1968 omc 120hp dual carbs

[goatman1]

im missing the dual single barrel carbs on the manifold but i have the 140hp merc carb
im thinking of just grabbing a couple flanges and plumb it to each barrel of the 140hp carb
my question is this
the merc carb is 2 barrel, does each barrel act the same or does each barrel have a special job?

 

[goatman1]

was trying to find the cfm of the carter rbs carb but cant find it
even looking for the cfm of a merc tks i got nothing
found a cfm formula for engines
cid 153 x max rpm 4500 x efficiency(typically 80%
550,800 cubic inches
divide by 3456 for 159.375 cubic feet per minute (cfm)
this 120hp engine needs a 160 cfm carb or 2-80 cfm carbs

a 3.0l merc would need a 190 cfm carb

 

[makomark]

The MERCARBs are younger cousins to the Rochester 2G's...the smallest 2G will flow ~ 275CFM so that would the preferred configuration.

The left side works like the right side when everything is correct...in either.

 

[goatman1]

ok, so each barrel could serve 2 cylinders through the divided intake manifold off of one set screw and it would be fine unless one of the ports got fouled.
thank-you

 

[goatman1]

looking at manifolds and am thinking i might be better off with a new manifold
the firing order is 1-3-4-2
i think the dual carbs were only one year/1968
1 carb did cylinder 1&2 the other carb did cyl 3&4
the manifold below from 1964 to 1972 changed what cylinders were served by each barrel
1 barrel did cyl 1&4, the other barrel did cyl 2&3


the air fuel mixture would flow back and forth between 1&4, 2&3
1982-1985 omc just dump both barrels into the intake
do you think they did that so 2 cylinders werent starved of fuel if one barrel got gunked up?

 

[makomark]

I don't have a definitive (independent) source but can tell you the stern drive community got rid of the "top plate" manifolds on the GM inline engines because they were prone to warping...and that would lean to a lean mixture...which leads to engine rebuild/replacement....the replacements were all cast a 'complete units' for the intake and exhaust passages - some had the service ports on the ends (water passages) and some were all one piece...

 

[goatman1]

thank you