1975 stinger 75hp , 78 sea nymph v, 150psi all 3, only 30mph WHAT? 13 3/4 x15 p,HELP

[sharpeye Mike]

Here in Canada, the oval port came out in 1994

[Revenge22]

i thought it was 92'? as that was the fastrac mid starting year, or early 1993

[phillnjack]

oval port first made late 1993 for the 1994 season. you could get one from about aug/September. here int eh uk we could get them in November and they were on display at earls court jan 1994.. strongest 3 cyl omc ever made. and no fastrak was made in 1990 on v4 and v6 and went on some 1991/2 Evinrude/Johnson 3 cyls 56.. some 3 cyls were still using the old brackets in 1996, mainly the commercials and the 65rs...
in 1992 the flatter flywheel went on the 3 cyl along with black top carbs . so from outside it would look like the oval port motor...

[99fxst99]

"Are you 100% sure the timer base isn't hung up on a wire or sticking?"
This happened to my 1976 Stinger SE motor. Worked myself silly before I found this out, thanks to another racer who experienced the same thing.

[phillnjack]

original poster posted his stinger 75hp on a 15 1/2 ft long fishing boat..
for the rpm that its rated for 5500 and a 15 inch pitch prop 30 mph is giving him 6 to 7% slip.that is perfectly normal with alloy prop, could get it down to 5% with stainless and possibly 32mph ??
Without modifying these motors that is what they done in the real world. its not a racing boat and definitely not a lightweight hull.
So 30mph for that boat and that engine with that prop is about right.
A 17 pitch stainless and 5500 is possible for about 34 /35mph.
if he wants 40 mph then he needs the 56 inch motor that is far more powerful.... the 75 was crank rated and only gave about 65hp at the prop.
The 75 in a shorty was only good on lightweight boats, and again nothing like a 56 70hp.
With the 75hp it is critical that its propped to get 5500 rpm as if its lugged it will destroy it quickly.
a tacho is needed and make sure you can hit 5500 with normal load.
If this engine is realy reaching 150 psi compression and not just a gauge reading high, then the head must of been shaved a good bit, as even the
optional factory high compression head would only get it to 140psi. cold cranking no extra oil down the bores.
Normal psi is around 125 for the 70 and 75hp 49 ci motors.....
The later 56 was again around 125 to 130 and the very latest of the 56 the oval port motor is 140 psi + in the 60 and 70hop variants
and around 125 in the 50hp variant..(due to thicker head).

phill

[Revenge22]

the 70 and 75hp 49ci motors are all almost always 135psi+ after i've rebuilt them. thats on 2 gauges dead accurate.

[sharpeye Mike]

the 70 and 75hp 49ci motors are all almost always 135psi+ after i've rebuilt them. thats on 2 gauges dead accurate.

Yeah, that's about right

[phillnjack]

not without skimming the head they are not...

if they have higher compression heads or skimmed then yes, but standard then no... 120 to 130 max psi from factory.. normaly 125 ish.

remember if you skim the head or deck the block you will raise compression.
Most people when doing a full rebuild will also be giving a skim on the head and or deck to clean then up like new. or even use sierra gaskets makes a difference of around 5 psi.

[Revenge22]

not without skimming the head they are not...

if they have higher compression heads or skimmed then yes, but standard then no... 120 to 130 max psi from factory.. normaly 125 ish.

remember if you skim the head or deck the block you will raise compression.
Most people when doing a full rebuild will also be giving a skim on the head and or deck to clean then up like new. or even use sierra gaskets makes a difference of around 5 psi.

good points there. I always get my blocks decked for me. I did have one terrible machine shop (long story) do a cheap horrendous hone job, and 2 cyls were around 130psi, middle #2 was 115psi. got lucky and those #'s went up some with running. come to think of it, most of them with lots of hours have maybe 100-110psi, so you are probably right on the money with a stock block when new.