Tohatsu 3.5A Carb

[bengtlarsson]

A week ago I bought a 2006 Tohatsu 3.5hp 4 cycle. The previous owner had bought and installed a new carburetor, the receipts was included, so I assumed it should not take too much time to get the motor to run well. I have spent several days assembling and disassembling the new carburetor. It has a tendency to flood, regardless of how I adjust the float level. The motor starts but needs choke to run and will die anywhere near idle.
The new carburetor is marked: 3BRBBD7 whereas the old is marked 3BRAUC7.
I have moved floats and float needles between them, adjusted the float level and checked that the float needle is closing by turning it upside down and blowing into the fuel inlet - the needle is closed. After installation on the engine the results are the same - flooding within a minute.
The carburetors looks to be identical but my main question is: Will this "B" type carburetor work in this application? I am out of ideas anything that I should try?

Thank you,

Ben

 

[pvanv]

Re: Tohatsu 3.5A Carb

There isn't anything significant about year of manufacture for servicing any Tohatsu/Nissan outboard. Exact model/serial is everything. I'm assuming you have an MFS3.5A, right? The current carburetor for your motor is part number 3GV032000M, which is the carb that comes on the 3.5B. That carb is the proper carb for any 4-stroke 3.5... both the MFS3.5A and the MFS3.5B.

If you need choke, you are not flooding... you are leaning out. There is no significant tolerance to the float height setting; it's either spot-on, or it is wrong. The float should be parallel to the carb casting when the carb is inverted. Sounds like you have confirmed that the needle valve is seating correctly, so if the float height is correct, and fuel is not spilling from the carb, you are not flooding.

What your symptoms indicate are a classic case of varnishing in the low-speed passages. That is caused by deposits from the fuel being left to evaporate from the carb. This will happen with any pump gas that we get in the US.

The proper way to correct that issue is by a thorough and proper carb cleaning: Remove all bits from the carb (don't forget the slow speed air bleed jet, main jet, and emulsion tube "nozzle"). Set any rubber bits (such as the float needle valve) aside. Then soak everything in real carb dip (available in 1-gal cans at auto parts houses) for 4 hours at room temperature. Any less, and the varnish will not be softened adequately. Much longer, and you risk etching the aluminum castings. Then blow out all passages with generic carb spray. Don't be stingy with the spray -- you need to flush any varnishing out of the tiny, almost microscopic passages. Then reassemble the carb, paying particular attention to proper float height. While at it, verify that the fuel filter (in the line before the fuel ****) is clear.

In the future, always, Always, ALWAYS run the carb out at the end of each and every day, to reduce varnish build-up.

Also make double-sure that your oil is not too full. Not even a drop. Half-way between marks on the sight glass is fine. Use only 10w-30, non-synthetic, FC-W rated oil.

 

[bengtlarsson]

Re: Tohatsu 3.5A Carb

Paul,

Thank you for your fast and thourough response.

Yes this is a 3.5A model. The carburetor was delivered on May 18 of this year and have had no gas in it more than for these tests. The fuel line coming in to the fuel **** has been redone and is not standard. I can find no fuel filter in the tank or in this line. I did flush out the tank and carefully filtered the gas that I poured into it.
My plan is to get the engine to run correctly on the stand and see what other parts I will need before I place one order, including the original gas line, new water pump kit etc, so this fuel line will be replaced including the filter.

The carb is flooding, spilling out fuel from the venturi, which must be related to the float level adjustment, and I agree that it does not make sense that once the engine start it needs various amout of choke to keep it running. Have you ever heard of a defective new carb.?
Thanks to your confirmation that this carb should work on this engine I am motivated to get on it again and solve this mystery.

Regards,

Ben

 

[TOHATSU GURU]

Re: Tohatsu 3.5A Carb

I've never seen a defective new carb. It is possible that the fuel line from the tank to the carb has been improperly routed. But, experience says it's just a dirty carb that is not being cleaned correctly.

 

[bengtlarsson]

Re: Tohatsu 3.5A Carb

Thank you for your comments.

Today I finally found the problem and it may help other folks that think they have checked out everything. One of the two small rubber tubes (1/4" OD), that is connected to the top of the carb and let air out was clogged up airtight! I never paid attention to those as they where simply hooked up for the test run.
After blowing them out with compressed air and adjusting the float level the motor now has a stable idle and seems to behave well. The final step is to lake test it. I live in Phoenix so it will be another Month before the temperature has dropped enough for a boat trip.

Regards,

Ben

 

[TOHATSU GURU]

Re: Tohatsu 3.5A Carb

That would be a weird one.

 

[ellie2]

Re: Tohatsu 3.5A Carb

Am about to do a carb clean. Friend came over a few nights ago and was messing with motor but I wasn't out there. There is one rubber tube running from the top of the carb and appearing to go nowhere, just dangling. Any idea if this is supposed to attach to something? When I look at the blown out diagram it doesn't give me any idea. Am thinking this is the same tube you are talking about in below post for air intake....

any help appreciated!

Thank you for your comments.

Today I finally found the problem and it may help other folks that think they have checked out everything. One of the two small rubber tubes (1/4" OD), that is connected to the top of the carb and let air out was clogged up airtight! I never paid attention to those as they where simply hooked up for the test run.
After blowing them out with compressed air and adjusting the float level the motor now has a stable idle and seems to behave well. The final step is to lake test it. I live in Phoenix so it will be another Month before the temperature has dropped enough for a boat trip.

Regards,

Ben

 

[pvanv]

Re: Tohatsu 3.5A Carb

Am about to do a carb clean. Friend came over a few nights ago and was messing with motor but I wasn't out there. There is one rubber tube running from the top of the carb and appearing to go nowhere, just dangling. Any idea if this is supposed to attach to something? When I look at the blown out diagram it doesn't give me any idea. Am thinking this is the same tube you are talking about in below post for air intake....

any help appreciated!

It's a vent. Typically the tube is routed through the lower cowl, to the outside.

 

[ellie2]

Re: Tohatsu 3.5A Carb

Great. Found the little hole to stick it down into. Carb is now cleaned and back together. Thanks!

It's a vent. Typically the tube is routed through the lower cowl, to the outside.

 

[JStrom]

Re: Tohatsu 3.5A Carb

It seems to me that the moral to this story as with many others, don't get any particular (SINGLE) idea about what's wrong and refuse to deviate from that. In other words, don't tear down an engine as a first step. You might just be out of gas.

 

[LudwigO]

Re: Tohatsu 3.5A Carb

It seems to me that the moral to this story as with many others, don't get any particular (SINGLE) idea about what's wrong and refuse to deviate from that. In other words, don't tear down an engine as a first step. You might just be out of gas.

Nah, never happened to me once ... or has it? Perhaps even twice ! But that was with an old Fergie tractor.. :facepalm::joyous:

 

[alantunney]

Re: Tohatsu 3.5A Carb

Captain, I have the exact same engine. Mine is flooding. it starts and doesn't do better than idle at full throttle. If I pull the choke it stalls immediately. and if I drain the carb while running it will go up to full throttle just before it runs out of gas. any ideas, thanks, Alan

 

[pvanv]

Re: Tohatsu 3.5A Carb

Odds are the needle/seat/float is not closing properly. Remove the carb and check. A good cleaning won't hurt. If necessary, install a carb kit while in there.

 

[ellie2]

Re: Tohatsu 3.5A Carb

Update: so even after cleaning my carb it still wouldn't start. Replaced spark plugs, cleaned out all lines, cleaned out fuel tank, did a carb kit, messed around with seafoam etc. Finally bought a new carb and installed and it started up right away. Go figure. Wasted 2 months of messing around with it, lol.

 

[JStrom]

Re: Tohatsu 3.5A Carb

Update: so even after cleaning my carb it still wouldn't start. Replaced spark plugs, cleaned out all lines, cleaned out fuel tank, did a carb kit, messed around with seafoam etc. Finally bought a new carb and installed and it started up right away. Go figure. Wasted 2 months of messing around with it, lol.

I've been told by two different Nissan Distributors that there are no such things as "carb kits" for these motors anymore, that you simply have to order the parts individually from the ones that are still available. What are the kits that people are mentioning?

 

[Sea Rider]

Re: Tohatsu 3.5A Carb

Gee, it's a 4 stroke, not 2. Soooorry, different horses...

Happy Boating

 

[pvanv]

Re: Tohatsu 3.5A Carb

Not true.
The NSF/MFS3.5A carb kits are:
CARBURETOR REPAIR KIT: 2HP ~ 3.5HP 4-STROKE
Description
Carburetor repair kit. Kit includes gaskets, float & needle valve.
Fits:
2hp, 2.5hp & 3.5hp 4-stroke models.
Part # 3AB871220M
$30.42

The OP referenced the 4-stroke 3.5.

 

[JStrom]

Re: Tohatsu 3.5A Carb

Gee, it's a 4 stroke, not 2. Soooorry, different horses...

Happy Boating

I see that now. Not sure why some people don't understand how it wasn't evident that not everyone picked up on the different engines. Not true? really? I have a 2 stroke 3 1/2 hp model and THAT is what I was referencing. No carb kits available.

 

[Sea Rider]

Re: Tohatsu 3.5A Carb

If it's a 3.5A model, it's an old horse, there must be some spare parts left overs while stock lasts. Plenty for same 3.5A2-B2 which are the last updated verions of the A model. Complete kits or spare parts are available too. Lucky for Ellie 2, to find a new carb for an older model, has saved his summer.

Happy Boating

 

[pvanv]

Re: Tohatsu 3.5A Carb

I see that now. Not sure why some people don't understand how it wasn't evident that not everyone picked up on the different engines. Not true? really? I have a 2 stroke 3 1/2 hp model and THAT is what I was referencing. No carb kits available.

Incorrect.
CARBURETOR REPAIR KIT: 2.5/3.5A2 & 3.5B 2-STROKE
Description
Carburetor repair kit. Kit includes gaskets, float & needle valve.
Fits:
2.5A2, 3.5A2 & 3.5B 2-stroke models.
Part # 3F0871221M, $28.33

This is available from any dealer... Including several forum members... including me