Carburetors

Maybe a dumb question but why do some OB's have a single carb and some have multiple carbs?

2 stroke engines breath lots of air. So a bigger displacement engine needs multiple carbs. If they just put a big one on, it would't respond well at low speeds. Has to do with the venturi size. Also, multiple carbs allows a straight airflow manifold.

Makes perfect sense....do four strokes use multiple carbs or is it just the two strokes? Im assuming four strokes use less air than two strokes...???

Can anyone tell me if the 35hp carb (late 70's-early 80's) has the restrictor in the venturi like the 25hp carb?

same reasons motorcycles use one carb per cylinder (with a few exceptions).

air and fuel flow.

A 150hp V6 no matter if it's 2 or 4 stroke would require a huge carburetor, and a carb that big would need a whole lot of "work" to make it idle and/or start acceptably. So, it's cheaper to manufacture with 3 twin throat carbs. Fuel flow is important too, with a single large carb, the jet would need to be so large to pass enough fuel that it makes it much harder for atmospheric pressure to force fuel through it, resulting in POOR cold starting and low speed performance, plug fouling, etc.

On small 2 stroke stuff, the starting and idle quality is much better with one carb per cylinder. You will see that on higher end outboards, multiple carb 2 or 3 cylinder. On lower end motors, single carb feeding 2 or 3 cylinders is common (Mercury tower of power is a good example)<--and those things were notoriously hard to start back in the day (I remember them well). Nowadays they're mostly novelty. Inline 4/6 cylinder and Chrysler had some 5 cylinder inlines as I recall.

4 stroke stuff is designed for good idle and low speed running so they almost always have one carb per cylinder on 3,4,6 cylinder stuff. Twin cyl stuff can get away with single carb. On the twin cylinder stuff they are usually very small engines (25hp and under) which will run pretty good and smooth with a single carb.

if an engine has multiple carbs, they need to be synchronized if they're ever off/apart (and they probably will be at some point in their lives). Synchronizing makes a huge difference in all areas from starting to full throttle and everything in between.

Thank you todd...that cleared up a bunch of confusion i had on the subject

I'm currently working on a friend's yamaha F50 that sat for about 3 years. Doing the carbs on it. Inline 4 cylinder, 4 carbs

I will also say this: Not every "mechanic" can do carbs.....

these carbs are probably the most complex set of carburetors I've ever done in my life! And....3 different sizes of main jets, 2 different sizes of pilot jets, primer (choke) is only on two carbs with passages and hoses that feed the other two, etc etc. I don't like these carbs but having done them before I knew going in that they're no fun.

if you clean your own carb(s) make sure of what goes where because even though it might have 2,3,4,6 carburetors that doesn't mean that they're all the same...

I did a yamaha 150 V6 2 stroke a while back, triple twin throat carbs. Not knowing any better just pulled it all apart threw everything in a bucket, few hours later blew it all out and started to reassemble. Upon blowing them out, I noticed several different size main jets and it was a sizable difference in the orifice size between some of them (6 jets). Installing them incorrectly would result in poor running and with these bass boat guys who run WOT all the time, possibly burnt up powerhead. Good idea to look at a parts breakdown & see where they are supposed to go. Just matching them from where you took them out of puts a dependency on the last guy who installed them.