Problem with Carburetor?

[Rob Roy]

Before retrieving my for winter I disconnected the fuel line & ran the engine to empty the carbs. One of the cylinders stopped firing almost immediately while the rest carried on going for a minute or three as they do. With hindsight, this has happened before and I wonder if there is something amiss which I should be fixing.

The boat has been used regularly & I haven't experienced fuel starvation problems - the engine has been idling and running smoothly. However, I have not gone beyond the local 6 knot speed limit this year so haven't revved above 2500 rpm recently. My engine is a 1999 Yamaha 50 hp 4 stroke and any suggestions/advice would be appreciated.

[Don T]

Many possibilities,
1. If the boat hadn't been used in a while the fuel may have evaporated.
2. The pilot jet for the idle system sits higher than the main jet (sometimes) so if one jet is plugged it may quit sooner, not pulling all the fuel from the float bowl. Same holds true if there is water in the bowl.
3. If the carbs have been worked on, one may have an improper float level.
4. The carbs may be out of sync and the cylinder that quits first is doing more work when idling.

These last three would probably cause idle roughness, but they are possible.
That's all I can think of at the moment.

Note:
I recommend opening (loosen only) the drain screw in the side of the float bowl and draining all the fuel. Also draining the fuel from the filter / strainer.

[The Mutt]

One possibility is that the cylinders need to be tuned to match each other, the easiest way is to use a Gunsons Colortune.



Warm up the motor, replace one spark plug at a time, run the motor and adjust the rev and mixture looking down the Colortune at the colour that the fuel is burning, match up each cylinder so that each burns at the same colour, after each spark plug has been done redo incase one throws any other a bit out, throttle/ fuel mix adjustments can do that.

I've used one for years on cars, etc, it now take about 15-20 minutes for me to tune up an outboard, if they don't adjust then go looking for blockages, leaks or cracks in hoses.

When tuned correctly and outboard runs very very smooth.

PS. I have never tuned a fully electronic ignition, I assume there would still be throttle/mixture adjustment to ballpark the settings before the electronics takes over.

Glenn

[ROAD Soldier]

Maybe nothing is wrong. First I would check the path the fuel takes. If the sold fuel lines on the engine itself go from top to bottom on the carberators and the bottom one is longer it will hold more fuel thus run longer. I dont know for sure because I dont own a yamaha so you will have to check yourself.

[Rob Roy]

Many thanks for all your helpful replies - I know where to start now. I'm planning to disconnect the fuel line again and, a few seconds later, when the first cylinder quits, I will stop the engine and drain the carbs (when cool!). If I could drain them into four separate jam jars I may well identify the problem (with a little help from the suggestions you have posted). However, orderly draining of the carbs looks difficult with all that clutter in the way. The weather, which is not very enticing here at this time of year, doesn't help much either but I'll keep you posted.

Thanks again

[kmclemore]

One possibility is that the cylinders need to be tuned to match each other, the easiest way is to use a Gunsons Colortune.

I totally agree, Matt. I've used a Colortune on my sports cars for years - I have two and I put one in the front cylinder and one in the back (4 cyl cars) and then adjust the carbs - they run very well thereafter. One important caveat, however, is that you should never run high RPM's or put any load on the motor whilst using them - they are for tuning only. Also, they work best when it's somewhat dark, since you can see the color of the burn much better then. For a 2-stroke, you can expect a somewhat more orange burn (as opposed to blue with a 4-stroke)... read the directions that come with the tool for more information on what to look for.

One other note - before using the Colortune, make sure your carbs are in balance (air flow is the same thru all at any given RPM) - for that you need a manometer and I use a Uni-Syn carb balancer, though there are other brands out there. If the air-flow passing through each carb is not identical the engine won't put out full power and will also run imbalanced (some cyl's 'pulling' more than others).

[The Mutt]

kmclemore, with the outboards that I know of there is a mixture screw only without individual carb controls for each cylinder, no need for airflow adjustment making life easier.

You and I seem to have a attraction to the same carb and flow meter, for me they live under the bonnet of Range Rovers.

FYI my nick name is The Mutt not Matt, stems from High School as no one could tell my ancestry, the only thing 100% is that I was born in Australia .. I have a certificate to prove it, to this day there has been no attempt to find where anyone in my families history migrated from. My wife knows her ancestry in Australia goes back to Free Settler and Convict, travel back further to England and you got to The County York at the time the Vikings were in control.
Sort of makes it easier to understand why she burns so easily in the sun.

Glenn

[Don T]

Hello,
I can almost guarantee there are adjustments for synchronizing the carbs. Usually threaded rods and ball end attachments. On my Tohatsu 50, not only do the rods adjust but so does the link ball on each carb's throttle arm (movable eccentric plate). With that adjustment is is possible to get the entire range of movement perfectly synchronized. I ended up making tubes out of PVC to give a good seal to the Uni-Syn and access the lower carb throat. I was going to do a treatise complete with pics on the procedure but never got around to it. Now of course it's a moot point as that motor has not been in production for a while.

[kmclemore]

kmclemore, with the outboards that I know of there is a mixture screw only without individual carb controls for each cylinder, no need for airflow adjustment making life easier.

Sorry on the name, Glenn. As for the carbs, though, yes, you can adjust the airflow independently. It is not done via screws like on British carbs, but instead it is done via adjusting the carb interconnects... the rods that connect each carb to the next and that controls the butterfly's... these can be lengthened or shortened to balance the car airflows.

I ended up making tubes out of PVC to give a good seal to the Uni-Syn and access the lower carb throat.

Ditto - I made mine with PVC and a foam collar on the end to seal them.