Re: Carburator issue (was Yamaha 650 Choke)


kb1jec@xxxxxxxxx wrote:
> After much fuss I managed to start the 650 today, via kickstart. I
> cranked and cranked the electric starter with the "choke" (carb.
> starter system) in the ON position. The fuel selector valve has RES,
> ON and PRI positions. RES and ON are vacuum operated from what I
> understand. It wouldn't start in the ON position and when I tried the
> kickstarter I felt very little resistance. However, after a few
> kickstart tries in the PRI (prime) position, I felt hard resistance in
> the kickstarter and a few kicks with 3/4 to full throttle started it
> up. I then switched the fuel selector back to the ON position and
> reved it up a bit (it was running rough) until it warmed up and held
> idle.

Not enough cranking vacuum to open the automatic valve in the pet***.

There is a little rubber diaphragm in the pet*** that opens the
automatic fuel shutoff valve in the back of the pet*** when there is
enough engine vacuum to suck on the diaphragm and overcome the
reistance of a small spring. If your idle jets and idle passages are
plugged up, the previous owner probably adjusted the idle RPM too high
to compensate for the lack of fuel air mixture coming through the idle
mixture ports. So, there wasn't enough vacuum to open the automatic
shut off valve. The PRI (prime) position bypasses the automatic fuel
shut off valve.

Notwithstanding the above remarks, you may still have a lot of rust and
crud in your gas tank and this may be plugging up the pet***. The only
way to know for sure is to drain the gas tank, remove the pet***,
flush all the rust and crud out of the gas tank, disassemble the
pet*** and clean it out and clean the rust out of any filters attached
to the pet***. If the tank has a lot of flaky rust in it, you may want
to acid etch the inside of the tank and apply a Kreem epoxy coat or
apply a
POR-15 coting to seal the inside of the gas tank.

> Everything was fine from there - I took it out on a 15 mile ride and
> got it inspected. On the way back, however - it started to run a
> little rough and finally sputtered and died. I stopped and tried
> cranking w/ electric and kick start but no luck. As soon as I switched
> the fuel selector back to PRI I managed to start it right back up.
> Then I put it back in ON and it ran fine the rest of the ride home.

Maybe some crud came through the ON orifice of the pet***. That crud
will go down the fuel line and sit on top of the float valve and keep
it from shutting off the flow of gasoline and keep the float from
maintaining the correct fuel level.

If the fuel level in the float bowls is too high, the outside air
pressure will have an easier job of pushing fuel into the carburetor
venturi. But, if your idle jets and idle ports are as plugged up as I
suspect they are, the fuel isn't coming through the idle ports, it's
drooling up past the jet needle.

There is about 4 to 6 thousandths of an inch gap between the jet needle
and the needle jet (the brass tube that the tapered needle goes up and
down in as the vacuum slide rises and falls)and the excessively high
fuel level drools past the jet needle.
> ---
> *Before I tried starting it in PRI I pulled the plugs and noticed they
> were black and dry (not wet like they had been exposed to oil/fuel).
> Otherwise the plugs appeared normal.

I suspect that's from a high float level and gasoline drooling past the
jet needle...
>
> *PRI allows fuel into the carburator even when the engine is not
> running, whereas the RES and ON positions on the fuel valve are vacuum
> operated (ie only run fuel w/ engine running or cranking). This is
> what the manual says.

Yup, it's a good system for riders who allow their machines to sit
unridden in warm climates from days or weeks or months and the fuel
evaporates out of the carburetors. The PRI position ensures that there
will be enough fuel in the float bowl to start the engine even if the
battery is a bit weak.
>
> *The entire ride was with the carb. starter system/"choke" in the up
> (ON I assume) position. Pushing the lever down would make it stall.

Raising the "choke" lever pulls the tiny little air valves in the
starting enricheners up and allows air to bypass the throttle
butterflies, which should be closed as far as possible to cause air to
flow through the enrichener instead of pass the throttle butterflies,
defeating the enrichener.
>
> *There is no major rust in the fuel tank, just tiny spots here and
> there. I just changed the oil this past Sunday.

Ocassionally smell your oil, if it smells like gasoline, your float
bowls are running over and dripping gasoline down past the piston
rings. Oil diluted by gasoline doesn't lubricate well, and there is
even a fire or crankcase explosion hazard.
> ---
> I'm yet to try the B-12 in the tank to try to clean the carburator. I
> know it sat in the previous owners garage for 3 weeks before he sold it
> to me - whether or not it had gas in it I don't know.
>
> With the problem I have described, would a simple thing like the B-12
> carb cleaner help? I might call the prev. owner later on and see if he
> knows anything about this. After it's warmed up the bike runs just
> fine, so I don't know whats up with it.

It gets up to 100 degrees every day during the summer here where I
live. Gasoline will evaporate out of the float bowls in a few days and
the remaining gaoline will turn to varnish and gum, so I need to use
B-12 several times a year.
>
> (the diagram for the carburator and starter system is here:
> http://www.biker.net/650_service/650_svc_e4.PDF)

Look at the drawing for the Idle System. #3 is the pilot jet in the
float bowl. Gasoline gets sucked out of the float bowl by engine
vacuum. The vacuum sucks a small amount of air through the #11 pilot
air jet. The air and gasoline mix in the idle passages. Engine vacuum
sucks some of the idle mixture out the #5 bypass hole. The #7 pilot
screw fine tunes the amount of fuel air mixture that is sucked out of
the #6 pilot outlet hole by engine vacuum.

The further closed the throttle butterflies are, the more vacuum there
is downstream of the butterflies and the more fuel air mixture is
sucked through the idle passages.

The .pdf file warns owners that the pilot screw was set at the factory
and that they shouldn't tamper with it. That's for the Environmental
Protection Agency. The EPA demands that the idle mixture be set very
lean
so the engine won't pollute the air.

Unfortunately, the idle mixture is usually set so lean the owner
experiences hard starting and the engine takes a long time to warm up
in the morning when it's cold.

The pilot mixture system on your Yamaha resembles the simple pilot
mixture system used on older slide valve carburetors where turning the
pilot mixture screw CLOCKWISE richens the mixture instead of leaning
the mixture up.

If you turn the pilot mixture screws all the way IN until they stop,
counting the turns and fractions of turns, write down the number
somewhere that you won't lose it, then screw the idle mixture screws
all the way out and pull them out. If there is just the screw and a
spring, the screw richens the mixture when you turn it clockwise, and
the engine would be easier to start.

But, if there is also a little washer and a tiny rubber o-ring in the
hole the screw came out of, turning the screw counterclockwise richens
the mixture and makes the engine easier to start.

If you want to try the aerosol version of Berryman's B-12, you can
spray a little bit of it down the holes that the idle mixture screws
came out of and you can spray a little bit down the pilot air screw in
the carburetor's air inlet. There's a little red plastic tube attached
to the can of aerosol B-12 for that very purpose.

Some of the B-12 will come out the idle jet into the float bowl and
some will come out the idle ports in the carburetor throat. But, if you
spray too much B-12 into those holes, some of it may run down the
carburetor throat, down the inlet ports and down into the cylinder. It
will seep past the piston rings and go down into the crankcase and mix
with the engine oil, diluting it, and carrying any crud that it can
dissolve deeper into the engine.

The excess B-12 will evaporate eventually, but I'm just telling you
what would happen if you squirted too much B-12 down into the
carburetor.

Putting the liquid B-12 into the gasoline is less of a risk.

.

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