PROBLEM SORTED

[garry h]

Spent a couple of hours checking over engine and found that by removing the new oil filler cap i fitted she ran fine,no smoke and smooth. Seems she was not able to breath properly so back on the road again happy days 

[Iancider]

Ahah yes - the old crankcase breather problem - well spotted.  Old addage "if it ain't brok don't fix it!"    

[Moleman]

That's really good to hear Gary. 

['Mendip Wurzel']

I know crankcase pressure can make a difference but I have never quite understood why?

The crankcase pressure on my Lomax (2CV engine) is crucial otherwise the tickover is rubbish. It actually run with a negative pressure to stop oil leaks.   

[Bulldog Bri]

New one on me!

[Iancider]

It is not common on modern engines now because there is usually a specific breather rather than relying on a cap vent.  But if you have no crankcase vent the blow-by on combustion that gets past the piston rings can cause considerable crankcase pressure.  The easy way out for that pressure is down the valve guides when the inlet valve opens.  This now causes high pressure exhaust gas to flow the wrong way up the induction port.  It reduces the fuel-air flow and there is no oxygen in it so it suppresses combustion as well.  The engine will also burn a lot of oil when that is forced down the valve-guide into the combustion chamber and the rocker gasket cover will usually spew oil as well.  So it is a little thing to miss but big consequences if you do.   4 to 2 to 1 exhausr manifolds help because the exhaust venting gas causes a slight pressure reduction in the induction path ans will also reduce the crankcase pressure  On two cylinder engines you often seen the two exhaust pipes coupled together to achieve the same effect.

To test for it just pull the filler cap off with the engine running - it there is a big phutt there is too much pressure and if you feel a big pulsating draft with you hand near the opening it is a good indication that the piston rings might be past their best so even with venting it may not be enought to overcome the piston-blow-by.

[garry h]

Well explained Ian and agree with your last post aint broken leave it alone lesson learned 

[Lucky Ed]

I know crankcase pressure can make a difference but I have never quite understood why?

The crankcase pressure on my Lomax (2CV engine) is crucial otherwise the tickover is rubbish. It actually run with a negative pressure to stop oil leaks.   

How do you get negative crankcase pressure........ I could do with some of that

[Iancider]

How do you get negative crankcase pressure........ I could do with some of that

Fit a supercharger to the exhaust and point the tail-pipe backwards for more thrust of course!