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Calling all BMC/SU foamers


mtbtimr

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So we recently picked up some twin carbs off Samdash's 1300...

 

They have AUC870 and AUC871 on each of the body castings.

 

Our current single carb setup uses a single 1 3/4 su carb.

 

The issue: 

 

Currently...

We have the timing cover (AKA clutch cover AKA fly wheel housing) venting straight to the atmosphere through the v.

We have our rocker cover also venting straight to the atmosphere. 

The vacuum advance from the dizzy plugs into a little nipple on the front of the carb. 

There is no other vacuum port on the carb. 

All is well. 

 

With the new twin carbs...

There is a vacuum port on each carb, currently joined with a Y piece (presumably for the flywheel and rocker cover ventilation). 

There is no vacuum advance nipple on either carb. 

 

So should we somehow plug the vacuum advance from the dizzy into Y piece and leave the flywheel and rocker cover venting to atmosphere?

 

A bit of reading suggests the carbs are post PCV design or something...

 

Any one understand???  

 

 

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If your going to run an airbox your breathers would go into that. But you have individual filters right? If so tee them buggers together and vent them at the ground or if your keen tap the inlet manifold and fit a pcv valve. Having your crankcase under vacuum does help keep the oil in a British engine.

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There should be a vacuum port in the right hand carburettor, it may just be lower than the previous one and a bolt on metal type.

 

Remove the crankcase breather and cut open the top, remove the insides and weld back together, always blocked and full of tightly wound caked gauze. 

 

Then either run to atmosphere for max stink- or plumb up to carburettors via a T piece as well as the rocker cover. Insufficient breathing leads to leaks as stated due to high crankcase pressure/gearbox in sump dramas.

 

Make sure vacuum advance actually works also, number one fault with poor performing and over heating A series, dead advance unit

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"Not necessarily a good idea as the Vacuum is not controlled. In the HS type Carbs (and most others), the Vacuum take off for Vacuum Advance lines up with the Butterfly such that at Idle, there is next to no Vacuum for the Advance. This reduces the likelyhood of overheating while waiting at Traffic Lights and the like."

 

Overheating vs bad fuel economy.. seems like its best to go for the lesser of two evils...? 

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One of the carbs will have a tiny nipple between the butterfly and the manifold, that is where the vacuum advance line goes.

The two larger nipples are for the vents, you can use ether the rocker cover or the clutch housing one you don't need both hooked up, block the other one off.

The A series will leak every where without one or the other connected.

If the little nipple isn't in one of the carbs drill a small hole in the manifold and fit one there, it needs to be between the throttle butterflies and the head or the advance won't work properly.

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