Why have multiple carburetors?

[85ss150]

I recently bought an 85 Starcraft SS-150 with an 84 Johnson 70hp 3 cylinder E70ELCRD. This is my first 'big' outboard, my others being a pair of 3hp JW's, a 77 4hp and a 72 25hp (all OMC).

One of the first things that jumped out at me as 'different' is the 3 carburetors. What are the reasons for multiple carbs instead of one big carb? I can think of a few possible reasons but figured I'd pose this question to the knowledgeable people here.

 

[Fleetwin]

Re: Why have multiple carburetors?

One of the first things that jumped out at me as 'different' is the 3 carburetors. What are the reasons for multiple carbs instead of one big carb? I can think of a few possible reasons but figured I'd pose this question to the knowledgeable people here.

Performance.

 

[SolingSailor]

Re: Why have multiple carburetors?

Having experience only with small 2 stroke motors myself, I find this question very interesting.
Could it have something to do with the nature of the 2 stroke vs. 4 stroke engine? The vertical crankshaft?
I hope some knowledgeable members will explain this to us.

 

[Frank Acampora]

Re: Why have multiple carburetors?

Yes, Squeezing the most horsepower out of limited cubic inches. Multiple carbs do help with performance. However, manufacturers in cost savings have at least several horsepower ratings from the same block. The engine block is set up for multiple carbs so all horsepower in that range has them, whether or not they need them.

Case in point: Chrysler made all three cylinder engines from 70 to 90 horsepower with three carbs. When Chrysler became Force, the 70 and 75 were changed to a three cylinder engine with one large carb--economics dictated a single big carb was cheaper to use than three carbs and the 75 did not require three carbs to generate its horsepower. The 90 remained a three cylinder, three carb engine--because it needed them to develop the full 90 ponies.

 

[matt167]

Re: Why have multiple carburetors?

2 strokes have 'tuned' intake manifolds. Cylinders need to fire the same and the intake charge effects the way the cylinder will run. If they ran 1 carb then the intake runners would be different lengths so each cylinder would run differently.. Carbs can be paired which is why most twins have 2 carb, and a 4cyl has 2 carbs ( and a 6cyl generally has 3 ), but a 3 cylinder will many times have 3 because the third carb cannot be paired with a different intake length. There is a little more to it but that is the gist of it

 

[jimmbo]

Re: Why have multiple carburetors?

Mercury used 2 carbs on some of their 3 cylinder engines. It wasn't the best engineering.

 

[John Frum]

Re: Why have multiple carburetors?

On a premix engine multiple carbs give more even lubrication.

 

[82rude]

Re: Why have multiple carburetors?

Mercury used 2 carbs on some of their 3 cylinder engines. It wasn't the best engineering.

don't I know it!had a 60 mariner with 3 cyl and 2 carbs .ill running beast.sold it to a friend and it blew up a year later.not sure how many motors are manufactured with carbs now but I would hazard a guess that it wont be too long and you wont find any atall other than used just like cars and trucks today.

 

[tpenfield]

Re: Why have multiple carburetors?

My understanding is that the 2-stroke engine process requires crankcase pressure in order to induce the fresh fuel 'charge' into the cylinder at the bottom of the stroke. So, in bigger engines, it is more effective to have a carb and reed valve setup for each cylinder, rather than an intake manifold to distribute the intake across multiple cylinders.

My new-to-me 7.5 hp Mercury has a single carb for 2 cylinder, but it is a small outboard, so not as big a deal.

 

[Bob_VT]

Re: Why have multiple carburetors?

On a premix engine multiple carbs give more even lubrication.

The only lubrication the bears and rings get is from the fuel oil mix. This answer is right on the money and the outboards are really designed for performance too.

Don't kill too much time on the design.......... keep the carbs clean, the impeller fresh, use TCW3 oil, check the lower unit and clean maintain all of the electrical connections. That boat motor combination is great.

 

[85ss150]

Re: Why have multiple carburetors?

Lots of great info here. Thanks for the replies! Now, the next guy to Google this question will no doubt get better results than I did.