Yamaha Two Stroke Primestart

[pstephens46]

What we need is someone with a two stroke that has a working Primestart to measure the pressure pulses so we can know more or less what they be.

Where would that lead?

[boscoe99]

Where would that lead?

If someone has a Primestart that is not doing its job it would be something to check. No pressure pulses, or weak pressure pulses, might cause the device to not do its job. Could be a kinked or internally failed hose. Could be something worse. I don't know.

[99yam40]

from what i remember in troubleshooting my C40, when in the normal position there was no liquid fuel being pumped into the line that ran to the intake.
but when placed in the on position there was liquid fuel.

I placed a piece of clear tubing in there to see what was going on.

I asked about this happening back in 08 I think and no one could answer if that was normal or not.

So I would think that when in normal position the pump is not doing anything.
The motor just sucks enriched air/fuel mixture into the intake as long as the needle is letting fuel past

I drew some diagrams of the system back then and posted them when asking about them.

I think it was Rodbolt that said I was trying to make too much out of it, thinking too much.
I you would like I can see if I can find them again to post up here on this thread.

here is a link to that thread
http://www.yamahaoutboardparts.com/f...ystem-question

[dray0151]

Here you go Yam I saved these and posted to this link.

[rodbolt17]

yall be whay whay over tinking.
it is a simple pulse pump with a heated wax pellet connected to a needle. as the pellet expands it simply shoves that tapered needle into a seat and shuts off the fuel.
yep it is actually that simple.

[boscoe99]

yall be whay whay over tinking.
it is a simple pulse pump with a heated wax pellet connected to a needle. as the pellet expands it simply shoves that tapered needle into a seat and shuts off the fuel.
yep it is actually that simple.

The question is about the needle being retracted (valve open) yet gasoline is not getting to the carburetor. If the pulse pump is not getting a sufficient pulse pressure from the crank case, the diaphragm is so worn that it can not pump gasoline, or perhaps a reed valve is stuck closed or open, would that not result in the Primestart being unable to do its job of providing extra gasoline when the motor is cold?

Yamaha only addresses the needle being open or closed as far as I can tell. What about the rest of the system?

[99yam40]

Actually the prime start enrichment system gets its fuel from the carb fuel bowl (Center one on my C40).
I am not sure if the diaphragm is needed to pump fuel thru the needle open port, or if the intake suction would pull it along with the air or not.
the more I think about it I would think the liquid fuel would be better off being pumped(under pressure at the needle port

[boscoe99]

Actually the prime start enrichment system gets its fuel from the carb fuel bowl (Center one on my C40).
I am not sure if the diaphragm is needed to pump fuel thru the needle open port, or if the intake suction would pull it along with the air or not.
the more I think about it I would think the liquid fuel would be better off being pumped(under pressure at the needle port

Four strokes don't use the pulse pressure pump. Makes me think they use engine suction/vacuum (hate those words) only to move the fuel along. Two strokes on the other hand use the pulse pump. Two strokes typically have less compression and therefore less suction/vacuum (hate those words) I would think. So they might need a pump to assist.

[boscoe99]

I have seen this done on some Yams. The screw mechanism is normally locked from the factory with some type of sealant. Sealant is broken loose without damaging the PS.

https://www.youtube.com/watch?v=toYTwg2XR-0

[walleye1]

The only time I have had to change that diaphragm is when the engine has running issues, like popping through the intake from the timing being off.

[boscoe99]

The only time I have had to change that diaphragm is when the engine has running issues, like popping through the intake from the timing being off.

Yes, just like a back fire can damage the main fuel pump diaphragm I would think it could damage the PS diaphragm. Folks that are having PS cold start issues seldom get the word to check the diaphragm for damage. Maybe damage the reed/petal valves as well.

Would be nice to have a way to remove the carburetor with the PS and then apply pressure pulses to it to see if it pumps gasoline. I once knew a gal that could probably do it but she is long since gone.

[walleye1]

I new a gal like that, I kept her.

[boscoe99]

I sent mine away. She kept rupturing my diaphragm.