XS400: Fuel in Oil

Last year my girlfriend purchased a 1981 Yamaha XS400 as her first
motorcycle. Since then, its been my job to get it running as well as
possible.

All seemed fine until the bike would no longer start. I clean the carb
and checked ignition and found the (paper) air filters to be soaked
with oil. They also smelled like gasoline.

I didn't know how oil was getting up into the carbs and to the air
filters untill I drained the oil and found out there was much, much,
much more in there that there should be. It was also thin and smelled
like gas.

I put in a clean oil filter and fresh oil. We rode the bike for about
10-15 miles that day and it seemed to be running very well. Since the
engine was now running and starting easily, I began to rebuild the
brakes. I finished the brakes a couple of weeks later (took a while to
get parts) and decided to test them out, but the bike wouldn't start.
Once again, there was way too much oil, it was very thin, and it
smelled like gasoline.

So, somehow gasoline is draining into the crankcase. I think this is
happening even when the bike isn't running. I remember someone
mentioning a similar problem with their XS400 related to a check-valve
that is specific to this model, that when it fails fuel begins to drain
into the oil. I cannot find this part on Yamaha's parts diagrams on
their website.

Does anyone know what this check-valve is called or what the part # is?


Thanks much,

Jason

 

If just one of the two float valves in the two carburetors sticks in
the open position, the float bowl will overflow and gasoline will run
down the intake tract into the cylinder, and then drip past the psiton
rings into the crankcase, raising the oil level.

Then, if you are able to get the engine started, the crankshaft is
spinning in the excessively high oil level. The oil gets whipped up and
some of it is pushed out the crankcase breather which is in the top of
the valve cover. There is usually an oil/air mist separator that is
supposed to condense small amounts of oil vapors and allow the oil to
drip back into the crankcase.

But, when the oil level is high and diluted with fuel, the mist
separator is overwhelmed. Oil goes through the breather hose into the
airbox and ruins your paper air filter and probably fouls your spark
plugs too...

Now, back to that float valve that leaked. Maybe the float itself had a
leak in it so it didn't float high enough to raise the float valve to
the shutoff position. Maybe the float valve has gum and varnish from
evaporated gasoline making it stick. Maybe there is rust in the gas
tank, petcock, fuel hose, and in the float valve seat...

Consider this, too. Most modern motorbikes have a vacuum operated
automatic petcock which doesn't have an OFF position. Instead, there is
a vacuum diaphragm that pulls an internal valve in the petcock open,
allowing fuel to flow down the fuel hose to the carbs when the engine
is running. When the engine is shut off, the internal valve is closed
by a spring. If there is a lot of rust and dirt inside the petcock, the
internal valve can't seat properly and fuel continues to flow while the
motorbike is parked. If the fuel system is really dirty, fuel flows all
the way down to the crankcase...

Go to www.partsfish.com and register. Select your Yamaha XS-400 and the
year. Look at the carburetor parts fiche for the float valve, and look
at the fuel tank fiche to see what I mean by a vacuum-operated
petcock...

If your petcock has an OFF position, you can still have problems with
the selector valve seal leaking...

A company at www.oldbikebarn.com sells carburetor rebuild kits and
petcock rebuild kits...

 

I had problems with my bike sitting and not starting... fuel had gummed
up in the idle/pilot jets. Got it running, then drained the oil, it
flowed like water and smelled like gasoline. While taking the tank
off, I had to plug the fuel line since the petcock wouldn't shut off.
It had a slow leak, probably gummed up/rust stuck in it, something.
But basically the slow fuel kept leaking through the petcock,
collecting in the oil.

So it could be that... I'm looking into a petcock rebuild kit, might be
simpler than pulling the carbs apart. Speaking of all this, do people
ever clean out petcocks? Would pulling it apart and looking for rust
hurt anything? Would I be able to just slap it back together?

So yeah, Jason... take a look for fuel leaks.

PS. Woot for krusty

 

Does your petcock have an OFF position? If it does, it's a manual
petcock.
But if the markings are ON, PRI, and RES, it's an automatic petcock...

If your gas tank has a lot of water and rust in the bottom, you can
keep cleaning out the petcock and the carbs over and over again and
still have the gas in the oil problem, and carburetor flooding as the
rust comes down and blocks the float valves...

It is especially a hassle with automatic petcocks that are supposed to
shut off when the engine's not running. If there's a lot of rust and
crud on the seat of the automatic valve, cleaning the front side of the
petcock out where the selector lever is at won't solve the rust
problem...

If you take the front side of the petcock off to clean it out, and
don't clean the back side where the automatic valve is at, you're
wasting your time. You have to drain the gas tank anyway if the
automatic valve is blocked by rust. If you're going into the petcock,
and have to drain the gas tank, you might consider laying the motorbike
up for a week or two while you order a kit to Kreem the tank and order
a petcock kit...

If you don't know about Kreem, google for it in this NG, we've talked
about Kreem many times...

Or maybe you could just always use a surgical hemostat to clamp the
rubber fuel hose shut if you have a leaky automatic petcock. Those
things have other uses besides just holding roaches, yannow...

 

I had disassembled and cleaned the petcock while I removed and cleaned
the carbs. I did not know the exact function and purpose to the
petcock, except to switch between the main and reserve tanks.

While we are at it, does PRI stand for prime (as in priming the carb
with fuel), or for primary (as in primary versus reserve fuel supply)?

Thanks,

Jason

 

PRI stands for "prime". If you've left the motorcycle sitting unused
for a week or a month, the fuel will have evaporated out of the float
bowls and the battery might be a little weak, too. So the PRI position
opens a passage direct from the gas tank to the fuel line and allows
the float bowls to fill up...

If you have left the motorbike sit so long all the fuel has evaporated,
you make be getting into more problems than you expected when you try
to start the engine after the layoff...

If the selector handle on the petcock doesn't move easily, don't force
it, the rubber seal has stuck to the back of the selector knob.
Instead, use a small Phillips screwdriver to loosen the two screws
holding the knob to the petcock, carefully move the knob to the PRI
position and
re-tighten the screws holding the knob on. The petcock might weep
gasoline for a few minutes, but you can hold a rag under the drips. All
this extra effort will save you a trip to the Yamaha $tealer$hip to buy
a new seal...

You still might have a hard time starting a bike that has sat for long
enough to evaporate the gasoline out of the float bowls. The battery
might be weak, so you should charge it up before trying to start the
engine...

Another problem you will run into if you've left the bike sit so long
that the gasoline in the float bowls has evaporated is that the vacuum
slides will stick and if you should manage to actually get the engine
started, the first time you twist the throttle open, the stuck slides
might cause the rubber vacuum diaphragms to tear. If you've left the
bike sit that long, you might consider removing the airbox inlet boots
and squirting a little aerosol carburetor cleaner on the vacuum slides
and working them up and down with your finger to be sure they are free
to move. A torn rubber vacuum diaphragm may cost you about $80...

Once you're sure the slides move freely and you have a fully charged
battery installed, turn the petcock knob to the PRI position and turn
the master idle knob all the way down so it doesn't open the throttle
butterflies at all...

Then you can put the choke on and start the engine and nurse it with
the throttle until it heats up enough to readjust the master idle knob
so the engine idles at the specified speed...

If the engine won't run with the choke off, you have plugged up idle
jets. I've described how to clean out idle jets hundreds of times.
Google for "kaybearjr@aol +idle jet" for those posts...

 

My petcock is vacuum operated, 94 Yamaha XJ600. It's definately got a
bit of rust in the bottom of the tank, due to water getting in and
sitting for a while (going to take care of that next). But yeah, I
plan to take the petcock apart and make sure there's not rust in it. I
haven't checked yet, cause I'm not sure when I'm going to do it, but
I'm assuming I'll take it all apart, not just the front. Might as well
if I'm going to drain the gas and have everything apart. My question
is; will I further damage the petcock seals just by opening it (knowing
that it's already leaking)? I don't know when I'll be able to get a
kit, and I don't want to make it worse in the meanwhile. Should I just
open it up and at least inspect for dirt when I have the tank off?

 

Petcocks work fine on motorbikes that are used every day, but the
rubber parts dry out when the motorbike sits idle...

I have found that if there's a lot of crud in the gasoline, or the
motorbike sits for months unridden, the rubber seal will stick to the
back of the selector knob and the knob won't move. The way to get the
knob to move freely is to loosen up the two screws in the front of the
petcock, let a little gasoline dribble out and then tighten the
screws...

Gasoline getting to the dried up seal will soften it so the leakage
will usually stop in a few minutes...

If I don't do this, the rubber seal will tear and leak...

The petcock repair kit that I saw on www.oldbikebarn only cost about
$9.00, but I suppose that special handling and shipping would double
that price...

 

I just checked oldbikebarn, XJ600 (94, yamaha). I only see 2 results,
one for 24.95 and one for 32.95. Did I do this wrong? How can I find
that 9.00 one? 30+ seems like a lot for a few o-rings.

 

The petcock repair kit I looked at was for a manual petcock for a
Kawasaki KLR-250. It only had the rubber seal with 4 holes in it and an
o-ring...

And $25 to $30 for a few o-rings really is a lot of money when you
consider that they cost pennies to manufacture. Then the middle men and
the distributors get ahold of the o-rings and mark them up double at
each stage of distributor...

One of my Yamahaha riding buddies came out of the local Yamaha
$tealer$hip saying, "Well, I got the screws I needed for my Virago.The
dealer has a minimum price on all parts. Nothing costs less than $2.00.
I got four little screws. $8.00, plus tax. Now I can't even afford to
buy a hot dog for lunch at the convenience store!"

Moral of the story: if you can *find* a cheaper supplier than Yamaha,
or a mail order house that charges you for shipping and special
handling, go there!