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BSAC convert
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Discussion Starter · #1 ·
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
 

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did it with no boots on
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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:)
 

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BSAC convert
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Discussion Starter · #5 ·
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
 

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The man who makes nice "SHINY" things
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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
 

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...
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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.
 

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BSAC convert
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Discussion Starter · #8 ·
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
 

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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.
 

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The worlds slowest sailor.
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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
 

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The worlds slowest sailor.
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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.
white means the engine is running lean and this causes overheating.
black means the engine is running rich-to much fuel or not enough air.

if the engine fault only happens when the engine is hot it may be leaking air past the crankase seals as the crankcase expands.
 

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BSAC convert
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Discussion Starter · #12 ·
First time Ive used the multi-quote button so please bear with me if I get it wrong.

Firstly thanks for all the ideas so far. Please keep them coming! We really need the boat over summer (students cant really afford hard boats to often) so its a priority at the moment though we cant really be throwing too much money at it either unfortunately.

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
This is what I was thinking though I havent a clue how to check the coolant flow (the telltale runs smoothly and isnt overly warm). What does a decoke involve?

Thanks

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.
May be worth a try but is this likely given that the engne has run fine with these plugs for a long time?

Thanks

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.
It doesnt sound like a misfire but the loss of revs is very jerky. Do you know of any ways I can test for an electrical problem like those you are suggesting?

I'll check the plug temperatures later today.

Thanks for your help

Chris
 

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The man who makes nice "SHINY" things
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Chris have you a Yamaha dealer up your way that can have look at your engine,

When was the last time you had the engine serviced?

John
 

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BSAC convert
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Discussion Starter · #14 ·
Chris have you a Yamaha dealer up your way that can have look at your engine,

When was the last time you had the engine serviced?

John
It was serviced a couple of months before the problem arose (nearly a year ago) and has been back in at least once (maybe more) to try and find the fault. They were unsuccessful so we have been replacing some parts in the hope that this would sort it.

In the last year alot of money has been thrown at the engine and we're at the point of giving up on it and trying to get a new one out of our university AU. They are pretty stingy though and getting this engine working would be the better option.

Thanks
Chris
 

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The worlds slowest sailor.
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First time Ive used the multi-quote button so please bear with me if I get it wrong.

Firstly thanks for all the ideas so far. Please keep them coming! We really need the boat over summer (students cant really afford hard boats to often) so its a priority at the moment though we cant really be throwing too much money at it either unfortunately.



This is what I was thinking though I havent a clue how to check the coolant flow (the telltale runs smoothly and isnt overly warm). What does a decoke involve?

Thanks

2 strokes have the lubricant mixed with the fuel to lubricate the moving parts.when it has done its job it is burnt with the mixture and exits the engine causing the blue smoke and nice smell. some of the burnt oil however is deposited on the piston ,head and all through the exhaust system.
this builds up over time and reduces engine efficiency untill the poor old thing just won't go.
remove exhaust and have a check.also check exhaust port.
we used to soak exhaust in caustic soda and tap the big bits out with a suitable heavy object.
 

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did it with no boots on
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or take exhaust off and set it on fire,burns the carbon, when cold tap it and ash falls out,
ssteve:)
 

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cwood said:
It doesnt sound like a misfire but the loss of revs is very jerky. Do you know of any ways I can test for an electrical problem like those you are suggesting?
If it's electrical it will drop out (miss) on (usually) one cylinder. You can hear it in the engine note. A constant miss causes a drop in pitch and the power will drop suddenly. An intermittent miss is bursty whith the engine juddering as the power comes in and out.

I'll check the plug temperatures later today.
I would not bother worrying about plug temperatures, it's more a tuning thing. But do a plug chop when the problem starts and look for a colour difference in the plug tips. If it's missing, one of the plugs will be black, wet and smelling of petrol.

Unfortunately there is no easy way to test for temperature related electrical breakdown. We were buggering around with our old Mariner for about two years swapping bits about until we finally isolated it to a tempermental ignition pickup. On the bench the resistance tested fine.
 

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is the vent on the fuel tank fully open and clear, is sounds like its building up a vacume in the tank,you could also try to pump the priming bulb as the engine starts to lose power to eliminate a weak fuel pump.

hth, John
 
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