Yamaha 40V 2 stroke loses power

https://www.youtube.com/watch?v=GuGZ3cmSPSE&index=27&list=FLBF6CEegwFUJ0g8dhUQKZhg

Hello everyone! I write from Italy for the first time and I congratulate everyone for this beautiful forum! I write because I have a problem with my Yamaha 40 CV 2-stroke VETOL of 1997 that has the same problem of video on youtube. The Swedish owner of the video engine, could not fix it. Until today, no mechanic has managed to solve my problem that unfortunately does not always present itself. In fact, during the tests in the tank with the test propeller, the engine has always been fine. The fault occurs only during the sea trials and even in this case, it happens that the engine is good for some time and sometimes go wrong again. without warning. I replaced all the petrol tubes, filters and Jacks, I cleaned the carburetors without ever giving up. We replaced the control unit but we did not have any results ... A very strange thing: the stroboscopic pistol works up to 1500 rpm and then goes off, but the engine goes up to 5300 rpm. What could it be? Do you experience a similar defect? can you give me an advice? Thank you so much for the answers!

I had the same issue. Check the timing with an inductive light, does the spark jump? Number 3 will fire on #2 and visa versa? Turned out to be a rectifier issue.

Thanks for the reply! Honestly it seems to me a little strange that this problem may depend on the rectifier. Could you explain better how the rectifier can affect the spark? Thanks again!

Im not an electrician and ac/dc is my Achillies Heel. Spoke to a mechanic who told me that the system is looped together and that all current flows thru each componant. While the rectifier will continue to charge as it should, if it is faulty it can draw energy that is needed to fire other componants. Whether this is ture or not I dont know. All I know is that it was the problem. No other companant had any effect on on the ignition firing properly.

BAS said:

Im not an electrician and ac/dc is my Achillies Heel. Spoke to a mechanic who told me that the system is looped together and that all current flows thru each componant. While the rectifier will continue to charge as it should, if it is faulty it can draw energy that is needed to fire other componants. Whether this is ture or not I dont know. All I know is that it was the problem. No other companant had any effect on on the ignition firing properly.

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Thank you very Much for the answer! I Will check my outboard's rectifier, i Hope to find the solution...

When the problem occurs, do a cylinder drop test on it to see if you can find out what cyl is acting up.

Thanks so much. Unfortunately, the engine does not always do the problem, we tried it indoor in the tank with the test propeller but it never lost power despite the various attempts. The same thing happened at sea. The engine is fine for a week and then suddenly the problem arises. The morning is fine and the afternoon no. The only strange thing found in the test tank is that with the inductive light we can detect the spark only at the minimum. If we accelerate beyond 1500 rpm, the signal disappears and the inductive light no longer detects anything. The engine, however, revs normally up to 5500 rpm pushing the test propeller with the right power. it's a mistery, obviously we tried three different inductive light instruments! What do you think about?