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Tohatsu 5 hp - stars/idles fine - runs fine in neutral - sputters and dies 1 minute after in gear

G

gmac2

New member
Hi There - I am trying to help a friend troubleshoot a 2007 Tohatsu 5 hp (Model MFS 5BS). He's had it for a year and it ran fine until recently.

Problem is it runs for a few minutes and then sputters and dies.

I checked oil - it was a little high so I drained some. Now it is not above top line at all.

I removed and cleaned carb. Not much gunk at all, but did notice oil on the air intake - side closest to fly wheel where tube from crank case comes back to top of intake.

After lowering oil level and cleaning carb, I took it back to the water and it started great - idled in neutral and ran at mid range rpms in neutral for 10 minutes. Thinking I had solved it - I put it in gear and it sputtered and died within 1 minute. A white puff of smoke and small backfire come out of air intake when it dies.

What's up??

Thanks!
 
Check compression first,followed with an open air gap spark test. Next troubleshoot the fuel system,starting at fuel tank pickup tube screen and moving forward. Sounds like a fuel delivery issue, Try pumping primer bulb",Manual fuel pump" when engine is running in gear, Is there a fuel filter? Is it plugged ?Vent on tank open? Collapsing fuel line? Faulty bulb? Does it pump up properly,firm?
 
What method was used to clean the carb? Is the float height correct? Is the float needle perfectly free and clean, and not binding?
You may have a thermal ignition problem -- that only happens at higher temperature.
Odds are, however, that the issue is still fuel-related.
 
Thank you Johnny and Paul - We first suspected a bulb issue on his boat - so we moved to mine - where I run a 6 hp Tohatsu with no problems - the bulb stays firm after being pumped - and during the time the engine is running - and past when it sputters and dies. I took the carb off, removed jets and needle/float. I sprayed all with carb cleaner and dried with air compressor. Needle and float appear to move freely with no binding. I am not sure how to check to check if the float height is correct. If it is a fuel delivery issue, I think it is happening inside the motor.

I did have a heck of a time getting the fuel tube off the carb and there was white residue on the inside of the fuel line from fuel pump to carb (photo). Maybe try a new fuel pump?

The carb had a few species in it - but no gel and was really pretty clean.

It sounds like you guys are not thinking it has anything to do with oil coming back up into the air intake? The second attached photo show the inside of the air intake - with the oil coming in on the left side in that picture.

I just ordered a compression tester and spark tester - as well as a new plug and in-line fuel filter. Will update when I get those items.

Thanks!
 
Thank you Johnny and Paul - I tried to reply but think I messed up - so sorry if 2 show up.

1. I have a compression tester and spark tester on the way.
2. We thought it may be a bulb issue on his boat - so we moved to my boat (where my 6 hp Tohatsu runs fine) and the bulb firms when pumped and stays firm while motor is running (and even after it sputters and dies under load).
3. There is a fuel filter in line inside motor housing - will replace that. Will also replace spark plug - and gap new one at 0.9
4. Fuel lines look fine - but did see white residue on inside and on fitting coming out of fuel pump. Will try to attach photo.
5. I took carb off - took out jets and float/needle. Sprayed all parts with carb cleaner and blew out with air compressor.
6. Needle spins/moves up and down freely. Not sure how to check correct float height.
7. Does not seem like you guys think the oil coming back into top of air intake could be part of the issue? That does not happen when in neutral (even at mid-higher RPMs), but appears to happen when in gear/under load.
8. I will check compression/spark, replace plug and in-line fuel filter and give it a try.

Thanks,

Geoff
 
trying to upload pics showing white residue on fuel pump and oil coming into air intake
 
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Compression will not tell you much, since the motor has cranking decompression for easier starting.
Liquid oil in the crankcase vent would mean a lot of crankcase pressure, which usually means either unusually leaky rings or oil still a bit high.
Carb spray is not aggressive enough to dissolve the organic varnishing that may be in the carb.
Depending on the exact model of your personal 6 hp motor, you can swap a few parts to help isolate the issue. The carbs interchange no matter what, so that would be a good way to swap-in a known-good part and re-test.
 
Good idea - I'll give that a try. I also drained a little more oil to get it between the lines on the dip stick. What should I use to attack the varnish in the carb?

Thanks,

Geoff
 
gmac2 said:
Good idea - I'll give that a try. I also drained a little more oil to get it between the lines on the dip stick. What should I use to attack the varnish in the carb?

Thanks,

Geoff
Click to expand...
Real carb dip, available at an auto parts house. It will need at least a 4-hour soak at room temperatures, then a blow-out with carb spray.
 
Problem solved. Can't say I followed the scientific approach and isolated one variable at a time - but I checked compression and got about 40 psi, but as Paul said - if there is a decompression in there that may not mean much. I checked spark - looked good - but replaced the plug (gapped at 0.9 mm) and the fuel filter inside the cowl. When I put it in the water to test the bulb firmed noticeably faster (less than 2 pumps instead of 4 or so). Started first pull and ran like a champ under load and idled at min throttle even in gear. I am pretty sure the full filter was clogged up. Thanks for the help.

Geoff
 
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