Tohatsu M90A Low Voltage and Rough Cold Start

[Arnie Davis]

I purchased a boat that has a 1996 M90A Tohatsu outboard. I appear to have two problems that could very well be related.
Problem 1:
The engine runs great once it is warm but is a cold start nightmare. I removed the carburetors and soaked them then installed new floats with new gaskets. This has helped some but as temps get cooler the cold start issue becomes worse. I have also replaced all the spark plugs and that has had no effect. I purchased a service manual and checked carburetor synchronizing and timing specs and they all appear to be within tolerance according to the manual.
Problem 2:
I seem to have low voltage (11.1v) with the ignition switch off and all accessories off. I have confirmed this with a Fluke meter and the voltage readout on my depth finder. At full throttle it seems to charge at 12.1-12.4 volts and at idle it drops below this. I can charge the battery overnight and achieve 13.8 volts then the next morning when I arrive at the fishing grounds and shut everything off but the depth finder I am back at 11.1 volts. I have checked all fuses, wire connections and replaced the battery but still 11.1volts with nothing on.

I am stumped and any help provided would be greatly appreciated.

 

[MattFL]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

If you take the battery off the charger and without anything connected the voltage falls back below about 12.6 volts within a few hours then it sounds llike it is time for a new battery. A new battery will also provide more power to the starter, spinning the motor faster and likely making it easier to start.

On rereading, I see you replaced it. See if the voltage falls even when it's disconnected. If yes, get another battery. If it stays up disconnected but falls when connected then you have ssomething constantly taking power and you need to figure out what and fix it.

 

[pvanv]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

Yep. And confirm that your electric choke setup, and warm-up lever are operating correctly.

 

[Arnie Davis]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

Very good and thank you for the responses. If my battery is good and my electric choke and warm up lever are functioning correctly where should I look from there? I'm thinking rectifier or CD unit?

Thanks again.

 

[MattFL]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

Is it a 2 stroke? If yes check the compression when it is cold. In my personal experience worn 2 strokes with slightly lower compression can be harder to start when cold especially after not being used for several days. Be sure you're choking it and giving it a little throttle. If you think it is flooded then use no choke and a lot of throttle, just be ready to pull back on the throttle the instant it fires.

As a last resort starting fluid will make it fire right up, but is dangerous and can rinse oil off the internal parts if you use a bunch. But if there is even a hint of spark and compression then starting fluid will make it start right up. Open the throttle and shoot a quick shot into each carb, quickly close the throttle and then try to start it normally.

 

[Arnie Davis]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

We made out to the fishing grounds yesterday and I did as you guys suggested. Disconnected all battery connections except for the depth finder and had 12.4volts. I also noticed some green corrosion in my negative battery cable that I did not notice previously. The cables are long and I guess for mid and front mount battery locations. I plan on cutting them shorter this week in hopes I can get the corroded portions out the lines. If not ? I will just replace both battery cables altogether.
The engine ran really rough and was hard to get started once I reconnected the battery as we decided to switch fishing locations. But after about a mile of full throttle it cleaned out and I noticed I had 14.3v at the voltage meter so I think I?m headed in the correct direction now.
It is a 2 stroke and I checked the cold compression this morning and have 128lb - 130lb of compression on all three cylinders. I left the gauge in each cylinder for 30 minutes or so and the pressure on all cylinders did not drop through the wait period either. The plugs looked great and have similar burn patterns and show no signs of fouling. With the deal I got on this outboard and it being almost 18 years old I question myself as to how long it would function anyways. There really is no way for me to know how much time, wear and abuse it has been through. I have no history on it but it sure does run great at mid and full throttle!
Is 128lb-130lbs compression considered ?o.k.? for a 2 stroke that is 18 years old? It naturally has to be somewhat lower than what it was in the beginning of its life span. I guess a better question would be should I be preparing for a power head rebuild and/or replacement soon?

 

[Sea Rider]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

Hard to tell not knowing a thing about previous osnwership, does the boat have an hour meter ? Have 2 stroke Tohatsu's with more than 2 k hours worked on a daily basis and still running pretty strong, but that's well maintained and looked after.

Happy Boating

 

[MattFL]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

You will need to check a service manual for the compression numbers. Note that the cylinder did not hold pressure that long. The compression tester has a check valve. The cylinder itself probably won't hold pressure more than a few seconds at most. It is normal for any carbureted motor to run a little ragged until it warms up. If it runs bad at low speed even when fully warmed up then check to see if you possibly have a sticking float in a carburetor.

 

[pvanv]

Re: Tohatsu M90A Low Voltage and Rough Cold Start

Your battery is still suspect. You want more than 13v when charged at rest -- ideally at least 13.2v. I would also replace the battery cables.

Going through the carbs properly may help your cold starts and warm up. Diagnose the motor -- fuel, electrical, and mechanical systems. If the link-and-sync is mal-adjusted, you may need to go through the whole process, including starting with Proper carb cleanings.