Hi

Just changing over the breather arrangements on my 1310. I have a timing cover breather venting to air or catch can ( if track day), catch can is always unpressurised - and a rocker cover breather(RCB)

old system connected RCB to one of the HS4 carbs and blocked the other carb hole.

new model - RCB connects to both carbs with a y piece ( no PCV) valve)

i had the carbs off and on resetting idle noted a significant drop off of idle with oil cap off (vented) - not too much of a surprise but

checked vacuum with palm of hand over oil fill - and it is quite strong

my question is if the vented oil cap is not letting in enough air - will the rocker breather start to suck air from the carb tube?

i know this situation is at idle with a high vacuum and could reverse at higher revs? but wondered if there can be too much vacuum??

Vacuum also helps keep the oil on the correct side of the seals. Pressure will push oil out.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

I run a vented cap on the 1360 and an unvented one on the 1412 stroker. Vacuum is not a problem, whether running to the SU vac port or a manifold PCV valve and a Dellorto.
As said above the vacuum helps keep the oil in, on a road car anyway.

If there is NO vacuum under these conditions at idle, your rings and bores are past it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.