Frustrating Yamaha 50 hp 2 Stroke Problem, help!

[Zcarpman]

Hi Guys,

New to the forum but I am having a very frustrating issue that I cannot resolve on my 2003 Yamaha 50 hp TLRB 2 stroke outboard. The engine is on my 17 ft. Carolina Skiff flatbottom boat. We bought the engine backll in 2009 and ran it hard for commercial fishing purposes and it was tremendously reliable, never had one issue. The boat began having issues after one winter of sitting. The problem from the start has been the same, the engine starts up and idles awesome in the yard in neutral, revs up just fine, and seems like it is good to go. Once on the water under load, the engine still starts up and idles fine but immediately when throttled up begins to bog down and almost die unless you bring the throttle back down.

To me this seemed blatantly like a carburetor issue / fuel issue and the engine was not getting the fuel it needed. I've now become an expert on dismantling the carbs and cleaning everything. This spring I'm determined to get the engine back up and running well but it is proving difficult, this is what I've done so far:

1. Replaced all three spark plugs
2. Replaced inline fuel filter inside engine cowling
3. Replaced all fuel lines from tank to engine
4. Installed and replaced new water separator
5. Replaced engine fuel pump
6. Replaced cold start priming mechanism on second carb and rebuilt priming system
7. Bought carb rebuild it, fully disassembled carbs, chem dipped all metal parts over night
8. Rebuilt carbs with new kit
9. Scrubbed and cleaned all jets and parts related to carbs
10. put carbs back together and reinstalled

I blew out both jets and the carbs really looked fully cleaned when I put everything back together and I was feeling certain that the boat would finally run good. I dropped it in today with the priming system set to "oN" and the engine started first turn and sounded good. Went to throttle up, same issue, immense bog into almost die, into throttle down. I also, noticed that when the throttle is in gear, if I put my hand in front of any of the carbs to restrict the air flow , the engine perks up and sounds much better and runs stronger. As soon as I take my hand away, same problem. So, to me it seems that the mix is lean, but I have NO idea how that would be since the carbs aare fully rebuilt. Does anyone have any ideas? Maybe the reed valves? Head gasket? Electrical issue? Powercore? Do the carbs need to be retuned??? Anything would be much appreciated, cheers!

 

[boscoe99]

Carburetors can be extremely difficult to get cleaned. There are minute passageways within the carburetor that clog very easily.

Was a heated ultrasonic cleaner used with a good cleaning fluid such as Yamaha carburetor cleaner?

Were the passageways and orifices rodded with soft copper wire to ensure they are free of debris?

Was a water jet used to verify that the passageways will allow the free flow of gasoline? Spray carburetor cleaner can also be used.

In lieu of blocking the air inlet, spritz some gasoline into the air intake system when the motor starts to bog down. If it picks up and runs that is telling you that the carburetor is not clean. The mid-range circuits are clogged.

 

[Grub54891]

Have you checked the reeds?

 

[Zcarpman]

I have not checked the reeds, that is one thing that I have not checked. I thoroughly cleaned the carbs with carb cleaner and dippedall metal pieces overnight in chem dip. They may still be dirty but I'm not sure how! Is there anyway to check if its the reeds?

 

[99yam40]

Well the prime start system should not be turned to the on position.
it should be set to the normal/automatic position

 

[Zcarpman]

Yes, normal / automatic positoin sorry.

 

[Zcarpman]

im really trying to think through what else it could be other than carbs, i know it could be potentially still carb issue but not sure how to rememedy it if so.

 

[Scott Danforth]

I pass wire thru all carb passages to ensure they are clean. just dipping alone doesnt clean them

 

[Grub54891]

You should be able to see the reeds when the carbs are off. Sometimes I use the boroscope to get a better look.

 

[boscoe99]

How does holding a hand over the carburetors fix a reed valve?

 

[Zcarpman]

I'm thinking a good game plan could be to buy new jets for all three carbs, take apart carbs again, make sure everything is totally clear. Also, thinking maybe one of the internal fuel lines is potentially restricted, going to replace all internal fuel lines as well. Also, thinking of replacing the reed valves for all three cylinders while I'm at it.

 

[Grub54891]

How does holding a hand over the carburetors fix a reed valve?

It don't fix them. The reeds prevent the fuel from spraying back out the carb on the downstroke, and force the fuel up through the ports to the top of the cylinder. By placing a hand over the carb, you are preventing some of the blowback, and helping the fuel to go on the right direction. I'm not saying the reeds are bad, it's just something to look at while you are in there. Never hurts to check everything while you have it apart.

 

[Zcarpman]

What exactly should I look for on the reed valves once I have the carbs off?

 

[Grub54891]

Cracks, chips, missing reeds. On occasion, you will find some that don't close properly.

 

[racerone]

There is no mention of a compression test.----Has water pump been maintained ?---Does spark jump a gap of 3/8" or more on all leads , yes or no?

 

[Zcarpman]

Just replaced the thermostat and the water pump last week, pumping and pissing good now, I have not done a compression test yet, is there a specific compression test tool you guys recommend? Also, any insight on how to test the spark would be awesome!

 

[Sea Rider]

Just use a any compression gauge up to 150 Psi, remove all plugs, connect hose to cylinder and pull rope hard min 4 times in a row, write down each compression reading. Perform same on all cylinders. Some test with cold, some with warm engines, don't know if will read same though.

Buy a spark plug tester, install as indicated, check bulb flash intensity while pulling rope hard, perform same on each spark plug. Report your findings.

BTW, tester tests Ignition Coils outputs, not spark plugs condition.

Happy Boating

 

[Zcarpman]

Bumping this back with an update, I know it is an older thread. We have made some strides with the engine but it is still not perfect and we are stuck again! We just replaced the fuel pump and all internal fuel lines. The carbs are now CLEAN, ran copper wire through everything. Replaced the jets, cleaned thoroughly. We took the boat out on a lake and finally got to run it. Starts up first turn, idles well but sometimes grabs and will die, but once warmed up it will idle and run good. The engine finally has some punch. I gets on plane and will run nicely up to 16 knots about 3/4 throttle. From there if you bump up to full throttle, the engine starts to almost "skip" and studder but doesn't bog. If you bring it back down to 3/4 throttle it runs nice again. It will run all day at 3/4 throttle. Sometimes after running it will be a bit sloppy on start up ie. turn the key, fires up and grabs / bogs and dies. Next turn give it a little throttle from the start in neutral and its all good.

I'm really not sure what's going wrong at this point but we feel like its really close to be being back perfect. I'm thinking maybe a spark issue at full throttle?? Maybe a timing issue? Going to take the plugs out and try and replace them and then maybe seafoam / decarbon to see if that helps at all.

If anyone has any ideas that'd be awesome!

 

[99yam40]

well since you have not told us what prop you are using and what kind of shap it is in or what RPMs the motor is topping out at before it starts skipping, you may be hitting the rev limiter and the motor is trying to protect itself.

this grab you keep talking about may be a lean sneeze.

did yoou follow the link and sync procedure in your service manual when reinstalling the carbs after cleaning?
needs to be done each time you put them back on

 

[bansil]

is it back firing? if so through intake or exhaust