Arwen's meanderings

Hi everyone and welcome to my dinghy cruising blog about my John Welsford designed 'navigator' named Arwen. Built over three years, Arwen was launched in August 2007. She is a standing lug yawl 14' 6" in length. This blog records our dinghy cruising voyages together around the coastal waters of SW England.
Arwen has an associated YouTube channel so visit to find our most recent cruises and click subscribe.
On this blog you will find posts about dinghy cruising locations, accounts of our voyages, maintenance tips and 'How to's' ranging from rigging standing lug sails and building galley boxes to using 'anchor buddies' and creating 'pilotage notes'. I hope you find something that inspires you to get out on the water in your boat. Drop us a comment and happy sailing.
Steve and Arwen

Thursday 14 May 2020

Taking out and cleaning the carburettor from a Tohatsu four stroke 3.5 hp outboard engine

Below you will find information to help you, should you decide you want to give it a go.  There are photographs from my strip-down along with video material. Then there are two excellent blog sites where someone shows how to take the carburettor out and then how to strip it down. Between my material and these external blog sites, you should have everything you need to know. 

Taking the carburettor out of the engine compartment

The external website that shows this procedure really well is found at 

Below is my lesser contribution and at the very end is a short video of the process as well to help you visualise what is involved at a better level.

Firstly the video: 

This is at my associated YouTube channel where you will find a playlist with the other videos in my 'servicing my outboard' series. 

Step One - remove the cowling, pull out the kill cord, have the engine in neutral. Use a 10 mm socket to unscrew the three bolts that attach the petrol tank to its plate and the engine block. Lift the tank up carefully and move it to the rear of the engine compartment and let it rest there. Watch that the brass collars in the petrol tank bolt holes don't fall out an get lost!

Step two: drain the carburettor of any fuel and likewise with the fuel pipe. To do this, place a large wad of paper towel beneath the carb and open up the drain screw so that fuel trickles out onto the paper towel. the drain screw is the bottom screw seen in the centre of the lower carb in photo above. When the fuel is drained, gently pull off the fuel hose to expose the brass pipe above the drain screw - let the fuel trickle out of this pipe. Then tuck it down out of the way. 

Step three: pull the thick black hose gently off the top of the carburettor and tuck it to one side out of the way.

Step four: Loosen that silver hex bolt on the far right hand side of this photograph and carefully lift it and the plate it holds down, out of the way. Watch out because the choke cable beneath it will suddenly spring up - so be prepared for that. You can then wriggle the silver wire out of the hole on the black plastic plate. Take plenty of photos as you go so that you know how to put things back together again afterwards. It should now look like the photograph below. 

Step five: there are a couple of other black tubes that lead from the carburettor down underneath it. These tubes exit the engine compartment beneath. You can push these tube gently back up through their holes from below, ready to gently pull out in a few minutes.

Step six: it is now time to remove the two bolts that hold the carb in place. Undo these carefully. The front air filter type unit will suddenly come loose. When you fully extract the bolts, there is a nut within this unit that may suddenly fall out - so be prepared for it. Similarly, be prepared that there are three gaskets at the other end of the carb where it joins the intake manifold pipe that may also come loose and suddenly drop into the bottom of the engine compartment. 

Step seven: in the photograph above you can see a long thick wire on the top left hand side which is bent in a couple of places and sits in a plastic plate on top of the rear end of the carb. This is the throttle control. Now that you have the carb loose in your hand, you will need to twist it almost through 90 degrees to get that wire out of its hole. At the same time, don'f forget to pull the remaining two tubes out of their holes in the base of the compartment.  If you have been successful - you should now have the carburettor in your hand and the engine compartment should be looking something like these photos below

Cleaning the carburettor

Again, Green Panther did a really good summary of how to clean the carb. I was not as successful as he/she was in cleaning mine but I put my contribution below. 

You should now have the carb in your hand. I put any loose bolts in an old baking tray for safe keeping. I also put the carb in another old baking tray where I then worked on it. In this way, anything that suddenly sprung out, or fell out, was caught by the curved sides of the tray. 

Step one: I just had a good look around my unit and took photographs from all sides and angles of how the external parts were arranged. I also referred to the parts catalogue for the engine - distributed by Tohatsu so that I could get clear in my head what was what.  The link to this catalogue is below: 

I cannot stress enough that this is the parts catalogue for my engine - a 2011 model. you will need to check the age and what catalogue you need for your specific engine. 

(I also have a copy of the service manual for my outboard, but I can't find the pages any more of where I found it and an internet search has proven fruitless). 

Step two: turn the carburettor upside down and you will see the square base with two screws securing it. Undo and remove the screws and carefully lift the square base off the carburettor. You should now see what is in the photo below. I soaked the bottom of the square base interior in carburettor spray cleaner for 40 minutes or so. It came up really clean. 

This was the staining of the bottom of the float bowl before carb spray cleaning. I used some Q tip cotton bud thingies to remove stubborn stains.

Step three: Warning - don't lose that rubber bung. Mine fell out easily and if you lose it - the carburettor will keep flooding (as I understand it). The central brass tube with the hole and the screwdriver slot is the main jet. You can unscrew this. Mine refused to budge, even after soaking for some time in carb spray. It is brass and you need to get the correctly fitting flat tip screwdriver blade. I almost stripped mine trying to get it out and that is why I gave up. Green Panthers site gives a much better description of how to clean this jet - remember the link is above. 

Step four: I removed the black float seen in the photograph above this one. I wanted to get at the area below it to clean it out. You need to pull/tap out the pin that holds the float in place. Don't tap it all the way out and be careful - because there is a little flaot device that hangs off it and this regulates the amount of fuel going into the carb - do not let it drop off - take plenty of photos. 

Step five: the slow air jet is the hole on the right hand side of this open chamber and again you need a flat tipped screwdriver blade of the correct size to slide down the hole to remove the jet. Mine refused to budge but I managed to get carb spray down there and I then let it drain out. I'm hoping this has done the job. if it hasn't then I will be taking the carb out again, ordering spare jets and will be taking it all apart again!!

Again, I would refer you to GreenPanthers site as he/she gives a far better breakdown of what was done.  Below is the parts diagram for the carburettor to help you. 

Reassembly was the exact reverse of taking it out. Use those photographs you took to help you reassemble it.  

Tohatsu MFS3.5B carburetor diagram parts

Now, a very public health warning! Learn from my mistakes!

I put everything back in with no problem at all. I was able to reuse the original gaskets because I was careful in the way I disassembled the carburettor from the manifold. However, I made a schoolboy error on the the very last action of putting it back in. Posts previous to this detail that mistake. However, simply put, my very last action was to give one last half turn to one of the bolts securing the carb........and it sheared. The right hand bolt screws into a blind hole and I had over-tightened it by a half turn. 

This was the result!

This disaster meant I had to take off the manifold and send it to a kind gent down in Cornwall who offered to drill out the offending broken bolt stub using specialist equipment.  I detail how I removed the manifold, which is straight forward enough, as long as you pay due care and attention and take lots of photographs in a previous post which can be accessed here:

And that's it. I hope this helps. Don't be put off. GreenPanther proves that removing, cleaning and reinserting the carburettor is straight forward as long as you take your time, take plenty of photographs throughout the process and have the correct tools and spares ready to hand. 

Good luck, have fun, learn loads and most importantly, enjoy. 

A reminder - the external sites of GreenPanther are found at:

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