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Old 15 May 2004, 14:03   #1
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Country: UK - Scotland
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Uprating outboards

When I bought my RIB I got a good deal on a 92 yamaha 60hp, (basically I ran out of cash) and thought if it did a year or so I would buy a 70hp. Well the engine is in even better nick that I thought so it is pointless putting out the cash for a while on a new one, even if I had it.
So......since it was a 70hp I wanted I had a look through the manual to see what the powerhead differences were and they seem to only be carb and timing adjustments.
It does state the carbs are different but I suspect it is just jet sizes inside that are different.
Has anybody tried this? If it is just a matter of changing several jets and retarding the timing to get 10hp then why not? It just seems a bit too simple which is why I am wondering.....................
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Old 16 May 2004, 15:12   #2
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Think you will find that the cost to covert it probably makes it not worth your while. Certainly Mercury used to ensure that the cost of carbs and reeds would make in uneconomical.

Are you sure it will perform faultlessly ? and what happens when you come to sell it ?

Pete
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Old 16 May 2004, 16:33   #3
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uprating

Its prob more than just jets, the float bowls are probably different giving different float heights for the floats in the carbs, this will affect fuel delivery etc
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Old 16 May 2004, 17:35   #4
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The carbs have different numbers on them in the manual but all the settings, including float heights are the same. I suspect, but will check tomorrow with my usual dealer, that the carbs are identical but the jet sizes are different.
The CDI unit is the same as well. All other info is identical and I have been unable to find any differences other than jet and timing alterations.
For the cost of a set of jets and altering the timing it is worth trying, I just can't believe it is only this alteration.
Still if nobody can point out something else it is worth having a go...
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Old 16 May 2004, 18:02   #5
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more power

Hi,

I don"t know so much about mercury/mariners, but I can tell you a thing about the evinrudes/johnsons. In the small range of outboards ( it's allready several years ago)6-8 and 9.9-15 they made a restriction in the air pass after the carb. and they put in a smaler SPROEIER (that a dutch word for the small thing which the fuel goes thrue) and thats the only differance between these engines. Don't know if it's the same for you but its worth checking !!

excuse me for the languege,
the best regards Arne
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Old 16 May 2004, 19:00   #6
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hi rupert,
i would say it was highly unlikely you would gain 10hp simply by changing the jets and retarding the timing...
increasing jet size from the optimum for your current engine setup would simply make it run richer which is likely to be detrimental rather than of benefit - you would not be getting increased mixture as you would with larger bore carbs, just too much fuel... retarding the ignition would also decrease performance in the absence of other modifications

i don't know anything specifically about your engine but i would suspect that yours has different porting to the 70hp version - also on 2-strokes the expansion exhaust shape and length is critical to determining the entire power-band... i would say the differences in jetting and timing you see between the 2 engines are far more likely to be just minor optimisations following porting and exhaust or other changes (which are the real factors actually causing the power increase)
i would guess the 70hp probably would also have a different timing curve programmed into the cdi -

i'm just guessing here and it could be possible it just has bigger bore carbs but almost certainly there will be other differences to the ones you list
best
andy
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Old 17 May 2004, 05:34   #7
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I am satisfied that the basic powerhead is identical to the 70hp. The CDI also seems to be identical and is listed in my manual as the same part.
The carbs certainly look the same with different main and pilot jets, not sure if this is why they have different numbers or they actually are different in oher ways. I suspect they are the same castings but some inserts may be different. The worst case is that the carbs need changing to 70hp type.
This idea actually came from the dealer I bought the outoard from but I thought until recently the cost would be prohibitive until comparing the engines in the manual specs.
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Old 19 May 2004, 03:44   #8
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60Hp / 70Hp:

My experience is with Japanese engines (Yamaha and Tohatsu mostly)

The three cylinder motors are sold in differing power ratings, but are pretty much the same engines - 40/50same - 60/70same - 80/85/90same

The differences between the 60/70 are (in order of importance):
1. Port sizing / Cylinder block [not just sleeves, the whole block]
2. Carburettor venturi diameter. (normally the same carb. No. though)
3. Reed stopper height.
4. The plugs are normally 1 number lower (hotter) on the lesser Hp version.

CDI is the same and timing is the same (optimal).

It is not too difficult (expensive) to make more HP, you just need someone who is good at doing 2 stroke porting (like a 2/S motorcycle mechanic).

Stripping the engine completely and rebuilding it is quite good thing to do anyway (these are really simple engines and they run much better after a rebuild).

Make the ports as wide as possible (about 20-25% wider).
You may go 1-2 mm higher, although you risk losing low down power.
Don't ever go lower.

You may also want to increase the compression by skimming the head. 1-1.5mm is OK (good for about 5% more power).

Boyesen reeds also do help (but not as much as they claim though!). When using Boyesens, leave the reed stops in place (they instruct you to remove them)

The carbs will cost too much and won't make as much difference as the porting. You can bore the carbs, but It is too difficult to get the mixture right afterward - so if you want to change them, rather get some 70 carbs from somewhere.

Iíve got a Yamaha 70CES standard and a modified 60M (same engine) and the 60 puts out way more power than the 70.

Nice engines!
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Old 24 May 2004, 17:49   #9
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Anybody know where I can get a look at the microfiches for the 60 and 70? I would like to see what the part numbers are on the differences. A;lternatively does anybody know where I can get a cheap 70B powerhead that I can strip?
Still curious about the differences as knowing engine manufacturers they are likely to be minimal.
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Old 25 May 2004, 18:36   #10
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hp

Had a friend with a Tornado 5.5 lovely 55 Mariner on the back, again obsessed with more hp he traded up to a 60hp Yamaha, boat went slower afterwards, as his YAMAHA was heavier, affecting power to weight ratio, I wouldnt go skimming the head, it may increase compression, but reduce cylinder volume and could cause , plugs to close if the piston hits them, as for porting and back pressure in the exhaust system, I have a book on two stroke tuning, you wouldnt believe how complex and scientific the theory is. Also the cdi may be similar, but the pulse coil on the stator may not be pulsed at the same instant on the bigger motor, to change the timing here you may have to change stator, and or flywheel ,Have you thought about oil delivery and adjusting the pump to satisfy a bigger block.You may find its not economical to try to get more hp from the motor and may well be cheaper to do a trade up cheers gavin
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