Help! outboard starts, runs for a few seconds, then dies

[rocky739]

Hello all,

I have a 2003 Yamaha 50TLRB outboard that has never run correctly from the day we got it last year. It has always had dying, rough idle, low rpm issues and dying when accelerating/ taking throttle. The problems seem to be worse at times then slightly better others. All classic dirty carb symptoms.

I overhauled/ cleaned the carbs this spring, they were pretty nasty from sitting, and it seemed to run perfectly for about 15 minutes then started its old tricks again. I assumed that some dirt had re-clogged a pilot jet... but I have re-cleaned everything again and didn't find any debris that would cause it to completely die, all the circuits are open and clean. It seems to run fine at full throttle.

I have cleaned the carbs twice now, replaced all the parts included in the OE rebuild kit, checked float height, added in line fuel filters, changed fuel lines, cleaned the fuel filter, replaced the fuel pump, replaced the diaphragm and gaskets on the enriching circuit. All circuits are clean as well as the pilot jets, I set the pilot screws to 1.5 turns CCW.

The PO had advanced the timing to 10 deg at idle, manual states 6-8, I reset the stop to the factory spec (20mm) and set the arm length to what I assume was the correct length. The aftermarket manual is pretty vague on which marks on the CDI should line up when at idle and WOT, if anyone has a picture of this I'd love to see it. I adjusted the arm length to several different marks that seemed plausible with no change in the issue, I had planned on setting it with a timing light when its running... but...

...Now it will start and run perfectly for 3-5 seconds then die as if the key were turned off..

I checked the prime starter assy thinking it may be a choke issue but it tested 3.2 OHM, in spec, and moved about 1/4 inch when hooked to a 12v battery.
The float bowls are continually full of fuel as are the fuel lines, making me not suspect the pump or fuel lines, It has fresh 92 octane fuel with stail-bil marine added. It will restart immediately and run for 3-5 seconds again before dying as many times as you care to do it.

Guess I will pull the carbs again and triple check for issues or a mistake I made but I'm starting to suspect a electrical issue, CDI box?



I have been a professional mechanic (car and motorcycle not boat) most of my life and never been this stumped...
Any advice will be appreciated!

 

[boscoe99]

If it is any consolation, Yamaha carburetors have been known to whip many a good mechanics ass.

Use of a heated ultrasonic cleaner is essential. Rodding out the small passageways with a piece of soft copper wire may be beneficial. You might also want to try increasing the idle jet a size or two.

 

[99yam40]

being a mechanic that long I assume you have a service manual for your motor
and you have followed the link and sync procedure after installing the cleaned carbs
and adjusted everything to the spec in that manual, right?

Compression , spark, and timing like all motors need to be tested.
Have you tried moving the red lever to a different position to see if that made a difference?
are you sure it is in the normal position?

The pointer on CDI may not be the final indicator about the timing, had my C40 timing running way too retarded with set at the mark at idle . Had to replace CDI to correct it

 

[rocky739]

Everything is synced and adjusted according to the aftermarket manual, wish I had a Yamaha shop service manual that was a little more detailed.

I have soaked all metal parts in chem bath overnight and also soaked everything in 50% simple green solution to remove varnish. You can eat off the surfaces of the carbs. All passages have been blown out with aerosol carb cleaner and compressed air and flow freely, I checked the vents for obstruction in the carb tops also as it seems like issue where the gas isn't being allowed to be sucked through under normal engine vacuum (carbs not venting to atmosphere). My next line of attack will be to make sure there's no reason that wouldn't be happening. I may try a fuel can vs the tank to eliminate any chance its an issue there
I haven't done a compression test but don't suspect low compression as it starts so easy, but yes anything is possible.

I tried the enriching lever in every position, started in every position. Heck it even started after I removed the auto choke thingy!

I am going to check over all possible areas where there could be vacuum leak although I don't see how there could be enough of a leak to keep it from running, check the seal and make sure there aren't any issues effecting the fuel pump.

 

[rocky739]

Got to fiddle with the boat Sunday, after thinking about it and typing it on here I had made up my mind that it was a fuel delivery issue.
I checked the tank vent: open, float bowls: full, I knew carb vents were open. checked vacuum lines and fuel pump gasket, no problems, I checked the enriching valve and it closed when hooked to 12v battery.

I decided to change the fuel line from the tank including the squeeze bulb even though it would pump up OK I couldn't see any fuel being drawn through when it was running.
FIXED! I put the new line on and it idled perfect, after an adjustment on the throttle stop to set the rpm I took her out for a brisk spin and it ran nice picking up smoothly... WOOT!