Yammie 90hp carbs on 70hp yammie questions.

[Pjpoppet]

Apologies if this is the incorrect place to ask questions, but here goes, I own a 15 foot Bulls bay skiff, aka ( pioneer boats), they are rare here in Australia, originally I had it fitted with a Tohatsu M50 D2, I loved the motor, but in the search for more performance, I sold that and fitted a Yammie 2005 two stroke, I was extremely pleassed with the results, 32mph to 38-39 mph, now spinning a 18 pitch. Prop, now here is the question, I’m going to replace the reeds very soon, I strongly suspect they are all out of whack or chipped, a little hard to start, some popping, from the lowest carb, loss of performance ect... I about to pull the pin on a set of square bowl 90 carbs, my question is the throttle cable on the 70 is actuated on the bottom carb, but the larger carbs are actuated from the middle carb, does anyone here know a solution to this, needs to be a perfect solution, I am a automotive mechanic by trade, so I’m no novice when it comes to fabricating things, my thought is extending the current throttle rod, however my concern will be the angle that it will then operate on, I will really appreciate any one who knows or has ideas to this modification, can’t seem to find anything on the GOOGLE. Photo is before motor swap.

[Merc 2.5]

As I've never wrenched on one of these I'm not gonna try answer but if u can post a pic of current set up and pic of new carbs b helpful ?? Someome will chime in and welcome to the jungle (s&f)

[Pjpoppet]

Photos 90hp linkages compared to 70hpdisregard the red arrow

[Pjpoppet]

The only thing I can think of is perhaps I could swap the throttle butterfly shafts around ie: put the bottom 70hp shaft in the bottom 90hp carb providing they are the same diameter.

[Pjpoppet]

Just a little bump, to see if anyone had advice.

[Merc 2.5]

I dont have experiences with these. so dont wanna lead u n wrong direction. Hopefully someone will chime in

[Pjpoppet]

I appreciate you’re attempts of advice, seems this modification may be long forgotten by those who have seen it or done it, the main issue will more than likely be the location of the starter, on a 90 it’s on the other side hence Yamaha running the throttle to the centre carb, I’ll post up more photos when I strip it, carbys should arrive this week, CC Reeds next week, stay tuned.

[c-more]

I'm with you. I was searching for a similar question (will 85hp carbs work too?). There was loads of discussion on 70 mods a few years ago. . .maybe archived? I'm afraid the carbs may be limited value without tuner maybe even porting changes. There were also discussions on using the 40hp mid and lower unit under it. I'd love to just find a perfect 70CES.

[Pjpoppet]

After much tinkering, thinking drinking Peronis, this was the only solution that will work, especially if you’re 70 has the starter on the drivers side, I carefully filled off the throttle attachments after removing tham from the carbs, ( be careful you don’t force the butterfly’s out), the I swapped them around ie: middle carb to bottom carb, then I brazed them back in position as carefully as possible whilst having the shafts submerged in water so as to not warp/damage them, reassembled the shafts back into the carbs, refitted butterflies, remounted them, then modified the cam to lower carb actuator to line up perfectly, has to be spaced out about 1/3rd of an inch. I’ve had it running, now just waiting for the reeds which are coming today.

[Pjpoppet]

After much tinkering, thinking drinking Peronis, this was the only solution that will work, especially if you’re 70 has the starter on the drivers side, I carefully filled off the throttle attachments after removing tham from the carbs, ( be careful you don’t force the butterfly’s out), the I swapped them around ie: middle carb to bottom carb, then I brazed them back in position as carefully as possible whilst having the shafts submerged in water so as to not warp/damage them, reassembled the shafts back into the carbs, refitted butterflies, remounted them, then modified the cam to lower carb actuator to line up perfectly, has to be spaced out about 1/3rd of an inch. I’ve had it running, now just waiting for the reeds which are coming today.

p.s you have to use the air box off the 70, so you may want to add a few extra holes, I’ll try in as it is first though.

[Pjpoppet]

Water tested today, it bogs down more when you take off, I had to muck about with the timing a bit, only by ear, I have gained about 150ish rpm almost at around 5750 rpm, mid range she is a totally different boat, but alas no real top end speed gain, best was 38.8 mph, conditions were horrible though, prop is a merc vengeance 13x18, I might try the yammie stainless one again in 17p, get that rpm up more and see what happens, should of taken it today, also I think I will put the jets from the 70 carbs in the 90 carbs, it might help with the bogging down on take off, again many thanks for any advice offered.

[Merc 2.5]

Did u up the compression so bigger mains ain't killin it? Def lean ur idle out tho

[Pjpoppet]

Compression has not been altered hence thinking the smaller jets may work better, why lean out the idle? She actually idles better than previously. I’m thinking of match porting the inlet/reed plate to the larger carbs bore size including the air box, reason being as there was a 2mph difference between the air box fitted and removed yesterday. Yesterday was just a first shakedown run whilst I wait the the CC replacement reeds to arrive.

[Roflhat]

You need to open up the reed block diameter to match the 90hp venturis, more compression will help a lot. Can get some velocity stacks for it which will be better still than no airbox. Usual starting point on them is main jets from the 70hp carbs in the 90's, think theyre 155 or 160

[Pjpoppet]

Sorry for the late update, have water tested a few times now, I mucked around with engine height, but the vent plate was a little out of the water at speed and became a bit too unstable, mph 39, dropped her back down, calm conditions spinning at 6000rpm, still only 37-38mph, much more torque otherwise, jets from the 70 are fitted, pushing her up from 5650 to 6000, yeah sure you get the revs but she just flattens out, all noise and no gained speed, I still feel the 18 pitch is spot on, I probably need to try different variations, maybe larger diameter. Of pull her all apart asses all internals, raise the comp a bit and start again ?..