Will XJ750 Carbs work on a XJ650? - Page 2

 Albrecht via MotorcycleKB.com wrote:

Don't forget to clean the port in the bowl for the enricher as well, it
clogs easily if there is an crap floating around....

 

Are you up for a challenge???  I just can't seem to get my current carbs to
work.  I have put close to a couple hundred hours of labor into them.  I
think some evil spirit has cursed them.  Any help or further advice would be
greatly appreciated.

Here is what I have already tried:

==Thoroughly dismantled and cleaned the carbs three times (details on how
below).

==I borrowed a different set of carbs from a friend who has an `81 650.  I
cleaned them for him (part of the deal), and put them on my bike, and then
it started and ran great, perfect.  Because of this, I am pretty confident
it is a carb problem.  As near as I could tell, his carbs were identical to
mine (jets, etc.) and had the same model number on the bottom of the carb
bowls (both were Hitachi, model 5N8).

==When I clean my carbs, I take them completely apart, except for the pilot
screw set (as described on the XJ CD set, see below).  I do not dismantel
the rack, I keep the four carb bodies bolted together.  Each time I soaked
the carb bodys in part cleaner for 12 hours.  Blew compressed air through
all passages, replaced pilot and main jets with new jets (first time only).
I've paid special attention to the bypass valve and passages.  The inside of
the carbs are spotless.   I even boiled the carb body and bowls at the
recommendation of a friend.  They are very clean on the inside.

==When I clean the carbs, I can't get the “pilot screw set” out.  From what
I have read in the XJ CD's, this is three parts below the pilot jet.  How do
I get these out?  Do I need to take them out?  I've tried toothpicks, a
jeweler's screwdriver, and compressed air, but can't get them out.  Any
suggestions???  Do these parts screw out, or is it just held in by the
o-ring?  The “pilot screw set” consists of a washer, spring, and o-ring,
correct?  If these were dirty, would they cause the problem as I described?

==Checked for air leaks by spraying starter fluid around the intake
manifolds and boots (did not increase idle).  Because it did not increase
the idle, and my friends carbs worked, I don't think it is a vacuum leak.

==Replaced the air filter.

==Synchronized the carbs with a vacuum guage, on the bike.

==Set the fuel mixture on the carbs with a "colortune" spark plug.

==Once started, the bike idles and runs beatifully, very smooth.  Just won't
start when cold without a small amount of starter fluid sprayed into the
airbox.

==I have the original owners manual, service manual, and XJ CD set, and have
researched them thoroughly.

==I've checked the float level.

==I've checked the tank for rust, removed the rust, etched and coated it.

==Installed a fuel filter.

==Replaced the fuel line.

==Checked the fuel petcock.

==Had it at the local bike shop, they gave it back to me unfixed after 5
weeks, not fixed.

==The local bike shop checked the compression and said it was 120, 120, 130,
140.  Is this high enough?  I have been assuming since it ran OK with my
friend's carbs, it is high enough.

==Posted in various newsgroups before and got lots of feedback about fuel
additives.  Tried two.  Ran through a tankful.  Also tried getting up to
about 60 MPH, pulling in the clutch, dropping it down 2-3 gears, opening the
choke wide open, and popped the clutch (to create vacuum and suck any debris
out of the bypass valve openings.  …I don't think fuel additives are the
solution here.

 

noreply@noreply.com wrote:


Too much drama.


From what I can tell by looking at the online parts diagram (which doesn't
show the idle mixture screws at all) they are on top of the carbs, under a
metal plug that has to be drilled out.

There's an aluminum sprue sticking up, just forward of the diaphragm cap. Is
there a round metal plug in the sprue?

To remove the brass plug, you have to drill a pilot hole, taking care not to
hit the top of the idle mixture screw.

Then screw a small sheetmetal screw into the plug, and pull the screw and
plug out of the hole with a pair of pliers

  How do

If you can see the end of the pilot screw, it has a slot for a common
screwdriver.

The pilot screw adjusts the flow of fuel/air mixture through the single idle
outlet port that is all by itself. The other three outlet ports are by the
throttle butterfly, they are the transition ports that allow extra fuel air
mixture to flow when vacuum decreases as the throttle is opened.

But you need to get the pilot mixture screw out of the hole so you can clean
out the single ide outlet port. The engine will never idle correctly if this
port (and the passage feeding it) is plugged.


The single idle mixture port is probably plugged and your engine is idling
off of the three transition ports because you have opened the throttle
butterflies a little bit by turning the idle speed knob up.

Doing that defeats the starting enrichener. It needs all the vacuum available
so it can suck gasoline out of the float bowl.


It's low, but the engine will run if it has 90 pounds of compression. It just
won't have any power. I would do a valve job and re-ring an engine that only
had 120 to 130 pounds of cranking compression, but the bike would have to
look nice before I would bother with it.

Now that was a bad idea. The idle mixture ports are tiny, and if they are
plugged up, engine vacuum will not suck the gum and varnish out, you need a
strong solvent under pressure or ultrasonic cleaning.

--
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 It is good to be conversing with someone knowlegible.  And I appreciate you
looking and researching.

Your description of the pilot screw is correct.  I was lucky that mine were
not under a brass plug, but rather were covered with red silicone, I think
from the factory.  Apparently some years had the brass plug, while other
years they used red silicone.  I have had the pilot idle mixture screws out
and cleaned all of them, and made sure the outlet port is clear.  After
looking closer at the factory service manual that I have, and reading online
about cleaning the carbs, I was confusing the "pilot screw set" with what I
thought was other parts under the pilot jet, in the bottom of the carbs.

I am pretty sure the idle mixture port is clear, because I have blown
compressed air and carb cleaner through it.  Also, the butterflies were
closed when I remounted the carbs on the bike, and I have only made minor
adjustments to sync the carbs.  I have also tried reducing the idle screw to
close the butterfiles.  I can't get it to die by backing down the idle
setting, it just goes down to about 1000 RPM's and then sits there, idling
fine, even though I continue to turn down the idle.  Because of this, I
think it is fine getting fuel through the idle circuit.

I don't think the problem is compression because when I borrowed by friends
carbs, it started fine.

I think the starter bypass circuit is ok.  If the bike is warmed up and
running, starting to open the bypass circuit even a little causes the motor
start to sputter and eventually die (too rich I assume).  The starter jets
in the bowls are clear because I can easily shoot carb cleaner through them.

This is really baffling me.  I have got so many hours of labor in this bike
now, the thought of giving up now makes me cringe.  I've been looking at
used carb bodies on e-bay, but with that you are just getting someone else's
problems.

 

noreply@noreply.com wrote:


If the engine continues to idle with the throttles closed all the way and the
idle mixture screws turned all the way closed, perhaps the throttle shaft
seals leak air. You could spray some water at the throttle shaft seals to see
if they leak. If the RPM changes, you know it's a leaky seal.

The bypass plungers have been known to leak air. You can find out of they
leak air by putting your finger over the inlet hole, which is probably in the
inlet bell of the carb, on the same side as the plunger.


Maybe the pilot mixture screws are too far open?

The pilot jets are #40's, which is quite large for an engine that size. I
would expect the correct setting to be about 1/4 of a turn open from lightly
seated.

Another possibility is high fuel level in the float bowls, which makes it too
easy for the engine to suck up idle mixture.

  >This is really baffling me.  I have got so many hours of labor in this
bike

It seems to me like you should be able to fix the carbs you already have.

http://houseofmotorcycles.bikebandit.com/partsbandit/ShowSchematic.aspx?deptIde3498&machineId ‚14


#21 Jet, Pilot supplies fuel to the idle mixture circuit. The jet may or may
not be crossdrilled to add air to the fuel/air mixture at this point.    

#22 Jet, Main Air supplies emulsion air to the cross-drilled holes in #18,
nozzle, main, AKA "needle jet". The tapered needle moves up and down in the
main nozzle to control fuel/air mixture in the butterfly's midrange.    

#23 Jet, Pilot Air supplies air to the idle mixture circuit

If you remove the diaphragm cap and the diaphragm and slide and squirt
aerosol carb cleaner through the pilot air jet, it has to come out through
the pilot jet, the idle mixture port and the three transition ports.

If it doesn't flow freely out of any of those four places, a passage is
plugged up.

The idle mixture circuit acts to emulsify fuel and air to make fine droplets
that the engine can burn when the throttle butterflies are closed, but the
idle mixture circuit continues to flow and ever-decreasing amount of fuel/air
mixture until the butterflies are wide open.

--
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