Tohatsu sputtering under load

[Salty Veins]

I have a tohatsu 40c I recently bought with a boat. It has good compression 119 top 120 bottom. It starts very easy. It idles smooth. It will idle away from the dock just fine but when given some throttle it begins to sputter. If I cover the air intake with my hand the sputter begins to stop and I can get on plane but don't feel like it's reaching max rpm. I had the carb rebuilt. I replaced the reeds because I could see a good amount of light through them. I had the local mechanic look it over and he couldn't find an answer. He said the electrical seems fine but I have my doubts he really checked. I'm at my wits end with this one and not sure where to turn next. Any help would be greatly appreciated. Thanks

 

[Sea Rider]

Welcome to Iboats,

How old is that engine ? Did you change just reed valves or the whole plate assy ? Did you put in new plate's gaskets ? Does the egine sputters right after passing idle and throughout the whole throttle range ? Is fuel circuit tank to carb immaculate clean, used fresh fuel for the test ? If all is ok, could be an elecctrical, issue as simple as a shorted Ignition Coil that fails when IC heats under load. See if you can barrow a used IC to swap and test.

Happy Boating

 

[Salty Veins]

It is a 1998 i believe. I did change the whole assembly and gaskets. I bought a new tank. All new lines from tank to motor all new on the motor. New filters. It starts sputtering about 1/4 throttle. It only stops when I cover the intake with my hand. I tried squirting fuel into the carb while it sputters and it only makes it worse. Aside from borrowing a coil is there another way for me to test it? Also is it possible it could have clogged exhaust?

 

[Sea Rider]

When you cover carb intake with hand, is black cover on ? Is the butterfly working properly, closes and opens freely specially when doing first cold start with choke pulled out/in ? Check by removing cover if butterfly doesn't get stuck at some point. If so squirt carb cleaner on axis, move buttefly back and forth untill it pivots freely, apply thin oil drops to axis on both sides to lubricate. Has carb been serviced recently ? Is float even with carb base when turning carb up side down ?

Clogged exhaust is too wide for that to happen. Never seen one. If with Ignition Coil issues, swapping it is the fastest way to troubleshoot spot on, heat related shorting isssues is the worst to diagnose properly with instrumentation.

Happy Boating

 

[Salty Veins]

When I cover the carb the black thing is on. I did a complete rebuild of the carb and the mechanic double checked my work. He had the motor for 3 weeks and did sea trials and was unable to find a reason for what it's doing. I don't know anybody with a motor like mine so I suppose I have to buy a brand new coil just to test it? That sounds like an expensive way of doing things.

 

[Salty Veins]

I am looking at the website where I've been ordering my parts and I can't find any part called a ignition coil. What else might it be called?

 

[Sea Rider]

That smyptom seems to be a fuel related issue. not an electrical one. Is fuel pump in opt operating state, that's all diaphragms in good condition, not severely balooned, cracked. Was carb soaked in carb clean dip for some hours, not just sprayed with carb clean products. Could have gum deposits on thin fuel passages that limits correct fuel amounts into combustion chamber, that's why when placing hand on air intake engine springs back to life...

When engine sputters you cover air intake with hand and engine springs back to life, for how long. Once at plane, will boat remain at plane for any extended time period ? Or after a while engine will start to sputter again ?

That part is called Ignition Coil. Max rpm can only be checked with an inducttion tach under load at speed on water trail.

Happy Boating

 

[pvanv]

Diagnose before changing expensive parts on an obsolete model. Does choke help, as did covering the intake?

 

[Lund_bC]

i am no expert as some on here are but i would take the carb apart again and clean again, and keep searching around the fuel line fittings and things for leak or something, making sure you don't have bad fuel. if it's not a main motor or your in safe place just keep running it with a bit of seafoam and stuff in it and see if the problem goes away. I have outboards that have a little quirk like a little bog at certain rpm that would vanish as i got up to speed on a plane and just couldn't find the problem but it never did get worse so i gave up and just chalked it down to it's personality... prob not the best way to look at things if you need to depend on a motor in the ocean or something, but the replacement value of an used outboard is sometimes just not that much greater than fiddling around all summer with it.