2012 Tohatsu 30hp 2-stroke jet bogging at WOT

[Positrac]

Greetings.

I just discovered the forum today out of need since I have a bit of what I believe is a fuel issue with my 2012 Tohatsu (Merc Sea-Pro) 30hp 2-stroke jet. The motor is on a new Solar Strela 420 Expedition and started life as a 25hp unit. I’ve changed out the carb restrictor gasket for a 30hp one and the timing is set at 25 degrees.

If I’m cruising along at 3/4 throttle and I open it right up it runs beautifully at WOT for about 10 seconds then backs off about 10% for a second. If I keep the throttle pinned after that 1 second bog it speeds right back up for 4-6 seconds at WOT then backs off for a second or so. It will keep running like that surging between WOT and off 10% all day with the tiller pinned. Pumping the primer bulb doesn’t change the way it operates. It’s like the float bowl is running out of fuel or the fuel pump can’t quite keep up.

Ive done all the checks I can. I’ve cleaned the carb a couple times. Checked the fuel lines and filter. Fresh 50:1 fuel with some Sea Foam in it. New spark plugs. The carb seems to be working as it should so it’s not an issue with the butterfly valve only partially opening.

I can see the float in the carb can be tweaked a bit but I’m not sure if that could be the issue. The motor was used a couple times per year since new and has very low hours on it. As a 25hp unit it worked great at WOT but since the changes this year as mentioned before it seems to be running out of fuel at what is now WOT. If I back it off slightly from WOT it works great so I think if it was still in 25hp form it would be able to keep up with WOT demands.

Its not mentioned anywhere when talking about the Tohatsu 25-30hp conversions that the reeds in the intake are a different part number between models. Could this be the issue? Something keeps making me think it’s the float in the carb….

I’m open to any suggestions and appreciate anyone taking the time to give me their thoughts on the matter.

 

[Positrac]

I know…it’s a head scratcher for sure…

 

[pvanv]

As I mentioned on that other forum, a leaky o-ring in the fuel line could cause that.

 

[Positrac]

Thanks Pvanv. I’m going to get at it these next days off and once I figure it out I’ll report back my findings. I’ve got some Boyesen reeds to put in so I’ll pull the carb, check and clean it, maybe see about adjusting the float and put it all back together.

 

[Positrac]

Cleaned the carb and rebuilt the fuel pump. Still having the same issue. Just like it’s running out of fuel at WOT.

I’ve had every hose apart and all look like new. The main fuel fitting on the motor seems fine.

I’m running a 5/16” fuel line that’s about 8’ long. Can’t see that being an issue. Primer ball works fine and motor operation doesn’t change if I pump it when the motor bogs.

 

[Positrac]

Cleaned the carb and rebuilt the fuel pump. Still having the same issue. Just like it’s running out of fuel at WOT.

I’ve had every hose apart and all look like new. The main fuel fitting on the motor seems fine.

I’m running a 5/16” fuel line that’s about 8’ long. Can’t see that being an issue. Primer ball works fine and motor operation doesn’t change if I pump it when the motor bogs.

Is there a difference in main carb jets between the 25hp and 30hp models?

 

[pvanv]

Both are 155 jets.

 

[MattFL]

Look carefully at the float valve area of the carburetor, make sure nothing is blocking the hole. I had a bit of rubber from a fuel line partially block the float valve on my Honda causing that carburetor to run out of fuel. Take the float out and blow air backwards through the hole and try to catch anything that comes out the fuel inlet (just so you know if something came out). Also how does the fuel filter look? I'm trying to think of what might be limiting fuel flow...

 

[Positrac]

Look carefully at the float valve area of the carburetor, make sure nothing is blocking the hole. I had a bit of rubber from a fuel line partially block the float valve on my Honda causing that carburetor to run out of fuel. Take the float out and blow air backwards through the hole and try to catch anything that comes out the fuel inlet (just so you know if something came out). Also how does the fuel filter look? I'm trying to think of what might be limiting fuel flow...

Sorry Matt, I was logged out so I didn’t see your post.
I’ve cleaned the carb out a few times now and I’m 100% positive there is nothing in the fuel passages.
I did find that the offer 2 jets for the 25 and 30hp Tohatsu though. A #155 and a #162. I have one of each on order to try. The Tohatsu 25 & 30hp models show the same carb main jet and everything I’ve read says it should be a 155 but maybe with this being a Sea Pro Merc did something funny.

 

[Positrac]

10 months later and still no resolve. I'm into this motor for far more than I could get for it so there's no stopping now. It still has the surging at wot and nothing has helped it. So far I've changed out on my own with no improvement:
Rebuilt the original carb and new aftermarket (Chinese) carb
Rebuilt original fuel pump and new aftermarket fuel pump
New 3/8" fuel hose and all new fuel line fittings including the motor side disconnect.
Went up in number drill sizes the original main jet until it started to run rough. It is back on a new #155 main jet.
Adjusted the fuel float in the carb up and down a bit.
Hose was plumbed in direct to the fuel pump to bypass the filter screen.
New spark plugs

Compression was in spec as tested by my local Tohatsu dealer. They have been unable to figure out why it surges and at $800 so far in labor alone I've given up on them. They triple checked timing and all linkage adjustments.

I have a new ignition coil with spark plug wires on the way to try as a last resort. The only thing left is to try that and new plugs for the second time.

 

[MattFL]

Is it possible you're hitting the rev limiter? If it doesn't have a tach, you can pick one up on Amazon for about $20 that will record the highest RPM, so you can see how high it's revving.

 

[Positrac]

Is it possible you're hitting the rev limiter? If it doesn't have a tach, you can pick one up on Amazon for about $20 that will record the highest RPM, so you can see how high it's revving.

Hey Matt, I have a Amazon knock-off tiny tach on it and my max rpm is 5,700 so I’m in spec. Not sure if these 2-strokes have a rev limiter on them either. But if they do, I should be fine. Nothing I can do to lower rpm either being a jet pump.