Poor idling and crankcase breather

[Kiwi 74 Alfa]

Had trouble getting the car to idle right. It would idle OK for maybe thirty seconds then die away. When this happened it appeared to be missing on one cylinder but I could not easily tell which one.

Had a look at the crankcase breather which is a tube that comes out of the front right of the cam cover and puts crankcase fumes into the air cleaner. It also has another breather tube about 4 mm ID which branches off from the main outlet. This puts fumes into the inlet manifold between the front carb and the block. This tube is obviously subject to full manifold vacuum. Have recently done up the motor and its running good with little blowby to the crankcase.

If I blocked off the main breather to the air cleaner at idle the effect of the small breather was that as it sucked any fumes out of the sump it gradually built up a partial vacuum. When this vacuum built up it was no longer bleeding air into the manifold and the motor started idling better and no longer misfired.

It looks like, at idle, this small breather is taking air out of the air cleaner and allowing it to bypass the carb and inserting it into the manifold causing the first cylinder to run lean and misfire. Only bleeds a small amount of air but, at idle, the motor is not using much air and is running at max vacuum so it is enough to make a difference.
Looks like the answer is a one way valve in them main breather line so that fumes can leave the crankcase and travel to the air cleaner but not reverse flow at idle.

Anyone else have this problem? Also, can anyone recommend a valve?

[Kiwi 74 Alfa]

Sometimes we are blinded to the obvious.
Why don't I just close off the small breather pipe.
Anyone see any problem with that?

[alfanos]

did you check if both breather/fumes rubbers are clear from the inside?

[Kiwi 74 Alfa]

Quote:

Originally Posted by alfanos did you check if both breather/fumes rubbers are clear from the inside?

[andy303]

Quote:

Originally Posted by Kiwi 74 Alfa Sometimes we are blinded to the obvious. Why don't I just close off the small breather pipe. Anyone see any problem with that?

[Mound Dawg]

In theory the throttle butterflies on the carbs should be completely closed when the engine is idling. This means that at idle there is no way for the crank case pressure to vent into the cylinders to get burnt off. The fumes and pressure will go into the airbox but won't be able to get past the butterflies so have nowhere to go.

This is what the small pipe is for. At idle, the pressure goes through this into the manifold at cylinder no. 1. As it vents in behind the throttle butterfly it goes straight into the cylinder and is burnt off there. At idle there shouldn't be much in the way of crankcase pressure or oil fumes (unless the piston rings have had it) so only a small pipe is required.

As soon as you open the throttle, the vacuum moves to the airbox so the larger breather comes into play.

You need to run both pipes, they both need to be clear of obstructions. If you run an oil catch tank, you can ditch the small breather but DO block off the pipe on the inlet manifold or it will be impossible to get a decent four cylinder idle.

Some cars DO have a vacuum take off for distributor advance but this is on the carburettor not the manifold.

[andy303]

At idle there is still some air being drawn through the carbs (obviously, or the engine wouldn't run) via the float chambers and the vent openings on the DCOE and similar carbs. It seems to me that there would need to be a restriction in the large breather pipe to prevent the line from the manifold being essentially open to the atmosphere, which would certainly upset any idle settings. On cars equipped with a PCV valve, this valve is spring loaded to allow only a small, calibrated amount of flow. Perhaps this is the function of the flame trap inside of the large breather tube.

At any rate the port on cyl.#1 on my car is blocked, and it idles nicely.

[Kiwi 74 Alfa]

Mound Dog said "In theory the throttle butterflies on the carbs should be completely closed when the engine is idling. This means that at idle there is no way for the crank case pressure to vent into the cylinders to get burnt off. The fumes and pressure will go into the airbox but won't be able to get past the butterflies so have nowhere to go."

That can't be right. The motor still uses air when idling. Either that or it isn't running. If breather fumes go into the airbox when the motor is idling they will get drawn into the carbs and into the engine. So, with the small air pipe stopped fumes will still burnt rather than go no where.

[Edgewood Alfa]

I recently found this small tube (blocked off with a common machine screw from the PO) on my '74 GTV w/SPICA, and wondered what it was for.

Where does it connect - is there supposed to be a "t" between the main breather pipe from the valve cover to the oil recovery unit?

thanks-
Chris

[andy303]

This is the PCV system on the SPICA equiped cars, below. The small tube on the breather fitting is connected to the fuel vapor recovery system on the Federalised cars. Note how the vacuum source for the Oil Vapor Separator is taken from the Idle Equalizer fitting.

I have read a number of other posts on the subject last night, and Mound Dawg's description of the small line being attached to the port on the carb Euro-intake manifold at Cyl. #1 appears to be the correct location. If I can find a suitable flame trap to insert in the large breather hose at the airbox I may try it.

[Edgewood Alfa]

Wouldn't the small tube leading to the intake manifold at #1 cylinder effectively do the same thing as the return to the idel air adjuster (#18)?

[andy303]

Yes, but the difference is that the vacuum "leak" is evenly distributed over all four cylinders, and not just one. The idle adjustment on the SPICA was a calibrated leak that had some degree of adjustable control. A line to a single cylinder shouldn't work, but as it was used by the factory is must have done.The question is how?

Another thought: When I adjust the carbs the airbox is off and the breather open to the atmosphere. In this case I think that the small line would have to be plugged during tuning.