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Yamaha 75 hp 2 strokes  Rate Topic 
 
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 Posted: Wed Oct 9th, 2019 09:30 am
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willem wikkel spies
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Mana: 
so I have 2x 75 hp Yamaha 2 strokes.
they do run well without any problems.

just to check if all were well, I decided to download the manual and see what is what.

I found that the 75hp motors are actually 85hp motors and that they have been choked.

so I did some reading through the manual to see where they ended up in screwing the end user.

there is not much difference.

both are 1140 cc motors.

they use different carbs, numbers only.
carb trout size is still unknown.

the 75hp carb jet sizes are about the same.
the main jet is 0,10mm bigger then the 85hp.
air correctio are the same.
emulsion tube on the 75 is 3,2mm and 3,4mm on the 85hp.

so how do you get 10hp more on a engine, with a main jet being 0,10mm smaller and 0,2mm bigger emulsion tube.

there has to be something else.
10 hp is not being hidden in the above.

so scrolling through the manual further.....

I found that the motors have different reed valve assemblies.
the 85hp allows the reed valve to open to 10,1mm with specs being 9,7mm to 10,1 mm

the 75 horse power the reed valves are set to a maximum of 3,2mm

so in essence, preventing the engine to breathe properly is done by choking the reed valve assembly.

lets see where this takes me.

do note, I have much knowledge regarding 2 stroke engines, their operation etc.
I have studied Jennings book on 2 stroke engines.
playing with 2 strokes, port timing etc. but not on my boat engines. so don't worry about these aspects.

it is mere a discussion tread.

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 Posted: Wed Oct 9th, 2019 09:54 am
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TimJan
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Mana: 
Hi

I have heard exactly the same about the old 2 stroke suzuki my dad had on his boat it was a 115 and they say exactly same engine as the 140 just reed valves are different.

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 Posted: Wed Oct 9th, 2019 01:10 pm
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yakuza
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Mana: 
Epic research Willem, do let us know how it goes after you adjust and or replace the reed valves, really interesting find

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 Posted: Wed Oct 9th, 2019 07:45 pm
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willem wikkel spies
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Mana: 
cool will keep you guys updated.

attached is the manual for the Yamaha 55,75 and 85 2 stroke engines.

Attachment: yamaha manual 55D 75A 85A SM.pdf (Downloaded 2 times)

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 Posted: Wed Oct 9th, 2019 07:55 pm
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willem wikkel spies
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Mana: 
just some off topic discussions while I get the list of aspects in order.

one cannot run a 2x stroke carb dry!!!!!
so for storage purposes, use the drain screw on the bottom of the carb and drain the excess fuel out.
this will prevent the fuel from evaporating leaving the 2 stroke behind and causing things to gum up!!!!

so who ever told you to run carbs dry is talking BS.

float level height is also very important when it comes to 2 stroke engines.
a very low float level will cause your 2 stroke engine to run lean on high rpm.

a very high float level will again cause a much richer mixture on high rpms.

so even if you did adjust the pilot screw correctly out at 1-1,5 turns it will not bring the lean or rich mixture back to where it should be.

when having problems with 2 strokes, start with the basics first.
then slowly work through all aspects.

I have seen too many guys jumping to conclusions when the most basic things have not been investigated.

2 stroke engine are a joy to work on.
not much that can go wrong compared to 4 strokes.
fault finding is also much easier.

they might be very thirsty but they do tend to handle crap skippers well.
servicing is very easy and not much to do.
no need to get a engine techie to do a service.

2 stroke engines have not changed much since 1968 when all was already known.

small aspects only.
new technology, of 2 stroke fuel injection was the last big change.

if we could have placed tune pipes on outboards then the coast guard would never be able to catch you.


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