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View Full Version : Yamaha V6 Excel: Trouble shooting the YMIS system


Markus
08-16-2007, 10:44 AM
A couple of friends have a Yamaha V6 Excel on a 24 foot single engine v-bottom. The engine runs smoothly and pulls to 5000-5500 rpm, but either it does not put out full power, or something is wrong with the hull, because their top speed is about 20 mph below where it should be.

What are the first things to check on the Yamaha Microcomputer Ignition System? The knock sensors?

I have spent far too much time playing around with the regular Yamaha ignition, but I have never looked up close at the YMIS. I will start by reading up on the service manual, but I would appreciate any ideas on what to look out for.
Jason Johnson
08-16-2007, 12:02 PM
Did it loose 20mph, or is it 20 shy of what they think it should be?
If it lost 20, Check basics first. Comp
fire
fuel
carbs
gear case
prop

Lots of times we assume the worst. I have never seen or heard of that computer screwing up, but it is possable. I think I know who has one.
Markus
08-16-2007, 12:15 PM
Did it loose 20mph, or is it 20 shy of what they think it should be?


20 shy of what they think it should be based on what the same boat does with engines of similar horsepower.

I will perform a full diagnostic, but it would be interesting to get some ideas about YMIS.
VectorPat
08-16-2007, 04:38 PM
Make sure they have 93oct fuel. The knock sensor will knock the timing back.
Markus
08-17-2007, 02:32 AM
They do run 93 octane fuel.

What is the easiest way to diagnose a faulty knock sensor? For example, if I disconnect a working knock sensor, will YMIS retard the timing or not?
VectorPat
08-17-2007, 06:54 AM
I am no the best on that ignition so all I can say is try it and see what happens. I am sure one of the oldtimers will know more. I heard from someone that if the wrist pins get sloppy sometimes the knock sensor will pick it up. You may just try watching it with a timing light.
Oh and make sure its get full throttle;) :)
Ray Neudecker
08-20-2007, 08:12 AM
Basically if you disconnect the knock sensor, it just no longer works. The throttle sensor needs to be ohmed all of the way through its range to be sure it is working correctly.
The fuel system and the boat would be the logical places to look for the problem.
Markus
08-21-2007, 06:07 AM
It seems like these guys will sell the engine, boat and everything instead of trying to troubleshoot it, so I will not get to play with it.

I will report back if anything changes and if I learn anything.