Throttle hesitation '83 VT750 Shadow

Got this annoying throttle hesitation problem on my old VT750. It happens
frequently when the engine's warm (not when cold) and has been idling a
minute or so, as at a stoplight, or after I've shut off the motor for a
brief time and restarted.

I turn the throttle and it just pretty much sticks until it gets to a
2,500-3000rpm threshold then suddenly roars into life. It means I have to
be extra careful w/ the clutch, obviously, and I have to be sure I've
gotten up to the threshold before I can jump out into a busy street.

Anybody have an idea what could be causing this?

 

I have a related problem. My 83 Honda VT500 Shadow runs just fine at
low speed. After a few minutes at 55 MPH on the freeway, it acts like it's
running out of gas. But If I open the throttle way up, it sputters
back to life. If I open up the carb drain plugs I get what should be
plenty of fuel flow to run the bike.
Ideas?
Thanks, mike

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That's why I opened the bowl drain plugs in the carbs. That should
include effects of all the above??? yes? I rigged up some plastic
tubing from the bowl drain and ran it up the side of the carb so I could
see the fuel level. Seems ok in the garage, but I never figured out
how to look at the fuel level at freeway speeds. I'm too young to die ;-)

Yes, I've tried this. Results are inconclusive. By the time I hold the
clutch with one hand, remove the key, open the tank, close the tank, put
the key back, restart the engine, all while dodging traffic at 55MPH,
the symptom is gone. I'll do some more experiments tonight on the freeway.

OK, I'll try pulling the idle jets. Can an idle jet have that much
effect at freeway speeds?

Yeah but...if the thing runs fine for several miles before choking up,
wouldn't that eliminate the air leak possibility?
Remember it runs fine all day below 40 MPH.
I'll try running it in a lower gear to simulate higher freeway speeds.
Wondering if the ignition system is overheating and cutting out at
higher speed??

Sure acts like gas starvation tho...

This thing has 83,000 miles of stopandgo garage sale travel on it.
Guess I should be glad it ran this long.

I have almost a whole other bike I can use to swap parts if I could
figger out what to swap...yeah, I know, swap the engine...

mike

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Hmmm, wonder if that explains the morning stall on my Vespa
GT200 (I need to really dig into the engine compartment -- find out
which of two carburetors I have)... Was trying to avoid coming to a full
stop at a light, and when the light went green, my wobbly "hit the
throttle" resulted in a stall... Suspect I was at the transition from
engine braking and coast (centrifugal clutch), and didn't have enough
"push" to keep running when the carb suddenly changed state.

--

 

Chrysler products used a "dual ballast resistor". A low
resistance circuit for the starter position, and a "normal" resistance
for run mode. Never saw a spec for the coil voltage, but did have the
start position half blow once. Was quite annoying -- engine would crank
properly, but not start... Then when you let go of the key, you'd get
one cylinder firing (my experience with that car was that I only needed
two cylinders to get it started -- even in the dead of a MI winter with
straight 30W oil! Chrysler reduction gear starters could pull a freight

<blink><blink> That sounds backwards... At least for Chrysler
systems -- the dual ballast resistor reduced resistance while cranking,
meaning a higher voltage to the coil (well, same voltage as would be
seen without a starter motor draining things).

--

 

The plot thickens. My vacuum cylinders are floppy.

I hooked up all manner of tubes and clamps
so I could capture the fuel level while
cruising at 55. I did catch the level being
low, but couldn't repeat it. And the fuel
level didn't come back up when I stopped.
Maybe I should not have tried to do this at midnight.

Results were inconclusive, but sufficient for me to
decide to pull off the carbs and clean up everything.

I also discovered that one of the exhaust pipes was
plugged up, but replacing that didn't help.

New problem.
When I push on the vacuum cylinder of the carbs
on the doner bike, I feel two forces. There's a
spring. There's also a velocity dependent force
of the piston accompanied by the sound of air
rushing out some orifice or other.

When I do the same thing on the bad bike,
the vacuum cylinders have the spring force ok, but
the bottom half of the excursion has no piston force.
The last half of the excursion does have the piston
force and the sound of air rushing.
This can't be good...
Both carbs behave the same.

Last time I had 'em apart, there was some cracking of the
diaphragm. Maybe I've developed a hole. Seems odd that
both carbs behave the same. I'd expect them to fail
one at a time.

Looks like I'm swapping carbs.
But the things are so darn expensive that I'd like
to understand what's going on and fix it. Resale
value like they are is zero...

mike




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