# Yamaha 15 has me and my mech stumped!



## sureicanfish (Sep 30, 2007)

Here's the deal, this motor is a 2003 yamaha 15 2 stroke. 125 psi compression on both cyls. It had a terrible miss and wouldn't take throttle. Once you hit half throttle it just bogs like it's out of fuel, go to full throttle and it stalls. The carb has been through the ultrasonic cleaner Twice, has new coils, new cdi, new plugs, entire new fuel system (tank, lines, bulb, fitting). Every testable component of the ignition has been tested and is well within spec, timing has been verified.

The cleaning cleared up the miss, that and adjusting the pilot screw, but the half throttle limit is still there. It just will not pick up. I have it mounted on my gheenoe and to get the boat up without porpoising I have the motor tucked down pretty good. My latest thought is that possible it's trimmed too far down and is somehow effecting the float level??? Anyone heard of such a thing?


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## Sequoiha (Sep 28, 2007)

Have you inspected the reeds,


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## sureicanfish (Sep 30, 2007)

Can't find any info on inspecting them other than making sure they're all there and closed? They look fantastic. Wouldn't bad reeds show on a compression test?


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## Snagged Line (Sep 30, 2007)

My understanding of the Reeds is to act as a check valve of sort to prevent what is below the piston(fuel+air) in the crankcase from exiting through the carb when the piston drops and creates a positive pressure in crankcase. 
The positive pressure that is created in the crankcase transfers to the top of cylinder through a port in lower side wall of cylinder when the piston top drops below the port opening...

Compression begins when the piston has raised above the height of port opening and traps fuel and air into space above intake and exhaust ports and compresses into top of cylinder. 
Also, as the piston raises, a negative pressure (suction) is created in crankcase that causes the reeds to be drawn away from their seats and allow more fuel and air to enter the crankcase below piston and wait for the next piston drop, so it transferred to combustion chamber above piston through intake port. 

If the reeds were not sealing as suggested, the crankcase would not pressurize with enough fuel and air to develop full power due to some of it venting back through the carb. I do not believe the compression test would be effected, only the fuel delivery to cylinder along with performance...



But I might be wrong........:001_huh:


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## sureicanfish (Sep 30, 2007)

Very informative! But, I'm still pretty sure the reeds are good. I know reed problems are more on the uncommon side of things and these look like brand new parts. They are sealed upon inspection and I don't see any evidence of anything coming back through the carb (into the air box). 

Also, I've replaced the fuel pump diaphragms and while the motor is acting like it's not getting fuel, squeezing the bulb has no effect on anything. The bulb does not suck flat either.


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## Snagged Line (Sep 30, 2007)

sure said:


> Wouldn't bad reeds show on a compression test?


 
I was not suggesting it is reeds, I just went the long route to say that I don't think a leaking reed would effect the outcome of a compression check................... But seriously, I do not know for sure...


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## Sequoiha (Sep 28, 2007)

You are correct Dennis


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## sureicanfish (Sep 30, 2007)

Reeds, look good to me


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## CB541 (Jul 20, 2013)

Fuel pump? Try pumping the bulb when it goes to bog. Also try a small spray bottle with premix that you can spray in the carb when it goes to bogging. If it clears up is a lean fuel problem. Dies out it to rich. Long story but I had a 25hp yamaha that gave me fits. Bought it used, purred like a kitten but would not get the boat on plane. Everything checked out, clean carb, excellent compression, ignition etc. Come to find out the main jet was too big. We believe it slowly eroded over years of use and with multiple cleanings. No signs of being drilled out. Put a new main jet and it ran perfect.


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## sureicanfish (Sep 30, 2007)

Fuel pump is new (diaphragms), pumping the bulb has no effect on the bog. I'll try the bottle of premix idea next time out and I've read of eroded jets before. This thing doesn't seem houred up enough to be eroding but for a couple bucks I'll try anything, thanks!


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## kanaka (Oct 3, 2007)

Have you tried playing with the choke when it starts bogging? If choke helps, its not getting enough gas. (air leak)


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## smooth move (Sep 25, 2012)

different motor, but my 15 Johnson was doing the same thing and it was a electronic ignition module under the flywheel. the module sensed magnetic contacts or sensors on the flywheel. replaced the component and set the gap with a special tool, then replaced flywheel and i'm back in business! like i said, different motor, but same problem.


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## sureicanfish (Sep 30, 2007)

yep, choke just kills it immediately, like it should once its up to running temp. i checked my float level idea in a clear bowl, definitely not the problem, so much for that idea. my next hope is to try replacing the main jet as CB541 eluded to.


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## kanaka (Oct 3, 2007)

Trying to look at the simple stuff. Fresh gas? Did you buy all the new parts you listed or were you told it was newly replaced?


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## sureicanfish (Sep 30, 2007)

kanaka said:


> Trying to look at the simple stuff. Fresh gas? Did you buy all the new parts you listed or were you told it was newly replaced?


i've bought everything new mentioned. motor was used, never ran on anything but the brand new tank, line, bulb, and fitting. all other mentioned replaced parts were bought and done by me. its so aggravating since its such a simple motor; i have awesome spark that doesnt cut out when running (timing light verified), all components are well in spec, carb is squeaky clean and i have compression.


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## chad403 (Apr 20, 2008)

*hey*

This might be dumb answer, However I had the same issue with a small mercury and it ended up being the timing advance linkage was not moving with the the throttle causing the exact same issue.


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## kanaka (Oct 3, 2007)

The timing light moving when the advance should be kicking in? See above. Needs to be under a load for that to happen? Tie off Geenoe to dock and go to WFO checking timing light?


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## sureicanfish (Sep 30, 2007)

well, after a little reading on the interwebs i convinced myself it could somehow be the brand new primer bulb, even though it wasn't sucking flat, and even though it pumped and primed like a primer bulb. got ripped off for a new one at Hall's and stuck it on, what do you know, it freakin runs!!!...better. Now i have a fuel pump issue as at 3/4+ throttle it sucks the bowl dry (no fuel coming out of the bowl drain after the crazy break down). in fact, towards the end of my trials today i had to constantly squeeze the bulb to force fuel past the pump and into the bowl to keep it running. I hit a blistering 22.5mph! hopefully i can see 25 once the fuel pump is 100%


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## kanaka (Oct 3, 2007)

Excellent!!! Have you rebuilt the carb or just cleaned it? My 90 Yami oil burner was doing the "suck the bowl dry" thing after 3/4 throttle when sitting in the sun and after fuel line, double clamps, primer bulbs I finally broke down and got the rebuild kits even tho everything looked fine. No issues now....
Double check fuel tank vent for "flash" blocking the vent hole.


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## azevedo16 (Aug 18, 2010)

sure said:


> well, after a little reading on the interwebs i convinced myself it could somehow be the brand new primer bulb, even though it wasn't sucking flat, and even though it pumped and primed like a primer bulb. got ripped off for a new one at Hall's and stuck it on, what do you know, it freakin runs!!!...better. Now i have a fuel pump issue as at 3/4+ throttle it sucks the bowl dry (no fuel coming out of the bowl drain after the crazy break down). in fact, towards the end of my trials today i had to constantly squeeze the bulb to force fuel past the pump and into the bowl to keep it running. I hit a blistering 22.5mph! hopefully i can see 25 once the fuel pump is 100%


If you have a brand new tank, take the cap off and take out the little rubber piece out of the vent.


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## sureicanfish (Sep 30, 2007)

Good tip, it is brand new so I'll check for that rubber thing tomorrow. Thanks man!


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## csmigels (Nov 2, 2013)

You said the carbs have been thru the ultrasonic, you can't clean varnish that way you need to pul everything apart all the jets and look for blockage,carb cleaner won't clean it either need mechanical cleaning ( you have to scrape it off) even just the smallest about can mess up the fuel flow, rember it all works on air moving across a hole to pull fuel, then check your needle and seat any leakage thru can cause a wild rich issue, you said that choke kills it instantly choke fattens up the mixture, if the MTR was running lean and the choke had a profound effect it would be a lean issue, sounds like a needle and seat or float height. good luck in my opinion a timing issue would only present it self under load.


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## mt0264 (May 8, 2010)

go to auto parts store buy gal of carb cleaner it comes with basket do as above stated it will eat everything that shouldnt be there, make sure you dont leave any soft goods they will be gone as well let set couple of hours wash with hot sope water rebuild carbs, sounds like you still have a blockage some were..


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## sureicanfish (Sep 30, 2007)

The carb is clean, I'll give someone 50 bucks if they find one microscopic spec of anything in there. Yes, it's been completely disassembled, inspected, cleaned, cleaning carbs isn't rocket science. Even the emulsion tube looked brand new, not a spec of anything in any of those tiny holes. Float level is good, needle and seat work as they should.


Not sure why it ran better after changing the primer bulb last time but the motor is back to running the way it always did, it just won't take throttle past 1/2 throttle. I've yet again replaced fuel lines and and primer bulb, checked the in tank pickup, and vented the gas cap, all with no effect. Re-verified that fuel flow from fitting to filter to pump is wide open and unobstructed. The primer bulb still doesn't suck flat and the choke still kills the motor when applied while running. 

I'm almost at my wits end with this simple little motor.


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## floater1 (Oct 31, 2007)

What fuel hose are you using if it the grey we have had a batch where ethanol has eaten a hole throught the innerliner and caused it to bunch up inside the sheating


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## sureicanfish (Sep 30, 2007)

floater said:


> What fuel hose are you using if it the grey we have had a batch where ethanol has eaten a hole throught the innerliner and caused it to bunch up inside the sheating


Black, regular old hose, been down the melting fuel line road before so no more lined stuff for me.


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## csmigels (Nov 2, 2013)

Here is a question I either missed while reading the thread or its not there. What happened between when it ran right and when you noticed it was not running right. Was it just sitting for while? Did it start acting up one day while in regular use? Did it start acting up while you were using it?


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## sureicanfish (Sep 30, 2007)

Bought used after a compression test and hearing it run, barely, on old gas and a dirty carb.


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## kanaka (Oct 3, 2007)

So, it won't rev past 1/2 throttle. If you leave it WFO, it stays at the same rpm's? 

Did you happen to check the gas line when running? I HAD a picture of my line and when it warmed up during the day, the heat actually put a kink in the line in a curved run between the bulb and quick disconnect. And speaking of that, is the QD loose? 
Is the fuel line the proper size? I've been guilty of putting 3/8 hose on when it was supposed to be 5/16. :blush:

Grasping at straws on this one, is the prop the right pitch?


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## sureicanfish (Sep 30, 2007)

Correct, it pretty much stays at the same rpm no matter how far past half throttle I go, I've checked all linkages, everything is good there. It has stalled and died a few times while trying to get it to full throttle, lately it just puds at half rpm though. The fuel line size is next on my list. I bought the kit from attwood that came with 3/8" stuff, fittings and line. I replaced the line with more 3/8" since that what came off, now I read anything under 115 horse or so should have 5/16" fuel line...never seen one of those kits for sale dangit! Trying that this weekend.


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## Ocean Master (May 20, 2008)

I personally wouldn't use the Attwood stuff even if it's not the problem


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## csmigels (Nov 2, 2013)

wal-mart is not the issue, fuel line size is not the issue, if is was a lean issue the choke would have an effect. let if run at what ever the highest RPM you can get it and read the spark plug.


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## kanaka (Oct 3, 2007)

Since the stuff from the gas tank to the motor is still "new", the primer bulb isn't a issue, YET. Eventually get a OEM bulb, it's worth the money. 
Anyway, now we have more info that the line is a kit, check the ends YOU tightened and see if you can twist the hose on the clamped barbs. If it moves, its a possible area to get a air leak and have less fuel going upstream. And it'll tell you that you're dealing with the wrong size.
Quick no cost checks: the arrow on the primer bulb pointing at the motor? Is there a short enough distance that the primer bulb is at a upward angle to the motor to help with the priming? Have you checked the timing to see if its advancing after idle? Still wondering about the prop pitch, if its too high, it won't rev.


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## kanaka (Oct 3, 2007)

Just in case you didn't see these....

http://www.marineengine.com/boat-fo...-50-power-with-throttle-open-100-Please-HELP-!

http://www.boatdesign.net/forums/outboards/mercury-70-hp-bogs-down-full-throttle-37336.html


http://forums.iboats.com/forum/engi...inrude-15hp-will-not-rev-to-wot-rpm-low-power
I think the key words were sticky reeds, clogged fuel filter, engine trimmed down too much....


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## CB541 (Jul 20, 2013)

Ever get it fixed?


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## 153 Large fish (Nov 30, 2013)

I have a 15 yamaha 2 stroke as my kicker motor...it runs real good at idle, but for full speed you have to adjust the choke just right...mine won't run correctly if you open the butterfly all the way...i don'tknow what causes it either. ..but mine does run better the more i use it...


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## sureicanfish (Sep 30, 2007)

Been too busy to play with it lately, I'll update with any fix I stumble upon


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## scott44 (Aug 17, 2013)

153 Large fish said:


> I have a 15 yamaha 2 stroke as my kicker motor...it runs real good at idle, but for full speed you have to adjust the choke just right...mine won't run correctly if you open the butterfly all the way...i don'tknow what causes it either. ..but mine does run better the more i use it...


This might be unhelpful but I hit mine with carb cleaner a couple times a year.I know...they say don't do it.Might not help you though,all depends on whats wrong.


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## sureicanfish (Sep 30, 2007)

Well, turns out CB541's main jet idea was the winner on this one!! 1000 internets to CB541!!

The boat runs awesome now, totally different animal, i hit 25.7, downstream, but it was sweet!!:thumbup:


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## CB541 (Jul 20, 2013)

Glad its fixed and I could help. I bet it was a heck of relief when that thing took off!! :thumbsup:


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## sureicanfish (Sep 30, 2007)

yes it was! my mechanic buddy has never seen that before so we both learned something.


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