86 Yamaha 40 3cyl. No Idle

sp543

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My 86 40hp Yamaha will run great and smooth at higher RPMs and when you just start it, will run for 10 seconds or so fine then cough and cut out. Will run well and longer as long as the rpms are higher than idle. It will also cough when bringing it back down from higher rpms and die. I cleaned out the carbs twice, checked the bowls after it coughed out and they have fuel so the floats arnt sticking. Would this be a timing issue? Has what looks like strong spark, blueish white. If you continuously hit the choke switch on the ignition it will run at idle so it should be a fuel issue, but I meticulously cleaned out the carbs, flushed all lines with clean fuel, cleaned out the fuel filter, replaced fuel pump. Im getting rather frustrated because usually these engines run forever and have few issues and I keep throwing parts at this one and nothing seems to help. Also, how hard is it to rebuild the lower units on these, or at least replace the clutch dog piece? Reverse seems to be engaging but forward is not fully engaging, adjusted shift rod many times to no avail. Thanks
 
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sp543

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Re: 86 Yamaha 40 3cyl. No Idle

Also, compression reads just under 120lbs on all cylinders which seems pretty good.
 

99yam40

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Re: 86 Yamaha 40 3cyl. No Idle

Clean the carbs properly and adjust like the service manual calls for
may need to use sonic bath to get all passages clean
 

sp543

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Re: 86 Yamaha 40 3cyl. No Idle

Tore carbs apart again and was going to soak them, took that little rubber stopper thing off while removing all plastic and rubber pieces and found that small jet, was all gummed up and blasted a can full through the 3 and got them all cleaned and is now idling and starting with no hesitation and no choke, fires up within a quarter turn
 

99yam40

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8,851
Re: 86 Yamaha 40 3cyl. No Idle

good to hear it is running well again, but it would be a good idea to follow the service manuals steps for link and sync after reinstalling the carbs every time.
Good to have all 3 carbs closing and opening exactly the same and all linkage set properly
 
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