05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

[vt17]

I've had this motor for ~4 years, it never idled well, but started and ran flawlessly otherwise. I wanted to go to 6HP, so put on a new carb, new fuel filter, and fuel lines between the filter/pump/carb. The motor had been in my basement for about 5 months after last season ended.

I fired it up with the new carb and it started instantly. It ran well and mid-high RPMs, but idled poorly still. It also seemed to smoke a bit. I pulled the plug and it was very carbon-like. I put in a fresh plug, and after a few minutes of running it was as bad as the one I took out. For the record, the gas I put in was treated non-enthanol fuel. I'm pretty meticulous about my fuel choice/treatment to prevent issue. The bowl on the 4HP carb I removed looked very clean.

Am I right to think I have a piston ring issue causing me to burn oil and have bad/dirty combustion? I don't think this motor saw a ton of use before I got it, but I'm not sure of its prior maintenance. It came on a sailboat I purchased.

What should my next step be? Thanks in advance!

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Check the oil level. if it is even a drop over full, you need to lower it. The dipstich should show oil between the marks when it has been screwed in all the way and removed. Might as well change the oil while you're at it. If the compression is within specs, the oil ring is probably OK too, so I would think that you are probably over-full on the oil. Run only non-synthetic 10w-30 FC-W oil. It only holds about 3/4 pint.

 

[colbyt]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

I think is holds 10 ounces at least that is what the 3.5 takes.

Was it stored in the proper position? 4 strokes are picky about that.

You can download the user manual for free from Tohatsu if you don't have one.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Thanks very much for the helpful responses. I have not yet changed the oil for this season, but was planning to do that next. I just took a look at the manual, and know that when I have changed the oil I have used manual recommendation of 450mL. That comes out to just over 15oz ...so it sounds like I probably have been filling with more oil then I need. Especially if I was making any more last season! It is always store up-right, but I was curious if some oil might have leaked into the cylinder through the off-season.

Is it best to measure the oil right after the engine has been running or after it's cooled down?

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Never blindly add oil. Always go by the dipstick. Otherwise you do not actually know how much is in there. Hot or cold -- either is OK.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Update: Oil level was already on the low side, but I changed it, and got the level between the marks. I'm sure the last time I filled it I put in more than it needed based on how much I put in this time.

So the problem persists. Poor firing at low RPM hence poor idle. Still seems like too much smoke in the exhaust as well. I have not yet checked compression - is that the next thing to do I assume?

Also of note is that the engine has always had a tendency to rev to higher and lower RPMs in neutral when the throttle is set low. Even once warm, if I set the throttle so it's running fast enough to run smoothly it will rev up after awhile, or potentially die down. I thought all of these symptoms were pointing to carb issues initially, but now I've ruled that out with the new carb. Would any common thermostat or ignition issue cause anything like this?

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Could be almost anything, but the unsteady idle is likely just too low an idle, or a minor vacuum leak. What does your trusty shop tach show for idle RPM in N and in gear? Have you put a spark tester on the plug wire to check for the overall spark condition? You can test the ignition system with a good-quality analog (not digital) ohm meter.

Excessive smoke could be too rich, or oil (or water) getting in where it shouldn't. A compression test and a leak-down test will tell you a lot about ring and head gasket conditions.

If the combustion chamber is carboned-up, you could try using a decarboning treatment. That will really make a ridiculous amount of smoke as the carbon comes off, but can really work wonders on smoky chambers.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Thanks Paul -

Based on my friend's Tohatsu I'm not down near too-low-RPM territory though I haven't pulled a number. I assumed spark was fine but at this point I'm going to get through everything. After ruling that out I'm going to run the leak down test. I'm now thinking that may expose my issue. I've always thought my OB sounded noisier than others and now I'm thinking I may have an exhaust (or intake I guess) valve leak. The sound is a sort of repetitive hiss sound that I'm thinking might be allowing oil into the chamber. Seems like this could explain everything? The leak down should hopefully isolate it if I have a problem.

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Good plan. A leak-down test can reveal a lot.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

So I ran the leak-down last evening with (I guess) good results. Good meaning I don't have any leads.... I only saw about 2% leak which sounds like it should be very good? I heard a slight amount of air under the valve cover, and though the oil fill hole. Both rocker arms were loose, so I'm pretty confident about the cylinder position ...well, and the low leak reading would indicate that it had to be correct.

Since the plug is still quickly fouling and I suspect a good bit of oil in the chamber perhaps that is just from storage. I'm considering running a bunch of sea-foam through it to clean it out and see where I'm at. Maybe I just need to clean it well and run it. Any recommendations other than sea-foam to clean up an oily/carboned chamber?

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Tohatsu sells "engine tuner" which is a spray that is specifically intended to dissolve the carbon.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Will give that elixer a try, but thinking I'm still battling another problem.

Still no luck on the clean running at low RPM. Now I'm wondering if I have a weak spark. I'd assume at lower RPM the spark would be weaker, so if there was a problem with marginal spark it may only be an issue at low RPM? I put a spark tester on it and could see a faint spark at the "Small Engine" setting ...but couldn't see one at the 20,000V mark. What should I expect to see when pulling the engine over by hand? I'd think it should be at least as fast as the engine running at a low RPM.

 

[colbyt]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

What should I expect to see when pulling the engine over by hand? I'd think it should be at least as fast as the engine running at a low RPM.

Idle speed on the 3.5 is 1300 in neutral and 1200 in gear. I have not met a rope starter yet that moves that fast.

 

[TOHATSU GURU]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

The best thing to do is to pick up a service manual and run the tests as outlined.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Been occupied with other boat projects, but as launch is a week away I decided to quickly revisit the issue. Found in a Catalina website thread that someone had a 9.8HP Tohatsu with a missing carb gasket. Based on the description I felt I had the same issue and pulled up a parts diagram. Sure enough, someone had reinstalled the carb incorrectly. The diagram shows no gasket between the spark arrestor and the carb (mine had one) and shows a gasket on both sides of the spacer between the carb and engine (mine had a gasket on only one side of the spacer). Assumption is that the mixture is getting too much air at idle with a bad seal there.

Sound like this could be it?

 

[pvanv]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Could be. You don't care what is in front of the carb (at the silencer), but you do need a gasket on each side of the thermal spacer for sure.

 

[vt17]

Re: 05 Tohatsu 4HP - new carb but still poor idle, instantly fouled plug, and smoke.

Thanks to all for the comments. Three days before launch I now have a real idle. It was indeed the missing gasket that was causing the problem. $3.98 for two new ones and I'm running great!