Upgrading from a yammaha 70 to 90 yammaha

Gledhill · 21 June 2016, 22:22

Hiya lads I currently have a 5.4m aircraft from Liverpool power boats running a 70hp yammaha I have been offered an immaculate condition 90 yammaha both 2 stroke autolubes. The 70 being a 1999 model and the 90 a 2004 model with only 130hrs on the clock do anyone no weather the wiring loom will be the same for each engine save having to remove the whole lot. Also the boat at WOT is currently running at around 36mph with the 70hp. Does anyone no what sort of speeds could be reached with the 90hp. Thankyou

Bigmuz7 · 21 June 2016, 22:35

I would 'expect' wiring to be same ... more is better .. grab it ,, its a rare beast

phantom 19 · 22 June 2016, 00:24

Wiring is the same. Controls are the same (70's used to be pull to open but they changed them to push to open late 90's)

I prefer the 70, having spent the last 20 years of my life working on both of them for a living.

Gledhill · 22 June 2016, 06:45

So would it be straight swop then mate I wouldn't have to change the wiring loom or anything else could keep my controller and loom off my 70?? For what reasons do you prefer the 70 over the 90 please?? I'm really undecided weather it is going to make a big change to the boat. Thankyou

Davie · 22 June 2016, 09:52

The control box wiring is the same but you may have issues with the instrument connections. There were various plugs around that era.

Gledhill · 22 June 2016, 09:57

Do you think there would be much difference in all round performance between the two I no the 70 is on a 1000cc block and the 90 is on a 1200cc or 1250cc block. Cheers

phantom 19 · 22 June 2016, 13:38

They both have their pros and cons. The 70 is lighter and more compact, it is an 8 or so year newer design and was designed for the autolube system from the start (although not fitted until 2 years after release) so everything fits nicely. It seems a bit more refined in use. The lower shock mounts are stronger on the 70 (not a problem really if you aren't racing or hammering)

The 90 is more rebuildable in the long term - the crank is one piece unlike the 70 which is pressed together, and you can get pattern cranks / conrods etc (not sure I'd use them tho)

Both the engines you mention have the better thicker gearcase which goes some way towards preventing the splits normally found on earlier 60-90hps.

You also know your current engine - do you know the really history of the 12 year old 90? I built a high performance 90 last year and it took five (admittedly older) 90's to get one crank that wasn't shot.

How fast do you need to go? I have a 70 on a 5.3 Humber attaque and it will do 42mph on GPS with a worked Laser2 prop, it cruises nicely at 25mph at 16-18l/Hr with Yamaha fuel management system. I had a 90 on it once and it was only a few miles an hr faster but used quite a bit more fuel (can't remember the figures now)

Gledhill · 22 June 2016, 14:22

The 90 has been apart of a sea scout group from new but has been stood for quite a number of years hence there's only 137 hours on the clock.
It was a friend who bought it off the sea scouts last year all the jets in the carbs were gummed up so he decided instead of re jetting the engine he would put a brand new set of carbs on converting it from the 75 to the 90 by just winding up the timing mark to the next notch. My 70 is a 1989 model and the 90 is a 2004 model he saying £1500 and I can swop them over just don't want to do it if there's not a significant increase in power I would like to be close to the 45-50mph mark at WOT.

phantom 19 · 22 June 2016, 15:55

Quote:

Originally Posted by Gledhill

The 90 has been apart of a sea scout group from new but has been stood for quite a number of years hence there's only 137 hours on the clock.
It was a friend who bought it off the sea scouts last year all the jets in the carbs were gummed up so he decided instead of re jetting the engine he would put a brand new set of carbs on converting it from the 75 to the 90 by just winding up the timing mark to the next notch. My 70 is a 1989 model and the 90 is a 2004 model he saying £1500 and I can swop them over just don't want to do it if there's not a significant increase in power I would like to be close to the 45-50mph mark at WOT.

Its not a 90 then it's a 75 running too rich and over advanced.

The carbs on a 90 are identical to a 75 but running larger mains. You need the high compression cylinder head and also a new exhaust tuner to convert it to a 90.

So unless you do that you are probably going to go slower.

And if it is an 89 not 99 then the control box linkage needs changing

Gledhill · 22 June 2016, 16:06

Do you no a rough price for cylinder head and exhaust system mate?? Thanks for your help

beerbelly · 22 June 2016, 16:35

I wouldn't have thought it was worth the chew and expense if your smaller outboards running well .after all your only going to gain a couple of knots.id stick with what you have. as an example my 5.6 osprey with 130 Yamaha only does 38 knots. and the hull is rated at a max of 42

Last Tango · 22 June 2016, 17:10

Quote:

Originally Posted by Gledhill

It was a friend who bought it off the sea scouts last year all the jets in the carbs were gummed up so he decided instead of re jetting the engine he would put a brand new set of carbs on converting it from the 75 to the 90 by just winding up the timing mark to the next notch. My 70 is a 1989 model and the 90 is a 2004 model he saying £1500 and I can swop them over just don't want to do it if there's not a significant increase in power I would like to be close to the 45-50mph mark at WOT.

Or he could have just cleaned the jets ????

If you're getting 42mph with a 70hp I'd think 50mph with a 90hp is a bit optimistic.

Gledhill · 22 June 2016, 17:19

No I've seen the carbs brand new in the boxes there still sat there now £920 he has just payed for them hence the reason why he is saying £1500 for the swop which I think is fair. I have been up to ullswater with the rib and was getting 40mph but in the sea with the tide etc etc I seem to get roughly 35mph but just after some more speed really 😃

Gledhill · 22 June 2016, 17:21

Just looked back over the post it was a typing error my engine is 99 model commissioned in 2001

Roflhat · 23 June 2016, 12:56

I would go for it, if you know the 90 is in good condition. There was a 90 head on ebay a while back, not sure if it's still there, was for use on a 75 to 90 comversion. Easy enough to swap, the exhaust takes a wee bit more time, need to remove the powerhead to get to it.

I believe the yam 70hp is 849cc

beerbelly · 23 June 2016, 17:04

Quote:

Originally Posted by Roflhat

I would go for it, if you know the 90 is in good condition. There was a 90 head on ebay a while back, not sure if it's still there, was for use on a 75 to 90 comversion. Easy enough to swap, the exhaust takes a wee bit more time, need to remove the powerhead to get to it.

I believe the yam 70hp is 849cc

but it isn't a 90 its a 75 with a set of new carbs and advanced timing he would still need to buy a head and do the exhaust or he wont be any further forward its starting to sound an expensive 3-4 knots improvement