MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Engine breathing

Been trying to tune the carbs and struggling to get a nice idle. When I remove the oil filler cap the revs increase significantly (and I can get a very nice idle!).

The engine seems to be sucking in quite a bit of air through there. Its an 18GB engine with the mushroom breather assembly if that helps.

Any ideas?
N McGurk

Mine is an 18V, but it is probably similar. The oil filler cap is a service item but usually ignored, so it might be worth replacing this first. What idle speed are you trying for? Most cars struggle to idle at "book" figures. FWIW I have mine at 850. After that it is a matter of following the tuning procedure, assuming the carbs are in good nick and you have no air leaks etc.
Michael Beswick

Hi Neil,

I am not familiar with the breather set-up on the GB engine but assume it must have a link to the induction manifold if you are getting suction rather than pressure in the crankcase. Is there a device to control the amount of suction (eg PCV) and is it working correctly?

I am interested in this topic because of a problem I have with excessive pressure in my 3-main engine.

I have posted aseparate thread on this

Regards,

Andy
Andrew Dear

I am not trying to achieve specific idle revs, just a smooth idle and was about to increase the revs when I noticed the difference removing the cap made.

I just changed the cap, it is the correct vented cap and doesn't seem to be the problem as the revs drop as soon as it is even close to fitting.

The engine has the mushroom crankcase emission valve. Any ideas what to check?
N McGurk

Check for excessive play in your throttle shafts. If you can move them up and down, they are probably allowing air into the intake manifold and leaning out your idle mixture. From your description, the PCV valve and breather cap are functioning correctly. RAY
rjm RAY

Up to 18GA use an open system: filtered air flows from the inside of air cleaner through the engine and out the draft tube. The draft tube supplies a slight depression at road speed, but none at standstill, so the flow is indefinite at standstill. There may then be a small reversal of flow, such that oil is deposited inside the airfilter under these conditions. Not a problem until the engine is quite worn.

18GB and some later 18G* engines use a closed system, with the mushroom valve. This is a sophisticated PCV valve; I think either Paul Hunt or Barney describe the function well. The flow is through a filtered & restricted rocker cap, through the engine into the inlet manifold under the control of the valve. Remove the valve, take the top and diaphragm and spring out, clean all, replace diaphragm if necessary. There is a check valve and a metered orifice inside, which must be clear and free. Removing the cap or wrong cap upsets the vacuum characteristics of the valve. If the engine speed increases a lot, it usually means that the mixture is rich; if it stalls it is probably but not necessarily lean. But in either case the valve is not quite right.

Later 18G* and 18V use a different closed system. Air flow is through a filtered/restricted cap or charcoal canister/rocker restriction into the rocker cover, through the engine, and out the side cover oil separator into the carbs at the constant depression area.

FRM
FR Millmore

I'm with FRM on this one.
The valve is the same/similar to the one fitted to my 1300 and I have a devil of a job getting it to idle properly and it stalls when coming to a halt because the valve stays open and a weak mixture results.
Strip it, clean it and try a new diaphragm, making sure the small hole in the top is not blocked.

Essentially the valve is open when the pressure in the manifold is near to atmosphere (or as close as the metering spring pushes the valve open) which typically happens when accelerating and cruising. At idle and overrun the pressure in the manifold is much lower (ie vacuum) and hence the valve should close as a result of the atmospheric pressure 'pushing' against the outside of the diaphragm.
Try setting the carbs with the spring removed and the valve in place, it'll be easy to set up and will confirm if the valve is suspect. Another fix is to remove it altogether and vent via a catch tank to atmosphere.

MGmike
M McAndrew

This thread was discussed on 10/05/2012

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now