Weber 45's - Air By-Pass Screw

[ian balson]

Does anyone know the purpose of the Air By-Pass valve on the Weber 45DCOE152.

 

As far as I understand, it feeds air into the carb on the engine side of the throttle plates.

 

Is this somthinig I should be playing with or is it a factory pre-set?

 

I have a problem with my Idle revs, I can't get them below 1400.

 

Thanks

Ian.

[Roger King]

First, disconnect the throttle cable. If the revs then drop the problem is in the cable. This is very common.

 

If your carbs are very out of balance you may have a situation where even though one carb is fully shut, the other is still flowing enough air to allow tickover, albeit on two cylinders, so check the balance.

 

If the above don't work you may have a problem with the butterflies not seating accurately in the housing or there may be a problem with the alignment of the two carbs causing them not to shut properly. It is often best to remove the carbs and then remount them carefully after attending to any problems.

 

The bypass screws let air around the butterflies. In an ideal world they should be fully shut (don't force them too tightly against their seats). If you have a situation where one barrel is flowing more air than the other barrel ON THE SAME CARB, you can open the relevant bypass screw an appropriate amount to correct this. You will need a flow meter to measure this and it can only be used to correct small imbalances. If there is a major imbalance you may have a twisted throttle shaft or other problem that needs rectifying.

 

The only time that both bypass screws should be open on a carb is if they are being used to set idle speed as an alternative to the idle speed screw. This is not normally done and there should be no reason to do it on a Caterham.

[oilyhands]

These are set at the factory to balance the airflow between barrels on the same carb, unless you can measure the airflow on the engine side of the throttle plate with a manaometer, then dont mess with them.

 

I doubt they are cuasing your erratic idle unless they have come undone completely. I would check that the throttle quadrants are returning correctly and not binding.

 

Oily

[ian balson]

Thanks Chaps,

 

The throttle cable appears loose at idle, but I'll check. If I pull the linkage where the throttle cable attaches, the idle drops, so I guess the throttle plates are not fully closing?

 

So using a DT Synchronisation tool won't allow me to balance the individual barrels on a single 45, I would need a manometer to achieve this ?

 

Thanks

Ian.

[oilyhands]

A manometer would allow you to balance barrels across a single carb by unsing the air bleeds and between the two carbs, generally though, unless the spindle is bent, just balancing between the two carbs is enough, you can get pretty close with a trained ear and 6 inches of rubber hose (oooh stoppit etc.etc.)

 

Oily