Triumph Repair: engine oil blowing out of dipstick tube-1978 MG Midget, crankcase ventilation, crankcase pressure

Question
I have been the owner of a 1978 MG Midget (1500) since 1980.  It has had a small oil leak for some time (mainly from the front seal) but the leak was only a few drops and only a nuisance.  

My problem is that I had the oil changed recently and when I returned home (less than 15 miles) I smelled burning oil, popped the hood, and noticed a large amount of oil blowing back through the dipstick. After checking the oil level and noticing that it was too high, I brought the MG back to the mechanic to correct the oil level, which he did and I was confident that my problem was corrected.  However, upon returning home, the oil blow back through the dip stick was still occurring (The oil level on the dipstick was correct).  Therefore, I brought the MG back to the mechanic to determine the cause. (This is a very reputable shop but with without much MG experience.) Now I am told that this blow back is normal for a MG Midget and that I should have a gourmet around the dip stick, similar to the MGB.  Please advise.  I have never had this problem before.  Could driving it 15 miles, averaging 35-40 MPH, with an overfilled crankcase cause damage that would result in this problem?

Note:  I did have the carb changed out to a Weber but that was 15 years ago.

Thanks,
David

Answer
None of that would have caused this.  More than likely the grommet was lost when the oil was changed.  Also make sure the dipstick is fully seated.

Oil blowing out the dipstick is caused by too much crankcase pressure from cylinder blow-by (weak rings).  You can make sure you crankcase ventilation is up to snuff, and that the valves/hoses on the emissions is working, but try a new grommet first.  You don't have to get a factory original, you should be able to find an o-ring or rubber washer of the approximate correct size.