Possible Carb Issue on 1994 Yamaha C75

Captain Kokanee

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Jul 12, 2013
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Hi guys,

Last year when on the lake I noticed dramatic drop in power on our C75 when under heavy load (pulling a tube) and it just couldn?t get onto plane. We have a very light fiberglass boat and without load it would be on plane in seconds and run perfect at WOT. Unfortunately because of work and college I am just approaching the problem now two weeks before our annual weeklong family reunion on the lake. We bought the boat 8 years ago but I will say overall the engine looks to be in immaculate condition.

To identify which of the three cylinders were causing problems we pulled the spark plug caps while the engine was running one by one. The motor began to stumble when either the top or bottoms caps were pulled, middle=no change.

quick compression test =good
sparkplugs/spark=identical and fine
basic fuel filter=small build up of small black particles, possibly from fuel lines (a tank of 90/10 gas overwintered in it 3 years ago)
fuel- new, ethanol free and run with yamaha ringfree as usual

Am I correct in assuming it has a clogged jet? If the particles on the fuel filter are degraded rubber, the same type of lines are past the filter which is not good. I read through pretty much every thread for carbs on iboats and this seems to be the likely problem. Also do I need to clean all carbs since it may be the idle jet clogged on one and the main on another?

Thank you so much for any and all help and sorry for the long post
 

robert graham

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Re: Possible Carb Issue on 1994 Yamaha C75

Maybe drain float bowl of middle carb onto a clean white rag to get an idea of debris quantity in that carb....could be a plugged jet. Best to clean this carb and others just to be sure, so a lean fuel mixture doesn't do serious damage to pistons....good luck!
 

Captain Kokanee

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Re: Possible Carb Issue on 1994 Yamaha C75

Thanks for your help Robert! Just drained the float bowl. The results were not optimal but I don't really know what is normal. This is the first time they have been drained since we purchased the motor. about 10 black specks the size of coarse sand 1 larger light amber sliver and some amber dust. -I'll attach a picture below, it might help someone else as well. (pic 1 is the middle "bad cylinder", the second is top cylinder for comparison though its not zoomed in quite as much.) We also replaced the middle spark plug and ran it for a good 10 min before pulling it and it appeared to have some fresh oil on the end. No change when pulling spark plug caps.

I am hesitant to break down the carbs and be stuck with a non functioning motor at the lake. The repair manual is arriving weds and that doesn't leave much time for troubleshooting for something I personally have not done before. I guess I'm just trying to judge if it is getting enough fuel to prevent major engine damage of if it can still safely be run just with lower power.
 

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99yam40

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Sep 7, 2008
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8,851
Re: Possible Carb Issue on 1994 Yamaha C75

if jets /passages are plugging the cylinders affected will run lean and that causes things to get hot enough to melt pistons.

Clean everything up and inspect and flush lines and pump so, more junk does not hit the cleaned carbs as soon as you put it back together. Link and sync procedures are needed every time you put the carbs back on
 

Captain Kokanee

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Jul 12, 2013
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Re: Possible Carb Issue on 1994 Yamaha C75

Lines appear clean, as did the fuel pump. Looks like I am going ahead with the carb cleaning. Unfortunately I was a little too forceful in pulling off stuck fuel lines and a little plastic connector snapped. What do you guys think about gasoline resistant epoxy? Additionally I have heard a little conflicting advice upon removing the brass plugs, some say you need to for proper cleaning others say you can just ignore them (Carburetor Cleaning Methods - Yamaha Outboard Parts Forum).

Also anyone have the link and sync procedure? It turns out I accidentally ordered the Seloc repair manual and its going to take an additional 3-5 days to get the official yamaha repair book since they have to print it first.

Thank you guys so much for the help. I'll let you know how it goes.
 

Captain Kokanee

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Re: Possible Carb Issue on 1994 Yamaha C75

The carb rebuild was much easier then I thought it would be, however they were incredibly clean inside, and after carb cleaner and compressed air look like they had just come off the factory floor so I don't know if the problem was there.

I ran the engine on Wednesday after putting It all back together and it ran about the same; maybe a little bitter. I also did another compression test. Unfortunately when I tried to raise the motor this afternoon to go up to the lake I had no power for any function in the motor (choke, hydraulic tilt or starter motor) and the the main fuse looked ok. Battery is fully charged as well. I'm assuming I must have shorted something some where but it was running fine on Weds. Any ideas? Help would be greatly appreciated- I'm already a day late for the family reunion.
 
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