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MG MGB Technical - Engine blow-bye

When the rubber tube between the the valve cover is disconnect from the air filter on my 1963 MGB it run better, but when its together it runs rough and I smell burnt oil flumes. Why does this happen.
I know its a dumb question.
but thanks for any answers ???????
David

The oil and unburned gases are fouling out the plugs and messing up the motor. Nothing that will harm it, but not good either. This blowbye is a sign that the motor is getting possibly a little weak. Vent it to the open fresh California air, maybe as far as putting its own filter on it and leave it that way. I have seen several clients cars with the standard filter soaked in oil and a poor idle. SInce engine rebuilding is not needed yet, we just vent them away from the air filter without any real issues.
FWIW,
-BMC.
BMC Brian McCullough

David: If you still have the original engine, there should be a draft tube extending from the side tappet cover down along the block. The negative pressure generated under the car as it moves along draws the crankcase air through this tube and out, with "replacement" air coming through the air cleaner connection. Sounds like your draft tube and/or oil separator are blocked.
Andrew Blackley

Bye-bye blow-by.

I'm sure that is not correct, Andrew. The pipe to the air cleaner is so the engine can burn any fumes from the engine via a rocker cover vent, fresh air being drawn in via a filter in the front tappet cover. The port in the filter box being on the fresh-air side of the filter would allow dirt to be drawn into the engine if the air-flow was the other way. Some time later (early 64?) a more positive ventilation system was introduced that used manifold vacuum via a valve connected to the front tappet cover and a vented and filtered oil filler cap. Later still the vacuum came from restricted ports on the carbs and a Y-piece which did away with the troublesome PCV valve.

On the earliest system, which David seems to have, there should be no effect on the engine whether the pipe is connected or not, but fumes should be more likely with the pipe disconnected. On the face of it it sounds like there might be a combination of the earlier and later systems, which because inlet/carb vacuum is used will affect the mixture if there are any leaks in the system.
Paul Hunt

Paul: Sorry to disagree, but I am pretty confident that the information I gave is correct. The NEGATIVE draft tube was a common feature on most engines prior to the introduction POSITIVE Crankcase Ventilation.
Andrew Blackley

If I recall...the set up for MGA's, Magnette's and early B's all had the road-draft tube vented from the side cover, and a fitting from the rocker-cover to the non-filtered side of the aircleaner assy.

The side cover vent was not filtered on these cars and there is no oil-separator built into the cover, just a plate with a vent pipe.

As I understand it the concept was that the road-draft tube ventilated the crankcase at speed, and the rocker cover/ carb pipe provided some suction when stationary. The rocker cover pipe was to the outside of the aircleaner so that all that nasty air pulled up through the draft-tube would be filtered before being pulled into the combustion chambers.

I can think of no reason why pulling off the rocker-cover vent hose would make David's car run rough unless as Brian suggested there is just lots and lots of blow-by.

David...when you pull of the breather hose and open the filler cap how hard does it blow at idle and how much oil is in the vapor?

Cliff
Cliff Maddox

If there is a lot of blowby in the engine there will be pressure in the base if the vent system can not handle it. When the piston is on its up stroke exhaust blowby will be reversed and the gasses will be trapped in the combustion chambers after the exhaust valves closes and it will be present and will F**&^& up the air/fuel mixture and cause poor ideling. When the piping is removed there is more venting of the base and this does not happen thus better ideling.
Cec

Paul Hunt:

<<<Snip>>>
The pipe to the air cleaner is so the engine can burn any fumes from the engine via a rocker cover vent
<<<snip>>>

You'd better read this:

http://mgaguru.com/mgtech/engine/cv101.htm

<<<snip>>>
"To get a handle on your choice, you should understand how the original (pre 1968) vent system is supposed to work. The hose connection on the air cleaner is inside of the filter element. The metal tube extending downward from the tappet cover terminates at the bottom of the chassis and has a angled cut at the bottom end of the pipe. This is properly called a "draft tube". When exposed to the air moving underneath the car there is a slight vacuum created in the draft tube which draws air through the crankcase to ventilate it. The incoming air is fiiltered at the air cleaner, and nothing should ever pass back in the other direction from the crankcase to the air cleaner."

Blake

Paul,

Not the way I remember it, but I no longer have the MGA or Magnette and my 65 has a PCV system which is different again. You didn't mention if you have a car with the early system yourself, but if not perhaps someone else will take a peek under their bonnet and let us know what they see. I have been wrong before and now I'm curious.
Cliff
Cliff Maddox

Gentlemen...I had gotten a bit confused in reading this thread and my reply was intended for Blake (not Paul). My apologies and regrets. Cliff
Cliff Maddox

I haven't looked at the MGB but my 63 Mini had that connection to the dirty side of the filter and a filter inside the port on the tappet cover. It did not have a pipe extending downwards. Clausager shows the MGB pipe between the rocker cover and the front air filter can right on the outer edge of the base of the can. Given the size and shape of the filter element relative to the can I can't see how that puts the connection on the clean side of the filter. Also given the amount of depression inside the cans from the carbs I can't see how there would be any air flow from the can through the engine. And even if there were at speed how would it when stationary? And if it didn't then it would be pulling unfiltered air from the tappet cover and feeding it directly into the carbs, dirt and all, according to the description Blake copies here. I remain unconvinced, but will argue no longer.
Paul Hunt

This thread was discussed between 04/05/2003 and 07/05/2003

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