Yamaha 75 2 stroke power upgrading.

[Willem Wikkel Spies]

so this is my 2nd post

i have done some research on the forum, lots of discussions present but not something particular to this engine.

i have 2x 75 yamaha 2 stroke engines on my boat.
upgrading the engines to 4 strokes is not cost wise as we do stay about 550km from the ocean.

to dump R 250 000.00 thousand rand into 2x new motors.......

i also like 2 strokes for various reasons.
we do surf launching in south Africa from certain beeches, so if you did manage to dunk the engines in the drink, cleaning them and sorting them out is very quickly.

i have gone through the specifications of the 75a hp motors and compared them with the 85a hp motors.

there is not much of a difference between the two.

i will list it here the differences.

carbs are different "not much" main jets and pilots and air "small difference"
they both consume 34 liters of fuel a hour, this tells me there is not such a big difference.

the 85a carbs jetting is:

main jet is 165
main nozzle 3,4mm
main air is 180
pilot jet is 78
pilot air jet is 100

the 75a carb jetting is:

main jet is 175
main nozzle 3,2mm
main air is 180
pilot air is 80
pilot air jet is 100

the compression ratio, corrected 75hp is 4,5:1 and the 85 hp is 5,1:1

part numbers also list that the heads are different and the cylinder sleeves are different.
this gives me a indication that the 85 head is skimmed down further to get the compression up.
the cylinders have different exhaust port sizes and possibly intake port sizes. but the motors have not been opened up so i cannot see. i also do not have a 85a to which i could compare them.

reed valves are different in opening sizes:

the 85a hp reeds are different, opening is up to 9,7 to 10,1 mm
the 75a hp is between 2,8mm and 3,2mm

the above is basically the differences between the motor specs.

so, can you guys advise on what i need to do, to get the 10hp per engine back.

i have also checked and the gasket sets sold is the same from 60 hp to 90 hp, that's how we can buy them.

i obviously need reliability on it as we dont have a big boating community like you guys.
our coast guard is none existent.

i do have quite a bit of 2 stroke engine knowledge, so it is not that i am a newbie at all to boat engines etc,

i have been studying gorden bells book and jennings 2 stroke books.
but it is more about 2 stroke bikes which have expansion chambers etc.

waiting for your replies
thank you, it is much appreciated.

[Dave S]

Welcome to S/F I would not worry about the 10 hp ,,,, leave them alone.... so they last longer. I say this because.... from what I here motors are hard to come by there ..... cheep because of taxes. Best off having a set of nice props that give best thrust for your use. What kind of boat do you have? Cheers.

[Willem Wikkel Spies]

hi, thanks for your reply

its not that engines are hard to come by, its that it is quite expensive having them.
and our technicians are not always up to scratch.

i also dont know how much hours are on them, so when i do decide to check them all out i might as well just get it all sorted.

the boat is a seacat 18,6 cat boat.
powered by the two yammie 75hp.

the new hulls deck is raised to accommodate 2x new 100 4 strokes.

we have certain legislation which states that a hull needs to be level floating when turned upside down.
so at least a 60% buoyancy in them.

i worked it out, that the combustion chamber is 12% smaller on the 85a compared to the 75a engine.

so i know therein lies some power increase if it is adjusted to be the same size.

i would have to look at squish if i do decide to shave off some metal from the head.

let see, maybe someone else can advise on the differences.

.

[hellbents-10]

Hydrotech marine is who I would turn to on modding those engines. They sell nice phase kits for them.

[minister]

Changing the reed stops or opening them up to 85-90HP spec is the biggest bang for your buck. On my 60 (detuned 70) this gave me an extra 200rpm. noticable more top end power.
Look at the part nr. from the exhaust aswell. Yamaha two strokes seem to be restricted by a Carbs, head, reed stops and exhaust tube.

[Willem Wikkel Spies]

i have already opened the reed stops to the 85 specs.
now its down to shaving the heads to bump up compression.
i will need to look at the squish clearance as well.
with the heads off, i would be able to see what size the exhaust ports are and see if they need to be widened a bit for better breathing.

is the exhausts chocked in the chest area?

nothing on the web with pictures to see differences.

[minister]

I just checked the parts manual. Exhaust tuner is the same on the75hp and 85hp. no restriction there.
Restrictions are at:
- Carbs (not just jetting)
- Head: easy fix
- Crankcase (although ive seen te same thing with the 50G compared to the 60F and 70B and I can-t spot any difference between them)
- Reed stops

I don't think there will be much difference in the porting, as with the 50G. Maybe just some minor things that help it with the lower compression head

[Willem Wikkel Spies]

i have done quite a lot of research on the net to see what i can find on all of the engines listed.
the 75a and other 75 models and 85 a models were sold to African countries, most possibly due to poor fuel being present back in the old days and thus the lower compression ratios being 4,5:1 for the 75hp and 5,1:1 for the 85hp.
these are all pre mix models.

Yamaha has their own range for their us market with auto lube engines, it seems, the 75hp, 80hp and 90hp are different.
it is listed that the compression ratios for the 75 and 85 hp are 5,90:1 and for the 90hp 5,86:1.

so you should take note not to get confused between the models.

what is listed is that all of the above engines are 1141cc or 1140cc with a bore of 82mm x 72mm.


I'm just doing my homework to see where the real differences are in the models listed.

The 50G, 60F and 70B you listed are all 849cc engines so they are out of the equation.

as far as i know, there is no restrictions after the reeds when i did open up the reeds.
the 75a reeds have stoppers which allow for a maximum opening of 3.2mm where the 85a reeds has a maximum of 10,1mm.

this was obviously one of the restrictors.

all that i need to do, is:

1. open the engines and check what the compression ratio is, by looking at stroke.
get the heads shaved off to get compression up.
and getting the squish bands cut again.
2. check what the exhaust port timings are and opening them up.
3. check transfer ports and boost ports timing. these are the last to grind on if it is needed.
4. check for restrictions in the exhaust passages.
our fuel is not a problem, having 95 at coast level with 93 and 95 up here in Johannesburg which is 1600 meters above mean sea level.
i would only want the engines to breath as 90hp I'm not looking for much more.

if i could find a broken, worn out Yamaha 90 hp engine, one could see where the differences are between the 90hp and the 75 hp.

looking at the current carbs on the 75a and 85 a.
there are not much difference between them.

as listed above, the 85a only uses about 2 liters of fuel more per hour, most probably the 0,2mm difference in main nozzle size.

its just difficult in South Africa as our boat market is small.
getting the right guys to do it is almost impossible.
i would rather do it myself.

2 stroke engines are not new to me.
all i need is some specifications and some guidance in achieving a certain goal.

and this is where you guys come in, if i may, tap some knowledge from your brains.

thanks a lot.