Any Ideas? (1998 Yamaha C40)

coleman kayak1

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Motor: 1998 Yamaha C40 Not Oil Injected; Mounted on an 18 foot pontoon boat

Since we repowered with this motor it has given us nothing but reliable year after reliable year. Unfortunately this year we ran into some problems.

At any speed above idle, the motor runs normally. We see the same WOT speed, runs just fine, etc. etc. Idle is where the problems start. Idling in neutral, it runs normally for a few seconds then "coughs" and almost dies, then recovers and runs, the repeats itself. In gear it does the same but usually dies. As you can imagine its a pain in the butt trying to dock or load on the trailer, much less troll (which we do a lot of).

So we did the usual stuff first, changed the plugs, ran seafoam through the gas, and replaced the fuel filter. No change at all. I was told by a reputable shop it was a lean condition, but the carburetors are all fixed jet. I used the propane torch to check for vacuum leaks, and didn't find any. We also decarbed it, which we do every year anyway, but it didn't help anything either.

When its coughing you can pull the bottom plug wire off, and it makes no difference. Of course rev it up and it falls on its face, but I can safely assume its the bottom cylinder giving us the problems. I held a card in front of the carburetor and it seemed to get more "stuff" on it then either of the other two, pointing to a sticky or bad reed valve? Weak diagnosis I know but its all I got to go on.

Just looking for some input. We're in the middle of redecking so we probably won't worry with the motor til next spring, but if I had something to go on we may tackle it this fall. Thanks in advance.
 

svxtech

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Re: Any Ideas? (1998 Yamaha C40)

I held a card in front of the carburetor and it seemed to get more "stuff" on it then either of the other two, pointing to a sticky or bad reed valve? Weak diagnosis I know but its all I got to go on.

Good diagnostic trick, seems worth investigating further.
 

coleman kayak1

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Re: Any Ideas? (1998 Yamaha C40)

Good diagnostic trick, seems worth investigating further.

Why thank you, me and seloc figured that one out :)

My question is would a sticky reed cause these symptoms? I don't wanna put the money into the new gasket much less go through the trouble of pulling off all three carbs unless i know it at least COULD be it.
 

svxtech

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Re: Any Ideas? (1998 Yamaha C40)

Sure, had one just last week, found it with a piece of fuel line to isolate the back pressure though. same concept :)
 

99yam40

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Re: Any Ideas? (1998 Yamaha C40)

I agree check pump 1st to make sure it is OK, but sounds like it is running lean on a cylinder or 2 like they said instead of rick like a hole in pump would cause.
Clean the carbs properly and then do the link and sync like manual says after reinstalling, there are passages and jets that can get trash or build up in them. Even pieces of dried up fuel(varnish) or deteriorating fuel lines. Even water droplets can cause problems in the carb bowls
 

coleman kayak1

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Re: Any Ideas? (1998 Yamaha C40)

Will check out the carbs, but would like to eliminate all other options before we go that route, I am cursed with carburetors :). Not saying thats not the issue though, as it very well may be.

As far as the fuel pump, I would think that would cause a problem at high speeds as well as low speeds, perhaps even moreso? I had a bad pump in a pickup years ago and it would idle but when you got on it it would cut out. I did check all the fuel lines and they were in pretty good shape.

I'm thinking for a start I'll use a piece of hose as svxtech mentioned to reconfirm the reed valves. All the gaskets are around 20 bucks.
 

mphelle8vld

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Re: Any Ideas? (1998 Yamaha C40)

The fuel pump in your pickup did not operate off of the crankcase pressure on your number three cylinder. A pin-hole in the diaphram will introduce raw fuel into the crankcase fouling the number three spark plug at idle and showing more "stuff" on your card.

That said, it's never a bad idea to clean the carbs and check the reeds while you have them off, anything to learn more and get familiar with your motor.
 

coleman kayak1

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Re: Any Ideas? (1998 Yamaha C40)

^ I never thought of it in that sense. I think pulling off the pump and checking for fuel in the vacuum passage is in order. if thats the problem I may skip the carb thing, but I don't know yet. Thanks all for the ideas keep em coming I can use all the help I can get :)
 
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