Yamaha 50hp 2 stroke "stuttering" problems

[BumbleAbout]

Out on the water last week, I had an afternoon where the outboard went from 3500rpm (happily on the plane) to about 1800rpm and it sounded very rough. When the throttle was pushed forward it did not want to rev past 1800rpm, but (only as a quick test) I put it in neutral and blipped the prime lever, it would happily rev more. Could this have been water in the petrol (I wondered about dirt in the carbs, but thought that the prime lever would also have failed to rev then) ?

[Downhilldai]

Could be muck in the carbs.
When you open the throttle in neutral, the engine is not under load, so will rev more freely.
Also check the fuel line, tank vent, primer bulb, etc. It has been known for people to forget to open the tank vent.........

[BumbleAbout]

Originally Posted by Downhilldai

Could be muck in the carbs.
When you open the throttle in neutral, the engine is not under load, so will rev more freely.
Also check the fuel line, tank vent, primer bulb, etc. It has been known for people to forget to open the tank vent.........

Definately not the tank vent this time

New fuel line (including a new feeder tube inside the fuel tank), new primer bulb. The strange thing is that it all recovered after an overnight mooring - the next day the engine revved smoothly - apart from a higher speed run at 4000rpm, when it seemed to stutter; run roughly for a couple of minutes and then recover again. I also whipped off the outboard cover and when I tried to rev the engine by pulling down on the lever which connects all 3 carbs, it died when trying to rev above 1800rpm again ...

If I am getting rubbish in the carbs is this a good time to change the fuel filter ?

[Downhilldai]

It could be one (or more) of a number of things and blocked fuel filter would definitely give these symptoms, so if it hasn't been recently changed, a good idea to get a new one on there. Best to try a process of elimination, by changing one item, trying it out, then move onto the next, if that didn't solve it.
I'm curious as to why it wouldn't rev from idle though - I don't suppose you were operatng the chokes, rather than the throttles?
Are all the connections ok on your new fuel line?
Alternatively, you may have a dodgy electrical connection, which is cutting out from time to time - these can be difficult to fix, as they seldom occur when you're looking for them.

[al40]

If it's not dirt in carbs then....

Is it an autolube? Could be low oil level in which case the ECU can rev limit the engine (I assume by killing one or more of the spark plugs). This would make it sound rough. If oil is at the critical level, you can see this problem coming or going depending on how rough it is and relative level of engine.

Similarly, some engines also do this on an overheat situation.

If it's always going down to 1800 rpm on the dot, this sounds like it could be electronically limited!

[BumbleAbout]

It might be/ have been dirt in the carbs - but it stopped having a problem after that afternoon and the following day it only happened once (at WOT). So I'm hoping that it is dirt rather than something else !

It is an autolube, but the oil was only down to about 2/3 full - I am going to reduce the oil level the next time we are out (a few weeks away) to check this !

1800rpm was what it would stay at on at WOT, it dropped down to 600 rpm (and then died) if the throttle was at a lower position - I wanted to get back to shallower water, so stayed at 1800 !

[Easyrider]

I would check ALL of the electrical connections and make sure there isnt any water in them.

[doggypaddle]

it wont rev if you operate the carb linkage, the timing staror needs to rotate to advance the timing too......