[karlster]

Hi Guys, this is my first post on this forum but I have been really impressed by the responses I have been reading so far.

I recently bought a 1996 scout with a 96 yamaha 50horse 4 stroke. It had not been run for more than a year and the carbs were coated with white powder inside.

It took several thorough cleanings and having the local yamaha shop sync the carbs to get it running satisfactorily. The mechanic said idle spec was 750 plus or minus 50 rpm. He couldn't get it to idle smooth at 750 so he set it at 800. I ran it for several hours (max rpm attained was 5200rpm) and then parked it for the week. Next time I started it #4 cylinder won't fire.
(I am calling the lowest cylinder #4 as that is what the plug wire diagram notes it as)

FWIW, It is running very well on the top 3 cylinders.

I have had the #4 carb apart and everything looks fine. The plug is wet. I get a strong spark, I have swapped plugs, I have swapped #1 and #4 coil wires and the top three cylinders still run perfect but #4 is totally dead.

I tried to check compression but i cant get my rubber cone seal comp tester to seat in the #4 cylinder due to interference with the motor housing and my threaded tester only fits 14 and 18 mm plug threads ( The yamaha has 10mm).

I don't know what to think at this point. I have a spark, it would appear I have fuel as carb is very clean, and plug is wet with gasoline. I have confirmed coil will fire #1 cylinder so it should be fine on #4.

I did run seafoam through the fuel system in the first tank of gas. It has new spark plugs properly gapped.

My best guess is that I have stuck rings from carbon buildup and the resulting low compression keeps it from firing

or

My #4 carb has a stuck float.

I should mention that I have only run it under very light load since discovering the problem. I was reluctant to take it outside the harbor on three cylinders to try opening up the throttles to see if it would clear out #4 so it has only run at 5mph and at home I have only run it in a garbage can full of water while trying to diagnose and remedy the problem.

Would running at WOT throttle be worth considering to see if it runs properly at a higher power output???

What do you all think?

Can low compression on one cylinder be remedied with a fuel additive or spraying a particular solvent in the cylinder?

What other symptoms would a stuck float have? (I tried running it with the fuel line disconnected thinking it could run on the fuel in the bowl for a 10 or 20 seconds yet could no longer overflow but same results -- dead #4)

Any ideas are greatly appreciated.

thanks,
Karl

[rodbolt]

that engine uses whats known as a waste spark sysrem.
it fires 1and4 and 2 and 3 in simultaneous pairs.
have a leak down test run as a compression test isnt very accurate for that engine.
have you performed the resistance tests as per the tune up spec guide.
i would lean towards fuel as an issue if 4 can jump a 7/16ths gap.

[karlster]

Originally Posted by rodbolt

that engine uses whats known as a waste spark sysrem.
it fires 1and4 and 2 and 3 in simultaneous pairs.
have a leak down test run as a compression test isnt very accurate for that engine.
have you performed the resistance tests as per the tune up spec guide.
i would lean towards fuel as an issue if 4 can jump a 7/16ths gap.

Thanks for the reply,
I didn't know it was called a waste spark system but I gathered they fire in simultaneous pairs. That was what led me to swap the plug wires for 1 and 4 to verify that the problem was consistently the #4 cylinder.

I haven't performed any factory specified tests as I ordered a factory service manual but haven't recieved it yet. I have a manual I bought at westmarine that covers a multitude of yamahas and mercurys but the attention to detail on specific models is quite disappointing.

I may pull apart the carb one more time and verify I am not missing anything but once the factory service manual arrives I will do any and all diagnostic/troubleshooting tests they recommend.

What types of fuel system issues would cause a wet sparkplug (extremely rich) other than a sticking float and needle valve assembly? The yamaha mechanic set the idle mixtures for me when he synced the carbs and the #4 carb was 2 1/2 turns out. I tried it in a variety of settings with no improvement in results.

Keep the ideas coming, This is driving me nuts.

Thank you,
Karl

[rodbolt]

get a copy of the tune up spec guide as well the 1994-2006 is a very good spec guide.
an easy way to tell if its flooding is to simply run it with the airbox off and watch it. you can also partially cover the #4 carb opening and make it rich or remove the stopper on the intake rail to make it lean for testing.
but first I would verify spark and compression.
or verify cylinder sealing with a leak down test.

[karlster]

Originally Posted by rodbolt

get a copy of the tune up spec guide as well the 1994-2006 is a very good spec guide.
an easy way to tell if its flooding is to simply run it with the airbox off and watch it. you can also partially cover the #4 carb opening and make it rich or remove the stopper on the intake rail to make it lean for testing.
but first I would verify spark and compression.
or verify cylinder sealing with a leak down test.

Rodbolt,
Thank you for the further suggestions. I will post back with my results, whatever they are.

I am a bit challenged to find a mechanic who really seems to stay on top of the game with these motors.

The shop that originally sold my boat when new has been a disappointment in their accessibility and interest level in making repairs.

The most highly recommended shop in the area for Yamaha only services motors on the boats they sell. They did recommend another shop to have check out the motor and I got on their work schedule for October 12.

In the meantime I will continue to look for something fairly obvious I may
be missing.

thanks again,
karl

[karlster]

Originally Posted by karlster

Rodbolt,
Thank you for the further suggestions. I will post back with my results, whatever they are.

I am a bit challenged to find a mechanic who really seems to stay on top of the game with these motors.

The shop that originally sold my boat when new has been a disappointment in their accessibility and interest level in making repairs.

The most highly recommended shop in the area for Yamaha only services motors on the boats they sell. They did recommend another shop to have check out the motor and I got on their work schedule for October 12.

In the meantime I will continue to look for something fairly obvious I may
be missing.

thanks again,
karl

I pulled the intake manifold plug and let the vacuum pull some gasoline out of a squeeze bottle. It immediately started running on the #4 cylinder. Pulled the carb apart again and realized I had never pulled the emulsion tube behind the main jet. It was all gummed up.

After cleaning it I am now idling on 4 cylinders!

Thanks for the input

Karl