Tohatsu 4 stroke 4 hp -- Just Won't Idle

[Kip Hansen]

This is an eleven-year old Tohatsu MFS4B 4 stroke 4 hp outboard -- it has seen a lot of service in the Caribbean.

Has has idling problems for years.

What I have done:

1. Cleaned carb thoroughly with spray carb cleaner: (many times over the years): NO JOY

2. Soaked carb in carb cleaner and blown out with carb spray: NO JOY

3. Adjusted throttle cable giving throttle lever full range of motion -- full open to full close. : NO JOY

4. Checked fuel pump -- manually operating fuel pump pumps fuel.

5. Replaced carb with New Carb -- NO JOY


I am out of options -- have tried everything I can think of.

When new, this engine would idle all day so quietly that once it was actually left running for hours without anyone noticing.

Anyone here able to help?

 

[pvanv]

Still sounds like a carb issue. Spray cleaner is not adequate. If the new carb sat with fuel in it for a week, it will need cleaning too.
Verify oil level. Not a drop over full.
You could check timing, but unless the woodruff key sheared, it is likely OK.
When it stalls, is the plug wet or dry?

 

[MattFL]

Check the valve clearances. If they've tightened up and are hanging open then it won't idle well if at all.

Also when you say won't idle, can you elaborate?

 

[Kip Hansen]

Paul ==> The brand new carb would not idle the from the moment it was put on -- I was expecting it to solve the idling problem and, honestly, stunned when it did not.

That leads me to believe there is some other problem -- but I have exhausted by knowledge base of twenty years.

I will check the oil level -- which is at least "enough" but I had not considered "too much" as a possible problem. Would that cause failure to idle? If so, what's the mechanism?

I haven't a clue how to check timing -- although I can see that as a problem that would allow an engine to run at high speed but not idle. I can't find any info on how to check timing on on a simple 1-cyclinder 4 stroke like this. Can you advise?

 

[Kip Hansen]

MattFL ==> When choked and started, the engine runs at high speed from the throttle "start" position. Warming up and pushing in the choke shuts the engine off if the throttle is turned much below the "start" mark -- and engine runs way too fast when it is running -- doesn't slow appreciably.

Valves are a bit beyond my current skill set -- I guess I could handle them if I had a real service manual.

I have to admit I am stumped -- was sure it was a long-term carb problem.

I'm sure this engine has had the prop hit things over the years -- stopping abruptly -- which COULD have partially /sheared the Woodruff Key -- not sure how far into the engine I have to dig to check on that.

Appreciate all the help you can give me.

 

[Kip Hansen]

Paul ==> I will check the plu to see if wet/dry when engine stalls out.

Thanks

 

[Kip Hansen]

Paul ==> Plug is black and "sooty" (maybe oily) when engine quits. When I wipe the plug contacts with a paper towel, it comes away black.

Engine speed seems to be entirely controlled by the choke pull -- pushing in the chokes slows the racing engine (even when throttle is on dead slow) and when pushed all the way in, once engine is warm, engine stops - chug chug dead.

Oil level, engine as level as I can judge, with plug/oil stick screwed all the way in, is between the marks on the little stick.

 

[pvanv]

Of course, activating the choke also increases throttle, so that is expected. If the plug is black and wet, you may be flooding due to choking. I have checked valves on a lot of these, but never had to adjust them ever. Getting a Factory service manual would help. Not an aftermarket manual. Double check that you have both base gaskets and the plastic thermal spacer installed at the carb base, and that they are in good shape. A vacuum leak could cause your symptoms.

 

[Kip Hansen]

Paul ==> One thing -- the replacement carb (the new carb) has two small hose barbs on the top which the original carb does not have. I don't have any hoses to connect to them. Should I seal them in some way? Can they be the cause of a vacuum leak?

 

[Kip Hansen]

Paul ==> Hmmm.... you mention the gaskets . . . according to the exploded parts view, I should have 3 gaskets on the motor side of the carb -- the insulator gasket sandwiched between two regular gaskets. I have only two. I also have two gaskets on the air-cleaner side. I will try making up the gaskets as shown in the diagram. They must have been mixed up some time in the past (there has been a lot of carb removal anmd cleaning over the years).

Re: Service Manual -- do you mean I should avoid the non-official digital manuals, such as

https://www.tradebit.com/filedetail.php/281602922-tohatsu-mfs-4-5-6?a=516802 ???

I see Clymer manuals and SELOC manuals online. The only "genuine: Tohatsu service manual" for sale.is an obvious digital COPY, but it looks official:

https://ebay.to/2WaBVuV

Thanks for your continued help,

 

[pvanv]

Paul ==> One thing -- the replacement carb (the new carb) has two small hose barbs on the top which the original carb does not have. I don't have any hoses to connect to them. Should I seal them in some way? Can they be the cause of a vacuum leak?

Those vents are supposed to have hoses to go out through the lower cowl. This type of carb is for the 6C, so you did not get the 6A2 carb. Don't seal the vents off.

 

[pvanv]

Paul ==> Hmmm.... you mention the gaskets . . . according to the exploded parts view, I should have 3 gaskets on the motor side of the carb -- the insulator gasket sandwiched between two regular gaskets. I have only two. I also have two gaskets on the air-cleaner side. I will try making up the gaskets as shown in the diagram. They must have been mixed up some time in the past (there has been a lot of carb removal anmd cleaning over the years).

Re: Service Manual -- do you mean I should avoid the non-official digital manuals, such as

https://www.tradebit.com/filedetail.php/281602922-tohatsu-mfs-4-5-6?a=516802 ???

I see Clymer manuals and SELOC manuals online. The only "genuine: Tohatsu service manual" for sale.is an obvious digital COPY, but it looks official:

https://ebay.to/2WaBVuV

Thanks for your continued help,

Yes, 2 gaskets and the spacer on the manifold side. None on the spark arrestor side:
That may be an illegal PDF of the FSM. Dunno. [h=5]SERVICE MANUAL - TOHATSU - 4/5/6 B&C 4STR[/h]

Description



Covers 4hp, 5hp & 6hp "B" and "C" 4-stroke models



Part #: 003-21034-3
List Price (MSRP): $41.67

 

[Kip Hansen]

Paul ==> You've done it!

Your advice to: "Double check that you have both base gaskets and the plastic thermal spacer installed at the carb base, and that they are in good shape. A vacuum leak could cause your symptoms."

I am extremely grateful, and very embarrassed, IT WAS THE GASKETS!

or rather the lack of the single second gasket on the manifold side of the carb!

In my "Double Check" -- I checked the Owner's Manual to make sure I had the gaskets in the right order -- and loo and behold, there should the the THREE items on the manifold side (as you say above) and none on the spark arrestor side.

I have no idea how long I (we -- son and I) have put these back on incorrectly after cleaning the carb.


I am astonished that the location of that single gasket makes so much -- all the world -- of difference ---- but it does and now the engine runs perfectly!

Thank you so much!

 

[pvanv]

Glad it worked out...