Anyone any good with engines?

[cwood]

Hi all.
Our club boat has been idle most of this last year as it developed a problem last season which we have not been able to sort out.

It works fine for perhaps 15 minutes at full power when it begins to loose revs (from around 5000 to 3000). The loss of revs isnt very smooth but will pick up after a bit when easing off the throttle and then bringing the power back up.

The engine is a yamaha 90hp 2 stroke (1995 I think)

So far we have replaced the fuel pump, fuel filter, carbs, reed valves and some fuel lines to no effect.

The immediate thought is that it is a fuel supply problem or maybe an electircal fault with one of the sensors perhaps.

Any ideas?

Chris

[Simon A]

may be worth checking the water system for any debris, could be getting too hot.
HTH
Simon A

[Ivon]

Sounds like a problem with the CDI/ECU to me.

[ssteve]

have you changed the spark plug? is it the correct one? blow out all fuel lines and check fuel tank breather,if all above ok could be ignition system module breaking down when up to working temp
ssteve

[cwood]

Thanks for all the suggestions. The spark plugs have also been changed. We have considered the ICU but its an expensive part. Does anyone know how we can test the current one?

We are also considering a heat problem - or perhaps a faulty temp sensor but are also unsure how to test for these.

Cheers

[Kent Tooling]

when was the last time you changed the fuel bulb as these tend to break down in side.

Check the rubber bush in the prop as this might be slipping?

Do you have an external fuel filter if so have you changed this as their might be some muck from the fuel tank getting in to the carbs? if you have not i would deffently fit one with a clear bulb and with a tap on the bottom then you can see the fuel is clean.

If you have any further questions you can contact me, i am no engine mechanic but have had many Yamaha engines so i have a little knowledge on that make.

John

[David G]

Going back to spark plugs a moment.

My outboard manual lists a spark plug for normal operation but then lists an alternative is the unit seems to run cold and a further alternative if it tends to run hot..........might be worth going for a hotter running plug. This would cost about a tenner compared to the costs of some of the other components you are thinking about!

Regards,

David.

[cwood]

Originally Posted by Kent Tooling

when was the last time you changed the fuel bulb as these tend to break down in side.

Check the rubber bush in the prop as this might be slipping?

Do you have an external fuel filter if so have you changed this as their might be some muck from the fuel tank getting in to the carbs? if you have not i would deffently fit one with a clear bulb and with a tap on the bottom then you can see the fuel is clean.

If you have any further questions you can contact me, i am no engine mechanic but have had many Yamaha engines so i have a little knowledge on that make.

John

Thanks for your suggestions. The fuel bulb and line is pretty new and plenty of fuel is reaching the carbs. The external fuel filter has been checked and changed though it looked fine. Same with the smaller internal filter. A sample of petrol from the filter looks clear and there is no trace of water in it.

Thanks again
Chris

[MattS]

When you say not very smooth, do you mean that one or more cylinders is misfiring? It sounds electrical. Stators, ignition pickup and coil break downs are more common than ICU failures on 2-stroke engines IME.

Plug temperatures can be checked by doing a chop test. Get the engine up to temperature, put it in full power for a minute and cut the engine (pull the dead mans). Whip the plugs out and have a look at the insulator and electrode. A tan colour means they are about right. White means the engine is running hot and you should fit a cooler plug. Black means the engine is running cold and you should fit a hotter plug.

[mala]

if it runs for 15 mins ok at full power then its unlikey to be a fuel or ignition problem.
possible overheating due to poor coolant flow or more likely carbon build up in the exhaust system and ports.
after 12 years of 2-stroke mix it may need a de-coke.

2p