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Old 14 April 2024, 17:23   #1
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Yamaha 75 hp 2 Stroke Wont go above 3300 rpm Under Load

We tested the new XPro 535 today and all went very well. The initial run of about 2KM the engine reved fine and the boat easily reached over 30 knots.The revs were at about 5600 (Not full throttle) and all running fine

Then on the third run the engine would only get to about 3300 revs.

This is a new built boat so has new fuel tank vents open. New/Fresh E5 Fuel, New 8 mm bore fuel line and new Yamaha connector.

When I got the boat home on the the Trailer the engine would easily reach 6000 + rpm on the lever and also on the Throttle in gear.

This is a 2005 Yamaha 75hp 3 Cylinder Oil injected 2 Stroke.

Any help would be appreciated as to why it would not go over 3300 rpm under load after it had been going so well

Thanks
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Old 14 April 2024, 19:29   #2
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How does it run at idle?
How does it run at 3300rpm, when being limited?
How does it run just below at say 3000rpm?
When were the carbs last rebuilt?
Is it misfiring or just being ignition limited?
What happens if you pump the fuel bulb right before accelerating.

The black box could be failing, the carbs jets could be plugged, along with a few other things.
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Old 14 April 2024, 20:20   #3
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Originally Posted by Peter_C View Post
How does it run at idle?
How does it run at 3300rpm, when being limited?
How does it run just below at say 3000rpm?
When were the carbs last rebuilt?
Is it misfiring or just being ignition limited?
What happens if you pump the fuel bulb right before accelerating.

The black box could be failing, the carbs jets could be plugged, along with a few other things.
Heck that is a lot of questions.

I think I have tracked it down to it going into reduced rpm mode or limp mode. From what I have read it does this for 2 reasons. Too hot or 2 stroke oil low. My oil level is fine but I also understand the oil sensor can be an issue also the float in the oil tank.

I also understand that the temp sensor can cause issue. I now certainly have some things to work on. Issue is I have not much time before I was intending to take it on our second Scotland trip
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Old 14 April 2024, 21:45   #4
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The symptoms fit with getting too hot, especially given that it was fine after you got it home and it had some cool-down time.
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Old 15 April 2024, 03:42   #5
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The symptoms fit with getting too hot, especially given that it was fine after you got it home and it had some cool-down time.
Yes I will investigate that. The issue with all boats is if you need to test it on the water but do the work at home it becomes a real PITB and us is usual time is not on our side on this one
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Old 15 April 2024, 09:59   #6
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So doing a little more investigation I checked the Thermostat and that was working fine. Started opening at around 58 degrees C so I decided to see if I could test the temp sensor that goes to the ECU.

It was quite easy to get out but as I gently tried to unplug the black with the pink wire broke at its plug. I was not putting any real force on the pink plug. The break point is corroded. Now as these I think measure resistance at different temps and report this back to the ECU I am wondering if this bad joint could have been messing with the resistance.

Here is a pic of the broken wire on the part.

What do others think
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Old 16 April 2024, 05:44   #7
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Cut the wire off and check the ohms reading, per the manual. If good put a new connector on and take it for a test drive. Of course replacing the sensor might be warranted. Sounds like it's time to check lots of electric connections.
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Old 16 April 2024, 11:54   #8
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Cut the wire off and check the ohms reading, per the manual. If good put a new connector on and take it for a test drive. Of course replacing the sensor might be warranted. Sounds like it's time to check lots of electric connections.
I checked the Ohms rating and it shows open circuit. I put it in boiling water and it also showed open circuit. I think not only did this have a bad circuit but is knackered also.

I have checked the other similar connectors and they seem good.

I an now waiting on this part. I have ordered from 2 sources but none seem very quick and I am up against a timeline to repair, find the issue, test and travel to Scotland
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Old 20 April 2024, 10:33   #9
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I have received the new temp switch and checked the old one and new one by bringin them up to the boil. Both switches switched on at about 87 degrees so these are working. My engine deffo was not that hot when the issue occured.

I have replaced the fuel filter and new spark plugs and have a set of new coils and plug leads and caps on order. Learning a lot about this engine as I try to find this issue
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Old 20 April 2024, 15:14   #10
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Read the description on the eBay listing below, sounds exactly the same as your issue.

https://www.ebay.co.uk/itm/186391490381?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=9QViw0BxTjG&sssrc=4429486&ssuid=DtgLIK_3RG u&var=&widget_ver=artemis&media=COPY
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Old 20 April 2024, 15:52   #11
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Read the description on the eBay listing below, sounds exactly the same as your issue.

https://www.ebay.co.uk/itm/186391490...mis&media=COPY
It does indeed. I have got some improvement on mine today. So now going to test it on the water later in the week. Just have to keep trying to work things out. Confident I will get there in the end. I am not pushing myself now as will use the old Narwhal Scotland trip
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Old 11 May 2024, 06:25   #12
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Many thanks for those that helped on this thread. I managed the second water test of the new XPro 535 rib yesterday and the outboard went very well. I just need to lift the idle speed a little. Whatever the issue was I am fairly certain I managed to fix it

These are the things I did

1) replaced the little in engine fuel filter
2) resoldered the corroded break in the temp sensor wire to the ECU
3) replaced the spark plugs
4) tightened the HT lead connections at each end
5) tested each plug for spark
6) lifted the tick over speed a little
7) Squirted some carb cleaner in the intake

Not sure why it happened or which of these might have fixed it. But it ran very well in this second test.

Hopefully this little thread might help others ��
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Old 11 May 2024, 11:04   #13
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Not sure why it happened or which of these might have fixed it. But it ran very well in this second test.

Hopefully this little thread might help others ��
Do you have confidence in this engine now? You say you donít know if or how an intermittent fault has been fixed.
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Old 11 May 2024, 19:01   #14
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Do you have confidence in this engine now? You say you donít know if or how an intermittent fault has been fixed.
I am fairly sure it is fixed but you are correct you need to build confidence in any boat. This is what we will do over this summer before we tow it around the Country.

The engine is supposedly one of the best Yamaha made so that is a very good starting point.

The issue could have been as simple as a spark plug failing under load/heat or more likely a loose HT lead. Even with a brand new engine you need to build confidence
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