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Author Topic: '04 90hp Yama OB intermittent idle stall  (Read 222 times)

February 20, 2020, 09:38:11 AM
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Boatdawg58

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'04 90hp Yama OB intermittent idle stall
« on: February 20, 2020, 09:38:11 AM »
Warm up video (link below) and typical motor missing at 1:40 then stalls.
New spark plugs look fine. 125lbs compression each cylinder. Cleaned carbs. Drained fuel tank, cleaned lines and changed fuel filter water separator. Checked ground wires and inspected motor wiring for corrosion and marginal connections.
Ran strong for a few hrs on the bay but stalled a number of times when trying to load it on the trailer, now occasionally struggles to idle and stalls with what seems like an intermittent miss.
Possibly a bad coil or power pack that's starting to fail when it warms up?
Hate to guess and just throw parts at it. Any input is appreciated.
Thanks. Mike

https://youtu.be/dfEIJdQ9P2A
2002 165 Osprey
2002? 90hp Yamaha 2 stroke

February 20, 2020, 10:38:00 AM
Reply #1

fitz73222

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Re: '04 90hp Yama OB intermittent idle stall
« Reply #1 on: February 20, 2020, 10:38:00 AM »
Audio can be deceiving but it does not sound like its running on 3 cylinders. Get a pair insulated or fuse pulling pliers and start with number 3 on the bottom. Pull the boot off and see if it's firing to the plug and also listen to see if the engine speed doesn't change or slows down. If it doesn't change and you have fire, it's either a fueling issue or a bad lower crankshaft seal. To determine a fueling issue, take off the front breather box cover and loosely hold your hand or a rag over the carb throat. If the engine speeds up, it's running lean on the cylinder, it it slows down that's normal. Next, go to number 1 cylinder and do the same thing to check the upper crankshaft seal, and fire or lean condition. (Usually you can see oil running down from the top on the block under the flywheel if that seal is bad). Then do number 2 last. This might get you pointed in the right direction. Try to this with engine running as slow as it will run without stalling.
1973 Aquasport 22-2, twin 115 Mercs
2000 Baycraft 175 flats boat, 60 Bigfoot Merc
1968 Boston Whaler 13, 25 Yamaha (project)
1966 Orlando Clipper 13, 9.9 Merc

February 21, 2020, 10:16:00 AM
Reply #2

Boatdawg58

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Re: '04 90hp Yama OB intermittent idle stall
« Reply #2 on: February 21, 2020, 10:16:00 AM »
Thanks Fitz, I went right out and tried the things you mentioned.

>>Progressively pulled each plug cap while at idle after a warm up and there was a significant and distinct change as it dropped each cylinder.
>>Then at idle I progressively plugged each carb throat starting at 3, then 1, then 2. Again there was a significant and consistent decrease in RPM and performance as each carb was choked down.

Leaning toward the possibility of a failing coil or pwr pack, I have an in-line spark tester coming to visually observe the intensity of the supplied spark at the plug, and if any particular cylinder is dropping spark when the motor intermittently coughs and stalls at idle (as seen in video link above).

 I'll follow up to let the next guy with this issue know what I found. In the mean time I turned the idle screw up a 1/4 turn to keep it running about 50rpm higher at idle and it'll be a bit happier when loading.

As a side note I bled some fuel out of the bottom of the water separator drain and it was completely water free. I also realized that the parts dealer gave me a set of NGK B8hs plugs instead of the B8hs-10 that the Yam spec calls for. The diff supposedly is that the "10"  is better suited for the broader gap of .039 for the Yama spec plugs.

It's probably all the idle speed testing but the new plugs are looking about 1/2 fouled already. A new set of B8hs-10 gapped at .039 will be here today along with my in-line visual spark tester.

Did I read somewhere that these oil pumps don't meter consistently as they age??? If that's why I'm getting fouled plugs it'd be easy enough to just premix as I fuel up. Inconsistency in an oil pump seems to be asking for catastrophic trouble.

Maybe it's grasping at straws on this old Yam but it runs so well otherwise that I can hope for an easy fix to a smooth low rpm idle. 

2002 165 Osprey
2002? 90hp Yamaha 2 stroke

February 21, 2020, 01:42:43 PM
Reply #3

fitz73222

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Re: '04 90hp Yama OB intermittent idle stall
« Reply #3 on: February 21, 2020, 01:42:43 PM »
This engine runs fine at higher speed and only stalls at idle? Does it start right up after it stalls or do you have to do something different? Even if it is dropping 1 cylinder, it still should run on 2. Each cylinder has it own coil so even if one is going it should still run.
1973 Aquasport 22-2, twin 115 Mercs
2000 Baycraft 175 flats boat, 60 Bigfoot Merc
1968 Boston Whaler 13, 25 Yamaha (project)
1966 Orlando Clipper 13, 9.9 Merc

February 22, 2020, 09:49:10 AM
Reply #4

Boatdawg58

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Re: '04 90hp Yama OB intermittent idle stall
« Reply #4 on: February 22, 2020, 09:49:10 AM »
It restarts pretty much right away. Ran really well otherwise. My tach is toast so I'm wondering if it was just idled down too far or if the set screw backed off last time out. The idle speed seems about right until it coughs and stalls like in the video between 1:40ish and 2:20

Been too windy to get it back out and see if cranking the idle screw up a 1/4 turn will result in an unacceptable shift speed.

New plugs and spark tester came in and I'll post if it looks like it's dropping spark when it coughs/stalls at idle.

Any feedback on the oil pump possibly running too rich or just inconsistently as they age???
2002 165 Osprey
2002? 90hp Yamaha 2 stroke

February 22, 2020, 03:24:30 PM
Reply #5

fitz73222

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Re: '04 90hp Yama OB intermittent idle stall
« Reply #5 on: February 22, 2020, 03:24:30 PM »
Oil injection is bullet proof, not the issue. Idle speed is adjusted in the water, in gear, so you need a reliable tach to make the adjustment. As engines age, they change so it may be time for the idle speed adjustment. I have 1000 hours on my 60 Merc Bigfoot and the older she gets the more adjustment she needs. Engine runs great but gets a little cranky at idle if it's idling for an extended period of time. I bought her new in 1998. If it blew up tomorrow it owes me nothing.
1973 Aquasport 22-2, twin 115 Mercs
2000 Baycraft 175 flats boat, 60 Bigfoot Merc
1968 Boston Whaler 13, 25 Yamaha (project)
1966 Orlando Clipper 13, 9.9 Merc