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Old 13-03-2020, 16:15   #16
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Re: Yamaha 8HP 2 stroke outboard

Try running it then disconnect the fuel line a run it dry. The leave for the period that normally gives a problem the reconnect prime and start if starts you know the tank is a problem.
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Old 13-03-2020, 17:25   #17
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Re: Yamaha 8HP 2 stroke outboard

You should be able to verify it’s the tank overpressurizing and flooding the engine by leaving the cap loose, if the problem goes away, your onto something.
The gas boiling out of the carburetor is called a vapor lock, vapor locks are usually in fuel lines and often fuel injected engines, the Lycoming aircraft engine is famous for it.
I have never seen an outboard vapor lock and doubt it’s possible, they just run so cool that I don’t see it happening, being raw water cooled engines they have to run cool, above some temp and something precipitates out of the salt water and clogs up water passages.
Looks like lime, I think it may be calcium carbonate but am not sure, it’s not salt.

The CARB compliant we are talking about is California Air Resource Board, California has set standards in excess of Federal standards and they are so big that I guess as a manufacturer you need be CARB compliant to sell in California, but not allowing vapor emissions is what these one way valves are about, so instead it overpressurizes the fuel system, overrides the needle / float and leaks raw fuel into the water.

They are also responsible for those silly complex valves on portable gas tank spouts, you know the ones that are so cumbersome to use that you end up spilling a whole lot of fuel trying to use the tank.

I’m no Lawyer, but that is my take on CARB anyway, I believe they have the purest goals, but somehow the implementation isn’t tested and it’s often a mess.
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Old 14-03-2020, 05:58   #18
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Re: Yamaha 8HP 2 stroke outboard

Vapor lock could be an issue

Also check the electrics that stop the motor

Be sure that you have s spark

Your motor may have an overheat sensor , this breaks the ignition
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Old 14-03-2020, 06:21   #19
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Re: Yamaha 8HP 2 stroke outboard

I had a similar fuel issue last year. I had purchased a new tank, one of the new ventless. I purchased the pressure valve separately from the hose, not the combo WM had. It doesn't spray fuel when you connect it and the tank is under pressure. The valve I purchased I think was developed by Mercury Marine. The pressure valve may be your problem. The diaphragm in the carb was damaged, I think, by the excessive fuel pressure. I have a Yamaha 15 2 stroke.
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Old 14-03-2020, 07:21   #20
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Re: Yamaha 8HP 2 stroke outboard

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I had a similar fuel issue last year. I had purchased a new tank, one of the new ventless. I purchased the pressure valve separately from the hose, not the combo WM had. It doesn't spray fuel when you connect it and the tank is under pressure. The valve I purchased I think was developed by Mercury Marine. The pressure valve may be your problem. The diaphragm in the carb was damaged, I think, by the excessive fuel pressure. I have a Yamaha 15 2 stroke.
When connecting to the outboard it is definitely spraying gas out of the connector when the tank is pressurized. I'm going to try leaving the cap unscrewed and seeing if i still have a problem. Also probably not a bad idea to get a spare fuelndemand valve as we are out cruising.
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Old 14-03-2020, 07:27   #21
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Re: Yamaha 8HP 2 stroke outboard

Echoing a previous post, you need both fuel and spark to get it to fire up. Need to systematically eliminate possible problems.

Did you test for spark (at the plugs) when it was hot and wouldn't start? Were the plugs wet? If wet, the fuel probably ok (poss. flooded). If no spark, need to trace back the issue on that side. It could be as simple as the kill switch that is malfunctioning.

If spark is OK, then check fuel system. Could be the fancy fuel tank, could be a collapsed fuel hose (seen this before w/some plastic lined fuel hoses). Is there fuel in the bowl/carb? If not, check for dirty fuel filter or fuel pump or back to fuel tank.
If fuel in carb, take carb apart, clean jets and check for stuck float.

The great thing about the 2 strokes is they are fairly simple and usually easily fixed. It can be frustrating at first to find sporadic problems, but the systematic approach is a great way to do trouble shooting.
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Old 14-03-2020, 07:31   #22
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Re: Yamaha 8HP 2 stroke outboard

The problem with my Yamaha 8 two stroke is that it is so reliable that I have never had to fix it

Only the normal impeller , spark plugs ...
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Old 14-03-2020, 07:37   #23
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Re: Yamaha 8HP 2 stroke outboard

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Originally Posted by Bill O View Post
Echoing a previous post, you need both fuel and spark to get it to fire up. Need to systematically eliminate possible problems.

Did you test for spark (at the plugs) when it was hot and wouldn't start? Were the plugs wet? If wet, the fuel probably ok (poss. flooded). If no spark, need to trace back the issue on that side. It could be as simple as the kill switch that is malfunctioning.

If spark is OK, then check fuel system. Could be the fancy fuel tank, could be a collapsed fuel hose (seen this before w/some plastic lined fuel hoses). Is there fuel in the bowl/carb? If not, check for dirty fuel filter or fuel pump or back to fuel tank.
If fuel in carb, take carb apart, clean jets and check for stuck float.

The great thing about the 2 strokes is they are fairly simple and usually easily fixed. It can be frustrating at first to find sporadic problems, but the systematic approach is a great way to do trouble shooting.

I haven't had the issue happen yet again. But when I was pulling the other day there was a gasoline sheen on the water around the outboard as I kept pulling. I'm assuming this is fuel coming out of the exhaust as it clears the cylinders and doesn't ignite?
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Old 14-03-2020, 07:51   #24
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Re: Yamaha 8HP 2 stroke outboard

The in line pressure valve just might be not functioning in. The pressure built up so bad that it was filling one cylinder with gas. I did a complete carb rebuild including the fuel pump/diaphragm, which is a seperate item, not in the kit. total cost was about $60 for parst, including plugs. You might try a carb rebuiid, and a new inline valve, your motor is older, I imagine. I don't know if the WM valve
is their own manufacture, or if they buy it from the original vendor. I sometimes question WM's own brand. Keep us posted how you make out.
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Old 14-03-2020, 08:22   #25
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Re: Yamaha 8HP 2 stroke outboard

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Originally Posted by felizcortez View Post
I haven't had the issue happen yet again. But when I was pulling the other day there was a gasoline sheen on the water around the outboard as I kept pulling. I'm assuming this is fuel coming out of the exhaust as it clears the cylinders and doesn't ignite?
Most probably, it could also just be leaking from the carb being overpressurized, but if appeared when you were trying to start its most likely what you suppose.
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Old 14-03-2020, 08:27   #26
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Re: Yamaha 8HP 2 stroke outboard

These two strokes are not CARB compliant nor EPA for that matter, as such they arenít designed to operate with CARB compliant fuel tanks etc.
If it were me instead of trying to add some kind of fuel pressure regulator or whatever in the fuel line, Iíd purchase a vented fuel tank, like they are designed to use.
This is the fuel tank sold by Yamaha in the Bahamas for their two stroke motors, you get one when you buy a motor of course, notice the simple fuel vent in the gas gauge fuel cap, itís a simple screw, vents both ways, isnít CARB compliant, but is what is supposed to go on a Yamaha two stroke.
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Old 14-03-2020, 10:01   #27
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Re: Yamaha 8HP 2 stroke outboard

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These two strokes are not CARB compliant nor EPA for that matter, as such they arenít designed to operate with CARB compliant fuel tanks etc.

If it were me instead of trying to add some kind of fuel pressure regulator or whatever in the fuel line, Iíd purchase a vented fuel tank, like they are designed to use.

This is the fuel tank sold by Yamaha in the Bahamas for their two stroke motors, you get one when you buy a motor of course, notice the simple fuel vent in the gas gauge fuel cap, itís a simple screw, vents both ways, isnít CARB compliant, but is what is supposed to go on a Yamaha two stroke.


The carb cans are an easy fix too.

The scepter vent valve is a simple rubber duckbill thatís easy to mangle with a pair of pliers. Now it vents normally and the vent screw plug still functions.

The moeller cans are almost as easy. Pull the vent plug out, run a small drill into the hole, put the vent plug back in.
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Old 16-03-2020, 08:46   #28
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Re: Yamaha 8HP 2 stroke outboard

I've been disconnecting the fuel hose and venting the tank before connecting the hose to the outboard. I haven't been able to recreate the issue since doing these things. I'm leaning towards excess fuel pressure now. Next thing I'm going to try is leaving the fuel hose connected and venting the tank while still connected.
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Old 23-03-2020, 08:12   #29
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Re: Yamaha 8HP 2 stroke outboard

I have the same engine.
this works every time.
with engine off, put shift lever in forward.
crank throttle wide open and closed 2x.
put back in neutral, choke, open throttle 1/4 way...give her a pull.
Bam.
ur underway.
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Old 23-03-2020, 09:25   #30
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Re: Yamaha 8HP 2 stroke outboard

I have a Yamaha 8lg. There is another possibility. I found the epoxy encapsulation
Of the CDI ignition module cracking. This is a common problem with older Yamaha modules. It drove me crazy with not starting intermittently. Not saying this is your problem, but worth unscrewing the module and flipping it over and look for cracks.

I live in California and found the the fuel tank cap on my old tank fits the new carb compliant tank . Both were bought from West Marine. The old cap has a vent that works.

Another thing in sealing the threaded fittings. I found that Permatex Aviation Form-a-Gasket No. 3, available at Napa, is bombproof for selling threaded gas line fittings. It will resist the tank pressure, even when swelled up from heat. I use it on all my gas fittings and no leaks.
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