Engine only runs at full throttle, Tohatsu m50c

[Marley88]

So last week, after running all day without any issues, my motor decides it will only run at full throttle. It was slow to start, but if I choked it and floored the throttle it would fire up and run just fine at higher rpm's. It may keep running at 3/4 throttle, but as soon as I back off closer to 1/2 it dies. Docking was difficult, to say the least. My first thought was that the low speed carb jets were clogged. Back home, I dumped the gas, replaced it with fresh, non-ethanol gas with seafoam, broke down and cleaned the carb thoroughly, replaced the spark plugs, and cleaned out the fuel filter. When I then started it up, it seemed to idle okay for a couple minutes, then resumed exactly what it was doing before. Seems like a problem with the fuel system, but could the stator also have something to do with such issues? I would think if the fuel pump was going out it would have the opposite problem, running at low but not high rpm's. The motor is a 2-stroke Tohatsu m50c, and had been running just fine for quite some time without any recent tweaking or adjustments. I'm out of ideas at this point, any advice or suggestions would be greatly appreciated! Thanks!

 

[pvanv]

That motor does not have a stator. Try priming the bulb to see if that helps. You could have a leaky o-ring at a fuel connector.

 

[Sea Rider]

Bad internal connector o'rings specially when extremely dried, hardened gives tricky issues, air gets in and engine starts missing. Connector o?rings are not sold separately, could buy same at an auto shop, will need to pop them out and match them with o'rings of same diam and thickness that can handle fuel, way cheaper than buying 2 fuel connectors or complete primer bulb assy.

Happy Boating

 

[Marley88]

Thanks for the quick replies! The primer bulb was looking pretty rugged, so I went ahead and grabbed a new one and installed it. It seems to be running slightly better, but still not right. now it will stay running at about half throttle, but no less. the sound it makes while running is a bit quieter than normal and slightly muffled, unless I rev it all the way or tap the choke. If its about to die, I can tap the choke and it will buy a few more seconds and sounds better as well. is it obvious that this is an air leak somewhere in the fuel system? I did notice the hose running from the fuel pump to the carb was looking pretty rough too, and could see a few drops of gas around where it attaches to the top carb

 

[Sea Rider]

How old is that OB ? Assume air tank vent was fully open when testing ? Are there other fuel hoses excesively dried, hardened, craked ?

Happy Boating

 

[Marley88]

I'm not sure I could recognize what a "missing" motor sounds like. Could this be the low pitched, muffled noise its making now? The external hoses look decent and are pretty tight and secure. When I took off the carb I did notice the internal hoses and fittings didn't look too great, and I wouldn't expect there to be any gas at all on the fittings when its running like there is. I did try to take off the cap and cleaned out the vent hole when running but it didn't seem to phase it. The motor is an 86'

 

[Sea Rider]

When you grabbed a new primer bulb, assume was the whole fuel hose unit along fuel connectors, right ? How's the behaviour of that 50 at idle, medium throttle. Rough, misses, not even. Said that works ok at full throttle according to post..You could have an electric issue, those 3 cylinder 3 carbed OB's are more difficult to troubleshoot compared to simple 2 strokes 2 cylinder one carb models. We're having tech issues with a new out of the box M50C, doesn't run even throughout the whole throttle range, seems CD related.

Happy Boating

 

[Marley88]

I'm not sure i completely understand your post. The motor is a two cylinder with 2 carbs. how could i identify an electrical issue vs a fuel issue? my next move was going to be replacing a couple of the internal fuel hoses and fittings and see what happens. I suppose i should probably try removing each spark plug wire while its running to see if i can isolate it to a specific cylinder or carb. Im not sure what sound it would make if it were missing or running on one cylinder. the idle is very rough and will not stay running for more than 2 seconds if i let off the gas, and it only sounds right when I rev it up pretty high. will try to post a video of it running when i get a chance.

 

[Sea Rider]

Sorry newer 2 strokes are 3 cylinder 3 carb models, that Ob surely is old to be a 2 cylinder, 2 carb OB. Never seen one.

Is tank and small fuel pickup internal filter immaculate clean, if so, fill gas tank with fuel, connect primer bulb assy to tank, with a sharp object keep pressed OB internal white pin connector halfway in, while priming bulb several times with bulb arrow pointing upwards check that fuel flow exits smooth, no restrictions whatsoever. Use a proper container to collect exiting fuel from OB connector.

Have you checked fuel pump's internal diaphragams cond, could be dried, balooned cracked. If small fuel lines are dried, cracked replace with fresh ones. Both carbs should receive same amount of fuel to work even. Are you testing on muffs, open water ? Report your findings on the fuel system to start with calmly.

Happy Boating

 

[Marley88]

So that damn job thing kept me occupied the past few weeks and i have not had much time for my "fun" work until today. I found out that my top piston is not firing at all until about 3/4 throttle. When pulling the top plug wire nothing changes while pulling the wire off the bottom plug causes the motor to die. As i increase the throttle to about 3/4, the motor hits a sweet spot and instantly goes from quiet and muffled, to screaming, which i believe is when the other piston kicks in. I got a spark tester and the spark easily jumps the gap and appears blue. Also checked the compression and it was 140 for both cylinders. Could this be a problem with the CDI unit or possibly still a fuel issue? I could always try cleaning the carb again and check some more connections and hoses but, at this point, I'm unsure where to go.

 

[dthx]

I have the same issue with my merc 25 hp...2 stroke 1 carb.....the forum and others decided it was something wrong with the cylinder bore...maybe scuffed....or a cracked piston.....the piston air has time to escape before igniting at low RPM...but at high RPM it doesnt and will combust and fire the cylinder...runs on one cylinder at low RPM.....the at high RPM they both kick in......

 

[Marley88]

I appreciate the input, but the compression checked out even better than expected so I assume my pistons are okay. Today I took apart the top carb and cleaned it again, and noticed a couple small cracks in the plastic elbow piece that the fuel hose attaches to. When running, there is a steady drip coming from that exact spot. I tried rigging my fuel hose to cover the cracks, which seemed to make it run slightly better, but there is still a steady drip from that area. The piece is part of the actual carb, so I cannot remove or replace it by itself without getting a whole new carb, which is not manufactured anymore. A local parts dealer I've gone through before says he doesn't have a carb for the 50hp model, but he does have a carb set for the 70hp model. He says the powerheads are identical, and simply putting the 70hp carb on will essentially convert my 50hp to a 70hp. This sounds almost too good to be true. Anyone have any experience or thoughts on this? Is it really that simple?

 

[pvanv]

Um... No. The 70A2 has a different block than the 50C.

 

[Marley88]

Good call, thanks! After comparing different part numbers on the different models, it appears the 60 and 70 have the same block, not the 50. Either way, I got it all figured out today and it cost me less than $1! That plastic, elbow-shaped fitting where the fuel hose connects to the carb seems to have been the whole problem. Looking at some used ones on ebay, I saw that a few of them had that plastic piece taken off and the hose can still slide on. I broke off the plastic piece, slid a new hose back on, and now she runs good as new. Going to grab a couple hose clamps to secure it better since the nub the hose slides onto is a bit shorter than the plastic fitting.. Hard to believe such a small drip would cause a piston to stop firing completely. Thanks a lot for the advice guys! Beers on me next time you're in St. Petersburg!