Yamaha 3hp 3MSHY (6L5) stalling after Carb Leak Fixed.

[cprodave]

I am working on a 3MSHY manufactured 10/99 that had a fuel leak between the Carb Bowl and the Carb Body. Gasket (#40 in exploded diagram) may have been original to the engine, i.e. 21 years old. Gasket felt more like hard plastic than rubber and was severely flattened, not rounded in cross section. I replaced the Gasket without altering any Carb settings nor Throttle Cable position and the Leak is now gone. However engine ran better (perfectly, really) before I fixed the leak. Now the engine (running in a tank) seems to need Choke longer than usual after startup, and after warmup seems to run better on Half Choke vs, Choke Off. Also engine wants to stall when switched from Neutral to Forward Gear. Note this engine is running 100:1 not 50:1 ratio (per 100:1 sticker on engine). It makes a decent amount of smoke so doesn't seem to be suffering on the 100:1 vs. 50:1 debate.
Before I potentially modify any Carb Setting(s) is there any explanation as to why fixing the Leak caused the engine to run worse? Ex: could the Vacuum from Intake have been sucking some air past the faulty Gasket? Therefore engine is getting too much fuel now and too little air? Any test I can do that might suggest proper adjustment?
Overall engine is in very good condition--no signs of saltwater useage, no physical damage, very clean outside and inside cover. So I would like to get it running perfect as well. Thanks in advance.

 

[99yam40]

just my thoughts, but it sounds like something plugged up or at least restricted the carb jets passages from opening them up

 

[cprodave]

just my thoughts, but it sounds like something plugged up or at least restricted the carb jets passages from opening them up

This explanation makes sense although I didn't see any debris in the bowl. I will do a Carb clean (including blowing out the jets etc) and reassemble/retry. Thanks

 

[QBhoy]

I’ve got 2 of these. Called a Malta here. Great things but need a proper carb clean often. Yours may have been previously adjusted to try and counter the issue you fixed

 

[cprodave]

Thanks for the input. I see you are in Scotland, I would like to visit your country when (if) COVID settles down. We have been to England and Ireland on vacation, but not yet to Scotland. Before I retired I used to work with a lot of folks from/in Greenock. They were very enjoyable co-workers.

 

[cprodave]

Thanks for the input. I see you are in Scotland, I would like to visit your country when (if) COVID settles down. We have been to England and Ireland on vacation, but not yet to Scotland. Before I retired I used to work with a lot of folks from/in Greenock. They were very enjoyable co-workers.

I forgot to include status update. I removed the Carb and disassembled as far as possible. I poked fire wires appropriately, blew out passages, did not see any debris. Reassembled and ran in water tank. The Fuel Leak is back! But this time seems to be coming from higher than the Bowl Gasket. Possibly Stuck Float, or Needle not seating properly. Still working on it. One question: I don't recall seeing a small rubber o-ring that a Needle (in my experience with other motors--mostly OMC and Mercury) seats against. From diagrams I have viewed, I can't tell if an o-ring is required there. Anybody know?

 

[QBhoy]

I forgot to include status update. I removed the Carb and disassembled as far as possible. I poked fire wires appropriately, blew out passages, did not see any debris. Reassembled and ran in water tank. The Fuel Leak is back! But this time seems to be coming from higher than the Bowl Gasket. Possibly Stuck Float, or Needle not seating properly. Still working on it. One question: I don't recall seeing a small rubber o-ring that a Needle (in my experience with other motors--mostly OMC and Mercury) seats against. From diagrams I have viewed, I can't tell if an o-ring is required there. Anybody know?

Hi again. Yeah you definitely need to visit here. The west coast in particular is just like nowhere else on the planet, when the sun shines. Stunning place.
About the engine. I had exactly this same issue on mine. Tried cleaning it a couple of times just by stripping down and cleaning manually. Problem persisted. Then eventually I used an ultra sonic cleaner. Problem solved. I was amazed at the crap that came out it and left in the solution in the cleaner. Despite being spotless looking to the eye. Can’t think I remember the rubber o ring either way to be honest. If you don’t get an answer, I’ll have a look at mine, if you like. Doesn’t take long to look.