2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

[jtp100]

I have a 2002 Yamaha 50 hp 4-stroke which I took out of winter storage recently. Recently alternates between running fine, and then missing. I have checked the fuel filter which is clear and the fuel is coming out of the fuel pump okay.

I have sparks on all cylinders. However, if I disconnect cylinder 2 or 4 when the engine is missing there is no change in the engine idle (when missing). (Interestingly, cylinders 2 and 4 run have seperate coils).

However, if I disconnect cyliner 1 or 3 the engine dies (as expected). I have alternated spark plugs for what it is worth with the same problem.

With the fact that there is no change in the rough idle with disconnecting cylinders 2 or 4 it makes me think these are the problem but I am unable to narrow down the problem further.

Does anyone have any suggestions? Thanks!

James.

 

[rodbolt]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

clean your carbs.
1&4 and 2&3 fire simultaneously.

 

[yamalube]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

I have the exact same problem, I removed and cleaned all 4 carbs, filters, ect. it is still doing the same thing. Could it be anything else..
Thanks

 

[rodbolt]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

still most likly the carbs.
ign on this motor sparks 1&4 and 2&3 evey time any is at TDC.

 

[jtp100]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

Rodbolt,

Thanks for taking the time to answer, and your help. I am in the process of rebuilding all the carbs and you wouldn't believe the dirt in the fuel bowls, and in some of the jets. I'm surprised it ran at all.

I am also surprised at the price of the o-rings!! Wow, who would think these little things were so expensive.

J.

 

[jtp100]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

So, I rebuilt all 4 carbs and in the process replaced all the rubber o-rings. (Wow! Are they expensive!) Found some dirt in the jets cleaned everything out with carb cleaner, and compressed air. They engine starts and runs reasonable well, and is excellent at speed. At idle it is slightly rough missing every so often. Not exactly sure what the problem is but perhaps a vaccuum leak? At least I can now use the motor! Thanks Rodbolt.

J.

 

[jtp100]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

Just replaced the spark plugs (with the old ones) and things work perfectly now. After rebuilding the carbs thought I would replace the spark plugs. Orignally had NGK plugs, but bought the 'equivalent' plug in Champion. Although they were brand new clearly something was wrong as when I put the NGK plugs back in the idle was smooth with no missing.

J.

 

[yamaha_50hp_4_stroke]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

I have a similar issue that I hope someone can help with.

I have a 2001 50hp four stroke and I am in the process of rebuilding all four carbs. I am using a Seloc manual to guide me but one issue seems unclear. The carbs have brass caps that cover the "pilot" screw (sometimes called the mixture screw). Apparently, this screw is set at the factory and it should only be adjusted after a complete carb rebuild.

Should I drill out these caps and replace the pilot screw? The Seloc manual does not give specific instructions on doing this. The manual says that they are set at the factory. However, the Yamaha carb rebuild kit includes a new mixture screw so I assume that I should be replacing it.

Does anyone have experience drilling out these brass caps that cover the pilot screw?

Thanks.

 

[rodbolt]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

use a 1/8th bit and drill carefully in the center. thread in a #10 selft tapping screw and pull the plug out by the screw with a pair of pliers.

try not to hit the idle screw when it breaks through but you most likly will.
a good starting point is 2 turns out from lightly seated.
you will have to use the tech mate vacum mate tool or an equivalent manometer to reset the throttles and the air fuel idle screws.
unless your just lucky .

 

[yamaha_50hp_4_stroke]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

Hi Rodbolt,

Thanks for the advice. I will use a drill press to create the holes in the caps. Hopefully, that will give me more control while drilling and avoid hitting the pilot screws.

I will post the results to the forum.

 

[yamaha_50hp_4_stroke]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

Just a follow up.

The brass caps have been removed from the pilot screws. I used a drill press to drill an 1/8th inch hole almost through each cap. Then, screwed a slightly larger sheet metal screw into the 1/8 inch hole in the cap and yanked it out with pliers.

I found that the #1 and #3 pilot screws were backed out 2 1/4 turns and the #4 was backed out 2 1/2 turns. The very strange thing is that the #2 was all the way in. I am guessing that this must have been a mistake at the factory??

Carbs are cleaned and will be going back in the engine tomorrow. Thanks for your help Rodbolt.

 

[rodbolt]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

the one you hit with the drill was the one threaded all the way in, look carefully at the screw and you will see the marks.
set it about the same as the other 3 for a starting point.

 

[yamaha_50hp_4_stroke]

Re: 2002 Yamaha 50hp 4-stroke all cylinders have sparks but cylinders 2 and 4 missing

Hi Rodbolt,

You are correct. That is the one that I hit with the drill bit when I was drilling out the caps. Thanks for the insight.

I got everything back together and the motor is now running great.

Thanks for your help.