2004 Tohatsu 18HP Auto Choke Problem?

[wlbacco]

I recently bought a boat that has a Tohatsu 18hp, 4 stroke motor on it. Model MFS18B2EPL. The motor runs a bit rough when idling and just doesn't seem to have the output I would expect from a motor this size. I have also been having a periodic issue where it will idle, but when you give it gas it wants to stall. This latter issue comes and goes. Some days it will run ok.

I recently changed the spark plugs, fuel filter, fuel pump and gone over the fuel delivery system. It seems to have plenty of fuel. I have also examined the carburetor and all seem very clean (no varnishing, etc.), float appears to be good and working properly, etc. As part of this I was testing the Auto-By-Starter (electric choke). When I apply 12V to it, it heats up and extends as you would expect. However, when the motor is on and running 12V is not being supplied to it. Continuity in the wire that runs from the CD Unit to the Auto-By-Starter is good. Should the CD Unit always be supplying 12V to this while the motor is running?

Also what RPM should this motor be idling at?

Any help you can provide is appreciated.

Thanks, Bill

 

[pvanv]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

Bill,

Yes, your ABS module tested OK, and is working. If it were not working, the carb would go super-rich as the motor warmed up, and did not require the extra enrichment of a cold motor. You probably don't have the correct meter to see the pulses going into the ABS when running (it's before the rectifier, and is not DC).

Sounds like you are lean at slow speeds. Although the carb "looked clean", it probably has slight/moderate varnishing in the low speed passages. That can only be cleared by a proper carb soaking in real carb dip. Recurrences can be reduced by religiously running the carb dry at the end of each and every day.

Page 16 of your owner's manual (which you can d/l from tohatsu.com) shows that correct idle is 900 in gear, 1,000 in N).

 

[wlbacco]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

Paul,

Thanks for the quick reply.

So the voltage coming into the ABS is pulsed. If I put an oscilloscope on it what should I expect to see? Could I also maybe use a meter on AC to see something?

If I try to lower the idle to 1000rpm in N, the motor will probably stall. It doesn't run smoothly until about 2000rpm.
Could old gas be causing this problem? Also what do you recommend as far as any gas additive for longer term storage of the gas.

 

[pvanv]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

Not sure what the waveform would look like. Not important, as your ABS is working.

You need to clean the carb properly, including a 4-hour bath in real carb dip.

Old gas is always bad, but your symptoms are classic of varnishing.

Never store gas more than 30 days. YMMV, but we have had OK results with K-100, especially the MG (marine/gas) version.

 

[wlbacco]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

I ended up putting an oscilloscope on the feed to the ABS. The voltage coming to it is AC (a sine wave) and was about 40V Peak-to-Peak. After running the motor for a bit, I pulled the ABS and it was extended as you would expect. I am convinced that the ABS is working.

I was chatting with a tech from Tohatsu about the issue and he indicated that there is an idle mixture adjustment screw located under a round aluminum or brass plug located in the upper left corner of the carb when facing the carb side of the engine. I see such a plug on my carb. He indicated that you need to carefully drill and pull it out to get at the adjustment or to clean. Have you ever heard of such a plug? I am a bit concerned about messing up the plug (and the carb) if this one doesn't have it or I am on the wrong plug.

Thanks again for all of your help. Bill

 

[pvanv]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

Bill,

Yes, your ABS module tested OK, and is working. If it were not working, the carb would go super-rich as the motor warmed up, and did not require the extra enrichment of a cold motor. You probably don't have the correct meter to see the pulses going into the ABS when running (it's before the rectifier, and is not DC).

Sounds like you are lean at slow speeds. Although the carb "looked clean", it probably has slight/moderate varnishing in the low speed passages. That can only be cleared by a proper carb soaking in real carb dip. Recurrences can be reduced by religiously running the carb dry at the end of each and every day.

Page 16 of your owner's manual (which you can d/l from tohatsu.com) shows that correct idle is 900 in gear, 1,000 in N).

Yes, the EPA requires that the pilot screw be plugged, so we mere mortals can't easily access it. Odds are it was set perfectly at the factory, and you are dealing with varnishing. Yes, it is possible to carefully remove the plug, and with the aid of a very good ear and a razor-accurate shop tach, change the setting of the pilot. It is of course a violation of Federal law (though I am not aware of anyone being arrested/fined because of drilling out the EPA plug).

If you do plan to go that route, clean the carb properly first. You need to do that before any adjustments anyway.

Then, only once you fully understand the standard procedure for setting the pilot screw, if you still really feel the need, you may proceed.

Odds are that after your proper cleaning, it will not need adjustment.

 

[rsnyder518]

Re: 2004 Tohatsu 18HP Auto Choke Problem?

I recently rebuilt the carb on my MFS6B and I removed the pilot screw (the screw behind the plug that you would need to drill out). I did this before I came to this forum for advice and found out that removing it probably wasn't necessary. But, if you decide to remove it anyways make sure you bottom it out until it lightly seats and count the number of turns before you remove it. Then you can at least always put it back the the factory set position as a starting point by bottoming out the screw and then backing it out by the number of turns you counted.