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Tohatsu 9.8 runs rough - very frustrated

C

cthoops

New member
We have a 2010 Tohatsu 9.8 outboard on our sailboat. When we bought the boat last year the prior owner said that occasionally the outboard would run rough, acting like it wanted to stall. For the first six weeks of the season it was fine, but then it started showing the same symptoms. We cleaned the carb with spray, replaced the fuel line/primer bulb, and it ran perfectly for the last several months of the season.

During the off-season we replaced the spark plug, replaced the gaskets, float, and needle valve in the carb, and put in a new fuel/water separator. There was a teeny bit of gas left in the 3 gallon external fuel tank that Mr. cthoops said would be fine, so we filled it up with gas, put in the stabilizer, and tried to motor out for a sail. It idled fine, we put it in forward, and within 2-3 minutes we started experiencing the same thing that we did last year.

Since then we have disassembled the carb and sprayed it out, we've put in new gas (high octane plus stabilizer), and we've replaced the jets, the in-line fuel filter, and the fuel line with primer bulb. No improvement. It idles fine, and then after 2-3 minutes in forward it runs rough and acts like it wants to stall while we limp back to the mooring ball. Twisting the throttle to give it more gas while it's acting like this doesn't help at all.

The mechanics at the marina said we have some gunk in the chamber that is covered by the EPA plug. They said all they have to do is drill into the plug, remove it, clean the carb again, replace the plug, and we'll be all set. I'm skeptical. If you can't access the chamber without drilling into and removing the plug, how would anything get in there? It's been almost a week and a half and they still haven't gotten to it, so I'm looking for confirmation that what they are describing will take care of the issue. Or, should we just disassemble the carb (without removing the EPA plug of course) and soak it in dip? I've been reading that the spray cleaner doesn't work, so perhaps it just needs to be dipped? Should we just buy a new carb?

Frankly, I'm starting to lose faith in the outboard and I'm almost ready to throw it into the Sound. Our already short season is over 1/6 over and we still haven't been able to go out.

Any thoughts or suggestions would be greatly appreciated.
 
I assume your model is MFS9.8A3 (year is irrelevant for servicing a Tohatsu outboard).

Spraying aerosol cleaner on/through the carb throat makes the carb shiny, but does nothing to clean the internal passages. A proper and thorough cleaning requires a full and complete disassembly (including jets and emulsion tube nozzle), followed by a 4-hour room-temperature soak of all non-rubber parts in real carb dip, and then a liberal blow-out of all tiny passages with generic carb spray to blow out any loosened crud in the passages.

When you replaced the plugs over the winter, what did the old ones look like?
Why did you change the primer hose? Was the bulb not pumping correctly, or was there evidence of an air leak at one of the o-rings? Is the tank venting properly? Did you inspect the fuel for any signs of water contamination?

Why did you replace the jets? Unless damaged, they can be cleaned and re-installed if varnished. Did you replace them with the identical jet numbers?

Is the motor oil level too high, even a drop above Full? If so, change the oil, setting the level at about 1/2 to 3/4 on the dipstick. Over-full oil could cause your symptoms. Use 10w-30 non-synthetic, FC-W rated oil only. Nothing heavier, and do not run synthetic oil. A complete change requires about 3/4 of a quart. Never fill it too high.

Your marina mechanics are guessing that you have a dirty carb isue; That is a common issue, but it is important to properly diagnose the problem before randomly throwing parts (and money) at the problem. Do you faithfully, religiously, run the carb dray at the end of each and every day? You want to do that to reduce the chances of varnish build-up in the tiny passages of the carb. Your problem may well be something other than fuel -- either a thermally-triggered electrical issue, or a mechanical problem, such as a stuck thermostat. Has anyone tested spark when the motor is malfunctioning? Has anyone checked the crankcase temperatures with an IR thermometer?

The brass EPA plug merely covers the access to the slot of the pilot screw. That setting is done by instrument at the factory, and seldom if ever needs changed; adjusting the pilot is tricky, requires a very good shop tach, and an experienced ear. If, for some reason, the screw setting does get disturbed, it is very important to re-set it back where it was. If it is set too lean, idle will be poor; if too rich, plugs will foul. I don't know anyone who replaces the brass plug after adjusting the pilot screw. It is indeed possible that some varnish crud is in the small chamber that the screw uses to meter idle mixtures, but that would have little (if any) effect on WOT operation. If there is crud there, a complete disassembly, soak, and blow-out will clear that varnish. Incidentally, it is a violation of Federal law to remove that plug. I've never heard of any enforcement action by the Feds, but it is within their purview.

There are 2 different carbs for the 9.8A3 -- one for manual choke, and one that is used on Remote models (for electric choke). They are not interchangeable. Either costs over $200, so diagnose the issue before investing in a new one. If you do have a carb issue, replacing the carb is the fastest (albeit most expensive) way to eliminate that problem.

Diagnose first, repair second.
 
Hi Paul,

To clean the carb, do you think I could do as I do with my motorcycle : from time to time I pour a liquid in the tank (don't know the English name) and I can feel the carb is more responsive?
Thanks.
Fred
 
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