tuneup question

I'm looking at an older bike that's been sitting for two years. Current owner
says it runs fine, except that it wants to die when downshifted. Ran fine
all the time before the two years in storage.

I'll admit to not being much of a mechanic, but why only then? If the carbs
were gummed up I'd expect trouble all the way around.

BTW, I've heard that automotive carb cleaners shouldn't be used on
motorcycles. Is this true? If so, is there a product that does work or do I
have to disassemble and do a manual cleaning?

Thanks for any help. I'm going to see the bike Monday or Tuesday and if the
downshift problem is an indicator of something serious it's be nice to know
before then .

 

Consider the development of the motorcycle carburetor. During the
1920's through the 1960's, it was a crude plumbing device that barely
mixed air and the gasoline into digestible droplets that the ignition
system could light off.

In the mid-1960's, Mikuni and Keihin began building carburetors that
pre-mixed air and gasoline by means of tiny air holes cross-drilled in
the tiny idle jets and the main jet holders.

The result was a carburetor that was very responsive to small throttle
openings, unlike the cruder Amal carburetors that would either suck too
little or too much fuel into the cylinders when the throttle slide
lifted.

And then along came the Environmental Protection Agency, with their
clean air obsession. Though motorcycles only constitute about 5% of all
registered vehicles, and only a fraction of a percent of all
motorcycles are in daily use, the EPA mandated that motorcycle
carburetors have their idle mixture screws set so the idle mixture was
too lean for easy starting, quick warm up and good throttle response.

Then the idle mixture screws were sealed behind anti-tamper plugs so
the owner couldn't adjust the idle mixture screws. The result was a
carburetor that is intolerant of the slightest amount of gum and
varnish in the tiny idle jets and idle mixture passages.

And, the modern constant vacuum carburetor equipped motorcycle is
ridden at about 1/8th to 1/4th throttle 95% of the time, so it has to
run on the idle jets and needs clean passages.

Try adding 4 ounces of Berryman's B-12 Choke and Carburetor Cleaner to
a full tank of gasoline and go for a slow ride to make the engine suck
the B-12 through the gummed up passages.

I've been using B-12 for 30 years now on all my motorcycle carburetors.
It costs less than $3.00 for a 15 ounce can of the liquid form or the
aerosol form, which is handy for cleaning disasembled carbs.

You'll know that the B-12 is working when the idle RPM increases when
you slow down to stop.

It's just dirty carburetors, Larry. You'd be amazed at how fast
carburetors gum up here in the San Joaquin valley, where the
temperature gets up to 100 degrees everyday in the summer.

 

My lawnmower was built in 1969 and has no tamper plugs but it
still gets gummed up every winter if I leave the gas in it, the idea that
the anti-tamper plugs are somehow causing gas to varnish up
carbs is preposterous. If you let gas set for 2 years in an unsealed
container - which is what a carb bowl is - it's going to varnish it up,
tamper plugs or not.

Try adding a 3rd to a half of a bottle into about a gallon of gas in the
tank, then run the bike until the tank is empty, then refill with the gallon
of gas your carrying with you in a gas can and ride to a gas station and
fill up.

Ted

 

I think that what B-12 is getting at here is that you're more likely to
notice crap in your carb becuase you will always be running off the pilot
jet and needle jet which are smaller and more finnicky than the main jet.

 

The idea couldn't be TOO preposterous, you thought it, didn't you?

No, the point is that modern motorcycle carburetors have tiny jets and
idle mixture passages, and the adjustment screws are hidden behind
anti-tamper plugs.

If the screws weren't hidden, amateur mechanics could adjust the screws
to allow just a tad more gasoline to flow through the outlet ports and
the carbs would tend clean themselves out as the machine was ridden.

 

Thanks to you all. I can now make an offer on the bike without worrying about
the "dying on downshift" problem.

BTW, the reason I'm looking is that my current ride, an old Yamaha SR500, has
developed a carb overflow problem that nobody can find. Someday when I've
got time, I'll list all the things that have been tried .

 

I found that the float bowl overflow problem on my KLR600 was caused by
a tiny bit of
gum on the float valve seat. I had to use a magnifying glass to see it,
and I cleaned the seat with a bit of emery cloth on the end of a
stick...

 

I have been trying to get a good setting on my 85 VT1100. According to the Honda
shop manual the initial setting is 3 turns out. I did this after removing the
anti-tamper plugs. I followed the instructions in the manual regarding proper
adjustment from the initial setting. I used a tach and set the left carb first
and the right carb second per the instructions. I am still having problems with
hesitation even after using the B-12. I've had several bikes in the past and
none of them caused me this sort of grief. I had an 81 Yamaha 650 Special, an 82
GL500 Interstate and two 83 V-45 Magnas. I bought an 85 VT700 two years ago and
it had the same problem although not quite as bad. I was eventually able to get
it running good. I sold it and bought the 1100 last fall. I like the 1100's ride
but the hesitation is very annoying. I've asked several owners of 85 1100's if
they have experienced the same thing and no one else has admitted to
experiencing this problem. Anyone have any suggestion? TIA

 

Assuming that the carburetors are cleaned out sufficiently, I recommend
raising the fuel level in the float bowl by 1 millimeter. That would
cost nothing except your labor.

Since float levels are adjusted with the carburetor upside down on the
bench, if the specified float level was, say, 14 mm plus or minus 1 mm
from the bottom of the float (which is on top when the carbs are upside
down) to the carburetor body gasket surface
(that's the aluminum surface of the carb, not the gasket), then you
would set the float level at 13 mm instead of 14 mm.

If that doesn't work, you might try installing idle jets that are one
or two size larger, assuming that they are available for your carbs.
They would cost about $5.00 each.

I haven't looked up the jet size information on bikebandit.com or
partsfish.com, but the shop manual recommendation of 3.0 turns out
would make the stock idle jet size about
# 35 or # 37, so you might try a # 40 or # 42 if they are larger.

Mikuni and Keihin jets for constant vacuum carburetors have holes that
are based upon
a # 100 round jet being 1.0 mm in diameter, so a # 40 idle jet has a
hole that is 0.40 mm in diameter.

You can see that the area of the hole would vary as to the square of
the radius of the orifice hole, like the area formula you learned in
high school algebra class, and the holes become so much larger so fast
the correct setting for the idle mixture screw rapidly becomes less and
less to the point where a carburetor with a # 45 idle jet requires the
idle mixture screws to be open only about 1/4 of a turn maximum.