Yamaha C 90 - 2 Stroke Prime Start Issues

[Riversea]

Initially suspected prime start issues. Carbs were all disassembled, checked and cleaned. Plunger & diaphragm both disassembled and checked. The stainless steel "leaf" valve under the diaphragm is NOT damaged and seems to be sealing. No damage and (seemingly) operating correctly. Disconnected L-shaped fuel delivery hose from prime start to manifold. Fuel flow from the prime start to # 2 intake, but ONLY when hose in picture is disconnected or clamped shut. What is the purpose of this hose? It runs from the # 2 carb to the spigot on the intake manifold between # 1 & # 2 cylinder. Blew in the hose in both directions. Free flowing from the carb to the manifold and from the manifold to the carb. Reed valve petals all intact. Possible that there is a "check valve" missing from the prime start assembly? Thanks for your help.

[TownsendsFJR1300]

What specifically is the engine doing or not doing?

[Riversea]

Thank you for your reply. The engine is extremely hard to start (cold) because the prime start is not functioning with the (red) selector switch in the auto (normal) position. With the pictured hose disconnected or clamped shut, the fuel delivery pipe from the prime start assembly, delivers fuel to the top plate (6H3-14146-10-00) of the # 2 carb and the cold engine fires right up.

[panasonic]

Good explanation of primestart here.

https://youtu.be/x35uqVMu2r0

That should be the vacuum pulse hose to operate the diaphragm in the primestart assembly. Why pinching it off makes the primestart work I don't know. Will have to read up on that.

What year and exact model is the motor?

Where in the He]] is Boscoe???

[99yam40]

I am not sure on that 90, but on my C40 there is a hose to the top side of a carb so that the Prime start can suck in air from there while furnishing fuel also.
in my search for how the system worked I found that in the normal position it did not suck solid fuel into the enrichment, it was a very rich vapor (mix of fuel and air).
only when put to on did it send just liquid

[Riversea]

To Panasonic's comment / question:
It is a 2001 (C90TLRZ), without oil injection.
The top hose in the current picture is the pulse hose to the Prime Start diaphragm.
Just to be clear, the hose that I was "pinching" was NOT the pulse hose; it was the small hose shown in the pictures at the top of this thread.
Still trying to figure out the purpose of that small hose.

[panasonic]

I found this on another forum. Somebody went through a lot of work to draw this out so have a close look and see if it looks right....I haven't had time.

prime start.jpgprime start 2.jpgprime start 3.jpg

[99yam40]

that sure looks familiar,
I am the one that drew that and posted it.
it is what I traced out on my C40TLRX

[robert graham]

Pretty darn good drawing.....I've owned several Yamaha outboards over several decades and never had a problem or issue with the Primestart....seems like a great little device so far....knock on wood!

[pstephens46]

I’ll go find Boscoe and harass him. Be back shortly...

[panasonic]

that sure looks familiar,
I am the one that drew that and posted it.
it is what I traced out on my C40TLRX

Well there you go...it is still kicking around the Internet. Looks like you put some good thought and effort into it.

Now to try and figure out what line the OP is pinching off to make it work...I am still not sure what line he is referring to.

[99yam40]

I cannot tell either, but my guess would be the vent line to the top of a carb, where the system gets air to mix with the liquid fuel.

but if the line is just disconnected and not blocked off, then it would be still sucking in air to mix with the liquid fuel.


It may be that the OP falsely thinks that he should see liquid fuel coming out of that one line when in the normal position, which I thought also at first.
I could not find anyone to give me an answer to that question, so I dug a lot deeper and came up with these drawings.

To the best of my knowledge there will not be a solid liquid flow unless you turn the red lever to the open position.( basically blocking off the air)

I used a clear hose in place of the one heading to the manifold behind the carbs so I could watch for liquid , and it only had liquid flow while in open position.
in normal you could not see anything, apparently it just sucks a super rich vapor in to add to the normal carb jets flow.

Rodbolt did tell me that even tho it looks like it just feeds on cylinder on the manifold, there are channels that feed all 3 cylinders bored into the manifold.

maybe Robert G could look at his motor and figure out what hose the OP is talking about to make sure the OP's line is hooked to the proper places.

thinking about this some more, there should only be 3 hoses hooked to the prime start.
the pulse line from crankcase to diaphragm,
vent line to top of one carb,
and the line feeding the enrichement fuel into the manifold

[Riversea]

I assume I'm the OP (what does OP mean?).
With respect to the hoses attached to the carb, there are three: 1. The 1/4" Pulse line from the crankcase to the diaphragm. 2. 1/4" "L" shaped hose from the prime start to the side of the top plate of the # 2 carb. 3. The small diameter vent hose from the prime start to the starboard side of the manifold.(pictured)
Anyway, the hose that I was pinching off is the one shown in the picture here, as well as the two pictures at the top of this thread. It is a small diameter hose (about 1/8") that runs from the top of the prime start to the barb on the starboard side of the manifold. I am confident that the placement of this small diameter hose, is correct, given the small diameter of both barbs. Not only does the fuel flow when the hose is pinched; it flows when the line is removed, either at the carb or at the manifold.
Incidentally, the fuel flow (at cranking speed) from the prime start to the upper chamber (plate) of the # 2 carb is NOT vapor, it is fuel, with some air bubbles present. This occurs with the valve in either the normal or opened position.
I appreciate the interest and the help of all that have responded on this topic and will continue in my attempt to solve this issue. Any additional ideas are appreciated.

[99yam40]

OP is original poster

It has been awhile, but looks like the PS is mounted to the carb and there is an internal passage from #2 bowl to furnish liquid fuel lines

#2. vent line from top of carb to PS to allow air to be sucked into the PS

is this the one you are pinching off?

#3 line from PS to the manifold, is this the one that you are seeing fuel flow out of when you pinch off the #2 mentioned above?

I was getting confused about what hose you were pinching and which had fuel flowing, I though they were on in the same which made no since to me at the time

I am thinking that when you pinch off #2 it would be the same as putting it into the open position, the fuel cannot flow back to the carb bowl when you pich it off so it runs up the other line to the manifold thru the normal passage.

never thought about this before, but the diaphram is pulling and pumping from #2 carb bowl,so maybe it needs to send it back to the carb it pulled it from when not being all used and that is not for sucking in air.

Anyway I did not see any liquid fuel heading to the manifold on my C40 when I put a clear hose in place of that #3 line when starting or running my motor cold. but I did when put into the open position..

If you find any more info on this system let me know, I never could find the info I was looking for. If liquid was to be delivered when in the normal position.

by the way My C40 does crank up and run well in cool weather, but being on the middle Tx coast, it does not get really cold here very often and when it does I do not go out in it anymore.
back when I was young I hunted and fished even with ice on the water, but I am not that mad at those fish and ducks these days