yam 90 dies at 4500 rpm

davediver · 30 August 2012, 15:02

hi all
i've been having some trouble recently with my 1999 yamaha 90 hp 2 stroke. it starts fine, idles fine and up to 4500 rpm the engine runs sweet as a nut. when rpm goes above 4500 it will run fine for anything between 1 and 5 mins before it dies. it doesn't cut out completely but the rpm drops and it sounds like its running with a cylinder down. when this happens i return throttle to neutral, wait a few seconds for the revs to come back to idle and check its back to normal by raising the rev bar and after 1 or 2 blips of the rev bar it fires as normal and away i go again sweet as until the 4500 rpm is exceeded.

i read a similar thread on here a couple of weeks ago but all the replys pointed to fuel supply problem and i am sure this is not, i have a 22l hulk reserve tank on board and it still does it with that attached so ruling out problems with the main tank, the carbs have been cleaned and rebuilt with a full yamaha service kit, a new internal inline fuel filter fitted, all fuel lines blown through and a new set of spark plugs. to be fair all this has greatly improved the starting, idle and running but still got the same problem at higher revs.

so after doing all the work on the fuel system my thoughts are with the electrical system but i'm a bit reluctant to buy any rectifiers, coils etc without being sure they are the problem so i was hoping someone on here might be able to point me in the right direction.

cheers

Dave

Markyboyo · 30 August 2012, 16:23

Have you changed the fuel pump diaphram? If that is failing slowly then it may be fine up to x rpm but as the fuel demand increases it suddenly finds it can't keep up.

Mark.

davediver · 30 August 2012, 16:30

hi mate cheers for the reply

the diaphram was replaced 12 months ago for what i was told an upgraded material as the original yamaha one's were prone to perishing so while i can't be 100% its not that i am fairly sure its ok

Markyboyo · 30 August 2012, 16:45

Have you tried giving the fuel bulb a good squeeze as soon as it starts to die at 4500 rpm. If it picks up straight away I'd change the diaphram.

Mark.

Bushrider · 30 August 2012, 17:06

Hi Dave ,
I had a week ago a similar issue with my 90hp 2 stroke - as well a 99 modell . When it occured I stopped the motor and removed the spark plugs . Then I found the lower plug was wet .
Then I found a bad connector of the lower ignition coil . The male connector was kind of black . I cleaned it as good I could on the water and after the motor was running well as always before .
Cheers - Jürgen

davediver · 30 August 2012, 17:57

hi mark yes i've tried pumping the bulb and no difference even waited a couple of minutes after to see if it picked back up but unfortunatley not but cheers for your input.

thanks jurgen think that may be my next plan to clean up all wiring connectors.

the maddening thing is though it only does this on the water i can't recreate the problem when running on wash muffs and no more trips out planned for a few weeks yet

mick · 30 August 2012, 20:00

I had similar trouble with one of my engines recently and it is the fuel filter that is split so at full reves it sucks air in and gets fuel starvation bypassed the filter no problems

David G · 30 August 2012, 20:27

Is it oil injection or pre-mix?
Is there any hint of an alarm?
Might be worth checking the oil flow sensor filter if it is oil injection.

davediver · 31 August 2012, 08:53

it is an autolube, oil level always kept near full and oil level on tach gauge always showing 3 bars and no alarms but it would make sense as i am getting the feeling it is in some sort of "SAFE MODE" all though no alarms.
does anybody know if the oil level sensor can be bypassed to test if this is faulty ?

David G · 31 August 2012, 15:27

It's more a flow sensor than level on my suzuki.
There was a mesh filter in it which was not that clean when I was experiencing similar problems.
Not sure how the Yam works; can't see anything that would measure the oil flow here:
1999 90TLRX Yamaha Outboard OIL PUMP Diagram and Parts
Assuming that's your engine of course.........

Sea Witch · 01 September 2012, 17:22

I had similar problem with my 40hp yammy , turned out to b water in d carbs - worth cleaning

davediver · 01 September 2012, 19:35

carbs have been stripped, cleaned and rebuilt with new gaskets and jets.
after a lot of work i am know 99% convinced it is an electrical not fuel problem possibly a dodgy connector or something more serious(££££'s) fingers crossed its the first.

Daibheid · 19 September 2012, 16:47

Saw a similar problem with the same engine which eventually deteriorated to 2 cylinders only running. It proved to be one of the trigger leads from the CDI box to the coils had corroded down its copper core into the sealed CDI box. An expensive fix and secondhand ones are scarce so I hope it's not your issue but it produced a lot of symptoms consistent with fuel starvation and always had a great spark when static/low speed tested.

Davie · 19 September 2012, 17:54

Run the engine with a flat out with a stobe light on each cylinder you will soon see what spark is breaking down however 1999 was the beginning of the prime start engines. Is this one? If it is, have you checked the small diaphram in the middle carb that will make you engine run richer than it should if its bust.