Yamaha 115 S115TLRX midrange struggling

[brodmann]

Just rebuilt carbs and replaced plugs and water pump all a couple of months ago. My routine for getting it ready for summer island hopping and just getting out and enjoying God's amazing creation on the weekends. Anyway, what I have is a slight annoyance and not really a problem. I have two boat ramps that I use that are each about 10 minutes from my house. However both of these boat ramps have about a 15 minute no wake zone in both directions before I can open her up and get going. Motor cranks up fine every time and idles great through the no wake zones. I usually have some music playing to make that more enjoyable. Anyway, once I get to the end of the no wake zone and start to move the throttle forward, it stumbles or simply doesn't increase speed at all and eventually shuts off. If I hit the choke one time as it starts to stumble, it takes off and I can get from 2000 RPM's to 5000 RPM's in no time flat. It's runs wide open perfectly. I had this same problem years ago with a '76 Evinrude 70 HP and I think I eventually changed either the low speed or high speed jets, but don't remember which one. Again, it's not an issue speeding up and slowing down the rest of the day, but it's just going from the 1500 - 2000 RPM no wake speed to opening up the throttle that causes a problem. Compression is 135 on all cylinders, boat doesn't ever run hot, good stream out of the pee hose, everything is fine other than this little irritation. I don't think about it too much, but if I let someone else drive my boat, they don't know and it sometimes shuts off on them. Cranks right back up, but might have to hit the choke to get beyond idle speed again.
Thanks in advance for any help you might be able to provide.

 

[99yam40]

could be the off idle ports are restricted.
small holes that add extra fuel as the throttle plates start opening.

or you just need to open up the idle screws a little to give it a little more fuel to help transition

 

[brodmann]

The Yamaha carburetors on my motor have idle enrichment screws. They allow more fuel to flow to the carbs at lower RPM's. I've got them turned out 1-1/2 turns which makes it crank right up even when cold. This is a great feature and it makes the boat able to idle through now wake zones with ease. Once the throttle starts to open, this feature goes away. So, I'm guessing I have a possible blockage in the low speed jet and this adjustment has compensated for it at idle speed, but the problem shows up when this "crutch" is removed. Could certainly be the case. Those carbs are such a disaster to take off and put back on, with some of the nuts being installed on the studs by feel alone since they can not bee seen once the carburetor has been slit over the studs and into place. I think I'll "punt" one this one and run a gallon of gas with a bottle of seafoam in it. Yeah, that's pretty crappy of me, but if it works, I'm a happy guy!!

 

[99yam40]

what are the specs for those pilot screws in your service manual,
or are you just guessing at where they should be set?
did you follow the link and sync procedure in the manual after putting the carbs back on last time?

 

[brodmann]

Link and sync and timing is all spot-on. As far as the enrichment screws go, it depends on what manual you look at. Yamaha says 5/8 to 1-1/2 turns out. Others say 1 - 3 turns out and if you need more than that you need to replace jets. When I bought the boat, the previous owner had just picked it up from the shop where he had it in for yearly service. The mechanic at the shop is a high-school buddy of mine, and does great work. He had set the enrichment screws at 1-1/2 turns out and that's where I've always set it. Turn them in any at all, and it's a real bear to get started. I may try turning them out to 2 turns out and see if that helps with the transition from low to high speed jets when accelerating. Some manuals call these "mixture screws", but actually they increase/decrease the amount of fuel/air at idle. They don't increase/decrease fuel or air, but actually increase/decrease the volume of the fuel/air mixture that the idle jet allows for. Hope that makes sense. If many folks read this, it will probably prompt a hot debate on exactly what these screws actually do. My buddy is a certified Yamaha tech and this is how he described their function to me. He's recently retired and sold his shop and told me it was because he couldn't go to the grocery store or out for a beer or two without having someone telling him about their most recent outboard issue, so I'll buy him a beer now and then or shoot a game of pool with him, but I have to really struggle hard not to ask him questions about my boat motor!!!

 

[99yam40]

yes he is correct, the pilot screws adjust the amount of a fuel with air it lets thru and adds that to the air running by the throttle plates.
heated sonic cleaner does well for cleaning carbs, better than just shooting the canned stuff into just some places
find a Yamaha service manual for your motor, not aftermarket ones.
they give a setting with a + - amount.
if good and clean passages and jets ,setting in spec should work well.
also, Yamaha Ringfree Plus can be run in a clean out ratio to help clean out stuff sometimes, but is has to get into all the passages/spots to clean them.

Most all carbs I have played with had off idle ports to feed extra fuel as the throttle plates start opening up past them