1984 Honda Magna 700 hesitation problem

Hello,
I have an '84 Magna 700C that has hesitation to it. Let me give you a
brief history. Bought the bike in '96. I'm in the Navy thus transfer from
time to time. In 2000 went to Japan. Bike sat in father's garage until 2004.
When I got it, it would start up, but not idle properly. I performed all
repairs that did NOT have to do with the engine directly, e.g. clutch and
brake seals, front and rear brake cleanings, fluid changes, cable
replacements. I put the bike in the shop for a full tune-up, fork seal
replacement, carb cleaning, and gas tank cleaning. My bike ran great after I
picked it up, just like old times. About two weeks later I hop on it to take
my daughter for a ride. I notice that it's slow to accelerate (my daughter
is only 80 lbs). It's been like that all winter. Whether starting out from a
stop, or accelerating on the highway to pass, it is slow. I cannot even get
above 9000 RPMs in first gear at full throttle. I've tried different gas and
carb cleaner. Nothing seems to work. I don't have any fluid leaks. Could an
exhaust leak cause cause this? Any help would be greatly appreciated.

Thanks,
Shawn

 

Thanks for the reply Mike.

All cylinders are firing. I've even removed one plug wire at a time while
the bike was idling and there was a noticeable drop in RPM. It even stalled
at one point. I was on the highway today doing 75. I could barely make it to
80. This is not normal. I remember doing 120 on this bike (8 years ago, when
I was 23 and indestructible). I noticed that when I had the bike at 75, then
opened the throttle all the way, I would actually start to slow down. There
was definitely NO acceleration, in fact, my body would move FORWARD on the
bike indicating that it was DEcelerating. HELP!

Thanks,
Shawn

 

Some thots...
I would investigate:
vapor lock-air needs to enter the fuel tank as gas leaves or a vacuum is
created.Vacuum gets strong enough fuel will not leave the tank or flow is
reduced. Check any tank venting.
blocked or partially blocked fuel line or fuel filter - dirt or kinked hose
maybe the extra weight pinched a hose?
dirty air filter or blocked air intake (leaves, nesting birds...)
PQ

 

Did you pull out the spark plugs and look inside them at the insulators
for color? If the insulator is all covered with black fluffy carbon,
the fuel air mixture is too rich. The engine would run with a heavy,
thudding exhaust sound, and be unwilling to take full throttle. If the
spark plug insulators are bone white and rough looking, the center
electrode and the ground electrode would probably be oxidized blue.
That means the fuel air mixture is too lean, the engine won't develop
full power and will overheat...

There's not much to go wrong with the electronic ignition systems of
modern motorcycles anymore. Some engines did have mechanical advancers
in the mid-1980's and the advancer could have a broken spring or one
spring would fly off, or the advancer weight would rust and stick in
the fully advanced position and that would affect the starting and mid
range power and the engine would grind on the starter when the bike was
hot and the mixture would seem to be too lean.
Check the parts diagrams at www.partsfish.com to see if you have a
mechanical advancer. The ignition advance curve is built into most
modern electronic ignition control units. They just count the number of
times the pulser coil sends a signal to the spark unit and that signals
the unit to advance the ignition according to RPM only, no vacuum
advance like on a car engine...

Years ago, when motorbikes had ignition points that had to be cleaned
all the time, you could blame many problems on the points, and the
carburetors seemed very reliable. As motorcycles all got electronic
ignition systems, riders didn't have to maintain the ignition points,
there weren't any. They paid less attention to the engine and the
carburetors would get dirty inside, especially the idle jets. Riders
compensate for dirty idle jets by turning up the idle speed knob and
using more "choke". As I have explained hundreds of times, modern
constant vacuum carburetors don't have a real choke plate, they have a
tiny carburetor in the side of each main carb and that tiny carb sucks
gasoline right out of the float bowl. But the tiny carb, called a
"starting enrichenment device" needs all the help it can get from the
fuel passing through the idle bypass ports and the idle mixture screw.
The idle bypass ports and the idle mixture screw have to get their gas
through the idle jet and the idle jet has such a tiny orifice (only
0.012 to 0.016 inches in diameter) it easily gets plugged up and
sometimes the plugging is so bad it can't be cleaned out using powerful
carburetor cleaners like the liquid version of Berryman's B-12
Chemtool(3 ounces per tank of gas, repeated 4 times)...

You can tell you need your idle jets cleaned when your engine is hard
to start, needs a lot of choking, is slow to warm up, and tends to
stall when you pull the clutch in to slow down. When you roll off the
throttle, the exhaust pipe goes fartBANG!

If Berryman's B-12 doesn't clean out your idle jets, diassembly is
required. I've explained the process hundreds of times. Google up
"kaybearjr@aol +idle jet"...

Other fuel, carburetor and air induction problems might include:

A clogged air filter that need to be cleaned or replaced;

A clogged external fuel filter in the fuel hose that needs to be
cleaned or replaced;

A clogged fuel filter *inside* the gas tank on the petcock standpipe;

A clogged fuel filter just above the float valves on the side of each
carburetor;

Carburetors far out of synchronization;

A fuel hose that is kinked due to improper routing (I had that happen
to me once, drove me absolutely nuts, took three weeks to locate the
problem);

Rust and crud in the petcock that has settled down into the fuel line,
blocking the float valves and the jets;

Water in the gas that sits in the bottom of the float bowls (you can
drain the float bowls into a clear glass container and see the water
settle to the bottom);

Vacuum piston or slide sticking. Sometimes evaporated gasoline will gum
up the
slides so they can't move at all. The carb can't suck much gas through
the needle jet, it has to run on the idle jets only, and the engine
starves for fuel;

Vacuum piston or slide diaphragm torn. It can't lift the jet needle out
of the needle jet. Same symptom as above;

Incorrect float level, the floats shut off the gasoline prematurely and
the
idle jets and main jets have to work too hard to suck up what little
fuel is in the float bowls. Floats don't just change their adjustment
that much, even over a period of years. They get out of adjustment when
a mechanic bends the adjustment tab trying to set the float levels;

Float leaking. If a brass float leaks, it fills up with gasoline and
settle to the bottom of the float bowl. The float doesn't shut off the
flow of gasoline into the float and that float bowl overflows. Gasoline
leaks onto the ground under the motorbike.

Conditions relating to the mechanical condition of the engine:

Worn out or stuck piston rings reduce the compression of the engine, it
won't develop full power. Rings stick when hot oil collects in the
piston ring grooves and hardens. Some chemical nostrum like Rislone,
Marvel Mystery Oil or Techron added to the gasoline will free up stuck
rings. This is more of a problem with 2-stroke engines, but an
oil-burning 4-stroke can have sticky rings;

Valves out of adjustment or burned. Even the intake valves can burn, if
the valve guide oil seals leak oil onto the hot valve head. This oil
turns into hard wet carbon and keeps the intake valves from closing.
The compression of the engine blows air backwards through the
carburetors. The engine can't hold in fuel/air mixture that it sucked
in on the intake stroke because the piston
blows it back out on the compression stroke. The engine pings and
detonates like crazy when this happens...

Blown head gasket between cylinders. If the pistons are on opposite
strokes, the piston coming up on compression leaks high pressure air to
the cylinder going down on intake stroke. That cylinder can't fill up
with fresh fuel air mixture. I saw that happen recently on a pick up
truck engine. Drove me absolutely nuts for three weeks troubleshooting
the problem...

Don't be afraid to do a compression test on your engine. Warm it up,
pull out the spark plugs, check the compression with the throttles wide
open so the rings will get enough air behind them to compress the air
in the cylinder. If the compression is less than 135 to 140 pounds,
something needs to be done, it's
worn or stuck rings or burned valves, or out of adjustment or sticky
valves...

Dribble a few drops of motor oil into the spark plug hole of the
cylinder(s) that is low on compression. Turn the engine over several
turns to spread the oil around. If the compression increases with the
oil sealing the rings, that's what it is, a ring problem. If the
compression doesn't come up, it's a valve problem...

Engine repair manuals will probably tell you that the specified
compression for that engine is around 175 to 190 pounds or so. If your
engine still have 135 to 140 psi, it will go 100 mph. It just won't get
there as quickly. And I wouldn't diassemble an engine that had 135 psi
compression, it's a lot of work to pull the engine out of the frame,
split the cases and re-ring a V-4 that's not worth a whole lot of money
at this point...

 

WOW!!! I think you covered every possible contingency. Thanks VERY much for
taking the time to type all this. Looks like I got some work ahead of me for
the weekend. I'll start with the easy stuff and work my way up. I'll let you
know what I find.

Thanks again
Shawn

 

One thing I didn't mention was regapping the spark plugs to about 0.025
inches. If the engine runs a lot better with a smaller gap, your spark
plugs might need changing. Something changes the internal resistance of
spark plugs, even the modern copper core plugs that can withstand a lot
of heat cycling. When the plugs change inside, for whatever reason, a
new set works so much better it's amazing. But by gapping the old plugs
tighter to see if that fixes the problem, you can save yourself about
$10.00. I have changed plugs twice on my pickup truck troubleshooting
problems. It takes 8 plugs for a dual plug 4 cylinder, and that turned
out not to be the problem...