Leaky YS engines? Just a thought

[Ihncheol Park]

I just had some thinking regarding those YS engines dripping (often more
than just dripping) fuel out of carb.

I am not sure how many engines are having symptoms like that, but I see
often at RCU YS support.

I am not an expert on YS engines as many people here at NSRCA List or YS
performance.  I have only been using YS around 5 years.  Until now I had 2
of 53, 1 of 63 and 5 of 91AC/FZs.  I just bought a used 140L.

So far, non of them leaking fuel as described by others.  Maybe I was just
lucky, did not run the engine that long, or didn't even look closely.

Anyway, after reading a few postings at RCU with 120s leaking fuel through
carb.
I looked at a few pictures of disassembled parts of 140L.
Also, looked a few part diagram (Exploded views of YS engines at Central
hobbies).

When I look at those exploded views and pictures of my 140L, basic fuel
delivery of YS 4-stroke engines are almost identical.  (140 has two
additional nipples on the back plate).  53/63/91/a few 120s don't have these
nipples on backplate.

Looking at the engine upright from front, there are two nipples right in
front of the pushrods.

The one on the left (let's call this #1 nipple) is to be connected to the
fuel (clunk) line from tank.
The other one (call this #2) is to the vent line with check valve.  (on
140L, this goes to one of the nipple on back plate, and the other on the
backplate goes to the vent line of tank with check valve on it.)

Looking at the engine from left side upright, there is another nipple (#3)
that connects to the carburetor body. (Most of YS 4-stroke engines, but
maybe not all come with a blue fuel line on this connected to carb body).

All YS engines including 2-stroke (not sure of DZ) are closed system because
of pressure within the system is necessary.

I probably know only a part of it.

When we turn the crank, the engine is sucking the fuel using the crankcase
vacuum created within the crankcase after the piston moving up and down.
(Not really understand here)

This vacuum is pulling fuel to the carbbody (the disk on the carbbody is
closing and opening the vacuum, timing for fuel delivery?).  The carbbody
has the blue factory fuel line connected to the #3 nipple.

If you look at closely, #3 nipple is going through the regulator housing,
under the plunger (space between the regulator screw and the plunger. You
can see the hole that connects through #3 nipple when you remove the
regulator screw right next to the plunger).  After removing the regulator
housing, you can actually see the diaphragm shaped like a contact lens if
it's been there some time, a new diaphragm seat on the housing almost flat
with it's center seating on the plunger.

On the crankcase side where the regulator housing goes, there is a small
space  that the regulator creates pressure for the tank. there is a small
hole in the front right of the space. This hole is routed to #2 nipple.

Under the diaphragm in the regulator housing, right next to the plunger,
there is another hole.  this one goes to the #1 nipple.

Removing the diaphragm, you can touch the plunger. The plunger goes in and
out, but stays out (crankcase side) because of the regulator spring.  Thus,
it is actually open. When the vacuum happens in the crankcase, the space
between the regulator screw and the plunger is also vacuum.  Therefore, the
plunger goes in to the regulator screw side.

Then because of the check valve, the small space on the crankcase side keeps
the diaphragm pulled to crankcase side.

At this time, the space between the diaphragm and plunger is vacuum, so that
the fuel is sucked in through #1 nipple to the space between the diaphragm
and the plunger.

The fuel then goes through the tiny space between the plunger and regulator
housing when the plunger that was pulled to regulator screw side bounces
back because of the spring filling the vacuum.

The vacuum is not happening always, so the plunger can bounce back.

When the plunger bounces (no vacuum), the diaphragm is pushed to the
crankcase side creating the pressure that goes through the #2 nipple to the
tank.

That covers how the fuel is reaching the carburetor.

--------------------------
Now.  Why do they leak when the engine is stopped?
I believe it must leak if you don't relieve the pressure in the tank.

When the tank is pressurized, fuel is being pushed to engine.
There is no vacuum created since the prop is not turning.

let's follow the fuel route.
>From fuel tank to #1 nipple because of the pressure in the tank.
Through #1 nipple, goes into space in the regulator housing between the
diaphragm and the plunger.

Remember the plunger pushed by the spring and where the fuel goes through?
The tiny space between the plunger and the regulator housing!!

Because the spring tension is stronger than the fuel pressure in the space
between the diaphragm and the plunger, fuel is going through.

Possible cause of lot of fuel dripping.
Plunger shaft is worn.  Plunger is very small and I believe it is brass and
not stronger than the regulator housing. It wears out.

It is also possible that old engine can have the regulator housing worn out
at the hole where the plunger is installed.

The bigger the hole, the more fuel goes through.

If the pressure pushing the fuel is stronger than the tension, fuel can not
go through well.  But, the plunger is not a very precisely flat with sharp
edge when you match with the housing.  Therefore, there is alway a small
amount of fuel leaking when the tank is pressurized.


Thank you for reading my thought.
Please, let me know if there is anything that I am wrong or not clear.  I
just thought about this within the last couple hours, and I am not perfect.

I wish I could explain with pictures.

Hope this helps,


Ihncheol Park












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