Volvo 164 B30A Double Butterfly Inlet Manifold
 

I am doing a rolling road restoration on a 1973 Volvo 164 B30A Automatic.

Last summer the cylinder head was off for a clean up and I noticed that there is a second set of Butterfly directly in the inlet manifold. They are directly actuated from the Strombergs.

The first set is the actual Stromberg 175 carburettor itself and the 2nd set is in the inlet manifold.

This set me thinking that in order to balance the Strombergs Carbs you need to deal with two sets of butterfly for each carburettor.

Does anyone know why Volvo used this double butterfly set up on the B30A ?

Is the 2nd set of Butterfly valves actually necessary ?

Or can they be removed ?

Hi Volvo Saint,

when I was trying to get a handle on the carbs on my '71 164 with B30A motor and thus twin Strombergs like yours I read in my Service manual (specific to my model) that the extra butterflies (secondary throttles) were meant to control the fuel mixture flow such that the intakes from the two carbs were shared/equalised and warmed by the manifold at low load. It's for emission control. Where you live you could possibly just remove those butterflies.

ie, The manifold has TWO cross connecting channels between the carb inputs. At low load the butterflies in the manifold (secondary throttles) are closed, which directs the air-fuel mix from BOTH carbs across the bridging channel to BOTH intakes. The point being to get the mixture a bit warmer, and uniform. See image attached.

When you accelerate, the secondary throttles open and the suction in each half of the manifold is such that it takes the mix directly into it's "own" three pots, and the sharing process stops.

Thank you for the technical details.

As I live in Malta the climate is constantly warm - so depending on the complexity of the removal this might be an option.

There is an inspection cover on the manifold, so I believe this is where you can see the 'balancing mechanism'.

ok i know there this is a second set of like mini tb's in the IM. Everyone is talking about taking thses out for a small gain of power, anyone have a step by step on this??

You don't have to balance the twin Stromberg carbs on a 164

because of the extra butterfly valves and the bypass port in the manifold

the balancing happens automatically in the manifold

I would beg to differ on that and would guess the actual balancing procedure would involve disabling the IM butterflies such that they close the bypass port on idle for balancing purposes. The carbs would then be balanced as per any other twin/multiple caarbi installation and the disablement of the automatic bypass/balancing system would be put back to normal operation after.

You may ask why i suggest this - if the carbs are badly balanced then on normal driving and particularly WOT one set of 3 cylinders will be working harder than the other resulting in excessive fuel consumption, low power and a rough running engine.

While i've not run a B30 with twin carbs, i have run many other cars with twin carbs and i certainly knew it when the carbs were out of balance! Usually about 50% of the normal fuel economy, sluggish and rough. In fact, one car i had was a 4 cylinder Toyota with two twin choke carbs - they were actually perfectly balanced when i got it and it went like stink when the secondary chokes opened up but still gave 42mpg with "sensible" driving.

You dont disable the secondary butterflies to balance the carbs.

The problem with the secondaries is that the spindles wear causing air leaks sending the mixture out of whack. Long term it is best to remove them and seal off the holes that the shafts used