Yamaha runs awesome, will not re-start after running???

[JLarimar]

Appreciate any support on this one.

I have a late 90's 50 hp Yamaha 4 stroke that runs like a champ and will start right away when cold. Once the engine is warm or I cruise for a bit and shut the motor off, IT WILL NOT RESTART. The starter motor turns the flywheel and the engine won't turn over and run until the engine cools down or sits for an hour or so. I did notice today that the water coming out of the engine was very hot. The water temperature on the lake was 46°F today. Could a bad thermostat prevent a restart on a hot motor?

Here is a list of recent repairs/maintenance that has not fixed the issue...
- fully charged batteries, running 12.9-13 volts
- cleaned all four carburetors, no issues with the floats
- cleaned and checked fuel pump, no issues with the diaphragm
- brand new spark plugs, change motor oil/filter, change fuel filter

Any help would be greatly appreciated here. Thanks again, feel free to ping me with any questions.

Your friend,
Conundrum in Carolina

 

[boscoe99]

Have you tried advancing the throttle to the wide open position just before engaging the starter motor to see if it will start then?

Being mindful that as soon as it starts you need to quickly retard the throttle.

 

[JLarimar]

Have not but could do. What’s the thinking?

 

[99yam40]

well, no start means no spark at proper time once heated up, bad compression when heated up, not enough fuel or too much fuel when heated up.
spark and compression should be easy to check, but the correct ratio of fuel to air is not that easy.
so, Bosco mentions adding more air by opening up the throttle to see if that helps.

hot motor could cause fuel to boil off causing an over rich condition

 

[JLarimar]

Thank you for the explanation. I will give this a try and report back. Also going to change the thermostat which has not been done.

 

[99yam40]

you can always pull the stat and test it in a pot of water to see if it opens and closed at the correct temperatures to see if that is a problem.

or buy a new one and hope that was the problem.

It is not normal for the telltale to be real hot on 2 strokes, but I know very little on 4 stroke motors so I cannot say if they should have very hot TTs

 

[QBhoy]

Is that the battery voltage when it’s running ?

 

[airshot]

You stated motor wont turn over and run...is it overheating and starting to seize up?? I would pull the plugs, get compression checked and look at cylinder walls for any scuffing.

 

[JLarimar]

Do not have a battery voltage gauge on the boat to tell running voltage. Added the battery voltage when charged to eliminate the battery as the problem.

 

[JLarimar]

you can always pull the stat and test it in a pot of water to see if it opens and closed at the correct temperatures to see if that is a problem.

or buy a new one and hope that was the problem.

It is not normal for the telltale to be real hot on 2 strokes, but I know very little on 4 stroke motors so I cannot say if they should have very hot TTs

That’s the plan to test stat

 

[boscoe99]

Have not but could do. What’s the thinking?

Trying to determine if it is flooded or not.

 

[boscoe99]

You stated motor wont turn over and run...is it overheating and starting to seize up?? I would pull the plugs, get compression checked and look at cylinder walls for any scuffing.

OP says "The starter motor turns the flywheel and the engine won't turn over"

When he says it won't turn over I suspect that he really means it won't start. Many, for reasons unbeknownst to me, use that term for a failure to start.

Some will also say it won't crank. To them meaning it won't start.

If the starter motor is turning the flywheel then to me that means the motor is turning over.

Good thing we all share the same language.

 

[SOG3]

Could be low compression, or possibly fuel system is vapor locking. JMO

 

[JLarimar]

OP says "The starter motor turns the flywheel and the engine won't turn over"

When he says it won't turn over I suspect that he really means it won't start. Many, for reasons unbeknownst to me, use that term for a failure to start.

Some will also say it won't crank. To them meaning it won't start.

If the starter motor is turning the flywheel then to me that means the motor is turning over.

Good thing we all share the same language.

Correct, won’t start but turns over.

 

[JLarimar]

Could be low compression, or possibly fuel system is vapor locking. JMO

Going to check compression.
Maybe allowing the motor to idle and pump water to cool is a good idea before stopping the motor in hopes to prevent vapor lock?

 

[airshot]

Going to check compression.
Maybe allowing the motor to idle and pump water to cool is a good idea before stopping the motor in hopes to prevent vapor lock?

Motor should never get that hot to worry about a cooling down period. If it is, then that is a sign of an overheating problem.

 

[99yam40]

the more fuel you burn the more heat is generated, so if the cooling system is not working well it can cause more heating while running high RPM

 

[JLarimar]

Have you tried advancing the throttle to the wide open position just before engaging the starter motor to see if it will start then?

Being mindful that as soon as it starts you need to quickly retard the throttle

UPDATE - ran the motor on muffs till warm, shut off, would not restart, opened the throttle and motored started!!

 

[JLarimar]

You stated motor wont turn over and run...is it overheating and starting to seize up?? I would pull the plugs, get compression checked and look at cylinder walls for any scuffing.

UPDATE - compression check was good (150, 130, 150, 130), the manual references 122psi as minimum, everything good here

 

[saltchuckmatt]

Well you know how to start it when warm/hot now but now you need to know why you have to....more work to do