CIS Mixture Control Unit Installation

[keynsham1]

I have just started cleaning up my CIS injection system as it is off the car. I notice that the mixture control unit is attached to the air box with six screws that have a spring underneath.

The instruction is to fully tighten them, and then back each one off by a whole turn. There us a gasket between the unit and airbox but I can only assume the spring installation is for venting if a backfire occurs through the airbox. Can anyone confirm this?

I am fitting a flap valve as recommended, but if the six mixture control unit screws were fully tightened then this would probably burst the airbox in the event of a backfire as no venting is available for the back pressure.

[E Sully]

While I can't say what the engineers design was, the mounting method is correct. Spring loaded, screws backed off 1 turn. I'm sure it helps prevent cracks in the plastic housing.

[boyt911sc]

Originally Posted by keynsham1

I have just started cleaning up my CIS injection system as it is off the car. I notice that the mixture control unit is attached to the air box with six screws that have a spring underneath.

The instruction is to fully tighten them, and then back each one off by a whole turn. There us a gasket between the unit and airbox but I can only assume the spring installation is for venting if a backfire occurs through the airbox. Can anyone confirm this?

I am fitting a flap valve as recommended, but if the six mixture control unit screws were fully tightened then this would probably burst the airbox in the event of a backfire as no venting is available for the back pressure.

keynsham1,

If ever a big backfire occurs in the CIS airbox, it happens inside and not outside the air box where the air flow metering unit is installed. When you get a chance, inspect how the metering unit is connected to the throttle body. The blast or backfire occurs at the combustion chamber/s delivering a great amount of pressure to the air box where the 6 runners come to meet. The force of the back pressure caused by a backfire is distributed "inside" the air box per se. The air filter and the metering unit are away from the blast area!!!!! And not effected by the backfire.

The pop off valve is installed strategically to relief the excess pressure buildup during an unforeseen backfire. The direction of the 'back pressure' caused by a backfire is directed to the pop off valve (if any), throttle body, CIS boot (pope hut), and downward to the air flow sensor plate. This is contrary to your expectation of lifting the air flow meter unit from below. The blast effect is actually coming from the top of the metering unit and not from below.

Tony

[keynsham1]

Thanks Tony,

I have looked at the airbox and realised that the metering unit is in a different chamber. I am installing a pop-off valve, although I would be interested to know how often this backfire problem occurs on a standard engine. My car is 36 years old and seems to have been backfire free so far!

I still don't know however why the metering unit is spring mounted. It has obviously been designed specifically so movement can occur. Even the gasket is adhesive on one side so it wouldn't be displaced, but all evidence would suggest that there is a vacuum sucking the metering unit down, and there is no obvious path for a pressure to be seen below it.

[Flat6pac]

If for some reason you get a misfire or a lean mixture causing a misfire, its the airbox.
The pop off valve is an airbox saver.
Bruce