Swap carby on Mercruiser 1986 inline 6 to holley carby

[Kev Paley]

Hi Everyone,
Wondering if anyone can tell me if I can swap the carby on my 1986 mercruiser inline 6 to a holley carby and what adapter plates do I need, where to get them from?

If anyone has done this before or can offer any advice be very much appreciated.
Thanks

 

[nola mike]

Hi Everyone,
Wondering if anyone can tell me if I can swap the carby on my 1986 mercruiser inline 6 to a holley carby and what adapter plates do I need, where to get them from?

If anyone has done this before or can offer any advice be very much appreciated.
Thanks

There was no inline 6 in 1986...
Anything's possible. But why do you want to switch, what are you switching from...

 

[achris]

Holleys in boats = boom!

Merc stopped using Holleys for that reason.

And I suspect your trying to put a 4bbl Holley on. Bad idea. Those inline 6 engines have a maximum turn rate of around 4300rpm, and they are a 4.1 litre engine. Even at 100% fill rate (totally impossible without forced induction), they can only draw a maximum of 340cfm. Even the most moderate of 2bbl carbs can provide that with ease. All putting a 4bbl on will do is make the engine a terrible idler, and use more fuel than it needs to. Leave the original 2bbl on and you'll have the best of economy and run quality.

 

[Scott Danforth]

nothing wrong with a Holley carb, volvo, PCM, Ilmore, and OMC used them for years.

however everyone is right, you dont need anything more than 500 cfm 2-barrel unless you do lots and lots of expensive head work on the inliner and you would need a custom manifold since the intake and exhaust are one unit.

if you want more power, upgrade the 250 cubic inch motor to either a 292 or jump to a 4.3

 

[Kev Paley]

Thank you for all your responces,and yes you all are correct i should have mantioned that i was thinking of a 2barrel holley and not the 4barrel i want to know where and what adaptation kit i would need and how difficult is this to fit , i would like more knots but i really want reliability most of all the boat is a carribean reef runner 1986, rebuilt to a off shore fishing vessel

 

[Scott Danforth]

If you want more speed, you need much more than a carb change. you need enough motor to jump up in torque significantly enough to spin a higher pitch prop.

to spin the inline 6 motor faster than 4200 RPM requires pulling the motor apart and building it to spin faster (balance the rotating assembly, lump-port heads, different cam, etc. it can be done, however it will cost big money

BTW, for the cost to build up a 250 cubic inch motor for higher RPM's, you can build a big block or a stroker small block

this is best a case of either live with what you have, or sell what you have and buy what you want.

FYI, Chris did the exact repower to a 4.3 MPI

 

[achris]

Anything for those 250 Chevy 6s is now very $pendy. I have a friend with a Reef Runner, and he has a 4.3MPI in his (same engine as I have) and he's a 40knot boat, also like mine. That's going to be the cheapest and most reliable way to get more speed.
Just adding a carb will not produce any more power. As Scott said, a LOT more needs to be done, but, you will get more SPEED at the cost of drive-ability. Read an article I wrote on this very subject not that long ago. But in particular, read the comments.... THAT is where the information is. A lot of people have personal accounts to verify what I wrote.

Why an increase in engine horsepower will hurt your ‘holeshot’, acceleration and midrange cruising.

So you want more horsepower, who doesn’t? But this is why an increase in top end power is actually a very bad idea in a boat. You’re off shopping for more horsepower. You buy a set of higher compression pistons, a ‘lumpier’ camshaft, a set of roller lifters and higher ratio roller rockers...

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