Yamaha 70 running at low RPM

[Birdo]

Hi all... first post and it's a plea for advice

I have a Yamaha 70 (1995). According to the tachymeter, it's running at approximately 2400-2500 rpm at full throttle. I looked on the Yam website and the current 70s have a range of 5-6k. Unfortunately, this has been the case since I bought it with the boat a couple of months ago (first boat and a boating/outboard newbie). I've been happily using it at this RPM without realising I had a problem (is this likely to damage it??)

The engine has been serviced (first thing I had done after I bought it).

Problem 2

There have been a few instances of where I've pushed the throttle upto max or close to max and it's been running fine until it suddenly loses power. The engine never stalls but I have to throttle back to lowest throttle (not neutral) and all works fine again.

Another thing.... how long should the fuel line be? The line on mine seems quite short... approx 2-2.5 feet. I wondered if this might be a factor? I'm going to change the fuel line/bulb and fuel connections this week I also want to fit a water separator onto the line (is this essential?).

If anyone recognises these symptoms and can offer advice, I'd be most grateful!!

(I'm going to order the clymer Yam manual to get to know this stuff and hopefully do any necessary work myself)

[Poly]

I guess the first thing you need to do is work out if the tacho is telling the truth... the easiest way to do this might be to give some sort of idea of speed you are getting - and the size of prop you are using - then the wise old sages here will be able to comment on whether they think the problem is the tacho or the engine...

[Birdo]

Thanks for the reply...

I don't have GPS or a speedo so I'm not sure of the actual speed. However, it doesn't seem particularly sluggish (hence the reason I wasn't aware of a potential problem before realising that the max rpm should be (or read) much higher). The trim gauge seems to work fine on the tachometer. The prop measures 13 inches.

[Tim M]

Birdo.

What does the engine sound like when its running at max revs? You should be able to tell roughtly what its doing by its tone. When its at max does it sound like a car thats being reved really fast or is it just a reasonably gentle (in an engine sort of way) noise?

Sounds from what you've said like your tachos nackered (or possibly not set up right; check the back to see if theres a little screw adjustemnt thing).

With regards to fuel line, no it doesn't matter if your line is really short. Might pay you to change the bulb though it it looks fairly old as they do tend to go. If your engine is an old fashioned 2-stroke fitting a water seperator/filter isnt essential as the engine should be able to handle a small amount of c**p in the fuel (unlike a modern computerised one). Saying that I would be inclinded to fit one; it will save you having to take the carbs apart when you get a bit of crap stuck in there from the bottom of your fuel tank.

[Birdo]

@ Tim... thanks for the reply

The engine suouds like it's running at high revs to me. It doesn't sound like it's struggling at all. The boat it's pushing is a 16' Eurosport dory (next boat WILL be a RIB lol), which I would think is quite heavy, so I'm sure that 2400 rpm would be really noticable?

Any idea as to why I get this sudden loss of power? (this is why I asked about the length of the fuel line - it seems to happen when the fuel tank is less than half-full - could it be possible that with bouncing up and down, that the line isn't holding enough fuel and it's sucking air into the carb? - probably a half arsed theory lol )

[mccabe]

I may be wrong but I remember I had a Yamaha 50hp and I thought that there was a switch at the back of the Tacho that altered the rev readings Ie it sound like his revs are reading half speed whilst the engine is reving at 5-6 k

[MikeCC]

My 115 had some similar symptoms.

Turned out to be the CDI. The manual should tell you how to test this.

As for the fuel tank, the pickup will be near the bottom of the tank, so half-full shouldn't cause any problems.

But do check that the breather is clear. If its a plastic tank, this might be a little screw on top of the guage that needs to be undone a bit. If there's no air getting into the tank. then after a while, no fuel will get out - the half-full thng might indicate this.

[seaquest1]

might be totaly irrelevent
but i have a 60 oil injection model had it now 10 years but seem to remmember that one time i was out the engine done same sort of thing
i had to cut the revs down to gaet it to run even .
once back to port checked it all and found out i never had enough 2 stroke in the tank it had a safty feature built in that when the 2 stroke went down it cut the revs that you could get ,to save the engine
just a thought
mike

[Biggles]

I had a Yam 50 that did that because of dirt in the carbs.

[Nos4r2]

If you know what you're doing with a good quality timing strobe you can use that to check revs.

Beware-if you've got a mainjet blocked with crap thats causing it then that cylinder will be being dragged along by the rest of the motor without proper oiling. Even oil injection engines rely on the fuel to move the oil around inside the engine.