John,
I have the same problem with our Vitesse, which has virtually the same
engine as your TR250. I have not been able to isolate the problem,
however, the main causes of oil burning are due to:
(a) oil getting past the piston rings. This can be checked by doing a
compression check with the engine at operating temp. If the compression is
below spec., then the check can be repeated having squirted a little engine
oil in each cylinder. If the compression reading rises appreciably, then
this is probably the cause.
(b) a blocked PCV valve. This can be checked by removing and seeing if the
valve can be heard to rattle inside the body. You could probably also
check by removing the pipe from the rocker cover and put your thumb over
the end to feel for a vacuum.
(b) oil entering through the valve guides. I don't know what the
diagnostic test is for this, probably just process of elimination. My
engine had done 90,000 miles when I had the guides and exhaust valves
replaced,. The guides (in fact all the valve gear was in pretty poor
shape).
In my case I tested for both (a) and (b), but neither seemed to be a
problem. I therefore assumed the valve guides were the cause. The bad
news is that the blue smoke on decelleration still persists. I wonder now
if I should have replaced the inlet valves as well - although the machinist
said they were probably OK. I have tried adding Morey's oil additive,
which seems to have a good reputation, but in my case it seems to have had
little/no effect.
Other more hand-wavy explanations I have heard are (c) oil dilution caused
by the engine running too rich and (d) oil pumping due to the oil control
piston rings not sealing correctly. I don't know how plausible these are.
I have been pinning my hopes to (c) for a while, but feel I am probably
clutching at straws. I will know for sure soon when I fit a set of
re-built carbs.
Hope this helps, Pat Edwards.
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