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MG MGB Technical - Idle changes with valve cover cap off

I have freshely rebuilt motor wtih a mild cam, rebuilt su carbs, header,stock alum. head. The motor runs fine (except not as much low end power as I thought I would have).I noticed that with the car idling if I remove the oil filler cap that the engine wants to slow down and try to stall. The valve cover is alum. with a vent tube that has afilter on it, no charcol cannister. Why would I have such a vacum under the cover? It seem that when I had a down draft weber and cam timing not set properly that I actually had positive pressure. Could this affect my low end power. Is this indicating something failling?
Ben

A new engine with a properly configured crankcase ventilation system should have negative pressure within the engine. A worn engine will have blow-by such that there is positive pressure in the crankcase/under the valve cover.

Does the valve cover vent tube have the metering hole in the end or is it fully open? This may explain the tendency to die at idle with the filler cap removed (which I am assuming is non-vented). I'm also assuming that there is no PCV valve and the tappet side cover is plumbed with a "Y" connector into the two carbs. Removing the filler cap in this instance creates a vacuum leak.

Never have seen a stock alum head, but the absence of low-end torque may be a result of a lean mixture (further enhanced by removing the filler cap). Also, a "bigger" cam usually (in my experience) needs more initial spark advance, and the aluminum head usually allows more total mechanical advance. Bigger cams, as you know, move the power band upward in the rpm range, and if installed "retarded" move it further. Was the cam "dialed in"?

Need a little more info.

Wayne
Wayne Pearson

Thanks for the response Wayne. By stock Aluminum head I meant no Porting or Polishing just checked by the machine shop for flatness and valve seating, which was out right from the box. The vent tube is fully open and the cap is vented. The Y is connected. The cam is dialed in with no advance or retard. The distrbutor curve was checked. I really can't run any more intial spark than my stock GT with cast Iron head. I did go up one size in needles as I am running K&N filters. Plugs indicate proper mixture.
bs stone

Are you using a PCV valve? With a filtered vent tube and a vented oil-fill cap, you essentially have a vacuum leak feeding the hoses off the Y connect to the carbs. There is no metered air flow.

Are the combustion chambers of the aluminum head the same as the cast iron one? If you replaced the pistons, are they high or low compression ones?

Wayne
Wayne Pearson

The pistons are 8:8 to 1, the chamber I think are a little larger cc than the cast iron. It is the msx head that all the suppliers sell. So I should just block off the vent tube on the valve cover and re-adjust timing and mixture ?
bs stone

If you're going to use a vented oil fill cap and valve cover vent tube open to atmosphere (with filter), then you need to use either a draft tube and not hook the carbs to the crankcase vent, or use a pcv valve to control the air flow from the crankcase to the carbs.

Depending on the combustion chamber size of the MSX crossflow head, you may have a lower compression ratio than you think - which can account for the lack of low-rpm torque. Or the fuel mixture is weak, or there's not enough spark advance, or the cam has too much overlap, etc, ...

Have to rely on someone experienced with the crossflow head to comment further.

Wayne
Wayne Pearson

You should always get a change in idle note when removing the oil filler. As previously mentioned engines from 1964 with PCV hold the crankcase at a slight negative pressure. Althoiugh there is a through-flow of air to purge the crankcase of fumes the fresh air inlet is both filtered and restricted. On non charcoal canister cars this filter and restriction is in the oil filler cap. On charcoal canister the restriction is in the port on the back of the rocker cover and the filter is in the canister. Removing the oil filler cap bypasses the restriction (and filter) and acts like a vacuum leak into the manifold.

Regardless of head if you have carbs with the PCV ports it makes no sense at all not to use them (and even less to fit a PCV valve). Fit a small filter on the rocker cover port if you had a canister and have removed it, or use the standard vented filler cap if not. You could block the rocker cover port and use a vented cap, but that is likely to cause problems in the future when sopmeone buys a non-vented cap for it thinking it is the right one. If you don't have positive crankcase ventilation you can get a build-up of condensation which causes internal corrosion, which is why MGBs had PCV from 1964!
Paul Hunt

I believe the cap is also vented so maybe I should just block off the vent tube, which should give me a slight vacum.

Thanks for all the help
bs stone

The vented cap is already restricted, as is the port on the back of the rocker cover, and it is that which gives the slight negative pressure. If you block the flow of air into the crankcase you will get no through-flow, hence no ventilation, and consequently the same build-up of condensation as if there were no ventilation at all!
Paul Hunt

This thread was discussed between 16/03/2009 and 20/03/2009

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