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Old 04 April 2008, 12:06   #1
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Yamaha 40HP 3 cylinder autolube problem

Hi there,

I have a problem, just wondering if anyone can help before I go to the dreaded 'Main dealer' (Not that I have anything against them, but will have to take the engien off the boat due to lack of legal trailer)

Engine is a Yamaha 40HEO, from between 1984 and 1991 as far as I can tell.

Starts and runs fine, but only on 2 cylinders - 3rd cuts in occaisionally but not very often. Runs up to full power over a couple of minutes and after a lot of coughing an misfires eventually runs very smootlhly.

It seems to be that the 3rd cylinder does not fire at all at with no spark present, have done the obvious so far including switch and new plugs, and switching coils - have concluded that I have an LT problem - but where?

Would be tempted to switch the ECU except it does run at full power. Any suggestions?

Roohairy
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Old 04 April 2008, 13:13   #2
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Edited, didn't read your thread properly.

Pete
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Old 06 April 2008, 06:24   #3
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Quote:
Originally Posted by roohairy View Post
Starts and runs fine, but only on 2 cylinders - 3rd cuts in occaisionally but not very often. Runs up to full power over a couple of minutes and after a lot of coughing an misfires eventually runs very smootlhly.

It seems to be that the 3rd cylinder does not fire at all at with no spark present, have done the obvious so far including switch and new plugs, and switching coils - have concluded that I have an LT problem - but where?
Triggers under the flywheel or ECU.

I assume you've checked that the sparking problem follows to whatever coil you switch the coil lead for #3 to? (It looks like thats what you'vesaid but not 100% clear)
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Old 07 April 2008, 07:08   #4
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Yes, HT coils checked by switching them around all ok. Have noticed that the engine tends to start and run smoothly for 20 secs then drops a cylinder, now someone has just told me that some yamaha 3 cylinders did this as a safety measure on detecting overtemperature, there is a sensor in a recess in the head so I assume that is what this is.
Does anyone know how this works, is it a switch, resistor etc and how can I fool it!
Trying to figure out what's wrong without buying a new ecu, 700 and they're not readily avaialble. The dealer who's helping me over the phone can't even find a manual for this engine, it's rare aparently.

Ruairidh
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Old 07 April 2008, 07:15   #5
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Diconnect the sensor and see if it stops the problem. Don't do it ifthe block gets hot enough you can't keep your hand on it.
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Old 07 April 2008, 07:32   #6
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Originally Posted by roohairy View Post
someone has just told me that some yamaha 3 cylinders did this as a safety measure on detecting overtemperature, there is a sensor in a recess in the head so I assume that is what this is.
At risk of adding to the rumours, I had a similar prob on my 2- cyl Yam when I went for a test run. Ran fine for about half an hour, and then started dropping a cyl (& returning it to life too) in a "switch on / off" style. Now, I know I was well over revving at points (need a bigger pitch prop) and had manually throttled back to the engine's max, so I wondered if it was cutting out because of excess RPM detected. The "half hour OK" might be the time to warm it up "properly"?

Assuming it's the same sensor I have, it's a simple thermal switch - just disconnect it as Nos says. The way mine is wired (granted older tech engine) it appears to be in paralel to the deadman, but now you say that I wonder if it just shorts one coil. Must follow the pink wire a bit further... I think some more experimenting is required!
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