1990 Yamaha Pro 50 2-Stroke - trouble starting, now it won't start

[mattpletz]

Hello everyone - thank you in advance for your input. Would love to hear some suggestions & ideas as I'm out of them now. I suspect a fuel issue, but can't sort it out. I'm a backyard repair guy for small engines at best.

MOTOR: 1990 Yamaha Pro50 2-Stroke, Model Type Code: 6H5 / Model 50D
Exploded parts view: https://ypec-sss.yamaha-motor.co.jp/ypec/ypec/b2c/html5/app/en_CA/parts-search/index.html

PRIOR PROBLEM & WHAT HAS BEEN DONE:
Bought it and had it out last month. Struggled to start at the dock - would fire but then die out within 1 minute. Once warmed up, it would run awesome at all speeds, but would sputter occasionally as if it was misfiring. Would cut out occasionally when trolling. Did NOT have any troubles with it bogging down. MY FIX: increased the idle speed and adjusted one carb that was out of adjustment with the other 3. Ran better, but still struggled to start & warm-up at the dock every time.

NEW PROBLEM: Won't start at all!
HERE'S WHAT I'VE DONE:
-Killswitch is present and in tact.
-Spark: Blue spark in all three plugs
-Compression: Good.
-FUEL SYSTEM (in order):
>Fresh fuel with stabilizer is in the tank. Tank is clean.
>Primer bulb is pressurized and delivers fuel, no blockages
>Fuel filter is clean, no debris or blockages
>Thought this was a fuel pump problem as the diaphragms were stretched, but replaced it today with a brand new pump and it still won't start.
>No leaks or cracks in fuel lines
>Carbs taken apart & cleaned (they were very clean, did not see any dirt or debris at all in any of the jets or bowl). When re-assembled, fuel is successfully delivered to each bowl when the fuel system is primed. No leaks.

STARTING THE MOTOR:
Killswitch is good. In Neutral. Flicked the switch for the choke & adjusted the choke lever to ON. Key turned and engine cranks but doesn't turn over. Crank battery is brand new & charged. Checked the plugs and all three are DRY.

Thank you in advance for your suggestions. I have no issues about taking it in for a look in the spring, but there's a certain amount of pride a young guy feels when fixing it himself (and with your help

 

[99yam40]

from what I saw on another web site(your link no workie), the 1990 pro50LD does not have a choke, it has the Yamaha prime start system.
Or some type of solenoid valve with a pump on the bottom carb
So I have no idea what you mean by Flicked the switch for the choke & adjusted the choke lever to ON

 

[mattpletz]

Forgive my terminology.
I meant the Prime Switch was clicked up (it returns back to the same position after being clicked up) with the fast idle lever up.
Sorry, I've never owned a Yamaha before, so this is new to me.

 

[CaptnKingfisher]

Sounds like no fuel is getting to the cylinders. You could try taking a plug out and dripping a little fuel into a cylinder, and see if it starts briefly. If that works them try dripping a little fuel into the carb and seeing if it'll start like that. If the first test worked but the second test failed then fuel is not getting from the carbs to the cylinders and you need to rebuild them. If the first and the second test both get the motor to fire briefly then you've got a fuel delivery problem and I'd start by testing fuel pump pressure.

Other thoughts, tank could be improperly vented, or if you have an antisiphon valve that could have junk blocking fuel flow. I hate troubleshooting fuel issues.. I will often use a portable outboard tank as a diagnostic tool to rule out my tank, vent, and incoming fuel line as potential issues.

 

[99yam40]

Forgive my terminology.
I meant the Prime Switch was clicked up (it returns back to the same position after being clicked up) with the fast idle lever up.
Sorry, I've never owned a Yamaha before, so this is new to me.
View attachment 352823

if this motor does have the prime start system, you do not need to do anything but turn the key switch to the start position to start the motor.

Since that electrical part they call a solenoid valve is different than the heater unit on a prime start system, I have no idea how this motor is suppose to work when starting

have you taken the cowling off and silencer off the font of carbs to see if this motor has choke plates or anything else that move when you hit that switch below the key?
do you hear anything operate when you flip that switch?

 

[mattpletz]

CaptnKingFisher - thank you, that test sequence is helpful. I will perform those tests shortly. I have tried using two separate fuel tanks. Both tanks were delivering fuel a couple weeks ago when I first ran the boat.

99yam40 - yes, cowling is off and the silencer. The switch does activate something when clicked, but it's not visible. But, I've never touched that switch when the boat was previously running.

 

[99yam40]

CaptnKingFisher - thank you, that test sequence is helpful. I will perform those tests shortly. I have tried using two separate fuel tanks. Both tanks were delivering fuel a couple weeks ago when I first ran the boat.

99yam40 - yes, cowling is off and the silencer. The switch does activate something when clicked, but it's not visible. But, I've never touched that switch when the boat was previously running.

so it sounds like there are no choke plates on the carbs or linkages that get moved when you activate that switch.
maybe this motor has some type of extra fuel injection operated with that switch.
Normal prime start systems I have seen use the crankcase pulse to operate the diaphragm pump on the system, but is only used when in the open position, not the auto
Opening the fast idle lever adds too much air for the normal prime start system.
finding an owners manual and maybe the service manual will help you understand the proper way to cold start that motor

 

[boscoe99]

A 1990 Pro50LD does not have use choke plates. It is a prime start model. Automatic fuel enrichment. Nothing need be done by the operator.

The toggle choke switch is on the 703 control box for those models that have carburetors with chokes. The toggle switch does nothing. Despite many saying that it helps to start their motor.

There is a PrimeStart lever on the from of the PrimeStart assembly. Is the lever in the automatic position? Turned fully clockwise?

The fast idle lever is normally down and fully stowed. Adding more air at the time there is normally too much air will only make a cold start situation worse. Having said that, try it up just a tad and see if the problem is the same, better or worse.

 

[99yam40]

A 1990 Pro50LD does not have use choke plates. It is a prime start model. Automatic fuel enrichment. Nothing need be done by the operator.

The toggle choke switch is on the 703 control box for those models that have carburetors with chokes. The toggle switch does nothing. Despite many saying that it helps to start their motor.

There is a PrimeStart lever on the from of the PrimeStart assembly. Is the lever in the automatic position? Turned fully clockwise?

The fast idle lever is normally down and fully stowed. Adding more air at the time there is normally too much air will only make a cold start situation worse. Having said that, try it up just a tad and see if the problem is the same, better or worse.

But why does the parts page list a solenoid valve assembly instead of a Prime Start assembly if it is a true Prime start system I am use to seeing?

 

[mattpletz]

A 1990 Pro50LD does not have use choke plates. It is a prime start model. Automatic fuel enrichment. Nothing need be done by the operator.

The toggle choke switch is on the 703 control box for those models that have carburetors with chokes. The toggle switch does nothing. Despite many saying that it helps to start their motor.

There is a PrimeStart lever on the from of the PrimeStart assembly. Is the lever in the automatic position? Turned fully clockwise?

The fast idle lever is normally down and fully stowed. Adding more air at the time there is normally too much air will only make a cold start situation worse. Having said that, try it up just a tad and see if the problem is the same, better or worse.

Mine doesn't have the fuel enricher PrimeStart attached to the carbs like I've seen. Pic to show...
Good point about the Fast Idle lever. I left it down and tried it up just a bit and same result - no start.

 

[boscoe99]

What is the complete engine ID? 6H5 L and then some numbers.

What country are you in?

 

[mattpletz]

What is the complete engine ID? 6H5 L and then some numbers.

What country are you in?

6H5 L 511732 - Canada

 

[99yam40]

Mine doesn't have the fuel enricher PrimeStart attached to the carbs like I've seen. Pic to show...
Good point about the Fast Idle lever. I left it down and tried it up just a bit and same result - no start.

have you tested spark to the plugs?
inductive timing light or inline spark tester would give indication if the plugs are getting spark or not

 

[boscoe99]

6H5 L 511732 - Canada

1990 model year motor alright.

I am not seeing any choke plates. I think that I am seeing the PrimeStart device.

 

[99yam40]

did you pay attention to what they called #41?

a solenoid valve assembly instead of a Prime Start assembly.
and he said he could hear something energize when he hit the switch, but saw nothing move, so it would not be a heater

 

[mattpletz]

Update: finally got some starter fluid. Motor fires instantly when sprayed into the carb throat on a cold start just by turning the key.

 

[99yam40]

Update: finally got some starter fluid. Motor fires instantly when sprayed into the carb throat on a cold start just by turning the key.

so it fires, but does it run?
at least if it fires only that one cylinder you spray the fluid into for just a second, you know that spark plug is working.
by the way that fluid has no oil for lubrication, so I would not use that on a 2 stroke motor.
maybe mix some oil and fuel and spray that into the carbs

 

[racerone]

Your starting issue has to do with the starter / choke primer solenoid.-----Are you pushing the key in and holding it in while cranking.----Or post a step by step of the procedure you are using to start this motor.------I had to coach an individual last year on starting an older motor.-----Had no idea on what to do.----Could start his van because all you need to now is how to push just 1 button.

 

[mattpletz]

PROBLEM SOLVED!
Turns out it's user error. There is a solenoid valve (#41 in the picture above) attached to the lower carburetor that activates when the toggle switch near the key is switched on. It injects a bit of fuel to help get it started. From what I've learned, my model is between models that had choke plates and then the PrimeStart system. The motor runs well again. I need to dial in the carbs yet and idle but, IT'S ALIIIIVE!
Thank you very much for everyone's input - I've learned some new ways to diagnose the fuel system and I'm very grateful for that.

 

[99yam40]

good to hear you found the info needed to explain how it should work.
where did you find it?
owners manual or service manual?