Tohatsu 18 hp 4 stroke auto chock problem

[Fishon dw]

My 2006 Tohatsu 18hp 4 stroke is loading up at idle. I have narrowed it down to the Auto-by-starter on the top of the carb. If I loosen the mount and lift it slightly, the idle will smooth right out. I have seen it called a PTC heater as well. Does anyone know how this unit works, and how to test it.

 

[pvanv]

Re: Tohatsu 18 hp 4 stroke auto chock problem

The auto-by-starter unit is pretty reliable, but can occasionally fail. It's an electrically-heated expanding unit. Information on how to test it is in the factory service manual, Part # 003-21041-0, $27.58, available from any dealer, including me. Basically, if you apply 12v to the wires, it will slowly expand, pushing the plunger out. In your case, it would seem that the plunger is stuck in the extended position. Unusual to fail in that mode. If your model is MFS18B2, and your ABS unit does indeed need replaced, you will need to order part number 3R3034100M AUTO-BY-STARTER ASSY $144.58, from any dealer, including me.

 

[Fishon dw]

Re: Tohatsu 18 hp 4 stroke auto chock problem

I bench tested the ABS unit, and the coil checked out at 19.5 ohms. It extends the plunger after heating for a couple of minutes with 12 volts applied, so it appears to be good. After remounting it, the condition was the same, smooth start and idle (choke on, ABS plunger retracted), then loading up after a couple of minutes as the plunger of the ABS is extended (choke off). Then if I manually lift the ABS slightly (chock on) it smoothes out again. One would think it should load up even more when lifted. Well, tomorrows first project will be going back through the carb very thoroughly, there must be a blockage somewhere.

 

[pvanv]

Re: Tohatsu 18 hp 4 stroke auto chock problem

Yes, you are headed in the right direction. The enrichment is when the plunger is retracted. Sounds like something is amiss in the enrichment circuit of the carb.

 

[Fishon dw]

Re: Tohatsu 18 hp 4 stroke auto chock problem

When I put this motor to bed last fall it was running great. So, two weeks ago when I hit the lake for a spring motor check out, it was very hard to start, and the idle was loaded up real bad, then smoother above idle, and then bad load up above half throttle. So I went after the carb. Went I tear down a carb, I take extreme care especially if it is a first time, which this one was. After find it in perfect condition and remounting it, just to find it with these same conditions, I pulled it down again on Saturday. This time I spent near two hour going through every port, channel, jet, and pin hole, which this carb as nearly 20. Everything open and clean. Remounted, and the same malfunctions. This is where you want to just throw it against the wall. I was checking the parts PDF and looking at the jet sizes to see if my boy had slipped in some big one as an early April fools joke, when I noticed a part called a rubber plug that is to cover the port to the idle jet. I knew I had not seen it on my carb's idle jet port, so I started a floor search of the shop work bench area. Eureka! Here was the small hat shaped plug. After reassembling it with the port plug, it ran smooth all the way from idle to WOT. This little plug must have fall out of the port over winter, and when I removed the bowl it slipped away before I seen it. I noticed on the parts list that all the 9.9, 15, and 18 hp carbs have this part, and probably others as well.