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Tohatsu MFS3.5S 4 stroke - possible fuel starvation

O

orduna

New member
MFS3.5A No 003-11085-0 Serial Number 036483XG

Owned outboard since new, lightly used, no previous problems, but due to illness, not run for about 8 months.
Drained tank, added fresh fuel, started on choke first pull, and then run happily at idle with good flow of cooling water. However, as soon as throttle opened, the engine would die, but again would run happily at idle. Found similar problems on this site and I have:-

  • Cleaned fuel tank
  • Checked fuel line for blockages
  • Taken apart carb, soaked carb bowl and all removable parts in carb cleaner for over 24 hours
  • Afterwards blasted all parts through with carb cleaner and then air
  • Float appears to be working normally
  • Fitted brand new spark plug

Again, started first time, run problem free at idle, but died as soon as engine revs increased. Re-did all the above, but still same problem. Engine will run constantly at idle with no problem, but as soon as revs increase.......dies!

I noticed in a thread posted 12-2-2014 on the same subject mentions "(I suspect your correct model is MFS3.5A) There is a long fuel filter screen integral with the fuel cock (hidden inside the fuel line from the tank). Has that been inspected?"

I must admit this is news to me, as the spares parts catalog describes the fuel tank to fuel tap hose simply as "fuel hose and spiral tube." No mention of an inline filter.

Any assistance and advice would be most welcome and appreciated.
 
Yes, it sounds as though you are too lean. Possible that there is still crud in the inaccessible, tiny passages of the carb. If it was allowed to evaporate with gas in the carb, that is likely. Verify the fuel needle is perfect where it seats. Also the fuel cock does indeed have a long inlet snout that has the filter screen. If that screen is restricted, you might get enough fuel to maintain the bowl level at idle, but not enough to run at speed. You will need a fair amount of patience and dexterity to get that apart and reinstalled. Make sure the tank is full (it's gravity feed) and that the vent is open.
 
Paul

Many thanks for your prompt reply.

I will remove the tube between tank and fuel cock and have an inspection. Strangely, the manual mentions checking the filter but provides no indication of its position, and I often wondered why this was the only outboard I have ever owned without a filter! Now I know, it is hidden within a tube!
 
Filter is clear and flow of fuel to carb is good. Therefore, I assume this indicates problem is still carb related and I will strip and clean again.

Apart from main nozzle, main jet and slow jet, are there any other jets that can be cleaned? On the parts list I can see reference to Pilot Screw that looks like a jet, but I am not sure if it can be, or should be cleaned.
 
The pilot screw is primarily for idle mixture balance. Its effect is minimal at WOT. It is set by instrument at the factory, and very difficult to reset correctly in the field. That said, if you [illegally] and very carefully drill out and remove the plug that caps off access to the screw -- without disturbing the setting, you can count and write down the turns-in to lightly bottoming, then remove it for access to the pilot passage. That will make it easier to flush the pilot passages. Then you can reinstall the screw to the same setting it was originally.

Did you use real carb dip (in the 1-gallon can)? If not, I would do that bath for at least 4 hours, then blow-out the carb with generic carb spray before reassembling.

Or buy a new carb. When paying for shop labor, eventually a new carb is cheaper than spending extra 100-dollar-an-hour labor on the original carb. At issue is that you have a better carb design that the new carbs, which are set for later EPA regulations, and are standard on the MFS3.5B. The original carb for the 3.5A is NLA.
 
Hi Paul

I think I will leave the pilot screw well alone until I have eliminated other possible problems.

Over this side of the pond carb dip simply does not exist, and therefore my only option was to squirt a large quantity of carb spray cleaner into a container to soak the various parts. I assume carb dip is stronger than carb spray? I can buy car-based fuel additives that you add to fuel tank and claim to clear carb jets and remove gunk etc. But I do not know if suitable for outboard.

I will strip the carb down one more time and soak and spray - I can almost do it blindfolded!

Thank you for your help and have a good week-end.
 
Just an update.

Despite repeated cleaning I finally had to admit defeat and took the engine to a dealer and from my description of the problem he immediately diagnosed the pilot screw. Without even asking the dealer quickly removed the blank plate with a homemade tool, cleaned, and adjusted the pilot screw and replaced blank plate within a couple of minutes without a charge. Outboard now runs smoothly at idle and open throttle.
 
Glad you got everything working. Be advised: The screw did not un-adjust itself. If the crud restricting it ever does clear, you will be too rich at low speeds.
 
Paul

Point understood.

I spoke too soon and the exact problem has returned. I have decided to thoroughly clean the carb by ultrasound.
 
Just to finish this thread I can confirm the problem was the "unadjustable" Pilot Screw. In Europe the blanking plug is designed to be removed with a special tool, but in fact with a little patience it can be removed with a screw driver, and a quarter turn was all that was needed. The engine has now been used 4 or 5 times a week for the past couple of months and runs perfectly. Hopefully, job done.
 
Glad it worked for you. The US EPA (who wants the leanest, lowest-emissions running conditions) requires the blocking plug, because "we" cannot adjust the screw correctly. Admittedly, it is difficult to set it by ear -- the assistance of a precision shop tach is a big help. Be advised: If the restriction in the pilot circuit ever does get cleared, your low-speed mixture will become too rich, and the plug will foul prematurely... should that happen, you can lean-out the pilot screw.
 
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