XJ750 Pilot Mixture Adjustment

I've recently bought an 83 XJ750 and am having considerable difficulity with carb adjustments - specifically 1 & 4 are doing fine, 2 & 3 are running rich.

I've had the carbs cleaned, replaced the needles and jets, had the carbs sync'd (vacum tube) twice, checked all the rubber boots for cracks, have made multiple float level adjustments and checked the levels against with what the manual says they should read on a tube, and while each adjustment nets some little bit of improvement I am quickly coming to the conclusion that the previous owner may have tinkered with the pilot mixture adjustment screws.

Unfortunately the Haynes manual says little more than "these are pre-set" and "do not touch", and dealers mutter archane words about horrible and dire consequences when asked. I would like to know what the factory pre-set is so I can check to see if that is what they are set to but this information seems to have a higher classification than most mortal riders can hope to attain, even Mr. Google has been of no help to me.

I have considered checking where 1 and/or 4 are set and seeing if 2 & 3 are at least in the general neighbourhood - is this a workable plan?

Does anyone know what the factory pre-set is?

Thanks

M

stageleft wrote:

> Does anyone know what the factory pre-set is?

No, even the factory doesn't know that information. The carbs may be
individually set on a single cylinder test engine, or they may be set
on a 4-cylinder test engine with a 4-into-1 exhaust system and an
exhaust gas analyzer.

That would explain why the pilot mixture screws on my FZR-1000 were set
at
0.12, 0.30. 0.40. and 2.5 turns out. If an exhaust gas analyzer is used
to test the emissions of an engine with multiple carburetors and a
common exhaust pipe, the best it can do is measure an AVERAGE of the
four idle mixtures.

But, you want a starting point, and you figure that the starting point
is to make all the pilot mixture screw settings the same. That's an
excellent strategy. You have a set of precision made carbs and they
should all be set just about the same. It's possible that the previous
owner adjusted the screws or even damaged the tips of the pilot screws,
so you might inspect the screws with a magnifying glass to see if one
or more is damaged.

I discovered that one of my Yamaha's pilot mixture screws had a
different length spring on it than the other three, but I had already
mixed up the springs so I don't know if it was on the screw that was
2.5 turns out. I eventually set all the pilot mixture screws at 0.25
turns out, after experimenting with 1.0 and 0.5 turns out. With the
pilot screws turned out
1.0 and 0.5 turns, the idle speed was too fast when the engine was
running on the choke in the morning, and the idle speed would increase
when the engine got hot and I would have to keep turning it down. Then
the next morning, the idle speed would be too low when the engine was
cold.

The problem with opening the pilot mixture screws too far is that I
wound up uncovering the unregulated transition ports. Read on,
stageleft...

Another reason for the center cylinders being set to run a little
richer than the outer cylinders has to do with intake tract length or
the fact that the center cylinders of an air-cooled engine need to
waste a little fuel to cool the them, since there's not as much air
flow.

But the center cylinder's pilot mixture screws shouldn't be set so rich
they
carbon foul the spark plugs...

When you adjust the pilot mixture, always remember that the strategy is
to
LEAN up the idle mixture by turning the pilot screws CLOCKWISE, and
compensating for the expected RPM increase by turning the master idle
knob
COUNTERCLOCKWISE. You're trying to achieve a smooth idle at the
smallest amount of throttle butterfly opening that makes the engine run
at the specified idle RPM.

Amateur tuners get all screwed up when the RPM doesn't increase as
expected when they turn the pilot screws COUNTERCLOCKWISE. The engine
speed doesn't increase, it may even slow down, the exhaust sound
becomes thudding and muffled, and the amateur tuner compensates by
turning the master idle knob
CLOCKWISE. Then he runs into all kinds of problems with the RPM hanging
up when he blips the throttle, and the engine RPM is too high when
running on the choke.

For many more words on the subject, google for "spectraltarsier@aol
+transition ports"