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MG MGB Technical - Is my oil filler cap vented ?

Hi all.

My 80 BGT (UK SPEC) has an alloy rocker cover with a large chrome filler cap. The cap has a hole pressed into it's spring loaded inner part but there is no sign of a vent hole on the outer part (ie the part you can see when the cap is on).

Does it vent through the gap between the inner and outer parts, or is it unvented ?...

Can anyone think of a way of testing this ?...

If it is vented, could the venting be too generous, hence weakening the mixture at high vacuum ?.
(I have a problem with pinking at low revs which can be cured by backing off the ignition timing, the distributor centrifugal advance doesn't have any play in it).

Would a PCV valve help ? (I have the Y piece feeding the HIF4's)..

What is the meaning of life ?.. (it's OK, I know that.. it is '42')

There is no sign of oil around the cap.

Thanks in advance (no pun intended) .. Don
Don

With early 3 main engines, if the oil cap is removed while the engine is running, there should be a rise in RPMs ---

Indeed, quick look at Moss catalog says:

18GA thru GH engines are vented
18GJ thru V engines are NON-vented
The 18G engine just has a steel cap, (whatever that means.)
glg

Clean it off and try blowing through it.
J Moore

Hi.

I tried blowing through it (still haven't got rid of the taste...) and it is clearly vented.

I reduced the hole in the inner part (it was about 3/16 in) to about 1/16 in, but the pinking remains.

I now have the distributor out and am wondering how to remove the centrifugal advance springs....

Thanks for the input... Don
Don

Don. Remove the points and condenser. Disconnect the vacuum advance can's spring from the post on the base plate. remove the two cross point screws holding the base plate down. Remove the centrifugal advance springs. This is for the 25D dizzy, but the 45D series is similar. Les
Les Bengtson

Don, LOL! Are your lips still black! It does have a funky taste, but know you know!
J Moore

Hi..

J.. It reminded me of an old girlfriend who was a biker, but that's a different story...

Thanks Les, my problem is how to unhook the springs..

I will order some new ones and may have to cut the old ones off.

Don
Don

A UK spec always had vented (and filtered) oil filler cap (and fuel filler cap) as it never had the carbon canister and emissions kit. On all UK models removing the oil filler cap should cause a rise in rpm as the venting is restricted and not free. An aftermarket cap could be non-vented, or too freely vented. As you can blow through it it is obviously vented, so as long as you get the rise in rpms if you remove it it shouldn't be too freely vented. If you get no rise it may be too freely vented, this would normally be taken account of at idle when tuning the carbs, but could well give a different mixture profile as the throttle is opened further.
Paul Hunt

Thanks Paul.

I will try the 'remove the cap and watch the revs' technique when I have got the distributor back in.

It seems that my engine is currently sucking in small quantities of unfiltered air, which is less than ideal.
The filler cap has a larger diameter (2 inch hole) than the standard one, does anyone know of a source of filtered and vented caps for alloy rocker covers, or some other solution ?.

Don
Don

I've come across a related problem. See the thread "Idle with pertronix". There is a lope in the idle that seems to have been exaggerated with the addition of Pertronix - but I'm not sure I remember how it idled before. At any rate, on this 74.5 US spec GT (rubber bumpers with HIFs and carbon canister) the idle speeds up slightly and smooths out almost totally when I remove the oil filler cap. This happens with or without the carbon canister hose attached to the rocker cover vent. The vent is not blocked. A 5/64" bit will go through, but a 3/32" will not - just as with other 18V valve covers. I recently put a new head on this car (a pre-smog head) and the valves are set correctly. Ignition timing is about 12 degrees BTC, plugs are clean and gapped at .035, and carbs freshly balanced. I experimented with leaning out the carbs to the point where blipping the carb pistons caused the engine to die, then I richened them up until blipping caused a very slight rise in rpm before a return to normal. I must emphasized that, except the annoying idle, this car starts very quickly and runs GREAT!
Allen

Allen - the vent in the rocker cover has a restricted apperture as you have found, whereas that via the canister has a large opening to the atmosphere so there will be negligible depression in the canister or its hoses. Thus removing the canister pipe makes no difference as the air still has to go through the restriction on the rocker cover, but removing the filler cap removes the restriction allowing a greater flow. Not sure how it is related to fitting the Pertronix (which makes great claims for an improved idle) but your description of a 'lope' could be due to a rich mixture which removing the filler cap 'corrects'. Rule-of-thumb mixture setting is first two turns of the jet nut (HS) or jet screw (HIF) down from flush with the bridge, then small adjustments to find the highest idle speed, then fine adjustment using the lifting pin to get that 'brief rise in idle then settle back' condition rather than 'stays up' (rich) or 'dies' (weak).
Paul Hunt

Thanks Paul,

I'm about to recheck everything!
Allen

This thread was discussed between 28/10/2003 and 05/11/2003

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