Tohatsu outboard - starting puzzle

[Marqus]

A week ago, the Tohatsu 8hp 2-stroke started and ran well. Now she won't.

What really puzzles me is:
- She does not respond to starting spray into the carb throat (no reaction at all, which is to me very a big deal), but ...
- She will run smoothly and confidently for 5 sec when fuel is sprayed directly into a cylinder via a spark plug hole.

My first thought was that the only thing that could block starting spray in the carb throat from getting through, are bad reed valves.
So I took off the carb and removed the reed valves - they look perfect, springy and seal well. So it's not them.

That's where I am now. Not sure what to try next - need expert help.


Extra Background:
Non-starting symptom appeared after dropping the lower leg, replacing the impeller, transporting her in the boot of a car for a day, on her back.
When problem arose I did the usual :
- fresh fuel and clean fuel line
- rebuilt carb with clean jets, float good, float needle valve good
- compression tested ok
- new plugs, good blue spark (jumps 5/16 air gap with plug tester)
In the past, whenever you sprayed starting spray into the carb she would fire briefly, even with no fuel connected.
My fisherman friend posted this puzzling scenario on his favorite forum but drew no useful replies.

Photos of the carb and reed valves below.

[Thumbs Up]

Your first photo shows the motor without the carb. But I guess that you know that. My guess is that you are flooding it. If that is the case try cranking it at full throttle with no choke until it fires. Maybe the needle valve is set to high or not sealing. If you can overflow the carb by squeezing the bulb it would confirm this. When you pull the plug, is it wet with fuel? I have the same engine. The main jets in these carbs erode after time also. I tried everything to get mine running correctly and a new main jet did the trick. That won't keep it from starting but will restore the correct mixture and have more power and use less gas. I even had a spare carb but it too had an eroded main jet.

[puffcard]

MooI got the same motor. Shame you aren't in the Bahamas I got a spare tested carb. You got spark, you got compression, are you getting fuel? When everything is together and you're pulling are you smelling gas, is it fresh? If smelling a lot of gas, does the primer bulb pump up hard and stop. If not float/needle valve bad and flooding. If no gas and bulb pumps hard, Check fuel connection at motor, they can look on and not be connected. If that checks, open the drain on carb, checking for gas.

If you certain the carb getting gas, try starting wide open and half choke. Should have a very strong gas smell. Still no fire there may something wrong about the firing/timing. Had that happen with a 15hp OMC. Also double check the plugs, I recently had two NGK outboard plugs go bad, had small cracks. If I clean them in acetone, they would run for a bit.

My bet is gas/carb. Get a good carb spray cleaner and use jewellers small brass wire to clean the jets. Go thru every hole.

Good luck.

[Dave_S]

It must be drawing air from somewhere (i.e. not through the carby) most likely on the piston side of the reed valves.

Seal off the carby inlet and pull the starter, see if you can hear/tell if it is drawing air from somewhere else. If you can see the reeds see if they are opening.


Is it a pulse fuel pump ? crankcase pressure line come off, cracked or broken ?

[RaymondR]

Try changing the spark plug. I had a lot of problems with intermittent hard starting and shut downs on a 9.8hp Parsun a couple of years ago until I replaced both spark plugs and fixed the problem. I suspect the problems were related to the resisters in the plugs they use with CDI ignitions.

[Dave_S]

Another good way to find the leak is to blow smoke in through the inlet and see where it comes out.

Feed back ?

[Marqus]

Thanks guys, appreciate your posts.
Been away from for a few days. After starting the thread I got no replies for 3 whole days so I thought it had just slipped away into the ether.

Thumbsup: Float needle is sealing fuel flow off; and yes the plugs are wet - so fuel mist is getting through. Main jet still looks pretty good with clean metal.

Puffcard: Primer bulb goes hard, and my test rig actually bypasses the connectors to keep it simple. Swapped in new plugs. Tried all combinations of choke and throttle until my arm fell off. Also thought of Timing, but no easy way for me to test it (is there a way?). Also, it was running a couple of weeks ago, and still runs with fuel directly sprayed into the cylinder, so I thought the timing ought still to be ok. But testing it would be good.

Dave: Vacuum pulse fuel pump seems to be working. Good tip about sealing off the carby throat and listening for air suction noise; will try that. I thought maybe the crankshaft seals let air in, but then, why would they suddenly deteriorate? Looks like a big job to check the two main seals.


Raymond: Swapped in new plugs early in the piece. No dice. But as I say, it runs well for a few seconds when I blow fuel mist into a cylinder - so the plugs must be working.

Dave: "Another good way to find the leak is to blow smoke in through the inlet and see where it comes out". Not sure I follow - is this looking for a suction (inwards) or pressure (outwards) leak?

A mechanically gifted sailor I met in a boat shop will be coming over in a couple of days to have a crack - will post back if we learn anything.
Thanks for your suggestions. At this stage I'm leaning towards a fuel issue, but ....

[liquidice]

Can you flood carb with gas until it overflows, and gas drips out from the sides of carb and then attempt to start with pull cord without additional pumping of gas until carb is emptied out? Sometimes that works for me, eliminating the question of fuel delivery at ignition. If it starts and dies, now we’re in business, and it’s a fuel pump issue. Sometimes the pump bulb can be bad or half-bad and you won’t be giving it the gas you think you are.

90% of the time it’s a fuel delivery issue, especially with a motor that suddenly stops running.

Another trick I’ve learned, if you suspect a clogged jet but don’t want to risk disassembling the carburetor, with carburetor on and the choke on, prime the motor, turn the throttle as low as you can so the motor won’t accidentally start, and put your hand on the carb throat, over the butterfly valve, completely blocking it with the meaty part of your hand. Then pull the starter cord three times. You will feel the suction on your hand and gas will pull through the jets. Then try your normal starting procedure. Good luck!

[Thumbs Up]

If the plugs are wet then you are getting too much fuel or they are not firing for some reason. Sometimes the problem can be traced to the last thing that you "fixed" before it ran. It could be the wrong plugs. You said you did an air gap spark test but did you test both wires at the same time? You need to rig up an air gap tester for two wires (they fire at the same time) Could be a bad spark module. Rebuilt the carb but are you sure the idle jet was put back in. Isn't there a rubber cap below where that screws in?....

[Thumbs Up]

An eroded main jet will look perfect. But when I got the new jet and compared them by eye I could see a size difference. But that wouldn't keep it from firing. When you do get it running, if it bogs down, try a new jet. I was amazed!

[Marqus]

Good news. A friend of mine pursuing help for this problem received a tip from an expert to the effect that, with the huge number of pull start attempts (100+), some 2-stroke oil may be have accumulated to form a sludge in the crankcase impeding ignition. So after adding quite a bit of clean fuel (no oil mixed in) to the crankcase to dissolve things it has now shown signs of life without making any other changes. So it's alive and will now be tuned up.

Thanks everyone for giving input to this thread - appreciated; all good stuff to know.

Liquidice: Thanks for mentioning a couple of really great techniques: flooding the carb bowl to sidestep fuel pump issues (good one), and the hand-over-carb-throat method to increase the vacuum that sucks fuel through the jets (even better).

Thumbsup - yes the rubber stopper for the idle jet is in place, and I will keep the issue of improved performance with a brand new main jet in mind once we have her tuned up and tested on the water.

Thumbsup, also very interesting that you suggest testing of two spark plugs at the same time - never thought of that; sounds good but I don't quite get the rationale. You say they would be firing at the same time - I didn't expect that so maybe I'm misunderstanding - any chance you could explain that one a bit more ?

[Thumbs Up]

Yes, the plugs both fire at the same time, but only one cylinder ignites because the other one is on the exhaust cycle. That way you don't need a distributor. But you need enough juice to fire both. A bad ignition module can be diagnosed with a double spark gap tester which you can make with three small bolts and a scrap of plywood. A weak spark can run the engine fine with no load but when there is a load it can run rough which can mimic fuel problems. That is why it is an important diagnostic tool.

[Marqus]

Originally Posted by Thumbs Up

Yes, the plugs both fire at the same time, but only one cylinder ignites because the other one is on the exhaust cycle. That way you don't need a distributor. But you need enough juice to fire both. A bad ignition module can be diagnosed with a double spark gap tester which you can make with three small bolts and a scrap of plywood. A weak spark can run the engine fine with no load but when there is a load it can run rough which can mimic fuel problems. That is why it is an important diagnostic tool.

All makes good sense and reveals something I didn't know about two-stroke electrics. Thanks for valuable input to the thread Thumbsup.

[Island Time O25]

A marine pro buddy of mine once told me the best way to keep the carb in a 2 stroke clean is to shut the motor off by way of separate fuel shut off valve and not by the motor's shut off knob. He said that 90% of non starts were caused by the gunk left over time as unburned gas in the carb evaporates. With the separate shut off valve all gas is burned leaving practically no gunk to accumulate. I tend to follow his advice as he has 50 years of marine industry experience and in fact grew up on an island and had to attend school via small dinghy with an outboard from age 7 to 15 when they finally built a bridge to the island he lived on in the middle of the river disecting his town in the old country.

[44'cruisingcat]

Originally Posted by liquidice

Another trick I’ve learned, if you suspect a clogged jet but don’t want to risk disassembling the carburetor, with carburetor on and the choke on, prime the motor, turn the throttle as low as you can so the motor won’t accidentally start, and put your hand on the carb throat, over the butterfly valve, completely blocking it with the meaty part of your hand. Then pull the starter cord three times. You will feel the suction on your hand and gas will pull through the jets. Then try your normal starting procedure. Good luck!

Remove the kill switch key while doing this, to ensure there's no chance of a backfire.