Honda Shadow VT1100C 1996 misfire problem

I have a Honda Shadow VT1100C 1996 problem, when I accelerate and on a
certain rev count the bike is miss firing (losing power) when give
more petrol it go past the miss fire problem. I did replace sparkplugs
change petrol and oil filters but problem still exists. What else can
I check to rectify the problem?

 

You might just have dirty carburetors because of deposits left in the
idle jets and passages as the gasoline evaporates while the motorcycle
isn't ridden every day.

When the idle jets are slightly plugged up, the engine will run better
at higher RPM, but when the idle jets get completely plugged up, the
engine will start and run at idle, but will stall when you twist the
throttle because it really needs the gas from the idle jets to
accelerate when the throttle butterflies are only slightly open.

There are carburetor cleaners like GumOut and STP that are available
in an aerosol can. They contain strong solvents like acetone, toluene
and xylene.

You can spray four or five ounces of the cleaner into a measuring cup
and then pour the liquid into a full tank of gasoline and just ride
until the carbs are cleaned out.

My favorite carburetor cleaner is Berryman B12 Choke and Carburetor
Cleaner.

It comes in convenient liquid form and a 15 ounce can costs about
$3.50 at Wal*Mart.

 

At what revs is it misfiring? No way of making a meaningful carb
diagnosis without this info.

Has the bike been laid up for any length of time?

 

As TOG had already asked, what RPM did it miss at ?

When you pulled the plugs, were the electrodes on the
old plugs all the same color ? What color ?

 

Air cleaner filter(s)?
I would dismantle the carburators, blow out the jets and clean float bowls.

Greg.P.
NZ

 

Misfire between 4000 - 5000 rpm

 

Misfire between 4000 - 5000 rpm

 

Given the relatively low revs of the Shadow engine, that's unlikely to
be pilot jets. As I asked before, has the bike been laid up for any
length of time? If not, and it was running fine before, then it's
*really* unlikely to be pilot jets.

It could be a dodgy electrical connection making and breaking at a
critical vibration period. Sidestand ignition cutout (if it has one),
and anything else in the ignition circuit, including the ignition
switch and where it plugs into the loom.

It would make life easier if you'd answer the questions put to you,
because if you don't, you're unlikely to get a solution.

 

That shows how little you know about CV carburetors.

At idle, or very small throttle openings, the engine gets ALL of its
gasoline from the pilot jets, two of the three transition ports are
blocked off by the throttle butterflies, and the jet needle blocks off
flow through the needle jet.

The engine sucks less and less gasoline through the pilot jets until
nothing is going through the pilot jets at wide open throttle.

The engine needs the gasoline to come through clean pilots jets and
out the three transition ports just downstream of the throttle
butterflies.

If the pilot jets or the transition ports in one carb are dirty, but
clean in the other carb, the engine will be mostly running on one
carb, and then the dirty carb will start sucking gas past the needle
at higher RPM and it will start running on both cylinders.

 

Well, when it misfires at 4000 ~ 5000 RPM, are you twisting the
throttle a lot, or just a little bit?

Your motorcycle has rubber diaphragms that lift a vacuum slide which
controls how far the jet needle is pulled up out of the jet needle.

The vacuum slide won't be lifted unless the engine is producing enough
vacuum
to use the fuel that comes through the needle jet.

This is to reduce hydrocarbon air pollution from unburned fuel.

Another possibility besides dirty carbs is that one of your vacuum
slides might have a small hole in the rubber diaphragm.

 

Wrong again. When you have a holed diaphragm, the diaphragm simply
can't lift fully. The bike starts arnd runs perfectly, but just lacks
power. It doesn't misfire at all.

Or so my Kawasaki 400 twin did, with holed diaphragms. Top whack was
70mph whatever you did to it. Replaced with nice ones from NRP: top
speed went up to 100.

Please note that my reply is based on personal experience, not
ignorant theorising.

 

Like I said, if it hasn't been standing for ages, it's not going to be
gummed jets. Secondly, you're just guessing that it's missing on only
one pot. Finally, 4000-5000rpm is not a 'very small' throttle opening.

I admit it's not easy to work things out, because the OP isn't
answering questions.

 

Bullshit. Yes idle jets can cause problems, but they're not very
likely to cause problems that start at 4000 RPM and go away
again at 5000 RPM. That was in the part of the message
that you snipped.

I like the idea of a vibration induced failure.

Let me repeat to the OP my questions about the color of each
of the old plugs. All the same color ? What color ?

 

The OP e-mailed me and said that he'd followed my advice and added
some carb cleaner to his gasoline. The problem disappeared after a few
miles.

 

Whatever got fixed, at 4000 RPM it wasn't the idle jet.

 

While cruising at 4000 RPM most MC's with CV carbs are still running
on the idle jet and the main jet is barely in play, because the
throttle is twisted only about 1/8th to 1/4th of a turn.

The straight part of the needle is still in the needle jet, blocking
off flow from the main jet at 1/8th to 1/4th throttle.

 

Ah, so you've changed your standpoint to "most".

Like I said, a Shadow 1100 isn't running on pilot jets between 4-5k.

 

If it runs smoothly at ~1000-4000 RPM and runs rough at
4000-5000 RPM, that says needle jet.

Needle jet works at ~15-80% throttle. Main jet above that.

 

That's on slide throttle carburetors. There's no direct correlation
between
vacuum slide position and the amount the throttle is twisted on a CV
carb-equipped engine.

When I'm cruising down the freeway at the legal speed limit, I've
barely taken up the slack in my throttle cables at about 3800 RPM.

So, what's the position of the the vacuum slides that are pulling the
needles out of the needle jets under those circumstances?

 

<snip>

KrustyUS morphs again.