Motorcycle Forum / General / Technical / June 2007

The mixture is too rich. Multiple possible causes.  

Agreed

Yes. Start by inspecting the sprue sticking up on top of the carburetor next
to the diaphragm cap. You should see an EPA anti-tamper plug there.

If you see a slot headed brass screw in a hole there, somebody has drilled
out the EPA plug and ignorantly tweaked with the idle mixture screws.

They are gas screws, the further you turn them counterclockwise, the richer
the idle mixture gets.

The ignorant shadetree mechanic thinks that the idle speed should keep
increasing as he turns the idle mixture screw more and more counterclockwise.

Once he gets past 3 full turns out, the idle mixture screw is no longer doing
anything, the idle mixture is too rich, the exhaust sound becomes dull and
thudding and the engine may even stall.

The shade tree mechanic responds to the stalling by turning the #9 throttle
set screw clockwise to raise the idle RPM.

This causes the throttle butterfly to open slightly and uncover a pattern of
three acceleration transition ports which are in no way controlled by the
idle mixture screw.

The idle mixture becomes ridiculously rich, the exhaust sound is "toot-toot
toot-toot toot-toot" until the rider opens the throttle enough to allow
enough air to
blow the excess fuel air mixture out of the engine.

Maybe he's happy that his motorcycle accelerates so well, but he's probably
puzzled by the fact that the idle RPM is too high, the motorcycle acts like
it keeps wanting to drive deeper into corners than he intended, or the idle
RPM actually hangs up at 4000 RPM for 15 or 20 seconds after he blips
throttle grip.

So, maybe he turns the #9 throttle set screw back down and just accepts that
his engine is prone to sudden stalling.

Nyahhh! The correct way to set the idle mixture screws is to start at about 2.
0 full turns out with the engine idling at the correct RPM. Then turn the
idle mixture
screws clockwise 1/4 of a turn at a time, and, when the idle RPM increases
turn the #9 throttle set screw counterclockwise back to the specified RPM.

Then turn the idle RPM screw another 1/4 of a turn clockwise and turn the
idle RPM down again. When you get to the critical point, the engine starts
running rough because the idle mixture is too lean, turn the idle mixtures
screws counterclockwise 1/8th of a turn and you're done.

When the idle mixture and throttle stop screws are set correctly, your engine
will start from cold without using full "choke", and it will idle without
messing with the throttle grip while you put your gloves on.

Your carburetors don't have a real "choke". There is NO flat plate that
increases engine vacuum. Instead, you have the #51, starter plunger. It's an
air valve in a passageway that bypasses the throttle butterfly. It sucks
gasoline directly out of the float bowl like a kid sucking milk through a
straw.

If your throttle stop screw is turned too far, there is less vacuum
downstream of the throttle butterfly and the "choke" doesn't work well and
the bike is hard to start.

There is a possibility that one or both of your choke plungers is sticking
slightly open. You can find out whether a plunger is sticking by putting your
finger over the choke air inlet in the carburetor mouth while the engine is
idling. If the idle speed changes, the plunger is leaking.

Another possibility is that whoever rebuilt the carburetors adjusted the
floats too high. The float dimension is from the bottom of the float to the
aluminum carburetor body, NOT to the float bowl gasket.

The float bowl gasket is about 1mm thick, and the additional 1mm of float
travel before shutting the gasoline supply off raises the gasoline level and
makes it too easy for the engine to suck gasoline through the idle jets.

Also, do the float bowls drip gasoline? Possibly the float valves (item #43)
are sticking from gum and varnish buildup that wasn't removed during the
cleaning and rebuilding process. Even a tiny speck of varnish on the needle
seat can keep it from sealing properly.

Check the jet needle to make sure that the clip is in the second slot from
the top, that nobody has added extra shimming washers under the jet needle,
and be sure that all the correct washers are stacked in the right order.

If the jet needle is too high, the engine will run to rich, because the area
difference between the jet needle and needle jet is too large.

Inspect your jet needle and the hole in the needle jet. Sometimes needles get
worn from banging against the needle jet. A needle jet hole that has been
worn egg-shaped by the needle is harder to notice.

http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=650832

9: THROTTLE SCREW SET

23: JET,MAIN #170 (This is not really the main jet. It's the pilot air jet.)

29: WASHER  

30: CLIP  

31: RING

32: NEEDLE

35: JET,PILOT #35 (This is a smallish idle jet, about 2.0 to 2.5 turns of the
idle mixture screw should work ok with a jet of this size.)
 
37: MAIN JET (112.5)

43: NEEDLE VALVE SET

51: PLUNGER,STARTER  

53: SPRING,PLUNGER

Hey, that's me!  Thanks to Albrecht, I got the courage to clean out my
carbs, and the bike is much better off now, although I'm still having
some minor issues getting it to stop running rich, and tonight I'm
going to follow Albrecht's description of setting the idle mixture
screws, found in this thread.  Thanks again, Albrecht.

By the way, another great resource for help with your XJ bike is the
XJ Owners mailing list at http://www.micapeak.com/mailman/listinfo/xj-owners.

-Ken