Yamaha 2-stroke 50HP 1980's bogs and not reaching max RPM

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Hello,

Hoping for some helpful tips to get my 50 HP Yamaha autolube 2-stroke outboard from 1985-ish going like a champ again.

Engine starts quite easily, it can idle, I can reach max RPM at idle (not in gear). But in gear, the outboard at first try from cold, today reached 1/2 to 2/3 of full RPM, for maybe 2 minutes, but then bogged down to low RPM perhaps a 5 knots boat speed, tops was maybe 15 knots.

We tried to stop and wait 10 minutes, then the engine would repeat a similar pattern but it would be even shorter time period before it bogged down, and we never reached the same RPM in gear as the 1st attempt.

The behavior of the motor has been the same during a few months (we bought it as a fixer-upper) so the fault seem predictable.

Any idea what could be common causes for it? Is it perhaps a safe mode the engine enters?

Lower unit does not feel more than luke warm to the touch after operation, the water jet seem strong. (impeller condition unknown however).

The motor has autolube, the oil tank is full, we bled it prior to use this season and the oil flowed easily.

Fuel filter was cleaned prior to use this season. Air bleed on fuel tank is fully open. Running on premium gas.

We have tried pressing the fuel bulb during running in gear and the engine still bogged down after 1-2 minutes so perhaps fuel pressure is OK.

So all seem in order occasionally, but just not for a long time period.

Any inputs is much appreciated.

Best,
Daniel, Sweden

 

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Motor is from 1987, 50DEO 6H5 L

 

[99yam40]

have you tried cleaning the carbs?
does it help to use the choke when the problem shows up?
have you checked compression on all cylinders?
restricted main jet or maybe something floating around in the carbs plugging things up.
even water down in the bottom of bowls could cause problems from time to time.

but if it is regular problem, you need to figure out if the overheat is stopping/limiting RPM.
do you have any gauges or alarms that will tell you if there is a problem?

have you checked the fuel pump to make sure it is not leaking into the crankcase

 

[1up]

All good tips to check off, thanks. Compression is not verified.

Judging by the general condition of the engine I would think it is a relatively simple fault. It looks in good condition under the cowling and sounds right idling at what would be crusing RPM. Eventhough it was a very slow and gradual throttle response in geae. Not so at idle.

It could be crud in the carbs.

Unfortunately no fault indication system active at least.

I thought of perhaps just overtemp due to bad thermostat? Leading to "limp" mode and cylinder to be shut down. Something else to check I guess.

Perhaps the only way forward is a prudent systematic approach evaluating cylinder(s) at fault, being it air, spark or fuel issue.

What would it indicate if it got better after pulling the choke?

 

[racerone]

They all sound great revving in neutral.---That means nothing.

 

[99yam40]

All good tips to check off, thanks. Compression is not verified.

Judging by the general condition of the engine I would think it is a relatively simple fault. It looks in good condition under the cowling and sounds right idling at what would be crusing RPM. Eventhough it was a very slow and gradual throttle response in geae. Not so at idle.

It could be crud in the carbs.

Unfortunately no fault indication system active at least.

I thought of perhaps just overtemp due to bad thermostat? Leading to "limp" mode and cylinder to be shut down. Something else to check I guess.

Perhaps the only way forward is a prudent systematic approach evaluating cylinder(s) at fault, being it air, spark or fuel issue.

What would it indicate if it got better after pulling the choke?

how about not enough fuel to the air going in

 

[1up]

how about not enough fuel to the air going in

That is what it seems like too. We removed the fuel hose with connectors and bulb for further inspection / replacement yesterday.

 

[racerone]

Compression test is the first thing to do.----Post the numbers here.----Then test spark.----Sounds like it might be running on 2 of 3 cylinders.-----Or there is blockage in a carburetor.-----Or a hole in fuel pump diaphragm.-----Orderly trouble shooting should find the issue in a matter of minutes.

 

[1up]

Compression test is the first thing to do.----Post the numbers here.----Then test spark.----Sounds like it might be running on 2 of 3 cylinders.-----Or there is blockage in a carburetor.-----Or a hole in fuel pump diaphragm.-----Orderly trouble shooting should find the issue in a matter of minutes.

Thank you for the condensed write-up.

Will update when there is news.

It is a rainy week ahead locally but the idea is to get the engine sorted for late season fishing and cruising.

 

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Howdy folks, after an evening messing with the engine and mosquitos here are the results:

Compression test conducted below operating temp, about 130 PSI for each cylinder. Minor variations. Maybe the number would have bwen higher should I have kept the starter on for a bit longer.

Spark plugs also replaced BR8HS, the store didn't have BR8HS-10. Only the connector tip at the boot side differ.

Result, first run with cowl off I got pretty food speed for a few minutes then it bogged down.
Lower unit did feel a bit warm actually after the run. So I wonder now if perhaps t-stat or impeller is properly doing its job. My hunch is safe mode due to "overheat" but that is my guess.

Anyhow, the engine doesn't run perfect either at low throttle after it has had its top speed run.

Photo is of the just newly replaced plugs;

Igniting end,
Right was living in bottom cylinder
Middle in middle cylinder
Left in top cylinder

Boot end,
Top living in bottom cylinder
Middle in middle cylinder
Bottom in top cylinder

Any further suggestions or conclusions are welcome.

 

[racerone]

Sparkplugs should all look the same.----Too much fuel on #1 and #.----No spark on 1 and 3.----Crankcase compression issues on 1 and 3.

 

[1up]

Thanks for feedback. One look oily, one look like it never got spark, plug for bottom cylinder. I feel I need to verify spark, testing leads first. I was alone at the dock today. With a helper I can ground the spark plugs and check for spark.

Perhaps as mentioned there are bigger issues. Still engine sounds and goes well those minutes it pulls before bogging. I am not giving up on her yet.

 

[racerone]

Need to do a proper spark test.----Does spark jump a gap of 3/8" ( 1 cm ) or more on a test device.---Sparkplugs are not used for a proper spark test.

 

[1up]

Need to do a proper spark test.----Does spark jump a gap of 3/8" ( 1 cm ) or more on a test device.---Sparkplugs are not used for a proper spark test.

Thanks for the details.

Will try some more with the engine.

 

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So I got one hour at the boat dock today. I opened the fuel pump to check the condition of the membrane. I could not see any tears or issues with it. Granted, it seemed paper thin though, but appeared intact. Of course a spring from the bottom fell out and a shim. I captured the spring, the shim fell down into the cowl. To be magneted / searched for later.

The thermostat housing appear to have not been touched since 1987 when the motor was put together judging by the paint on the housing mating surface. The aluminium? bolts was stuck a bit. One snapped.

So that'ts how it sits right now. I got a spark plug checker, inline variant with testing light. That's next on the agenda, for another day.

Is it easy to see if there is an issue with the membrane of the fuel pump? Just so I didn't miss anything.

 

[99yam40]

usually just remove the pump from the block and pump the bulb to see if any fuel come out the pulse port not along the edges.
with the bolts removed you might see leaks between the parts

 

[1up]

usually just remove the pump from the block and pump the bulb to see if any fuel come out the pulse port not along the edges.
with the bolts removed you might see leaks between the parts

Perhaps I overdid the check then. What does 'pulse port'refer to? I can see an 'In' and an 'Out' fuel connector on the pump housing where the fuel line comes from the fuel filter, the out side seem to feed the three carbs.

 

[99yam40]

it is the hole on the backside of the pump into #3 crankcase that furnishes the pulse to the pump diaphragm

 

[1up]

it is the hole on the backside of the pump into #3 crankcase that furnishes the pulse to the pump diaphragm

Interesting. I need to review that. Does that mean a vacuum line would go between the back of the pump and the crankcase? Not sure I saw such a hose.

 

[racerone]

Many pumps are mounted directly over the PRESSURE pulse port.