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2005 Tohatsu MFS4A - won't start

R

Rdubs

New member
Hi everyone
Recently bought a 2005 Tohatsu MFS4A2D motor. Prior owner bought it in a batch of boat parts, tried to get it to run but it wouldn't. So I bought it as a project, hoping it would just be gunk in the carbs.

NOTE: For all of the below work, I just had the motor on an outboard stand without any water covering the bottom of the outboard. I wasn't going to let it run without water, but figure it won't get damaged during the time when I'm trying to first get it to start. Need to get one of those tall 5-gallon buckets.

I drained the carb and put fresh gas in and tried to start it. Would not start, and I wasn't even detecting a chug like it was trying to or see any exhaust.

I spent a lot of time online trying to research what to try and with many thanks to Pvanv's posts, it seems clear that the order of things to check is first the fuel, then the spark, then the mechanical.


  • Fuel: I took the carb apart and cleaned it. The idle jet was completely blocked, I cleaned it out with a very thin piece of wire. Cleaned the main jet and nozzle. The float seemed to be moving easily.
  • Ignition: Bought a spark plug tester and tried to start. The spark tester flashed indicating there was a spark. Bought a new spark plug just to be safe and took the plug out and put it against the spark plug hole and tried to start, the spark was clear and bright.

After cleaning the carb and putting in the new spark plug, I tried to start it again. This time I heard a couple chuffs like it was thinking about starting and also exhaust began to be visible (a grey-white smoke). One item of note: the exhaust seemed to be coming out of the telltale / cooling water exhaust hole, I'm not sure that's supposed to happen that way.

So next I checked the compression. It came in at 30-40 psi. Originally I thought, "that's the problem!" but when I read the service manual and it said that motors with a decompressor should show only 50 psi. (Note: the service manual I have is for a MFS4/5/6 B/C, whereas I have a MFS4A. Can't seem to find what the difference is between the A and B/C models). The next thing I did was pour some oil in the cylinder and try the compression test again. This time, the compression went up to 70-90 psi (70 was with the motor in the upright position, 90 was a test with the motor horizontal so the piston was straight up-and-down). Normally such an increase in compression with the wet test indicates worn piston rings, but I'm not sure if the test still has the same conclusion if there is a decompressor - the oil could just be clogging up the decompressor outlet to produce the higher psi.

Anyway, would appreciate any thoughts. Also a special shout-out to Pvanv, thank you for sharing your knowledge on these forums, your explanation and detail is not just very impressive but very helpful.

Best,
'Dubs
 
Quick update: Tried to spray some starting fluid in the intake. Did not seem to have any kind of impact or difference. The starting fluid was a few years old so not sure if degrades over time.
 
What came out of the carb when you drained it? Was it several-years-old gas, of a few-days-old gas? Withe the dirty fuels we gat in the US these days, the fuel can't sit in these carbs more than a couple of days, or it will varnish the carb as it evaporates.

Never push a wire through any passage in these carbs. The passages are so narrow and long, that even a copper wire can cause scratching, which can lead to weird mixtures at some speeds. The low-speed jet is an air jet in the top of the carb, so I would not expect it to be plugged, aside from some dust. You likely have a completely-plugged main jet, and if so, the odds that the small passages in the carb are full of crud are high. What method did you use to clean the carb? If you did not completely disassemble it (removing all jets and emulsion tube nozzle), followed by an overnight soak in real carb dip, then a thorough spray-out using the better part of a whole can of generic carb spray, you did not do a thorough cleaning.

Yes, exhaust can come out of the telltale if you are not pumping water. Running a dry water pump, even for a few seconds, will destroy it, so better to put a
336600070MFLUSHING ATTACHMENT, 4B-40C/F2-30 (8 MM)$7.78
on it, and connect a garden hose.

After warmed-up, with the throttle wide open and choke open, the compression at slow speeds should be about 49 PSI at hand-cranking RPM (below 500 RPM). It should come up to about 138 when above 1000 RPM, when the motor is warm. A cold motor will read quite a bit lower, so that's not necessarily a problem. B and C versions have different ignition systems from the A2, and some linkage differences. The decompressor is a centrifugally-operated weight on the camshaft that prevents complete valve closure below about 500 RPM, so oil in the cylinder doesn't affect the decompressor, but it will cause an increase in compression. Odds are your compression is fine.

Ether-based starting spray isn't going to spoil in a closed can, so that isn't the problem. Looks like you verified compression and spark, so pull that carb, disassemble and soak it, then spray out each and every passage. OR... since the carbs interchange between the 4/5/6, so you can just buy
3R4032001MCARBURETOR ASSY, MFS/NSF6A2$122.35
and gave a good-running 6 hp version with a lot less work.
 
Last edited:
Paul - thank you for the very thorough reply.

When I cleaned the carb, I removed it from the engine then unscrewed and removed the idle jet, the main jet (the one you unscrew on the bottom) and the main nozzle. I did not remove either of the (two, I believe?) adjusting screws. I then blasted everything with carb cleaner and let the small removed parts soak for about 15 minutes in the excess carb cleaner spray-off, then ran a pipe cleaner through as many passages on the main nozzle and main jet as I could. I blew and looked through all of the passages on the main jet and main nozzle and they seemed clear. The Pilot Jet is the one I had a bad feeling about which was confirmed when I tried to push a thin copper wire through it, it was blocked but became clear after I pushed through and ran the wire around.

On the two adjustment screws: One of the screws on the carb (Pilot Screw Set, per the parts diagram) looks like it was covered with a cap of some sort and wasn't sure how to take it off, but in general I didn't think to touch the screws because did not want to mess with their setting. In hindsight, I wonder if the passages which the screws push into could be clogged.

The gas which came out when I first took the carbs off: The guy before me said he put fresh gas in, and I didn't see anything off with the gas. However there was some gunk caked on to the bottom of the carb bowl which I had to scrub off. I'm sure the gas had ethanol in it, it's hard to find gas without it at least where I am. At least every pump says "May contain up to 10% ethanol."

I'm not nearly the mechanic you are but I'm surprised that the dramatic increase in compression when I ran the wet compression test doesn't seem to indicate anything. The manual doesn't make an exception for if compression increases greatly during the wet test.

Part of me wishes the motor had a little more horsepower just in case I needed it - so your idea of buying the 6HP carb is a good call even if the problem persists with the new carb. Defender Marine has them for $110 but I wouldn't mind buying one from you if you still carry them.

One question: if the problem was the carbs are still gunked up because didn't clean them sufficiently, wouldn't the starting spray have let the motor fire up until the spray was consumed? The spray was kind of awkward to try to spray in through the intake screen, a bunch of it pooled up and dripped off the intake but I figure some should have gotten in. Since I did not detect any change in the motor trying to start, between that and the compression test results I thought for sure the fuel wasn't the problem.

Thanks much,
'Dubs
 
I assume you kept the stop clip engaged while testing with the ether. It can be tricky to use that fluid; too much and it won't fire. Without actually having the motor here, my best guess based on what you have gone through is still a carb issue. I would also get two new carb base gaskets, one for each side of the plastic insulator.

The small combustion chamber is made even smaller by adding oil; That, and the extra sealing on the rings and valves will increase compression significantly.

When dipping the carb, if the cover plug for the pilot screw has already been removed, you can count (and note) the number of turns until it seats, then remove it. It has a small rubber o-ring that might get softened in carb dip. When reinstalling, return the screw to the same setting. The idle stop screw will be fine in dip. A 1-gallon can of dip is about $24. I use it in a heated ultrasonic cleaner.

If you didn't dip the carb, and THEN blow it out with acetone-based cleaner, expecially since you found that crud in the bowl, it's likely still stopped up.
 
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