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  Topic Review (Newest First)
08-04-2012 08:26 AM
Re: tohatsu 3.5 won't run without choking

You open the manual and start running the tests on the ignition system or you can randomly replace parts hoping that the shotgun approach will yield a positive result.
08-03-2012 08:28 PM
tohatsu 3.5 won't run without choking

I have a Tohatsu 3.5 as well. I have an intermittent problem driving me nuts. Most of the time it starts reasonably easily. Once started it runs fine But once in a while it is dead as a door nail. Yes the fuel **** is open, the fuel tank vent is open, the starter kill switch is in place the choke is pulled. I am inclined to think it is electrical, but have been unsuccessful in finding the problem. Any suggestions?
08-02-2012 06:24 AM
Re: tohatsu 3.5 won't run without choking

Originally Posted by Sea Dawg View Post
Agreed, Seloc has a way with words to help you understand your motor. I would recommend it over any others.
The Factory service manual (either the Tohatsu or Mercury) is head-and-shoulders above the Seloc, generic, one-manual-fits-all, publication. For one thing, it explains how to disassemble the carb, which so far has not been done.
08-02-2012 06:21 AM
Re: tohatsu 3.5 won't run without choking

You did not adequately clean the carb. Carb spray itself it not aggressive enough to dissolve the organic varnishing in the low-speed passages. You must completely disassemble it (including jets, emulsion tube nozzle, and of course the needle and float). Then submerge in real carb dip for 4 hours at room temperature. THEN blow it out with generic carb spray, and reassemble, paying close attention to the float height.

You did not need to adjust the pilot mixture screw. But, since you did... It should not bottom out the spring; it should Gently seat in a delicate brass jet in the carb. The correct initial setting is about 1-3/4 turn out, and once the motor is running OK and fully warmed, you would then adjust it as on any other carb on any other motor. If you are not familiar with that procedure, or don't have a very good ear and a razor-accurate shop tach, get assistance from someone who does. Yes, you must pull the fuel tank bolts to make that adjustment... But since you are "never" supposed to change it, that's not a big deal.

There is in fact an inline filter screen between the fuel **** and the tank, which should be inspected.

08-01-2012 07:33 PM
Sea Dawg
Re: tohatsu 3.5 won't run without choking

Originally Posted by Sapperwhite View Post
Seloc Marine has a book on about every engine out there.
Agreed, Seloc has a way with words to help you understand your motor. I would recommend it over any others.

If choking it keeps it running it does sound starved for fuel. Float valve can stick or stuff can block flow even if the float and valve operate correctly. I had a head gasket leaking water into the combustion chamber on a 2 HP older motor and with choke on it would at least run. The evidence was in removing the spark plug and shining light on the piston to observe tiny droplets on the piston. Very little water, but gasoline will generally not leave this evidence so I replaced head gasket. The gasket gave me even better indication where the leak was.

Lastly, if the float or valve were not filling the carb bowl with gas due to sticking would expect a shot of starting fluid while it falters would bring up the rpm as the fuel shortage would be remedied and the intake path wouldn't be blocked. Hope this helps and let us know what you find.
08-01-2012 06:29 PM
Re: tohatsu 3.5 won't run without choking

I'm guessing you just pluck it out? If so, how do you get it back in? I'll dump the fuel and take a look. Thanks.
08-01-2012 05:30 PM
Re: tohatsu 3.5 won't run without choking

I had a Honda 2HP. It had a very small fine mesh filter inside the fuel line just at the tank outlet. You can't see it from the outside. Yours may be similar?

Paul T
08-01-2012 03:40 PM
Re: tohatsu 3.5 won't run without choking

Okay thanks. Well I changed the plug, cleaned out the tank, cleaned out the carb, cleaned out the petcock, cleaned out the lines, cleaned the air screen, blew out and cleaned the air lines that run to the bottom of the engine compartment and put fresh premium fuel in. I wasn't sure how many turns to put on the one screw in the back of the carb, so I screwed it all the way in until the spring bottomed out, and then turned it out 2-3/4 turns, which from memory, should be about right. I put the tank back on and now I can't get to the adjustment screw. Seems like a dumb design but oh well.

I started it right up and it actually idled great with the choke off. Problem is that it still will not run once you throttle up, unless you choke it about half way. I opened the fuel cap to see if that helped. Same deal. This is what it was doing before. Has anyone figured this out? I was pretty meticulous about cleaning out all parts of the carb and jets, although I could not figure out how to get the float off without breaking the clip that holds it on. I did my best to spray that out and confirmed cleaner was squirting out another part of the carb.

I got this motor as a package with a sailboat, and it has never worked right. This is the second time I've tried cleaning it. Oh and I recently read about an inline filter. That is one thing I did not find. Can anyone tell where that is and how to take it out, clean or replace it?

Any other advice here?
08-01-2012 06:19 AM
Re: tohatsu 3.5 won't run without choking

Nope. The 3.5 4-st is the "big brother" of the 2.5, and is already upgraded.
07-31-2012 09:08 PM
Re: Difference btw M3.5B and MFS4

Originally Posted by pvanv1 View Post

The OP was talking about the old 3.5B, with slide type throttle. Yours is an entirely different animal. We clean them all the time (few a week) in my shop. Must be done in carb dip. Period. Better yet, if your carb was abused by letting gas sit in it too long... for your MFS4... would be to install a new carb for the MFS6B (identical powerheads), and clean up the old carb as a spare. 50% more hp, too. We do it all the time.
Sorry to dig up an old thread, but I have a 2008 Mercury 3.5HP 4 stroke. Will this carb (MFS4) fit that as well? Thanks!
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