Also have trouble 1983 Shadow VT750

Bike was sitting for 5 years. Starts and idles well. I can accelerate
up to
3000 rpms and no more. Starts popping and boggs down but only above
3000 rpms. Bike has after market mufflers--tried to restrict (like
stock mufflers) but did not help problem. Carbs rebuilt 4
times--clean. Have syncronized carbs--no improvement. Any help would
be appreciated.

 

1. Plugged idle jet
2. Plugged idle passage
3. Stuck vacuum slide
4. Torn or punctured diaphragm
5. Stuck float valve
6. Float level too low
7. Plugged air jet
8. Loose electrical connections in ignition system
9. Plugged up air filter
10. Vapor trap in fuel line to pump
11. Clogged fuel filters in tank or carbs
12. Induction system air leak

 

Also, add intake valve deposits prevent intake valve from seating. This
causes spitting back out the carburetor...

 

where are you located?

 

Most of the time this is fuel relted problem. Want verification? Whle it
is bogged down, spray a little carb cleaner into the intake. Yamaha makes
a liquid carb cleaner that works darned good.

When you rebuilt the carbs did you spray your Berryman's through each
orifice? To be honest it sounds like your main fuel circuit is clogged.
Read the appropriate manual and figure out the main fuel circuit and make
sure it is free of any occlusion.

rj

 

Question?: Why would plugged idle jet affect rpms above 3000 (#1 and
#2). Bike runs well enough below 3000 but cuts out above. I have the
air cleaner removed and vaccum slide/diaphams working well (#3 and
#4). Air cleaner not a factor as it is not connected to test (#9).
Unhook fuel line and rebuilt electric fuel pump--working well (#10 and
#11) No induction system on ’83 (started in ’84) I like your idea
about intake valve deposits--any idea how to remove without teardown?
Bike has hydraulic lifters--possible sticky lifter?? Many thanks for
input--bike is getting frustrating to problem solve and it helps to
bounce ideas off you guys.

 

Central Minnesota

 

red wrote:

When you're cruising down the highway at 65mph and your throttle grip
is only turned 1/8th of a turn, the throttle butterflies are hardly
open. CV carbs have HUGE idle jets, compared to the idle jets on the
older slide valve carbs.

As I recall, the VT750 had about #48 idle jets. That's a jet with a
hole in it that's 0.48 millimeters in diameter. By contrast, my
GSXR-750 has #17.5 idle jets in its 33mm smoothbore racing carburetors!

So engine vacuum is high at small throttle openings and the engine can
be running off the idle jets any time the load on the engine is light.
The engine is running off of fuel air mixture coming from the idle jet
and idle passage at idle, and the flow of fuel through the idle
passages drops off until nothing is coming through the idle passages at
wide open throttle.

The idle jets and idle passages had better be squeaky clean to pass
enough fuel to the engine.

As the rider opens the throttle more and more, vacuum lifts the
slides and the jet needles come up out of the needle jets, allowing
more and more fuel as more power is needed. The hole in the needle jet
is HUGE, about 1/8th of an inch in diameter. It's almost impossible to
plug up the fuel hole in the needle jet, but the little air emulsion
holes in the sides of bleed type emulsion jets can get plugged up with
gum and varnish.

Your engine isn't running on the main jets until the throttles are
3/4th's to full open, and then only when engine vacuum is high enough
to lift the slides all the way. Badly plugged up main jets can begin to
affect engine performance from 1/8th throttle to full throttle, but the
way CV carbs depend on the idle jets, I would bet that you could ride
for hours on the freeway and still be running on the idle jets if you
never twisted the throttle past 1/4th of a turn...

Run about 3 ounces of Chevron's Techron concentrate through the
gasoline for four tanks of gas.

 

gotcha

mine has been frustrating as well, seemed to be running too rich (after
much consulting with Mr. Kritter), and backed down on the pilot screws.
but i have the same problem you do with it cutting out at higher RPMs.
i cleaned the carbs, checked the fuel pump, slides, etc. still don't
have it quite figured, yet.