Tohatsu M40C issues - possibly carb related

[jondavies]

My M40C has been acting up for the last couple of outings. Here's a summary of what happens.

Motor starts and runs fine when cold.
When the motor is hot/warm, it won't fire.
Open the choke, motor fires and then dies.
Close the choke, motor fires and then runs roughly.
If you put the motor in gear at idle the engine bogs down and stalls.
If you put the motor in gear with some revs, it runs fine but stalls when it goes back to idle.

At the same time, I've noticed a lot of soot deposited around the exhaust.

At first I thought this might have been related to a loose plug. However, the issue is getting worse despite a new set of plugs, which I correctly torqued. I'm now thinking it's a carb issue and I have a carb rebuild kit on the way.

Are these symptoms consistent with a dirty carb?

Thanks in advance!

 

[25432]

Re: Tohatsu M40C issues - possibly carb related

I had a lot of the same issues as you. Read tons of posts on here, bought a service manual from a dealer and went to work. The service manual says to never "dip" the carb.....I disregarded that and pulled mine off, completely disassembled it and let it soak in chem dip for 4 hours. After that was done I put the carb back together, set everything by the book and she ran MUCH better. I then went to work setting the throttle linkages by the book and adjusting the timing. After doing that and playing with the height of the motor I now have a damn fine running motor.

Oh yeah, insert "bought a tiny tach" up there somewhere as they are invaluable when it comes to setting it up right.

 

[25432]

Re: Tohatsu M40C issues - possibly carb related

And.....spark plugs are cheap, keep some on hand at all times.

 

[Sea Rider]

Re: Tohatsu M40C issues - possibly carb related

Check if your air butterfly is sticking inside carb when you have pushed choke all the way in after you have started the engine ? If so spray carb cleaner into butterfly shaft let soak, move with fingers back and forth to loosen and apply some thin oil on shaft's internal, extenal sides. How are your fuel pump diaphragms ? Is tank filter clean including hose.

Happy Boating

 

[jondavies]

Re: Tohatsu M40C issues - possibly carb related

Thanks for the suggestions.

I've been waiting for the carb kit to arrive but it has been back ordered. In the meantime, I decided to take off the carb and take a look inside. The bowl and everything else inside is absolutely immaculate (brand new in 2010). I can't be sure but it seems unlikely that the carb is the root cause of the motor trouble. For good measure, I cleaned it with carb cleaner, made sure everything was working smoothly and reinstalled it.

At the same time, I changed the plugs, checked the fuel pump gaskets, made sure the fuel filter was clean and switched to a new fuel source.

Unfortunately, none of this has resulted in an improvement in the motor's performance. I was able to get it to idle a little better by adjusting the idle screw but it's still sputtering and stalling after a few seconds without throttle.

Any other ideas? TIA

 

[Sea Rider]

Re: Tohatsu M40C issues - possibly carb related

If sputters and dies at idle seems a poor fuel/air mixture adjustment or badly cleaned carb. How old is that horse ?

Happy Boating

 

[jondavies]

Re: Tohatsu M40C issues - possibly carb related

If sputters and dies at idle seems a poor fuel/air mixture adjustment or badly cleaned carb. How old is that horse ?

Happy Boating

The motor was rebuilt in 2010 and had a new carb installed at that time. I'm guessing the engine dates from the 1990s.

 

[Sea Rider]

Re: Tohatsu M40C issues - possibly carb related

The motor was rebuilt in 2010 and had a new carb installed at that time. I'm guessing the engine dates from the 1990s.

How about adjusting fuel/mixture at idle along idle rpm too when mixture is correct. Was the new head gasket well torqued to 30 NM ? How about checking all bolts.

Happy Boating

 

[jondavies]

Re: Tohatsu M40C issues - possibly carb related

How about adjusting fuel/mixture at idle along idle rpm too when mixture is correct. Was the new head gasket well torqued to 30 NM ? How about checking all bolts.

Happy Boating

Ha! Your previous response (about the fuel/air mixture) prompted me to play with the carb adjustment a little further. I managed to get it running pretty happily. I hope so anyway. I need to do an in-water test to make sure but at least I'm making progress.

I assume the rebuild was done to spec as it was done by a dealer (you never know though).

Cheers (salud)!