MGB rocker cover pressure

[shaunmorris]

Hi,
About a month ago I bought a standard 1978 MGB, I have noticed that it uses a lot of oil and blows smoke under heavy acceleration but not when cruising at steady speeds. I should mention the engine was rebuilt about 500 miles ago and produces 94bhp on a rolling road. This evening I noticed that if I remove the oil filler cap with the engine running the tick over increased and the engine runs much smoother and if you place your hand on the filler you can feel the engine sucking your hand to the rocker cover. At this point I started to rev the engine and notice that without the cap the engine was not smoking when revved. Replace the cap and the tick over drops engine runs rough and starts to smoke when revved. question is does this point to worn valve guides ? is the engine sucking oil into the cylinders due to the increased negative pressure with the oil filler cap in place ?

Any thoughts or advise ?

Many Thanks

[shaunmorris]

just realised although I said the car was standard, the previous owner fitted one of those polished rocker covers with a vented cap. vent is very small could this be the problem.

Thanks

[shaunmorris]

Thanks, sounds like a plan, I have ordered a cap, I will let you know how I go on.

Many Thanks

[shaunmorris]

nta16, you got it spot on, put a new original filler cap on the car, went for 45 minute drive and no smoke !
PS my B is shiny red and you will see it on the road all year round.

Thanks