What's wrong with my new 6hp Tohatsu?

[24x7fisherman]

I bought it new about 3 years ago and started to use it last year. It has only about 10 hours of usage. I could not start it the first try. Cleaning it the carburetor has made it easier to start it. Sometimes it is easy to start it and sometimes it takes forever to start it. And sometimes it only starts with throttle wide open. Once it starts, it runs flawless. Yesterday it quit when it was running at low speed, I could not start it after that. I waited for about 20 minutes to restart it again. I think I might have fuel flooding problem. I did a compression test today, the reading is very odd like 5 psi. My cheap tester might not give the correct reading. Is it possible for a new motor to have such a low reading? and it runs very good once it starts. I will borrow another compression tester to try it again. Any input will be really appreciated.

 

[MattFL]

When it stalls or won't start hot, pop the fuel cap off (if using the built-in tank) to let some air in, or give the bulb a squeeze if you're using a remote tank. See if that helps. One day I had a similar issue, while trolling very slow the motor started wanting to stall and then would not start, as if it was running out of gas. Revving it and running it hard for a minute seemed to resolve it so I chalked it up to a fuel delivery issue or water in the fuel. The issue hasn't returned.

For cold starting, maybe this will help:

 

[24x7fisherman]

When it stalls or won't start hot, pop the fuel cap off (if using the built-in tank) to let some air in, or give the bulb a squeeze if you're using a remote tank. See if that helps. One day I had a similar issue, while trolling very slow the motor started wanting to stall and then would not start, as if it was running out of gas. Revving it and running it hard for a minute seemed to resolve it so I chalked it up to a fuel delivery issue or water in the fuel. The issue hasn't returned.

For cold starting, maybe this will help:

Thanks, the fuel cap was open. I might have bought a lemon, didn't know much about the outboard motor. I borrowed two compression testers from Autozone and Oreily to double check it, the reading is very low like 15 psi. I will do a leak down test to find out where the leaking is coming from. I just could not believe this could happen to a new motor. I thought it could be a problem with the carburetor or the fuel system like a bad fuel pump or fuel blockage.

 

[MattFL]

Check to see if it has an automatic compression release to make starting easier, common on lots of 4-stroke motors. If it does then you'll need to disable that before any compression or leak down test. The most common implementation is a small spring and weight mechanism on the camshaft. When the cam is moving very slow (pull start speed) an actuator will prevent one of the valves from closing fully to reduce compression. Once the cam hits a certain speed, the weights fling out, moving the actuator and allowing the valve to close completely. BUT along those lines, do check the valve clearances. Valves that are tighter than spec can lead to difficult starting. Also be sure you pull the rope like you mean it, my 6hp starts reliably but does require a good pull.

 

[24x7fisherman]

Check to see if it has an automatic compression release to make starting easier, common on lots of 4-stroke motors. If it does then you'll need to disable that before any compression or leak down test. The most common implementation is a small spring and weight mechanism on the camshaft. When the cam is moving very slow (pull start speed) an actuator will prevent one of the valves from closing fully to reduce compression. Once the cam hits a certain speed, the weights fling out, moving the actuator and allowing the valve to close completely. BUT along those lines, do check the valve clearances. Valves that are tighter than spec can lead to difficult starting. Also be sure you pull the rope like you mean it, my 6hp starts reliably but does require a good pull.

The Tohatsu's service manual has some detailed instructions of compression test which doesn't mention the automatic compression release. I guess since it has single cylinder so it's safe to assume it doesn't have it.

 

[racerone]

It may very well have a compression release on the camshaft.----Check valve clearance and valve operation.----Or hurry to the dealer where you bought it.----Should be able to sort it out.

 

[MattFL]

The Tohatsu's service manual has some detailed instructions of compression test which doesn't mention the automatic compression release. I guess since it has single cylinder so it's safe to assume it doesn't have it.

Many single cylinder motors have it. My lawn mower has it, my motorcycle. Glancing briefly at the parts diagram for the Tohatsu, the drawing of the cam sprocket looks like it could possibly have the little weights on it, it's difficult to tell from the drawing. Maybe you can find some pictures of the motor disassembled. Pull starting my own 6hp Tohatsu (bought in 2012) it feels as if it does have one. Although you would think the procedure in the service manual should be accurate.

 

[24x7fisherman]

Many single cylinder motors have it. My lawn mower has it, my motorcycle. Glancing briefly at the parts diagram for the Tohatsu, the drawing of the cam sprocket looks like it could possibly have the little weights on it, it's difficult to tell from the drawing. Maybe you can find some pictures of the motor disassembled. Pull starting my own 6hp Tohatsu (bought in 2012) it feels as if it does have one. Although you would think the procedure in the service manual should be accurate.

Please take a look at 2:06 of this video

I believe it looks like the same one. Now if it does have the automatic compression release, then how can I do the compression test? Taken
from the Tohatsu service manual, those are simple steps. It does mention that removing Exhaust rocker arm will have a higher psi reading.

 

[MattFL]

Yes the instructions give you both pressures, with and without the compression release. You should be looking for right about 49psi with the compression release active. Open the throttle all the way, make sure the choke is turned off and pull the rope like you mean it. If you get 15psi as you mentioned earlier, then this is significantly below spec, so the next step is either to see if you can adjust the compression release and/or check the valve clearances; or remove the exhaust rocker so the exhaust valve does not open and see if you're close to the 138psi they specify for that case. If the pressure is too low both with and without the compression release then you've got a sealing issue somewhere (rings, valves). If the pressure is too low with the compression release active (exhaust rocker installed) but is OK with the exhaust rocker removed then your compression release or exhaust valve clearance needs adjustment.

 

[pvanv]

Yes, the MFS 4/5/6 has decompression.

 

[km1125]

I do believe the service manual calls for an inspection and adjust of the valves after the first 20 hours or one month, so you might just need to do that. As long as you have that cover off though, you may as well perform the complete compression test as you'll have direct access to the exhaust rocker.

Those are great little motors. You shouldn't have any trouble with it starting when things are correct.

 

[24x7fisherman]

I do believe the service manual calls for an inspection and adjust of the valves after the first 20 hours or one month, so you might just need to do that. As long as you have that cover off though, you may as well perform the complete compression test as you'll have direct access to the exhaust rocker.

Those are great little motors. You shouldn't have any trouble with it starting when things are correct.

Thanks, if the exhaust rocker is removed, then will the valve spring be kept in place? I am aslo having trouble to find the flywheel “T” mark and cylinder mark by the service manual, please kindly let me how to locate them. It's so confusing for a newbie.

 

[24x7fisherman]

Yes the instructions give you both pressures, with and without the compression release. You should be looking for right about 49psi with the compression release active. Open the throttle all the way, make sure the choke is turned off and pull the rope like you mean it. If you get 15psi as you mentioned earlier, then this is significantly below spec, so the next step is either to see if you can adjust the compression release and/or check the valve clearances; or remove the exhaust rocker so the exhaust valve does not open and see if you're close to the 138psi they specify for that case. If the pressure is too low both with and without the compression release then you've got a sealing issue somewhere (rings, valves). If the pressure is too low with the compression release active (exhaust rocker installed) but is OK with the exhaust rocker removed then your compression release or exhaust valve clearance needs adjustment.

Thanks my friend, I am determined to find the answer for the problem.

 

[MattFL]

There is a keeper that keeps the valve spring on when the rocker is removed. If the spring comes off with the rocker then something is wrong. You may have to remove the pull start assembly to see the top of the flywheel and the marks that you're looking for. Before you go taking the rocker out, first check the exhaust valve clearance. If it's wrong then fix that first and see if that resolves the problem.

 

[km1125]

You don't really need to look for the "T" on the flywheel, just look for the timing marks on the side. I believe there are two, one for 5BTDC and one for 25BTDC. Either one will do for these purposes, because you just need to know that both valves are closed.

 

[24x7fisherman]

I found the "T" mark on the flywheel. Removed the cylinder head cover, measured the clearances of the intake and exhaust valves, they are just so tight that the clearance is thinner than 0.0024 inch (0.06mm). So I adjusted them by the manual, did the compression test, the reading is 75psi. but it's supposed to be 49.3 psi. I believe I have done something wrong. Now it has become very difficult to pull the start rope and could not start the motor. It feels like the piston refuses to move by creating a vacuum. Now the problem has been changed from not enough compression to too much compression. I need to continue to work on it to find a way to start the motor. Thanks for all your help.

 

[MattFL]

It sounds like you found your problem (valves out of adjustment). Double check your work, the exhaust may be too loose now preventing the automatic compression release from working. Pick a feeler gauge (or stack of gauges) in the middle of the acceptable range, it should be a very snug fit in-between the valve and rocker. If it just slips in and out super easy then the gap is probably too big. Edit --> Before you open it up again, pull the rope a few times to be sure the cam has gone around at least once or twice and see if that fixes the problem. The compression release may not be triggered, but the motor spinning over once or twice should allow it to work.

 

[pvanv]

Roughly 50 psi with decompression; about 140 without. Cold valve lash should be 0024 to 0055 intake, 0043 to 0075 exhaust. There is no way that you have too much compression. The piston does not create a vacuum, aside from pulling the air/fuel mix in. Your starting issue is likely a carburetor issue, such as a needle not always seating correctly. Since you bought it new, I very much doubt the lash is off. I have checked a lot of these, but never had to adjust any under 1000 hours. You are in warranty, so as long as it's not the fault of crud keeping the carb dripping, your dealer should be able to help you.

 

[24x7fisherman]

Roughly 50 psi with decompression; about 140 without. Cold valve lash should be 0024 to 0055 intake, 0043 to 0075 exhaust. There is no way that you have too much compression. The piston does not create a vacuum, aside from pulling the air/fuel mix in. Your starting issue is likely a carburetor issue, such as a needle not always seating correctly. Since you bought it new, I very much doubt the lash is off. I have checked a lot of these, but never had to adjust any under 1000 hours. You are in warranty, so as long as it's not the fault of crud keeping the dripping, your dealer should be able to help you.

Thanks Paul! It looks like I made a mistake by adjusting the valve clearance on the exhaust stroke instead of compression stroke. I have cleaned the carburetor multiple times which didn't solve my problem. Could you be more specific about the needle not seating correctly? Which needle? I just did the compression test, now it's 20 psi, it's low. But the engine is cold and my cheap tester is about 10 psi off. After I add some engine oil to the cylinder, it's 25 psi. Does that mean there is something wrong with the rings or piston? I did the leak down test as well, the difference is less than 5%.

I bought the motor from an online vendor 3 years ago and it had been stored in my garage for two years, and I was lazy did not register it. Called Tohatsu USA, they told me no warranty. I was not lucky when I open the box trying to start it the first time. Cleaning the carburetor has made it easier to start it. But it has never made me feel comfortable on the water, I always worry about not being able to start it again.

 

[24x7fisherman]

Just started the engine with 4 hard pulls. Let it run for 5 minutes then turned if off. Try to restart it, after more than 15 hard pulls, I gave up on it.