'85 Honda Shadow carburation (?) problem

My wife has an '85 Shadow 700 (VT700C) that I think is having
carburation issues.

These are the symptoms:

It starts and idles perfectly. A little cold-blooded, so it needs the
choke a little longer than I'm used to, but not excessively so.

During acceleration, it pulls well up through the gears and will hit
70 with no problem.

The problems start to happen at sustained highway speeds. At any speed
over about 50 it won't hold the speed steady and seems to "hunt", as
if the throttle is going up and down. It also won't accelerate well
unless it's downshifted a couple of gears so the revs are back up near
red-line.

Around town everything seems happy again.

I've changed spark plugs, air filter, and fuel filter to no avail. I
ran a tank of gas with Sea Foam in it and that didn't seem to help
either.

A couple of years ago it was running even worse and the problem turned
out to be a torn diaphragm in one carburetor -- the other one was OK.

Could this be the other diaphragm giving up, dirty carbs, or some
other problem? Maybe not carb related at all?

She hasn't ridden the bike much since she got her BMW R1200C and she's
wanting to sell it, but it seems like it would be better to fix this
problem instead of passing it on to the buyer (especially because my
daughter's husband is interested in buying it . . . ;^)

Any hints? Pointers? Clues?

Thanx,

-Don

 

That indicates lean idle mixture. Have you drilled the EPA plugs out so
you can open the idle mixture screws about 1/2 to 1 full turn?

Sounds like fuel starvation. The float valves might be getting sticky,
you might have dirty fuel filters in the petcock and the carburetors.
Check the carburetor parts fiche at www.partsfish.com to see if there
is an internal fuel filter in the carburetor above the float valve
seat.

Have you tried Berryman's B-12 Chemtool Choke and Carburetor Cleaner?
The
non-aerosol 12 ounce can costs about $3 at Wal-Mart and you put 3 or 4
ounces in each tank of gasoline until you've used it all.

The only way you'll know if the diaphragm is torn is to inspect it
visually.

 

I just suffered a torn diaphragm on my CJ360T. The tear was something
like a half inch long, parallel to and near the circumference of the
slide, and I almost didn't notice it. It may have been caused by
getting carb cleaner on the already aged and weak rubber.

Symptoms were:

* poor high-speed performance (top speed ca. 50 mph)
* exhaust pipe not very hot on the side with the bad diaphragm
* After five miles, a new plug on the side with the bad diaphragm became
carbon/fuel fouled
* poor gas mileage (little more than half of normal)
* carbon deposits in cylinder on the side having the bad diaphragm
* carb with bad diaphragm used more fuel

It seems that since the torn diaphragm fails to lift the slide up out of
the venturi, it acts like a choke to pull more fuel into the mixture and
cause an overrich condition (thanks kk) that causes fouling and failure
to fire.

I didn't attempt to repair the rubber, although maybe I should have
tried superglue or Goop.

I sent both slides to NRP Carbs in England (thanks Mark Olson and TOG)
to get new diaphragms at a cost of three weeks downtime and $130.

I'm getting great high-speed performance now.