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Old 16-02-2021, 05:34   #1
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2-stroke headache

I have no experience of 2-steokes so may be doing something obvious wrong here.

My tohatsu m9.8b starts fine with full choke. It almost immediately wants to die unless I'm fast to push the choke in.

When I push the choke in it revs up massively and then slowly comes down. And after a minute stalls.

There are three adjustments that I can see. Idle stop, idle mixture, and timing.

For a while I suspected fuel starvation. - tank air valve is open. Tank take off tube is off the bottom. Connector valve (tank side) is new. Fuel filter is new (improved when I replaced this). New Diaphragm kit in fuel pump. Carb opened up (bowl off) and cleaned out, jet blown clear.

The idle stop is screwed down full (fastest). Seems way fast to me but maybe that's just how it's meant to be.

Idle mix is set to 2 turns back from fully closed. Any tighter and it doesn't seem to want to start.

I think it will go for ever at full power, (not tested more than a minute) so pretty convinced it's a mixture/timing problem.

Have found the user manual online but no service manual or workshop book.

Anyone got any thoughts?
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Old 16-02-2021, 06:00   #2
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Re: 2-stroke headache

Motor age ? How many cylinders? Fuel-oil mix ratio? How old is the gas? What's the throttle setting when it stalls? Please describe how it stalls.

Given all the things you've tried so far, I'm thinking compression, timing, or crappy spark at low rpms.
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Old 16-02-2021, 06:12   #3
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Re: 2-stroke headache

Are you SURE the carb is clean?

It might be too much fuel rather than too little. When you open the choke it could be revving up because it's air-limited with the choke closed and there's excess gas in the carb. Is there a lot of smoke when it revs up?

There's a pin valve that controls the flow of fuel into the carburetor bowl and shuts off the flow when the bowl is full. Often, especially with ethanol fuels, it becomes worn and the engine will slowly flood while running.

The idle stop should not be screwed all the way down.

The mixture screw is unlikely to be the culprit I think, and it's tough to properly adjust it without the engine idling.
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Old 16-02-2021, 07:10   #4
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Re: 2-stroke headache

It sounds like it is a fuel issue and will agree, you need to clean the carb. Usually it is just cleaning the crud out of the bowl and the main jet for 2 strokes, then they are good to go.
If nothing else, drain the bowl (carb) and see is water/crud comes out. Sometimes this is all you need to get it running again. If you don't know what you are doing don't mess with the fuel mixture screw. Find the factory setting (typically could be 1 1/2 turns out) and reset the fuel mix. Do this by gently setting the screw in, then backing it out.
If this still doesn't work, then you need to drop the bowl and clean the main fuel jet (large jet usually in the middle of the upper carb body).
If this doesn't get it to run w/o the choke all the time, there are a few more things to check.
Look inside your fuel can to see if you have crud or a large mount of water in your fuel (happens often w/ethanol fuel).
Your fuel line from the tank to the motor may be faulty. Again ethanol fuel can collapse the lining in some fuel hoses causing fuel starvation (see attached pic of a bad fuel hose). Change the hose and check bulb.


Let us know how it goes.
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Old 16-02-2021, 07:11   #5
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Re: 2-stroke headache

There are two jets in a carb, one for the idle mixture and one for the running mixture. Are both clean? Are the internal air passages in the carb clean?
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Old 16-02-2021, 07:21   #6
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Re: 2-stroke headache

The behavior you describe: running smoothly and well at high speed; and poorly, or not at all at low speed, is EXACTLY what to expect with a clogged low speed jet in the carb.

These can be tough to get clean, they have tiny little holes, and a good blast of compressed air and carb cleaner can be needed.

The cause is usually either dirty fuel, or putting the engine away with the carb full of fuel. Over time the gasoline evaporates and the oil oxidizes to goo and clogs things up.

Keep your fuel filter in place and fresh, and run the carb dry EVERY time you finish with the engine.
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Old 16-02-2021, 07:27   #7
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pirate Re: 2-stroke headache

Check your float is working correctly and not pierced causing flooding.
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Old 16-02-2021, 07:51   #8
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2-stroke headache

Just dealt with this same issue on a 2-stroke 15hp Nissan (same as a tohatsu)
The choke issue tells you it is fuel related.
Choke pulled out = less air more Fuel= richer fuel mixture
Choke pushed in = more air less fuel = leaner fuel mixture.

If you pull your spark plugs and look at their wear/color youíll also get an idea of whether your running too rich or too lean.

Are you running the correct fuel/oil ratio? Youíll notice I burnt oil/fuel on spark plugs if thereís an imbalance.

Steps to resolve:
-check spark plugs for fouling (not likely the case here but itís by far the easiest troubleshooting step and will tell you a lot of info)
-Clean the carb with carb cleaner.
- then clean it again.
-Clean the in-line fuel filter (water likes to collect in there).
-Check your not pulling crud from the bottom of tank (if itís external).
-Check you have enough flow from
External tank if you have one. Those back flow preventer valves are t necessary and fail frequently.
- Check fuel hose especially if it is gray. Those gray hoses have a liner that is supposed to be ďimpermeableĒ to Ethanol fuels but they clearly are not and frequently get degraded and clog the line. I replaced with a standard black rubber fuel Hose (they work on cars just fine and are much cheaper) Actually I did this twice as the first one I ordered had a defective primer bulb. This was what was ultimately clogging my fuel system and with a new hose the motor immediately started up.

I also adjusted fuel mixture screws (over time they can back out due to vibration) to get a better run. No harm in playing with these as you will be able to hear where the engine is happiest. Turn (while counting number or turns) until you begin to sputter then go back to where it sounds happiest. Note than at idle might not necessarily be where itís happiest while under throttle. It is better to run too rich than too lean.

Another issue with 2 strokes is that they like to be run a FOT and that is how the fuel/oil ratio is designed. With extended periods of idling you can frequently get issues with unburnt oil in combustion chamber and that will slowly clog spark plugs. Where I am anchored a couple months of the year is a idle speed manatee zone and I have begun running motor at a 75:1 instead of 50:1 during those months and she seems happier. (This was advice from dirt bike users)

Best source of info for 2 stroke engines Iíve found is on dirt bike forums. They are all two stroke motors that typically get run very very hard.
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Old 16-02-2021, 07:56   #9
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Re: 2-stroke headache

Here's a link to a Tohatsu service manual for 1 & 2 cylinder Tohatsu OBs
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Old 16-02-2021, 08:09   #10
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Re: 2-stroke headache

SOLVED: I'd put the carb gasket on wrong so it was blocking the vacuum supply to fuel pump. I'm a doofuss that could have spent this sunny day with my sweetheart!

Next question is... I have the idle burbling nicely, but when I gun it, it revs up but misses a bit and smokes a lot. I'm guessing the timing setting is what controls the high revs as there's no mixture screw for high on the carb. So what's the method for tuning the timing? What am I looking out for?
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Old 16-02-2021, 08:16   #11
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Re: 2-stroke headache

Fantastic, thanks Bill.
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Here's a link to a Tohatsu service manual for 1 & 2 cylinder Tohatsu OBs
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Old 16-02-2021, 11:19   #12
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Re: 2-stroke headache

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-Clean the in-line fuel filter (water likes to collect in there).

What do you use to clean these inline filters out? Spray carb cleaner in backwards?
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Old 16-02-2021, 15:58   #13
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Re: 2-stroke headache

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What do you use to clean these inline filters out? Spray carb cleaner in backwards?

I don't know whether the encapsulated inline filters (with nipples on each end) can effectively be cleaned. if you can actually see gunge in it, you should replace it.
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Old 16-02-2021, 23:47   #14
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Re: 2-stroke headache

Quote:
Originally Posted by andypag View Post
SOLVED:

Next question is... I have the idle burbling nicely, but when I gun it, it revs up but misses a bit and smokes a lot. I'm guessing the timing setting is what controls the high revs as there's no mixture screw for high on the carb. So what's the method for tuning the timing? What am I looking out for?
Have you put new sparking plugs in it?
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Old 16-02-2021, 23:57   #15
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Re: 2-stroke headache

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Originally Posted by jimbunyard View Post
Have you put new sparking plugs in it?

No, they looked spotless when I took them out. The previous owner had replaced them 18 months ago, right before he put the outboard in storage.


Might that help with the missing?
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