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Ok guys I need help to get an extra 3mph from my rig.
my engine is a 1995 evinrude 60hp, 56 cu in longshaft model.
its got the lost foam cast block and is the oval port engine. so no porting mods.

Now at the moment I can get 5900 rpm with a 13x19 omc sst prop and hit 42.4 mph on
my boat.
with 2.42 gears so getting just 4% slip. I think thats not too bad.
the hull of my boat is no race design, but slip shows its ok (I think)
I need to get to 45mph and here is the problem.
what I wanted to do was simply bolt on a set of the 70hp carbs to give me that extra
bit of oomph.
problem is that in the uk I cant seem to find any carbs that are from 1993 onwards.
I would of liked to stay with the same type of carbs as what was factory fitted, but it
looks like I might have to settle on something else. mine have the black plastic tops.

what other carbs will fit on my engine ?
could i simply use any of the 56 cu in motor carbs or could this create a problem ?

and what exactly is the difference between the 60hp and 70hp carbs?
from what I see its just the size of the venture being about 1/4 inch smaller on the 60's and different screw in jets.
is there any different passage ways or would these be the same in both carb body's
of same model year ?
as I have thought about getting my spare set of carbs machined out to the size of the
70 venturi's if its that simple .
I know everything else on the engine is identical apart from carbs.

and does anyone know or have an idea as to what rpm I could get by going up to the bigger carbs ?

advice on this would be good guys, as that extra 2.7mph might just do what I need.

here is a view of the venture on a 60hp carbs, its very restricted. .





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sounds like the same power I have on a 14' checkmate.

You want 3mph out of strictly engine mods? Have you tried raising the engine?

the plastic top carbs should be just fine - but you CAN fit the stinger carbs on if you can find em

Have you played with jet sizes at all?
How about a meatier prop?

I have tried raising the engine ?
oh boy did I raise it and try it, what a disaster that was.
I made a set of jack plates with a 4 1/2 inch setback, in total I could raise the engine
7 inches up.
I had the bullet real high and centre of propshaft just 1 inch lower than the boat.
for this I was trying a omc silver streak /srx prop size 13 3/4x23 .
I did a speed run before I jacked it up with the same prop and got 39.1 mph with a
not brilliant holeshot.
after raising it up I got 38 mph dead and it took ages to get on plane, and I mean ages.

the old stinger carbs are for 49 cubic inch not 56 cu in as far as I know.
I would think my carbs are already pumping more juice than those.
the stinger was flywheel rated at 70hp and 75hp, so at the prop this would no way be 70hp.
unless modified.

I was thinking more about the later engines of around 1988 onwards, the carbs that used
the electric choke thing the same as mine.

any idea of what amount of rpm gain I would get from 70hp carbs ?
and what rpm are you getting on your checkmate and what prop are you using.

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here is a few pics of mine jacked up, this gives an idea of what I have tried.



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The 75 stinger (yes, 49cu.in), did have large carbs at one point. But as our boats aren't spinning crazy rpm, i don't think the slightly bigger carbs would make a difference really.

Our checkmate spins 6400 with a 23 pitch and goes 53mph. Wish i had a 24/25 pitch. We're jacked up pretty well as high as you too. Our boat was blue printed, and the 'pad' is round, not flat.

How big is your boat???

my boat is 13ft 3 inch, nothing like a race boat at all.
its a sort of tri-hull. looks very similar to a boston whaler 13 classic, but with slightly taller sides.
I know I am never going to compete against a full race boat, but my goal is 45mph.
Look at the vid of my tug below. then you will see why I am struggling to get to 45.
this is with a 15 pitch prop just to see what it would do with that, 36mph top end.
there is no other boat on the river the same as mine getting anywhere near me for speed.
most of these are doing low to mid 30's with 50hp engines.

I have a 13 3/4 x 23 omc silver streak (same as srx now its a polished finish) I was hoping that would be good up high, but my boat just did not like my jack plate /set back..
The boat is bow heavy and I realy need to get the front to ride a bit higher.
With engine straight on the boat I cant swing the 23 past 5500 rpms, and that gives 41.3mph
with a huge amount of slip 17%,
with the SST 19 I get just 4% of slip at 5900rpm and 42.3mph.
what I am thinking is if I can get 6400 rpms with the 19 then i could be way up on speed.
is your engine the 56cu in ? and is it longshaft with 2.42 ratio or short and small gearbox ?

.https://www.youtube.com/watch?v=nh9GCaU41l8.

Ours is a 56cu.in 'bridgeport', but everything else is the same. Big case with 2.416 ratio, plastic top carbs, metal reeds, long shaft, and I even run the airbox without the cover too! Stock head.

I don't know, maybe play with jetting/timing?

what is your compression ?
mine is 149-147-148.
what year is your engine ?
My timing is bang on, and carbs synced real good.
all electrics are perfect and all ignition is now good too.
power trim all good too.
but you have the bigger carbs and your getting 6400 rpms with a 23 pitch.
have you had a smaller prop to try ? and if yes what did that rev at ?
is your prop a over hub cleaver or solid prop.
I have thought about a raker, but get conflicting stories about them.
just think I need bigger carbs to get that little bit extra.
I would settle for 10% slip at 6400 rpms with a 23 prop.
what is diameter of your prop and make etc.
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I'll get back to you on most of those questions, but for now:
- through hub 23" chopper stainless. Unsure of other data/info at the moment
- I have the SAME black plastic top carbs as you
- Tried a 12-22" merc labbed prop once. Got like, 6700rpm and 55mph I think

6700 rpm !!!!
wow I wonder what revs it could climb to with a 13x19 prop then.
last week I tried a 12 3/4 x 23 ali prop. mine could only climb to 5000 rpm with that.
sounds like I seriously needs some bigger carbs for sure.
i found a set of carbs but they want £200 for them, that's about $410 Canadian dollars.
i am trying to see if he will take a set in exchange plus a bit of cash, as i have a spare
set of 60hp carbs that are 100% perfect, got a lot of spares as this came with a spare engine
that has a cylinder gone bad, i think the block is too far gone for a re-bore.
but 6700 rpm sounds very promising.
i don't actually know at what revs my limiter would come in at ?
i have had some say 6400 and some say 6700 ?

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