1999 Yamaha 2-stroke 75/90 upgrade.

[wavedancer]

Hi everyone,

I recently replaced my old Johnson by a '99 2-stroke 75 Yam.
I've been told that it is possible to upgrade to 90 hp. by modification of the carbs.
Does it means that the complete carbs have to be replaced or just parts if it ?
What about the cost of such an upgrade ?
Is there an alarm for exeeding max. rpm ?
I dont have a tach yet.

Thanks a lot.

[Bay Catt]

I would most certainly check with a yamaha technician on upgradeing hp. As far as I am aware there is way this can be done inexpensivly. And yes, you should definatly be alarmed with exceding max rpm's. Depending on if you are looking for greater power or greater speed you could always change your prop to a different pitch. Geater pitch will result in more speed, lesser pitch with result in more power. By modifying the prop pitch you rpms will also change either higher or lower. Be carefull though and check with yamaha on the pitch band allowable for you motor. I have found it best to stay within the middle of the prop band, it's a safe call. Correctly propping is a science, don't take a guess. The results could be disasterious.

[wavedancer]

Thanks Bay Catt,

I know that changing props can make a tremendous
difference but the prop I run is perfect.
I'd rather would gain that extra 15 hp. by modification of the carbs since the
75 and the 90 are exactly the same engines.
I'm just curious about how to do it and the cost.

[Bay Catt]

Your probably looking at upward of $2000 to modify/rebiuld carbs. Ask the tech guys at Yamaha about this.

[T-Bone]

The 70 and 90 are completely different powerheads. Your assumption of only a carburetor difference is incorrect.

I currently own a 1988 90 and the information I'm offering is based on the Yamaha factory shop manual that covers both the 70 and 90HP 1988 model year outboards.

70HP: 849cc disp, 2.33:1 lower unit gear ratio, 8 mounting bolts from power head to mid section, press-pin rods and crankshaft, 14 bolts between upper & lower crankcase halves, common bolts for cylinder head and cover, . . .

90HP: 1140cc disp, 2:1 ratio, 11 mounting bolts, cap-and-bolt rods & crankshaft, 20 crankcase bolts, separate bolts for cylinder head and cover, . . .

There are some similarities. i.e. the lower units will interchange, but they have different gear ratios. Stern mounting patterns are identical, though, so a 90HP unit will bolt up directly. However, you can't upgrade a 70 to a 90 without total replacement of the whole outboard.

[T-Bone]

Sorry to "pipe in" again, but I'll offer to "eat my post" if the information I provided is not accurate for the '99 model year 70 and 90HP 2-stroke outboards.

[wavedancer]

No problem T-Bone.

Everyone can make a mistake.
Thanks for Your effort to help me with my question
aldo it wasn't the right answer.

Regards,

[Norway73]

Hi,
I understand that you are interested in upgrading your 75 Yamaha to a 90HP which should be a nice little weekend prodject !!
Normaly there will be like a blocking in the Exhaust Maifold which you would have to open up but on the 75/90hp there are only different carb's on them. So you should try to look on the bet to buy some second hand carb's and fit them!!
Should not be so hard to get the right part's from a broken down engine,, you can use carb's from 1988 and up,,, Good Luck with you prodject!!

I currently run a 90hp which had been blue printed and put out about 115hp excelent engine to drive!!

[wavedancer]

Thanks Norway,

That's the answer I was hoping for.
Used 90 hp. carbs are easy to find since that motor has been very popular and still is.

Does the oil flow has to be adjusted or are there other modifications that has to be made ?

Btw: Do You live in Norway ?
I live in Belgium but my boat is Norwegian made.
It's a Triss / Ryds 520 ST. ( 17 footer )
Solid as a rock.

Regards,

Wavedancer.

[DAN75-90]

can be done easy

i just put 90hp carbs on my 75hp enduro and it worked a treat the 2002 75 enduro is the same as the 90hp got second hand carbys cleaned them and put them on i also put all the electrics on and changed it to forward control and power tilt and trim and electric start the motor now hammers i also changed prop from 15 pitch to 17 pitch so when you check all the specs on this motor it is the same as the 90 but have a mate with a 90hp on a boat that is very close to the same and we had a race it was pritty fare race as we both had around the same load and i blew him away was great that i had done all this myself and ended up with a better motor so to all those who say it carnt be done your wrong