Yamaha not running on all 3 cylinders

bigdee

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91 85hp yamaha sounds like it is only hitting on 2 cylinders. When I first start out on the water it tries to run on all 3 for a second or two then drops back to 2 and stays that way the rest of the day. I ran it this way all last summer but want to fix before the season starts again. I was just wondering if anyone has an idea of what to expect before I travel up there to take a look. I am prepared to clean the carburetors. How to I check the coil pack?
 

gm280

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If you seriously think you are running on only two of the three cylinders, have the engine running and a one at a time, remove the spark plug boot to each cylinder and see if the RPMs change. If the RPMs change, that cylinder is working and move to the next one after replacing the spark plug boot from that cylinder. Once you isolate the suspect cylinder, then start swapping coils, and wires from the bad cylinder to one of the working ones and such to see if the problem follows the part(s). Once you do that, you could find the bad part or parts. If that doesn't determine the problem, then it is time to look at the ignition system that fires the coils and such. Hope this helps you isolate your problem... Just be safe and use precaution when removing spark plug boots. They will bite you if you aren't using an insulated tool or good cloves to do that. JMHO!
 

bigdee

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If you seriously think you are running on only two of the three cylinders, have the engine running and a one at a time, remove the spark plug boot to each cylinder and see if the RPMs change. If the RPMs change, that cylinder is working and move to the next one after replacing the spark plug boot from that cylinder. Once you isolate the suspect cylinder, then start swapping coils, and wires from the bad cylinder to one of the working ones and such to see if the problem follows the part(s). Once you do that, you could find the bad part or parts. If that doesn't determine the problem, then it is time to look at the ignition system that fires the coils and such. Hope this helps you isolate your problem... Just be safe and use precaution when removing spark plug boots. They will bite you if you aren't using an insulated tool or good cloves to do that. JMHO!

Appreciate that gm. I have not had any problems since new (1991) so I am unfamiliar with this engine. I understand how to troubleshoot the problem but was wanting to know what to expect as most likely. The boat is in a remote area that I have to drive to so I want to go prepared. My list is, wiring diagram,meter, carbeurator cleaner,spark plugs,fuel hose and clamps. It starts off fine but as soon as I throttle up it goes to about 4k for 2 seconds then loses power but runs fine. I have ran it all day so I know it is not overheating. I need full power to wakeboard.
 

gm280

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Appreciate that gm. I have not had any problems since new (1991) so I am unfamiliar with this engine. I understand how to troubleshoot the problem but was wanting to know what to expect as most likely. The boat is in a remote area that I have to drive to so I want to go prepared. My list is, wiring diagram,meter, carbeurator cleaner,spark plugs,fuel hose and clamps. It starts off fine but as soon as I throttle up it goes to about 4k for 2 seconds then loses power but runs fine. I have ran it all day so I know it is not overheating. I need full power to wakeboard.

Sounding more like a carb issue then anything else. Maybe it is time to do a good carb(s) cleaning for the boating season. If when you throttle up it has problems, it could be the carb, fuel pump, tank, primer bulb or any number of carb/fuel delivery issues. So weed out the usual things and go from there.
 

bigdee

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Sounding more like a carb issue then anything else. Maybe it is time to do a good carb(s) cleaning for the boating season. If when you throttle up it has problems, it could be the carb, fuel pump, tank, primer bulb or any number of carb/fuel delivery issues. So weed out the usual things and go from there.

Will do. thanks again
 

99yam40

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Might check the fuel pump for leaking into the crankcase, that would flood the cylinder that the pulse port is tied to and cause it not to burn the fuel in it properly.

It would be best to figure out which cylinder is not firing properly to guess at what the problem was.

Compression, spark, timing, and fuel to air ratio is what to look at.
 

bigdee

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Might check the fuel pump for leaking into the crankcase, that would flood the cylinder that the pulse port is tied to and cause it not to burn the fuel in it properly.

It would be best to figure out which cylinder is not firing properly to guess at what the problem was.

Compression, spark, timing, and fuel to air ratio is what to look at.

Does the fuel pump have a crankshaft activated diaphram that could be ruptured? This may explain why fuel consumption is high. How do I check this?
 

gm280

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Does the fuel pump have a crankshaft activated diaphram that could be ruptured? This may explain why fuel consumption is high. How do I check this?

If your engine has a fuel pump like on mine, then no, there is no mechanical operated fuel pump but a diaphragm type fuel pump. As the crankcase creates a suction the diaphragm is pulled inward and then when the crankcase switches to a pressure situation as the pistons move up and down the diaphragm pushes outward. And with two check valves, that pulls the gas in from the tank and pushes it out towards the carb in the proper direction. So if there is a pin hole or a crack in that diaphragm, it will suck gas into the crankcase and the engine will run rich or even totally stop if it is bad enough...
 

bigdee

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thanks again gm. I understand small 2cycle engines but I have had little experience with OBs. Prior to the Yamaha I had a 1973 v4 Evinrude and in all those years they never broke down except for pump impellors, fouled plugs and fuel inlet problems!
 

Relic Hunter

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I am having a similar if not the same problem on my '99 90hp. My #3 cylinder goes dead after about 5 mins of run time. The plug shows bad fouling. I've switched plugs, coils, rebuilt carbs, and fuel pump, and results are the same. My repair manual has a list of system checks to perform, but I'm not sure of the best order of priority to identify the problem. Any suggestions on the proper order would be appreciated.
 

bigdee

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I got around to looking at this yesterday. While doing ohm checks on CDI green/white trigger terminal it showed direct path to ground. The other 3 trigger terminals check open.....so I assume I have a bad CDI. Checked prices online and was shocked at prices. I will check some boat salvage yards first.
 

99yam40

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Testing peak voltages, as service manual call for, into and out of the CDI is the best way to make sure where the problem is
You will need the proper test equipment though or take to a shop that has the equipment to test it with
 

bigdee

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Testing peak voltages, as service manual call for, into and out of the CDI is the best way to make sure where the problem is
You will need the proper test equipment though or take to a shop that has the equipment to test it with

Could not run engine. Boat in storage. Used procedure in service manual for checking ohms on CDI terminals. service manual says trigger terminals should read open to ground. 3 triggers read open and one reads .1 ohm.
 
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