2 barrel 2 banger

[JohnsonM50]

An Idea to re-ask, Ive been thinking about putting a 2 barrel carb on a '70s 31.8ci OMC twin. Ive been given some good ideas so am in progress. Ive got a 2 barrel from a 4 cyl OMC. that used 2 carbs. The throats at the rear are 1 3/8", same as a stock single for the 31.8ci motor. The venturi sizes are very close between the carbs, the 2 barrel appears to be equal side to side.
The single barrel stock carb feeds a common intake that both reed sets draw from so in effect the carb singularly feeds each cyl. Every charge to each cyl is from the full bore of the single barrel drawn back & forth thru the reeds.
If the above is correct, I theorize that if I separate the intake plenum so the each barrel feeds 1 cyl Ill essentially [short of working out bugs] have the same thing. If so then the performance.. where would be the gain? Im thinking the smaller more direct intake per cyl might cause an increase in velocity [this is a crossflow] and therefore send a better atomized, possibly slightly larger charge in. The carb would be drawing from each feed tube 1/2 as many times per barrel as the single barrel so might be more capable at top end. Im wondering if in alternate operation the feed jets would cause a siphoning effect on each other but doubt it because the air vent should cover that.
Worse case it doesnt work & I cut up a spare intake & sunk a bunch of time.
Im ready now...Point-n-Laff, Ask why!?, make remarks,or add wisdom Rest assured Im enjoying try-n it & theres more to the story than just stickin this on any motor. Thanks

[JohnsonM50]

so far...

[OUTBOARDER]

1-3/8" O.D X 1-1/4" I.D Alum tube will fit on butterfly side, drill thru hole for throttle shaft and make/get a 1-1/4" dia butterfly....................

oops also Drill out idle and transition holes thru sleeve

[JohnsonM50]

At 1st glance I thought to make a plate to attach to the intake but it became apparent that to thru bolt to whole thing was the only way. Ill have to go thru the carb of course, there appears to be orifices in the rear cutouts between barrels. Will separating the intake effect them is 1 concern, how to compensate the other. Im not concerned with idle/troll speed so can sacrifice purr like a kitten in favor the roar.



Thanks Anthony, I will do that if needed but am going to test 'as is' just in case.

[Powerabout]

If you want power at high revs you will need to use reeds on a vee block not the flat ones

[zul8tr]

Thoughts

Have you checked the size of the total venturi area of the 2 x carb compared to the total venturi area of the single carb you are replacing? It appears to me that the 2x has a whole lot more area than the single. While this may be good for max high rpm output (?) it most likely will be negative for lower to mid range performance due to poor throttle response from reduced air velocity at the venturi. The new 2x carb setup would require proper jetting to obtain the correct A/F ratio but such a larger venturi area then stock might make jetting out of reasonable range.

You note correctly that each cylindere feeds on the single carb and theorize that a 2X carb divided at the plenum will allow each cylindere to use each side of the 2x carb and that should provide a more efficient intake. While it sounds reasonable the intake air still mixes atthe reed cage. I think that unless you keep the intake seperate all the way to the cylindere intake ports the 2x carb will have about 1/2 the air velocity at each venturi as the stock single venturi setup based on the similiar geometry dimensions you note.

Since you are already on the way go for it and see what happens. Edison tried 1000 filiments before he hit on carbon impregnated cotton, but he missed it on his incorrect belief that DC power was the way of the future. It took Nikola Tesla (his former employee) to set the light and gave us AC, Edison was pissed - thank you Mr Tesla.

[JohnsonM50]

Thanks for the suggestions, I will keep them in mind as I run it. Ive thought about V reeds & might later, for now Im thinkin non steel reeds.
The venturi area is slightly less than the stock single barrel & this is to be tried on a .030 over 32+ci motor with squared port tops & lighter than stock spinning mass & bumped up compression 35hp.
Im sure I run the risk of a 'fail to launch' but Im ok with that. Once the intake is made I can try different carbs or whatever. Velocity or not may well be the maker/breaker of this experiment.

[Seagull 170]

Before you follow zul8tr thoughts, who I'd guess is bang on the money, & build a dividing wall.
See if you can seperate the butterflys so that they are actuated by separate cables, if you have a spare carb.
Cable up one choke in the normal manner to give normal low & mid range performance, & the second to open when it starts to come on song, maybe 1000 revs below max, then open both together to find the limits on the second choke.
If the second choke is first tried on a lawn mower type throttle it's easy to find the start point.
I've seen this done on small single cylinders fishing motors with two seperate carbs both slightly smaller than the origonal, the second only opening almost at the previous peak revs, with a 10% gain in peak revs with the same prop.

This is another experiment where it would be good to have dyno at home.

[JohnsonM50]

Before you follow zul8tr thoughts, who I'd guess is bang on the money, & build a dividing wall.
See if you can seperate the butterflys so that they are actuated by separate cables, if you have a spare carb.
Cable up one choke in the normal manner to give normal low & mid range performance, & the second to open when it starts to come on song, maybe 1000 revs below max, then open both together to find the limits on the second choke.
If the second choke is first tried on a lawn mower type throttle it's easy to find the start point.
I've seen this done on small single cylinders fishing motors with two seperate carbs both slightly smaller than the origonal, the second only opening almost at the previous peak revs, with a 10% gain in peak revs with the same prop.

This is another experiment where it would be good to have dyno at home.

I was also thinking the secondary barrel & could maybe do something with that, would be easy to cut the shaft. [it sure works as a V8-4 barrel] Thanks

[zul8tr]

Before you follow zul8tr thoughts, who I'd guess is bang on the money, & build a dividing wall.
See if you can seperate the butterflys so that they are actuated by separate cables, if you have a spare carb.
Cable up one choke in the normal manner to give normal low & mid range performance, & the second to open when it starts to come on song, maybe 1000 revs below max, then open both together to find the limits on the second choke.
If the second choke is first tried on a lawn mower type throttle it's easy to find the start point.
I've seen this done on small single cylinders fishing motors with two seperate carbs both slightly smaller than the origonal, the second only opening almost at the previous peak revs, with a 10% gain in peak revs with the same prop.

This is another experiment where it would be good to have dyno at home.

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That is a reasonable idea and worth a try. Like delayed secondaries on a 4 barrel carb.

I remember an older Mercury (car) I was tinkering with. It was a stock 312 c.i.V8 with a 1x four barrel so I decided to replace the stock manifold and carb with a 2x four barrel set up. It definately was too much venturie area at the low and mid range if not careful with the throttle but was a stomper at the higher revs. The only set up that worked was the stock factory setup progressive linkage system. The front 2 barrels of each four barrel were operated together and the rear 2 barrels came in at about 70% open throttle.