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tohatsu 3.5 won't run without choking

35K views 64 replies 29 participants last post by  TohatsuGuru 
#1 ·
Hi,

I have a 2002 3.5B2 3.5HP 2cycle that I purchased used last year. Worked great for 2 outings. Died the 3rd.

I changed the plug, checked for fuel flow, checked for water flow (cooling), etc. New gas, new oil. 50:1.

It starts great with the choke on. Whenever I move the throttle up or completely turn off the choke, it dies. Sounds to me like a fuel flow problem, but the line seems to be fine into the carb. Carb spray did nothing. My guess is that I need to get a rebuild kit for the carb, but before I start looking for one and go to the trouble, I wanted to ask for some friendly advice.

Thanks!
Johnny
 
#28 ·
That's not a great idea. Most carburetor cleaners will dissolve the rubber and plastic components of the average carburetor. The only safe method is to remove the carb off of the engine, take it apart, dip the metal parts in carb cleaner for at least three hours, wash off, blow dry and re-assemble.
 
#29 ·
I have a Johnson 3.5 internal tank on my 525, always shut the gas off, run the gas out of the carb, raise it and tilt it , place the sunbrella cover over the engine. I've done that for over 12 yrs. It starts on the third pull and runs strong. Try to find 100% gas, we just have a few stations that still sell it in our area. Only problem to date, watch out for the mub dobbers, they will make a nest in the strangest places like air cleaners or water exit tubes...
 
#30 ·
Mentioned elsewhere in this thread (and Sailnet) is the fact that alcohol (the 15% of E85) eats rubber. (try this experiment at home - pour some isopropyl alcohol into a dish, put a peice of black rubber - say a peice of an old windshield washer into the dish, let stand for 12 hours. Do you see any change in the rubber?)

The problem with this motor is most likeley that some rubber in the fuel supply (gasket, fuel or vacuum line) has partially dissolved and is either gumming up the carb, or causing a vacuum leak.

The FIX is to identify the damaged rubber component(s) and REPLACE it(them) - preferably with a like component that is not affected by alcohol. Rub a Q-Tip over suspect parts. If the cotton turns black, or gets little gooey black blobs on it, then you've found a problem. Then clean the rest of the fuel system (needle valve seats, venturi, jets, etc.).

The best preventitive measure is to run the motor dry, but then you may have to worry about corrosion. If this is a concern, then spray a can of motor fog into the carb before storing it.
 
#32 ·
Resurrected thread. Found that new carb for 4hp Tohatsu is $108. Rebuild about 40-50. I haven't heard anybody point out that the Tohatsu/Nissan is almost impossible to throughly clean. The choke butterfly, carb intake butterfly linkage and choke linkage are plastic and I have not been able to remove them from the carb body prior to soaking the carb in cleaner (and I'm not sure it won't melt in carb cleaner). Factory manual is weakly written for 'how to' but does give assembly order and specifications. The idle jet is under a brass plug and can be difficult to remove. The main jet often corrodes in its' threads and the brass screwdriver slot can give way before the jet releases. If. any reader has solutions to the above I'd sure like to hear them. I hate to replace the carb every few years and fortunately I've just found a source for alcohol free gasoline a couple of blocks from my house which may extend the time between cleanings.
 
#33 ·
Difference btw M3.5B and MFS4

Diamond,

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.
 
#56 ·
Re: Difference btw M3.5B and MFS4

Diamond,

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!
 
#34 ·
pvan1.....do you remove the plastic parts prior to carb dip? I found the manual did not refer to this and the plastic parts don't just 'pop' off. The choke butterfly appears to be press fit in a slot on the intake side and the levers for both throttle and choke are apparently press fit on pins. Is that the way it is??
 
#42 ·
The plastic actuators stay on. The procedure is a full disassembly, including jets and emulsion tube nozzle. All non-rubber/non-gasket stuff goes into the dip basket for 4 hours at room temp. Then an aggressive spray out with generic carb spray. Then careful reassembly, verifying float settings.
 
#44 ·
Negative. The Tohatsu filter is 10 microns. If contaminated or clogged, replace it. If you have a remote tank, a water-separator filter is a good idea. In any event, a baja-type filter funnel is never a bad idea. Start with clean, fresh fuel, and keep it that way.
 
#45 ·
If you can avoid the E gas, cool. If not, be religious in using only clean, fresh fuel. Always run the carb dry at the end of each day, and enjoy many years of clean carb.
 
#39 ·
Berryman's B-12 fuel additive "absorbes" the water and it goes out the exhaust. No affiliation, it just works for me for many years, no fuel related problems during that time. Suggest reading the instructions, it can melt paint and some plastics, good stuff!!, they will probably ban it in California soon, it works that well.

Dabnis
 
#41 ·
Looking for more detail than the factory manual offers in terms of carburetor disasembly if anyone cares to share their experiences. How can you throughly clean the carb body in normal carburetor cleaner dip if you cannot remove the plastic parts first? Do any readers have experience with having removed the idle jet under the sealed brass plug? Don't say "manual" please. The manual setting in front of me is full of specifications (which are needed) but short on techniques (as are most 'factory' manuals).

I did a search on ethanol free gasoline and most posters indicate that anything greater than 10% ethanol content plays havoc with our small engines and there are some who claim that even 10% content is damaging. Hard to quantify this but many local operators of small outboards are aware of the issues of 'old' gasoline. Most don't use ethanol-free gas which I have recently found to be available nearby.
 
#46 ·
Looking for more detail than the factory manual offers in terms of carburetor disasembly if anyone cares to share their experiences. How can you throughly clean the carb body in normal carburetor cleaner dip if you cannot remove the plastic parts first? Do any readers have experience with having removed the idle jet under the sealed brass plug? Don't say "manual" please. The manual setting in front of me is full of specifications (which are needed) but short on techniques (as are most 'factory' manuals).

I did a search on ethanol free gasoline and most posters indicate that anything greater than 10% ethanol content plays havoc with our small engines and there are some who claim that even 10% content is damaging. Hard to quantify this but many local operators of small outboards are aware of the issues of 'old' gasoline. Most don't use ethanol-free gas which I have recently found to be available nearby.
The factory manuals are primarily written for mechanics, so the authors expect you to have some carb repair skills already. It is possible, but not recommended, to remove the EPA-mandated plug to access the pilot screw. Very difficult to re-set that correctly in the field. I have done it in the controlled conditions of the shop, with the aid of precise shop instruments and a very good ear. Don't bother. After a bath in carb dip, and a blow-out with generic carb spray, you should be fine.

No outboards of any make are designed for more than 10% ethanol, nor more than 5% methanol. In addition to "cleaning" out the delivery network before you buy it (the cleaned stuff becomes part of the fuel, yuck), E gas dries a lot of rubbers and absorbs water from the air. Eventually that water can separate, causing corrosion in the lower areas of anything metal in the fuel system.
 
#49 ·
And you could run a motor with a rod knock, as long as you wear ear muffs. But that is not how it was intended to operate.
 
#58 ·
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?
 
#62 ·
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.

HTH
 
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#64 ·
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?
 
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