vacuum advance and rocker breather

[horrace]

Following on from my previous electronic ignition posts I now have the champion spark plugs in....
whilst leaning up the mixture I noticed if I remove the rocker breather which goes to the carb and blocked up the hole in the carb (with my finger) I could make the mixture more lean and it would still run nicely but if I remove the finger it would stop.
Plugs are a bit sooty black so I'm guessing its running a bit rich...
I already have a crankcase breather in the tappet cover on the side so do I need another in the rocker? and would it be better to not have the carb connected to the rocker breather pipe?
also the vacuum advance is not connected, would it be better if it was?
I have electronic ignition
hs4 carb
maniflow inlet and exhaust
1275 head
thanks guys

[oliver90owner]

Any hole into the manifold will allow air to enter, thus weakening, not enriching, the mixture. If in front of the venturi of the carb, it should make little difference to the air flow as long as the pressure drop across the air filter is at a minimum, I would have thought; if after the venturi/fuel jet it would weaken the mixture.

Sooty plugs are not good. Could be due to an over-rich mixture, a cold engine or wrong grade of plug.

The vacuum advance is there for a reason. Disconnected and open will clearly allow air in, weakening the mixture, although a smallish leak, in comparison with a breather pipe but possibly at higher vacuum.

The standard spark advance characteristics may not be ideal with any non-standard set up but were the best compromise of the day. Think here the difference in octane rating of modern fuels cf those when the engine was new, per eg.

Doubtless(?) bmc scotland will along with specific advice on your particular example.

RAB

[bmcecosse]

Yes - you want that rocker breather to the carb - it helps prevent leaks and nasty carcinogenic fumes and smells, and yes - you should have the vacuum advance connected. It's partly there to avoid the need for excessive static advance, partly to 'brighten' the engine when you open the throttle, and partly as an economy device for part throttle cruising. All in all - it's a good thing! Check the plug colour after a good hard run - shut off the engine and drift to a halt (safely of course) and whip out a plug or two to see the running mixture - without any idling. Don't get too hung up on idling - it's only about 1% of the engine running revs.