engine surging

This question probably sounds similar to others that have been posted lately,
but I'll ask anyway.
My 85 1100 Shadow has occassional surging of the engine when traveling at a
constant speed. It doesn't do it all of the time. I'd say about 50% of the time.
At any speed it will run smooth for a while then for no reason it surge for a
while then run smooth again, and so it goes, back and forth. I've installed new
plugs, air filter, oil but that hasn't helped matters. It also doesn't matter if
I ride sinlge or two-up with my wife. It is getting decent gas mileage...43-45
mpg. But that surging is annoying and I'm concerned that the engine is being
harmed in some way. Any help is appreciated. TIA

 

Plugged up idle jets and idle passages will cause an engine to lean
surge, and you will hear the exhaust pipe going "piffle piffle SNAP!"
or even "fart BANG!" as you roll the throttle off.

Go to Wal*Mart and buy a 15-ounce can of Berryman's B-12 Chemtool Choke
and Carburetor Cleaner. Buy the can that has all liquid in it, not the
aerosol can.

Carefully measure 4 ounces and pour it into a full tank of gas,
avoiding dripping it on the paint. B-12 contains xylene, acetone and
methyl alcohol. It dissolves gum and varnish quickly.

Go for a slow putt, using as little throttle as possible. This stuff
will make your exhaust stink, but it cleans the gum and varnish out of
your idle jets and idle ports.

If you notice that the engine is idling a lot faster, turn the idle
speed down using the master idle knob. If you aren't the first owner of
your Shadow, look at the carburetors to see if you can see the slotted
ends of the idle mixture screws.

The factory set those screws and sealed them under anti-tamper plugs.
If you can see the slotted end of those screws, you can be sure that
somebody has removed the anti-tamper plugs and tweaked with the mixture
screws and screwed up your idle mixture in the process.

The mistake they always make is thinking that the idle speed should
increase when they turn the screws counterclockwise. They discover that
the idle RPM not only doesn't speed up, it slows down and the exhaust
pipe sounds dull and thudding.

Since the idle mixture is too slow, they have to turn the master idle
knob to make the engine idle faster. Then the idle RPM hangs up when
they roll the throttle on and off. It takes 20 seconds for the idle RPM
to settle down.

Maybe they don't ride the motorcycle that much, it just sits in the
garage and gums up the carburetors as the gasoline in the float bowls
evaporates. A new owner buys the bike and notices that it's hard to
start and maybe he cleans out the carbs by running carb cleaner through
the gasoline and discovers that he went from lean surging to having an
idle mixture that is far too rich.

The strategy of setting the idle mixture screws is to adjust the idle
RPM using the knob between the carbs to get the engine to idle at the
specified RPM in the manual.

Then you turn the idle mixture screws clockwise about 1/4 of a turn at
a time. If the engine RPM increases, you're getting towards the right
setting, and you turn the idle knob down and turn the mixture screws
another 1/4 of a turn and turn the idle knob down again.

You want to get to the point where the engine idles at the right RPM
with the smallest throttle opening possible and the least number of
turns out on the idle mixture screws.

 

Thanks for the thorough explanation. I'll try using the Berryman's and check out
the idle mixture screws.

 

Don't know if this applies to your bike in particular, but some twins
can exhibit pretty dramatic surging problems if the carburettors (or
Throttle bodies on FI engines) are not properly syncronized.

 

Constant vacuum carbs that are out of synch will exhibit problems as
you roll the throttle on and off. You'll experience a sudden surge of
acceleration. One butterfly will be uncovering the transition port
before the other carb uncovers its transition port, so the acceleration
mixture on the first carb will be richer than the other carb.

When carbs are really badly out of synch and adjustment the engine may
even run on one cylinder until the other carb gets enough mixture to
fire. In that situation, the motorcycle is almost unrideable at small
throttle openings.

 

FB...
per your suugestion I put the Beeryman's in the tank and putted along as long as
I could stand it. There is still some surging after riding for about an hour at
normal speeds. Not as bad as before but still annoying.
Thanks for your input.

 

Did you find the heads of the idle mixture screws? You might want to
look at the carburetor drawings on www.partsfish.com to see where they
are. The diagram might call them "pilot mixture screws", I don't
remember for sure.

But, if you can see the heads of the screws, turning them
counterclockwise 1/4th of a turn might help.

I have described how to drill out the EPA anti-tamper plugs a bazillion
times, so you can google for those posts.

There is the distinct possibility that you have a buildup of calcium
carbonate inside the carburetors from hard water getting into the
gasoline when you wash the motorcycle.

Calcium carbonate is that white powdery substance you see in the bottom
of a tea kettle. Solvents won't touch calcium carbonate. Sometimes the
only easy way to get it out it to carefully poke a hole through it with
a fine needle.

Idle sets are made from brass, and brass is soft, so if you have to
poke a needle through a hole, be gentle.

Some people suggest using an ultrasonic cleaner when the carbs are
really plugged up.

 

I just checked and it doesn't appear that the plugs covering the pilot mixture
screws have been drilled out on either carb. I checked my Clymer service manual
and my offical Honda shop manual for the location.

 

Besides the possibility of partly plugged up idle jets and passages,
there might be fuel level problems caused by sticky float valves and
this is usually solved by the B-12 cleaning the crud out of the float
bowls.

Check the petcock diagram to see if there's a fuel filter inside the
gas tank and check the carburetor diagram to see if there's a tiny fuel
filter in each carburetor, above the float valve.

There is also the possibility that the petcock is partly plugged up
with crud that settled in the bottom of the tank, or that there is a
small vacuum leak somewhere that stops the automatic petcock from
staying open.

Sometimes gas cap air vents get plugged up and the fuel flow stops
because if vacuum build up in the gas tank.

Another possibility is that the fuel hose rests against a hot part of
the engine, boiling the gasoline in it, making a vapor lock.

One of the most frustrating fuel system problems I ran into was when I
bought a set of used carburetors from a friend. He was using thick
automative fuel hose, instead of the thinner motorcycle hose. When I
installed the hose, it kinked the hose, causing fuel starvation.

Honda Hawk GT V-twins had a strange problem with wind blowing up the
float bowl vent tubes when there was a cross wind. The air would
pressurize the float bowls slightly.

 

If I run with the choke open the problem doesn't seem to exist. I know that is
not good on the mileage or the engine to do that very long. I'm wondering if I
should pour a little more Berryman's into the tank. Would this hurt?

 

I think you stated it backwards. When the choke is "on" it is closed.
So you are saying by closing the choke some the surging goes away. That
means that your surging is being caused by a lean condition on at least
one of the cylinders. The lean condition may be caused by gummed up
fuel passages in the carb or worn airway passages (main jet, needle jet,
etc.)

 

The choke is actually a bypass passage that sucks gas right out of the
float bowl and the very rich fuel/air mixture then goes around the
butterfly. The more throttle opening you use, the less mixture is
sucked through the bypass because engine vacuum is lower with the
throttle open more.

Running about 8 ounces of B-12 in a tank of gas shouldn't hurt your
slow turning Vee-twin. The risk of running any kind of alcohol (B-12
contains methyl alcohol) is that it's a drying agent that dilutes the
oil film on the cylinder walls. Running the engine at high RPM with
alcohol in the fuel could wear the rings out really fast.

For a really quick clean up of the carburetors, you might try draining
the float bowls, and refilling them with a 50/50 mixture of gasoline
and B-12 from a plastic squeeze bottle.

Then let that mixture sit in the carbs for half an hour before
starting the engine and burning the mixture out. It shouldn't melt any
plastic parts inside the carbs at that concentration. Yamaha sells a
product similar to B-12 and they mix it with 50% gasoline to clean out
carburetors on marine engines that sit unused for months at a time.

 

While you may be right for this model of bike/carb (I forget what it was
now), technically that is not a "choke", but rather is an enrichening
circuit.

A true choke is a butterfly plate that blocks the intake path ahead of
the throttle plate, raising the vacuum there, and causing more fuel to
be drawn from the float bowl.

Yeah, I know. It probably says "choke" right on the lever... ;-)

 

Hey! That's MY line!

 

Sorry, Didn't mean to infringe. I'm fairly new to this NG. ;-)

 

FB:
After three tank fillups with doses of the B-12 the surging problem seems to
have disappeared. Thanks for your input.
Checkers

 

FB: In the above paragraph you mentioned cross winds causing engine surging. Do
you know how that was resolved? I am asking because I think that has been my
problem all along. Yesterday I was riding and there was some pretty strong winds
and I was having surging problems again. My surging has always been irregular. I
thought back to when I did and didn't have problems. I realized most of the
surging happens on very windy days when I'm riding with a cross wind. Riding
into the wind or with it at my back is no problem - just a cross wind. Any
suggestions. TIA
Checkers

 

The Hawk GT's had a problem with quartering winds that came from the
front and side.

Apparently, the wind would blow into the vent tubes that you may notice
coming up from the float bowl. Since constant vacuum carbs are running
on the idle circuits most of the time, and the EPA mandates a
marginally lean idle mixture, any unintended pressurization of the
float bowls lowers the fuel level and makes it even harder for the
engine to suck up enough gasoline.

I don't remember whether Honda re-routed those tubes into the air box,
or if they just moved them so air couldn't blow up the tubes. My
de-smogged motorcycles just run long vent tubes down behind the engine
to the road.

The marginally lean idle mixture mandated by the EPA can be easily
corrected by drilling out the anti-tamper plugs. The plugs will be on
top of the carbs, or underneath the carbs downstream of the throttle
butterflies.

On Yamahas and Suzukis, you can carefully drill a pilot hole in the
plug and then enlargen the hole just big enough to thread a small
sheetmetal screw into it. When you pull the screw with a pair of
pliers, the plug comes with it.

I have heard that Hondas may have a specially-shaped anti-tamper plug
that requires a special tool to remove (or maybe the idle mixture
screws have a special shape).

But, if you can get that anti-tamper plug out of the way and turn the
idle mixture screw about half a turn counterclockwise, the engine will
start easier, warm up quicker, have better throttle response and run
cooler in the summer.