Yamaha 15hp is pulling air on the fuel stroke

[starett]

"I brought my Yamaha 15mshv 2 stroke to the repair shop after I failed to repair my problem of not being able to start it. I did go through all the usel steps of cleaning carb replacing plugs new fuel check spark. There diagnoses was that on the draw stroke air is getting by the crank seal not allowing the draw stroke to pull fuel? Has anyone experienced this before or should i be looking at something else

 

[moonfish]

Re: Yamaha 15hp is pulling air on the fuel stroke

You have a compression tester you can use? They're cheap and they will tell you what you want to know. You could pull the spark plug and put your thumb over the hole and when you turn the motor over it should push your thumb off with the pressure and both cylinders should act the same.

Why didn't the mechanics fix the problem? If they think it's the seal, why didn't they get the seal and fix it?

The FAQs here are awesome

 

[starett]

Re: Yamaha 15hp is pulling air on the fuel stroke

The comprssion on both cylenders 125 psi the macanic explained to me that the motor needed to be sent to a machine shop to deal with the crank shaft pressing the pistons back on the shaft? Not quite sure if im not getting the shaft! seems to me the problum would be at the reeds?

 

[sigma]

Re: Yamaha 15hp is pulling air on the fuel stroke

What year is your motor? I want to look it up to give you some more info

 

[starett]

Re: Yamaha 15hp is pulling air on the fuel stroke

The motor is A 1997 15mshv.the macanic explained that the motor ran great when air was forced into carb at idel or runs at half throttle with out any help
but wont idel on its one

 

[sigma]

Re: Yamaha 15hp is pulling air on the fuel stroke

Not sure why he would have to take the crank apart to replace the crank shaft seals?????

Here is a pic

http://www.yamaha-motor.com/outboard/parts/home.aspx

 

[moonfish]

Re: Yamaha 15hp is pulling air on the fuel stroke

S, explain the problem more. You have it running, but it won't idle? Or what? Explain it all out.

 

[starett]

Re: Yamaha 15hp is pulling air on the fuel stroke

here is some history on the motor.originaly there was water in the lower clynder,replaced head gasket.went through the carb and replaced fuel pump diaframs,reed valves check OK.now the motor is hard to start once started it will only run at 1/2 (1500rpm)throtle and not able to gear it.But if you idel down it dyes,wont pull fuel from carb only if it is forced with air behind the fuel.This also has me very confused.only thing i can figure is on the intake stroke its pulling air insted of fuel from somewhere

 

[moonfish]

Re: Yamaha 15hp is pulling air on the fuel stroke

That's easily possible. Put some premixed gas in a squirt bottle and feed it some gas, start it and see if you can keep it running that way.

Also, another thread had the idea to put in some clear fuel line temporarily so you can see if there are bubbles in the gas, there shouldn't be but if it's leaking, you'll see them. I think most of my motor running troubles have been gas line leaks.

You can also unhook the fuel line after the fuel pump and turn it over and see if it's pumping. Hook some clear hose on there and it'll be easy to see.

Gas lines can be bad anywhere, in the motor, any connector, even in the gas tank itself.

You should get the manual for the motor. Diagrams are great and invaluable. You can also have problems if you messed up assembling the carb or there's a gasket missing or something. It happens, and it's all hard to convey over the net.