Motorcycle Shuddering When Cold and Engine Loaded

 2006 Honda VT600, carb'd (no EFI), has TPS wired to ignition control
module.

Had my valves adjusted about 1,000 miles ago.  As usual, the valve
adjustment turns it into a different motorcycle.  It sounds a bit
different, but it still has about the same performance and top speed.
Valve adjustments have always done this (it is a different motorcycle
when I pick it up).

Anyway, noticed a new symptom ...

When the engine is on the cool side (not fully warmed up) and I'm
cruising down the road and I give it a bit of throttle, the engine
nearly shudders.  There is kind of a low-frequency component there.
Not quite a lurching, but the shuddering is at a far lower frequency
than the engine RPM.

But after the bike is fully warmed up, it doesn't do that.  When I
open the throttle under load, it just makes the usual noises of an
engine trying to deliver power.

The bike is "normal" when warmed up fully.

My first thought was the TPS ... maybe not advancing enough or
something like this.

But I actually can't figure out how the TPS is going to affect the
behavior of the ignition controller, anyway ... it seems advance would
be tied to RPM and nothing else.

Any thoughts on the behavior?

It didn't used to do that when cold.

Thanks, DFC

 wrote:

Does it shudder?  You said "it nearly shudders."   So does it just
feel like it is about to shudder, or does it actually shudder?

 On Sat, 10 Sep 2011 21:31:31 -0700 (PDT), Tim

It will actually shudder.  The amount of shuddering is proportional to
the increase in throttle.  That is why I necessarily was thinking of
the TPS.

But only when not warmed up.  After warmed up, no issues.

It never used to do that.

DFC

 wrote:

I bet you shiver when you're cold too.

 

The usual problem with motorcycles that have diaphragm type
carburetors is that the tiny jets, ports and passages become plugged
up with gum and varnish from gasohol evaporating in the float bowls.

I recommend adding about 4 ounces of Berryman B12 Chemtool Choke and
Carburetor Cleaner or Berryman B12 Fuel Treatment to a full tank of
gasohol two or three times a year.

The Choke and Carburetor Cleaner comes in aerosol form and can be
squirted into the various carburetor ports and passages and jets for
direct cleaning.

In order to avoid removing carburetors from the engine for thorough
cleaning, I will sometimes squirt B12 into the pilot air jet which is
the smaller of two holes in the intake mouth of a diaphragm
carburetor.

This will get full strength B12 into the idle mixture circuits
rapidly.

The Fuel Treatment comes in liquid form in a 15-ounce can and you can
get it for around $3.50 at Wal*Mart.

Use a funnel to pour 4 ounces into the gas tank, since the powerful
acetone, toluene, and methyl alcohol may soften your paint and damage
plastic.

This is only about a 3% solution of gasohol/B12 but it also works
rapidly.

Go for a ride and watch your idle RPM increase.

That shows that the B12 is doing its job to dissolve the gum and
varnish that plague small carbureted engines fueled by gasohol.

Riders who attempt to adjust their own carburetors are often baffled
by their inability do do so.

One would expect that turning the idle knob would raise the idle RPM,
but sometimes this doesn't happen.

Sometimes the engine becomes hard to start or the idle RPM races up
too fast when the engine warms up.

*One cannot adjust a dirty carburetor!*