Flaming Carbs-Honda Shadow

Just rebuilt the engine on my 86 Honda VT1100, to replace the tranny.

Got a weird problem with one of the carbs, it puffs out smoke and the piston
inside the carb pulsates. With the air box off I can see flame inside the
carb !

The carbs are clean - and worked fine before the rebuild. I opened up the
carbs today and they look new. I can't fathom why the one carb would
pulsate and flame. Could it be the timing is off?

 

Not the *desired timing*, but inexpensive ignition systems on
carbureted engines usually fire a *waste* spark 180 degrees after the
*desired* spark occurs. Gawd only knows whether it will find a burnable
mixture in the exhaust gasses. It should not find a mixture which is
that rich...

So, what is it, a 60 degree V-twin with both rods on a common journal?
Or do it have a weird crankshaft arrangement to make it sound just like
a Harley Davidson?

The desired spark for #1 cylinder occurs when both intake and exhaust
valves are closed, very late on the compression stroke. Then the
desired spark for #2 cylinder occurs 60 (or 45) degrees later, again,
with the intake and exhaust valves closed...

Then, 180 crankshaft degrees after the desired spark on each cylinder,
the waste sparks occur, firing into the exhaust stroke when the exhaust
valve is open. You shouldn't normally be getting any spark while the
intake valve is open...

But your idle mixture may be so rich that burning isn't completed
during one power stroke. The excessively rich exhaust gasses may be
pushed back through the intake tract while both intake and exhaust
valves are open during the camshaft overlap period.

Are you the original owner or a second or third owner?

The usual cause for such mixture richness problems is that some owner
has changed the exhaust system, bought into the notion that he needed
to radically re-jet the carburetors, and went up about a dozen or more
sizes on the main jet because he didn't understand the jet numbering
system. Or maybe a previous owner didn't understand which direction to
turn the idle mixture screws while tampering with them. In that case,
the inexperienced home tuner turns the mixture screws counterclockwise,
expecting the engine RPM to speed up. When it doesn't respond as
expected, the engine RPM slows down and the backyard tuner then turns
the master idle knob to increase idle RPM and this uncovers the
transition ports nexts to the butterflies. In that case, the idle
mixture is too rich and burning continues in the exhaust pipe after the
exhaust valve is open.

You can tell if somebody has tampered with the idle mixture screws by
the simple fact that you can see the slot headed brass screws forward
of the float bowls if they are on the bottom of the carbs, or forward
of the diaphragms if they are on top. The factory installs aluminum or
brass anti-tamper plugs to keep owners from messing with the idle
screws, but guys like me have been drilling them out now for about 25
years.

If you are the original owner and nobody has drilled out the
anti-tamper plugs and tweaked with the idle mixture screws, or
radically re-jetted the carbs, I would suspect a leaky float valve or
floats that are set too high.

 

Sounds like you've got the cam timing off on that cylinder