Stall at high rpm!

[ecoflorida]

1996 Tohatsu M50D has developed worsening problem over the last year. At first though it was water in fuel and repeatedly drained fuel filter secondary (Racor) bowl and chained filters. Discovered a bad tank and replaced but problem persisted every now and then. Now runs fine at low and midrange but at mid range it seems to slightly down throttle over 1-3 seconds then dies. (see video attached). It cranks right back every time without hesitation or need to reprime bulb.
I have tried the bulb pumping testing as it happened twice but the weather was back and rough so we gave up. I will do that again. We do think we have eliminated electrical issues. Stop Circuit test was good and the CD unit has been replaced.

I am leaning more toward vacuum system failure. If I am correct vacuum negative pressure increases as rpm increases and that pills more fuel in the system. Wouldn't a small leak manifest itself at higher negative pressures and stop leaking a those pressures decrease? If that leak suddenly caused loss all negative pressure fuel flow would stop!

What about the reed valves? If they leak/fail at higher rpm wouldn't that stop the fuel flow without emptying the carburetor bowls and then allow the motor to crank once then vacuum pressure lowers and the leak/failure closes? I have read on a few other power sports forums that reed valve failure can indeed do just that and that reed valves need replacing very 100 hours (that's a bit extreme and Im sure is meant for racing considerations).

There is good chance that I have no idea what Im talking about too! I'm all ears now.

 

[Sea Rider]

That's a motor used in salt or fresh water ? Had same issue with a Tohatsu M40C, was due to both o'rings located on both fuel connectors sides were dried/worn, let air in on the fuel circuit and motor bogged down. Worked OK at any other throttle setting 3/4 downward.

Those o'rings doesn't comes as spare parts which is a crime, will need to buy the whole fuel hose assy, both end connectors or remove both o'rings with a needle, go to an auto/hardware store and buy 2 new ones with same diam for fuel usage. The tank's connector has a slight wider diam than the motor's connector.

Reed valves rarely goes bad on Tohatsu motors. If the motor was tested with other fuel line in excellent working state or with a new fuel line, never mind that.

Happy Boating​

 

[ecoflorida]

Sea Rider,
I really appreciate your reply.
The boat is used in fresh and salt! Is there significance there with this issue?

If I understand correctly you are talking about the fuel line quick connect fitting? Because of what you described I actually have the fuel hose from the tank to the secondary Racor filter and to motor directly connected without the quick connect fittings.

If it was a fuel supply problem by leaking o-rings before the carbs, allowing air entrainment, wouldn't the carb bowls need re-priming each time to crank after it shuts down?

This motor cranks right back up and would run unabated at mid-low range without a problem without having to do anything. But go back to WOT and it shuts down.

That's a motor used in salt or fresh water ? Had same issue with a Tohatsu M40C, was due to both o'rings located on both fuel connectors sides were dried/worn, let air in on the fuel circuit and motor bogged down. Worked OK at any other throttle setting 3/4 downward.

Those o'rings doesn't comes as spare parts which is a crime, will need to buy the whole fuel hose assy, both end connectors or remove both o'rings with a needle, go to an auto/hardware store and buy 2 new ones with same diam for fuel usage. The tank's connector has a slight wider diam than the motor's connector.

Reed valves rarely goes bad on Tohatsu motors. If the motor was tested with other fuel line in excellent working state or with a new fuel line, never mind that.

Happy Boating​

 

[Sea Rider]

You're welcome Ecoflorida,

The only issue witht a motor runing on fresh or salt water is that the latter needs to be flushed way more to lessen salt formations inside the entire cooling water passages. If understood correctly, have split the hose line in two and both ends tank's and Racor's exit maintains both quick connect/disconnect connectors ?

Other issue could be the fuel pump's internal diaphragms have turned bad, could be extremely ballooned and not doing their homework right at wot. Have the fuel pump been serviced, checked lately ? No need to install a new FP, internal parts are cheap and easy peachy to change.

Being a 24 year old motor, would you say that has low or high run hours on it ? Are there white lines formations seen around the cylinder head gasket, exhaust cover or base gasket contours ?

Happy Boating​

 

[ecoflorida]

Oh yea. This is an old motor with a lot of hours it. But it has been well maintained. I run it judiciously like all of my motors. Don't push it out of the hole and dont run it WOT all of the time. Salt water is flushed religiously. Gaskets are good. No salt seepage.
There are No quick connects at all on the fuel line from tank to motor. Hose barbs and stainless hose clamps at all connections. I did that exactly for the reason that you write about. Not going to pay for a whole quick connect assembly only to risk the same failure down the line.

I keep going back to a vacuum pressure failure. SO correct me if I am wrong. If your theory is correct, wouldn't the carburetor bowls be emptied and require priming again before it restarts? But if there was a sudden loss of vacuum at a higher RPM range wouldn't that stop fuel being pulled past the carburetor, resulting in shut down but carburetor bowls being left full to be able to crank at a lessor vacuum pressure?

 

[Sea Rider]

Can you post pics of whatever connection there's from tank's fuel pick-up tube to Racor's fuel entrance and from Racor's fuel exit to the motor's entrance. Are hose barbs some sort of metal clamps ?

When priming the bulb till firm with arrow pointing upwards there should be no fuel spills on the entire fuel line whatsoever, if sucking air the motor will miss specially at high rpm assuming the fuel pump's diaphragms are still in excellent working order which isn't known ?

Happy Boating​

 

[MattFL]

Can you reproduce the issue while someone is continually squeezing the fuel bulb? If not, then it's likely you have an air leak somewhere. Also remove the connector from the motor side of the fuel line and be sure that fuel flows freely when squeezing the ball. Some gas lines can collapse inside cutting off fuel supply, even thought they look fine on the outside.

If you have the capability, the best way to trouble shoot is to connect a different external gas tank and gas hose and see if that resolves the issue. If it solves the issue then the problem is between the motor and your gas tank. If it doesn't solve the issue, then the issue is likely inside the motor somewhere.

 

[ecoflorida]

SURE! See attached pics. All of your recommendations are reasonable but that does not answer the question I have posed about the carburetors needing priming if the fuel was restricted or limited prior to the carburetors. Yes the problem duplicates when someone is pumping the priming bulb. And My motor fires right up after it stalls WITHOUT needing to pump the priming bulb at all!

 

[MattFL]

Does it have an overheat alarm that works? I had an old outboard that had an overheat problem (1960's motor, stopped pumping water above idle), it would actually seize when it got hot. But almost as quick as I could get to the pull-rope it had cooled enough to free up and re-start. Is there any chance this is happening to you?

 

[pvanv]

Does it have an overheat alarm that works? I had an old outboard that had an overheat problem (1960's motor, stopped pumping water above idle), it would actually seize when it got hot. But almost as quick as I could get to the pull-rope it had cooled enough to free up and re-start. Is there any chance this is happening to you?

Very possible. A cheap IR thermometer can tell a lot. Could also have an electrical failure only when hot.

 

[Sea Rider]

Nope a 2 stroke Tohatsu M50D2 dosn't have an overheat alarm as there's no temp sensor installed on the cylinder head area, old technology with just a thermostat. Will be more inclined to having a fuel related issue or an erratic electrical component heating up and failing when at full throttle, the worse nightmare to diagnose properly in a timely manner, unless having new spare parts to swap and test till the culprit if found.

Would be good to check the thermostat condition if suspect the motor has overheating issues specially if has never been changed before and the motor running ocassionally in fresh and salt water. If found salt crusted, rusted, corroded install a new one along a new thermo gasket as well. Placing your palm around the cylinder head area will determine very fast if the motor is overheating or not.

During a restart the motor needs very little fuel compared to runing at wot where misses. Thanks, as the pics have shown what inquired, try the following :

Disconnect the motor's internal hose that sends fuel to the fuel pump, put a small container there, with tank's air vent fully opened or with fuel cap removed prime the bulb with arrow pointing upwards several times in a row, check if the amount of fuel fills completely the diam of the small hose when exiting towards the small container.

If the fuel flow is constant with 0 restriction the fuel system is working OK up to the fuel pump entrance, if hesitates or delivers a much less fuel exiting, the Racor water separator is not working properly, could be internally bit clogged with dust particles that have bypassed the internal tank's small mesh filter if still counting with one installed ? To determine if the whole fuel system is doing its homework right need to open up the fuel pump and check the internal diaphragms condition, if dried, hardened, ballooned install new ones. E fuels likes doing so as per experience seing such mess through the years. Report your findings...

Happy Boating​

 

[MattFL]

This might help:

 

[Sea Rider]

The M50D2 it's a simple 2 stroke motor which uses a simple mechanical fuel pump which sucks fuel from a portable fuel tank as the posted pic shows. That video is for troubleshooting fuel related issues on more complex higher HP 2-4 stroke motor which this one isn't.

In this particular case the OP needs to check if the right amount of fuel is being delivered from the tank to the fuel pump's entrance and the cond of the internal diaphragms as described to rule out for once and all if having a fuel related issue or not which isn't known so far.

If the motor still counts with the original fuel pump's diaphargm after 24 years uf use, don't expect to find them in excellent working shape.

Happy Boating
​

 

[MattFL]

The video covers basic things too, like how to tell if you've got an air leak between the main tank and motor, or between the fitting on the motor and the fuel pump. There is some extra info, but it covers the basics too.

 

[Sea Rider]

If you have an air leak somewhere in the entire fuel circuit will also leak fuel when priming hard the fue bulb while using a portable fuel tank or when the electric fuel pump has build enough pressure in the entire fuel circuit. It's a simple matter of looking where the fuel flows from...

HappyBoating​

 

[Sea Rider]

The motor accelerates quite well, there's a small revving hesitation heard seconds before shutting down. Question : if the tank's cap is removed does the motor still drops dead at full hammer down ? If doing same will need to troubleshoot the entire fuel circuit tank to fuel pump to start with.

For how long have been using that West Marine tank along the Racor water separator/fuel filter. Is the grey hose a compact fuel hose or it's the kind that has an internal sleeve ?​

Happy Boating

 

[ecoflorida]

The motor accelerates quite well, there's a small revving hesitation heard seconds before shutting down. Question : if the tank's cap is removed does the motor still drops dead at full hammer down ? If doing same will need to troubleshoot the entire fuel circuit tank to fuel pump to start with.

For how long have been using that West Marine tank along the Racor water separator/fuel filter. Is the grey hose a compact fuel hose or it's the kind that has an internal sleeve ?​

Happy Boating

All good questions and advice. You are correct about the little hesitancy just before shut down. Have not been able to get at it since the weekend. May be able to put it in the water tomorrow and test some of these theories. The grey hose is sleeved i believe. I have some new black hose to Im willing to switch all of that out to eliminate the fuel intake issue. You that WEST Marine has a vent fault? These EPA rules have ruined full tanks! They don't vent to the atmosphere maybe faulty compensation vent? Gotta get her running! Only three more weeks to duck hunt!

 

[ecoflorida]

All good questions and advice. You are correct about the little hesitancy just before shut down. Have not been able to get at it since the weekend. May be able to put it in the water tomorrow and test some of these theories. The grey hose is sleeved i believe That could be collapsing internally I suppose. I have some new black hose to I'm willing to switch all of that out to eliminate the fuel intake issue. You think that the new WEST Marine tank has a vent fault? These EPA rules have ruined full tanks! They don't vent to the atmosphere. Maybe faulty compensation vent? Gotta get her running! Only three more weeks to duck hunt!

P.S. I NEVER Run E Fuels in any of my outboards!!!!!!! NEVER NEVER NEVER. Rebuilt fuel pump three years ago "just because". Im going to double check the fuel line from tank to pump. Thinking more about it I do not think it is the tank because I think I would have noticed it collapsing and the time from throttle up to shutdown is always rather consistent from my memory ( have not put a watch to it. YET!) , and, I think the time to shut down would be longer as the Tank become more empty.

 

[Sea Rider]

Those internal sleeved fuel hoses doesn't work well, over time the internal sleeve will begin to degrade, break in pieces and restrict the fuel passing through the hose to the water separator/carb.

Been there and drove me crazy with a motor using such type of grey fuel hose. Dismantled the complete fuel line, removed the internal sleeve which was found with broken bits that restricted the fuel flow at wot, problem solved, that happened while using non E fuels.

Yep, those EPA regulations are giving problems with motors running non vented caps. If haven't tried it test motor as previous runs with removed cap to see what happens...

If plan installing a standard single piece fuel line don't make it any longer than the original Tohatsu fuel line assy, the fuel pump built into the lower carb is quite small compared to outer larger fuel pumps found in other motors which stresses much less the internal diaphragms when pumping fuel.

Happy Boating​

 

[pvanv]

You may want to verify the exciter and pulsers. Those could work well cold and fail hot.