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MG Midget and Sprite Technical - Wrong filler cap

Hello,
Could anyone please tell me what happens or what you would notice if you fit the wrong oil filler cap (ie) non vented as opposed to vented.

Thanks for looking. Wayne.
W Williams

back pressure would build up in tank so it would feel like fuel starvation.
if it is running lumpy stop - open and refit the cap see if it improves.
Mick struggling with the wiring

Mick - oil cap not fuel cap !
David Smith

I assume you're talking A series not a 1500? The 1500 caps aren't vented. I would suspect if you had sufficient crankcase pressure, ie worn engine, that the normal breather was unable to cope with you'd have all sorts of oil leaks. On the 1500s there's positive crankcase ventilation this is provided by a connection to the carbs. The slight vacuum provided by this stops oil leaks I'm not sure of the method used on the A series.

Bob
R.A Davis

It'll be interesting to see if there's a definitive answer to the OP question or whether it's one of those that creates speculation and arguement but the real answer is lost in the mists of time or the original purpose has been lost by developements (like the reason for the 'flame traps' on the RV8)

I don't know the answer only it's best to let the engine breath as designed and not to stifle the breathing for best performance
Nigel Atkins

oopppss that'll teach me to put my specs on when on the pc
Mick struggling with the wiring

fitting a non-vented cap to an engine that has crank case ventilation (vacuum drawing on the front cover's oil separator from either the base of the carbs, or from the intake manifold via a PCV valve), could, in the worst case, result in oil being extracted from the engine and ingested into the intake (messy and smoky), or, in some cases, on the valve cover gasket being sucked in, resulting in a suddenly leaned out air/fuel mixture (and an oil leak from the valve cover lack of seal, on shut down).

I have seen, and heard of both, from first hand experience.

Or, it could result in nothing bad at all (also seen that).

The difference depends on how much that particular crank case leaks air in the first place. The intention of the vented filler cap (or, on the later cars with charcoal canisters, the orifice in the valve cover that does the same thing), is to meter the incoming air so that enough is provided to avoid creating too much vacuum inside the engine, and balance that with a controlled vacuum "leak" on the intake air/fuel mixture (a leak is an un-intended leaning, this metered flow is adjusted for, so it is not really a "leak").

The crucial thing (as in most things in life) is balance. Too little vacuum (or positive pressure from too little breathing) results in oil leaks. Too much vacuum can cause trouble too.


Norm

"Although variety's the spice of life, a steady rhythm is the source. Simplicity's the crucial thing, systemically, of course." - Brian Eno
Norm Kerr

Cheers Norm

I can follow what you've put and even the Eno quote (on second reading)

It reminded me that I had a car that drank its own life blood for a while, Mobil 1 at that

it baffled one 'expert' for 5 days and took a real expert half an hour to find, even then Mobil 1 was too expensive to waste
Nigel Atkins

Not seen the drastic problems that Norm has witnessed but I would expect not too much trouble with the fitting of a non vented cap. If the cap is non vented then if air could not be sucked in anywhere else then no flow would be experienced in the engine so what? not sure that would happen as air would be drawn in through the rear scroll and ought to help cure leaks in this area

However there must be a reason why you ask? what problems are you having?
Robert (Bob) Midget Turbo

This thread was discussed on 16/04/2011

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