Carburetor Theory - 1978 70HP (3cyl)

[dtmc78]

Carburetor Theory - 1978 70HP (3cyl) ...
Looking for some general theory of a 2 carb system / 3 cyl ? Confused about the principle of operation; top to one, bottom to three, both to two?

Thanks in advance. - Dave

 

[nautical]

Re: Carburetor Theory - 1978 70HP (3cyl)

I have wondered the same thing....

 

[Laddies]

Re: Carburetor Theory - 1978 70HP (3cyl)

It was/is a poor design, part of cost cutting the engines do not idle well for any period of time.

 

[dtmc78]

Re: Carburetor Theory - 1978 70HP (3cyl)

Is it like a 4 ? Barrel carb on a car? Does all 3 use top, then when the demand is present does it then draw from the bottom carb (like the back two barrels)?

 

[Laddies]

Re: Carburetor Theory - 1978 70HP (3cyl)

Actually if you look carefully at the front of the crankcase you can see the flow pattern. both the top and bottom carbs feed the center cyl. thru the red blocks, each carb has a reed block behind it with 4 reeds on the top and 4 reeds on the bottom, the 4 reeds on top of the top block feed #1 cyl and the 4 on the bottom feed the center cyl. the bottom reed blocks 4 top reeds also feed number 2 cyl. and the 4 on the bottom of the reed block feed # 3 cyl. So 1 & 3 are each feed with 4 reeds and # 2 is feed with 8 reeds and I wonder why the don't idle well?

 

[jimmbo]

Re: Carburetor Theory - 1978 70HP (3cyl)

The carb/bearing block/reeds system works extremely well on engine with multiples of 2 cylinders, but on the 3 cylinder it does have some oddities.

Laddies. Have you had the reed blocks out to see if #2 is breathing thru 8 reeds? it might hsve been built with only 2 reeds per block feeding #2.

I guess that design sits up there right next to the Dockbuster

 

[dtmc78]

Re: Carburetor Theory - 1978 70HP (3cyl)

Thanks for the info. It helps to know the theory behind the system before you attempt to troubleshoot it. The merc service manual does a great job of telling you how to rip the thing apart and put it back together but it does not really help with general systems knowledge. Thanks again.

Dave