still having problems: '83 Honda VT750

ok, i eliminated the low speed cutout of the engine by putting a loop in
the fuel line, no more kinks.

however, i am still having problems at higher rpms when in neutral,
around 4000 and it starts to cut out. even more interesting, when i put
it in gear, i won't even make it that high. maybe 2000 rpm. so why the
diff when it is out of gear and in gear? and why the hell is it doing
it at all????

ok, now for the best stupid question (always save the best for last).
what does the clutch switch do? i can't find any relevant explanation
on google or in my clymer manual.

Thanks

tim

 

I suspect that you're having the same problem as everybody else that
complains about crappy running with a motorbike equipped with constant
velocity carbs. I've explained what to do about the EPA anti-tamper
plugs and how to clean out the idle jets and the idle mixture screws
about a bazillion times. You can google for "kaybearjr@aol +EPA" and
for "spectraltarsier@aol +idle jets" to see what to do...

Why does a motorbike with dirty idle jets crap out when it's in gear,
pulling a load? It can't get the gasoline it needs to accelerate into
the midrange. If you could get the engine past the transition from
closed throttle to about 1/4 throttle, the engine would run on the
needle jet and it would probably go down the road OK until you rolled
off the gas and closed the throttle, then it would stall on you as the
exhaust pipe went fartBANG!

The clutch safety interlock switch is supposed to keep newbies from
having the motorbike fall over on them when they try to start the
engine while it's in gear and the motorbike lurches forward. So you
have to pull in your clutch lever because that actually did happen to
some newbie somewhere...

My Kawasaki had a sidestand safety interlock switch and a clutch safety
interlock switch. The sidestand would flop up and down on rough roads
and shut the engine off. So far as the sidestand safety interlock
switch was concerned, I was parked with the sidestand down...

My Yamaha has all of those switches, plus a neutral interlock switch.
It knows if I put the motorbike in gear with the sidestand down. It
shuts off the ignition so I won't hurt myself with my newbie stupidity.
I've only been riding for 42 years, I don't know what I'm doing yet...

 

thanks for the info, i was hoping you might give me the right directions.

funny, i was wondering if it actually did anything. mine is set just
far enough away it doesn't contact the clutch lever anyway. starts up
just fine without the need to have it in, but it does have a neutral
requirement.

you really should be careful, maybe take a riding course before going
out. such inexperience kills.......

 

i forgot to mention it was stalling out when i came to a stop.......

 

That is a symptom of dirty idle jets, idle screws and idle passages...

While you have the carbs apart for cleaning, be sure to inspect the
rubber diaphragms for holes and tears. That problem seems to occur
fairly often on motorbikes that have been stored for long periods. I
suppose the diaphragms tear because the gasoline in the carbs
evaporates and gums up the slides. But riders don't know about all the
problems with gummed up carbs and they manage to get the engine started
and they open the throttle thinking the engine will start running
better after the rev it up, but the slide is stuck and the diaphragm
tears. Or maybe it tears when the engine spits back because it can't
get enough gas through the plugged idle jets.
Who knows for sure what happens? Those diaphragms get torn and they are
expen$ive to replace...

 

first time i had the carbs apart, the diaphragms looked ok, no holes or
tears. i did have trouble getting one to go back on, it seemed
oversized. i let it sit overnight, shrank back down to a manageable
size. the slides move fairly smoothly. part of the problem i realized
the other night is someone prior to me drilled out the plug on the rear
carb and adjusted the pilot screw. i need to get in the shop tomorrow
and reset it to the factory/book setting, and work from there.

 

On old cars with 2-barrel carburetors, we used to set the idle speed to
the specified RPM with the idle mixture screws out about 2 turns and
then we would screw each idle mixture screw in until the engine ran
rough and screw it back out 1/4 of a turn so the idle would smooth out.
We'd rev the engine twice, smile, say "She's a-takin' tha gas now,
Bubba," slam the hood and go grab a beer from the refrigerator...

Then, sometime during the 1970's the EPA exhaust emissions requirements
demanded that carburetors be sealed so we couldn't "tamper" with them.
Cars and motorcycles were affected by the EPA rules. EPA wanted really
lean idle mixtures to reduce air pollution. Performance enthusiasts
wanted off-idle throttle response. Commuting motorbike riders just
wanted their damned engine to start easily and warm up quickly on cold
mornings. So we started drilling out the EPA plugs...

It's strange that somebody would only drill out ONE of the EPA
anti-tamper plugs on your Honda. Maybe he thought the rear carburetor
was just out of adjustment, when it was actually getting gummed up...

The carburetors were set at the factory on some kind of machine and
then the anti-tamper plug was installed. The information you want
probably ins't in the manual at all. It will probably say something
about "factory set" or "non-adjustable" if it says anything at all...

The setting may be anywhere from 1/4 of a turn out to 3 turns out from
lightly seated. About all you can do is experiment with BOTH of the
screws when the idle jets, ports and passages are cleaned out...

What you are trying to do is find the setting where the engine idles
the fastest for the throttle opening, without having the throttle be so
far open that a lot of mixture comes out of the transition ports that
are normally covered up by the throttle butterfly...

If the idle mixture screws are turned too far out, the throttle
butterflies have to be too far open to compensate, so when the rider
twists the throttle quickly and closes it again, the engine gets a big
shot of gas from the transition ports and the RPM stays too high, it
takes 15 seconds or more for the engine to slow down...

So you can find the best setting for the idle screws by setting them to
about 1.5 turns open and setting the idle speed to specification and
then blip the throttle and see what happens. If the engine RPM hangs up
and stays too high, turn the screws IN equally on both carbs and try
again until the RPM stops hanging up...

If the engine RPM doesn't stay too high you can continue turning ONE of
the idle mixture screws out 1/4 of a turn at a time and see what
happens to the engine RPM. As you open the idle mixture screw, the
engine RPM should increase, so you have to turn the idle speed screw
down so the engine idles at the specified RPM. At some point, you will
see that turning the first idle mixture screw out further doesn't
affect the RPM at all, the exhaust sound just gets dull and thudding.
You want to hear a crisp sounding exhaust note, so turn the idle
mixture screw back until the exhaust sounds right and adjust the idle
speed screw so you get the specified engine RPM...

Then you can adjust the other idle mixture screw the same way. Keep
track of how many turns out the first idle screw is and you have a good
starting point...

 

well, that explains the plug......

believe or not, the Clymer book I have actually details how to drill out
the plugs and reset the "pilot screws". something like 2 3/4 turns out,
then do 1/4 turn increments after setting the idle.

of course, then it says i'm to plug it back up............just as soon
as i get around to buying some more plugs, i'll get right on it.......


snip

So we started drilling out the EPA plugs...

 

btw, thanks again for all your help


tim

 

2-3/4 turns out is a lot if the idle jet is as big as the parts fiche
at www.partsfish.com says it is. I was looking at a 1983 VT-750C...

#20 JET, MAIN (#118)
#21 JET, SLOW (#40)

Those are large jets. The engine is cruising on the slow jet when you
have the throttle open about 1/8th of a turn on the highway...

I know you're only kidding, but you'd have to buy the idle screw,
spring, washer, and o-ring to get the plug...

#5 SCREW SET A $11.99

Since you say you have the Clymer manual, what does it say about this
screw which is right under the carb mouth according to the drawing? Is
that an adjustable screw, or is it a plug?


#6 SCREW SET C $14.99

Hopefully you don't have a torn rubber diaphragm on either carb...

#13 PISTON COMP., VACUUM $116.99

If you dind that you need to buy repair parts, you might find them
cheaper at www.oldbikebarn.com

They sell carb repair kits with newly manufactured parts. I had to
order two little rubber bits for my petcock before I found out about
Old Bike Barn. It cost me $10 for the parts and $10 for shipping to by
them from Partsfish.com...

 

i'm showing the main jet in the manual as a 115, slow is a 40
the screws behind the plugs are referred to as "pilot screws" and are
adjustable. initial setting is 2 3/4 turns out for the '83 at 900 rpm.

the diaphragms seem to be ok, no noticeable tears or holes. i paid $200
for the bike, and have maybe another $200 in parts in it, so i can't
complain if i have to drop a little on it. maybe i can tweak the carbs
to something resembling functional soon.

i also want to go back and check my floats. the book isn't always clear
in the explanations or pics, so i'm glad you mentioned the necessary
height of the floats over the gasket. i want to make sure they are set
to the same setting before i check anything else.

i had been using bikebandit.com for most of my research, but i'm always
looking to price shop.

 

Actually, it sounds like I've confused you...

When the manuals say "gasket surface", they don't mean the surface of
the paper gasket, they mean the metal surface of the carb body that the
gasket fits against. If you measured from the bottom of the float to
the gasket with the carb upside down on the bench and the gasket was 1
millimeter thick, you might set the float level 1 millimeter lower than
nominal...

Usually the specification for float level will be something like 15mm
plus or minus 1 mm so setting the float to the gasket itself would be
at the lower end of the tolerance. The float level would be 16mm
instead of 15 mm...

That sounds weird, but remember that float level is adjusted with the
carbs upside down, so a bigger number is a lower fuel level in the
float bowl...

If the float bowl gasket is stuck to the carbs and you don't have a new
one, you can subtract the thickness of the gasket from the nominal
float level...

Tuners will play around with float level rather than buy expensive
slides or
needle jets if their off-idle transition is a little lean. They raise
the
fuel level in the float bowl by reducing the dimension from the bottom
of the float to the gasket surface. Like if the nominal dimension was
15 mm plus or minus 1 mm, they would set the float level at 14 mm by
bending the tang on the float up slightly...

That makes the fuel level in the float bowl higher when the carb is
turned over, and the engine vacuum has an easier job of pulling fuel
out of the float bowl...