89 Yamaha 4 stroke 9.9 HP Problem

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CatPontoon

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Feb 7, 2008
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I bought a pontoon last fall with a 1989 9.9 HP Yamaha 4 stroke. It starts easy, idles perfect and motors up right away. After a few minutes at any throttle setting above an idle there is a distinctive CLICK like a relay and it idles down. Never dies just idles down and stays that way for 30 seconds or maybe a minute then it clicks again and back to the correct throttle. I've worked on a lot of motorcycles but this thing has me stumped. Primarily because it's an intermittent problem. The only time I've had a similar problem it was low oil sensor on a 2-stroke on a big Yamaha. Any ideas? Any help is appreciated. Rick in INDY
 
JB

JB

Honorary Moderator Emeritus
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Mar 25, 2001
Messages
45,907
Re: 89 Yamaha 4 stroke 9.9 HP Problem

Howdy, CatPontoon.

Welcome to iboats. :)

You need to get the Manual and read up on that mad scientist's version of a non-electronic "computer" controlled carburetor.

I had one of those dudes for a while several years back. I don't have the manual anymore, but as I recall there are vacuum operated controls to adjust mixture and idle based on engine temp sensor and at least one more.

Sounds to me like one of those sensor/vacuum circuits is operating incorrectly part of the time.
 
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CatPontoon

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Feb 7, 2008
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Re: 89 Yamaha 4 stroke 9.9 HP Problem

Yeah I've poured through the shop manual, trouble shooting section etc. None of the symptoms I have are in there. I'm hoping to not have to play the "replace each item until it runs right" game. Especially when it comes to non-returnable electrical parts. Sounds like sensor for sure. I'm thinking if it's a coil it would just die and not idle down. Thanks for the reply! Anyone else fought this fight?
Rick in INDY
 
S

sdsaw

Petty Officer 2nd Class
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Aug 19, 2007
Messages
188
Re: 89 Yamaha 4 stroke 9.9 HP Problem

Hi CatPontoon,
These motors will retard the timing if they lose oil pressure or possibly overheat(not positive about this one). When I started having oil pressure issues due to a corroding oil, tank mine would not run above idle. You may want to check your oil filter to make sure it's not clogging up.
Hope this might help you.
 
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CatPontoon

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Feb 7, 2008
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Re: 89 Yamaha 4 stroke 9.9 HP Problem

Just an FYI for anyone with a similar problem. After much digging it appears this model (as do many others) has a low oil pressure SWITCH not sending unit. If the oil pressure drops to about 8psi it tells the CDI to slow it down to about 1,000rpm. The switch should be open at pressure and close at low pressure which grounds the wire thus telling the CDI to back down but lets you get home. So I'm guessing a bad switch or at worst a bad oil pump. But being an intermittent problem I'm thinking the oil pump is OK. I'll be swapping as soon as I get a break in the weather and post the results. Thanks for the posts.
 
R

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: 89 Yamaha 4 stroke 9.9 HP Problem

why switch anything. simply put the ohm meter on it and watch the swith. its either open or closed.
memory is fuzzy but I believe some older models also had an oil level switch.
however I cant remember offhand.
 
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