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Tohatsu 30hp 2-stroke jet bogging at WOT

Positrac

New member
HI guys. I just discovered the forum tonight 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 WOT. If I back it off slightly from WOT it works great.

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

New member
[FONT=&quot]Cleaned the carb and rebuilt the fuel pump. Still having the same issue. Just like it’s running out of fuel at WOT.[/FONT]

[FONT=&quot]I’ve had every hose apart and all look like new. The main fuel fitting on the motor seems fine.[/FONT]

[FONT=&quot]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 between main carb jets on the 25hp and 30hp models? I’m running at the 2000 foot elevation level for the most part not that I think that matters.[/FONT]
 

pvanv

Outstanding Contributor
When it craps out, I would immediately drain the carb to see how much fuel comes out. How do the plugs look?
 

Positrac

New member
When it craps out, I would immediately drain the carb to see how much fuel comes out. How do the plugs look?

If I turn the fuel mixture screw in I can lean it out and if I turn it out 3 turns it looks rich on the plugs. That doesn’t seem to change whether the motor intermittently looses rpm at WOT or not.

It only bogs for a split second then takes off at WOT again, so not sure you would be able to see a difference in the fuel amount in the carb. The fuel filter stays full. I even tried running with the fuel filter off but operation was no difference. I thought it could be a fuel filter issue starving the motor at WOT, but it wasn’t.

I looked at trying to adjust the float in the carb but there’s no adjustment there unlike some carbs.

Pretty frustrating actually.
 

Positrac

New member
Have you tried putting the 25hp restrictor back in?

Not yet. I had thought about it but I’d have to order one in. I could probably make a temp one out of brass shim stock. That would tell me I’m getting too much air with the non-restrictor gasket which would also tell me I need more fuel if I’m going to run non-restricted.

The main fuel jet looks to be the same part number between the 25 & 30 horse motors though. It sure seems like it is running out of fuel.
 

Positrac

New member
I see Tohatsu offers a #155 & a #162 main jet. I’ve got one of both on order and I’ll give that a try.

No info on them other than most literature says it should be a 155 in there. Maybe Merc did something funny.
 

Positrac

New member
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
Carb restrictor plate in or 30hp non-restrictor plate in made no difference.

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.
 

Lastmanstanding

New member
My only other question would be is there a difference between spark plugs for a Merc Sea Pro and a Tohatsu?
I have a Mariner 30HP 2 stroke which is a Tohatsu that has the exactly same issue as you have explained. I haven't found a solution either. I also run it at 600m altitude and at sea level and it plays up at both. It would be good to find the fix.
 

Positrac

New member
I have a Mariner 30HP 2 stroke which is a Tohatsu that has the exactly same issue as you have explained. I haven't found a solution either. I also run it at 600m altitude and at sea level and it plays up at both. It would be good to find the fix.
It turned out to be a coil or spark plug wire. They were the last thing to change out and it fixed the issue. So, after a year of screwing around including time in a Tohatsu dealer that proved pointless except for making my wallet lighter it turned out to be something very simple.
 
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