbs and blackcaps , i have checked a few books and there seems to be a few different sizes ie.220s 230s and 260s i have came across. Bike is fitted with 220s at the moment , as always thanks for any replies Brian

Hy brian!

It is also depending which valves you have [diameter from the inlet valves]
I got 41mm inlet valves and I go best with 280 mainjets.

But in my opinion best way is when you go on full throttle on a save road for a few minutes and then abruptly shut the engine off.
Take the spark plugs out and have a look at the colour they have.
The should be light brown – greyish – to white…..
On internet there are several homepages where you can find the right colour that spark plugs should have when mixture is right [not lean not rich].

For example:
http://www.spark-plugs.co.uk/pages/tech ... gnosis.htm

hope it helped….

Have a nice day and greetings from Austria

chris

Nortons are sensitive to exhaust and intake flow, so Chris is correct in recommending a full throttle plug chop. Typically if you're too lean on the main you have a hard time reaching 90 mph on the highway and if too rich you get black sooty plugs.

I think the stock MKIII used 230's? LAB will know...

maylar wrote:I think the stock MKIII used 230's? LAB will know...

I know the MkIII factory manual says they're "230" and the MkIII parts book says "220"!