Yamaha Seca II Carburetor Problem

I recently acquired a 1992 Seca II with low miles that had been sitting
around for about 10 years. I bought a tech manual, took apart the
engine, cleaned everything, replaced what needed replacing, but did not
take apart the carb. Put everything back together, and it would not
turn over.

Checking through everything, I noticed there was no fuel going into the
carb. When I covered the carb air intakes with my hand, there would be
enough vacuum to pull in fuel and start the bike. I replaced the air
filter box with a new air filter, and same result -- when I covered the
air intake slightly the engine ran but the moment I removed my hand
from the intake the engine sputters out.

Has anyone had a similar experience? I browsed through the posts but
didn't see anything, and the tech manual did not go over this in
troubleshooting, nor does it tell me how to adjust the carburetor at
all. I eventually did take apart the carb just enough to make sure it
was clean, but that did not help either. Any advice would be
appreciated!

 

No, nobody has ever had *that* problem. You are the *only* one.

Yeah, right. Just kidding.

It's the most common problem (other than a dead battery) with Japanese
motorcycles that have carburetors when they are allowed to sit unridden
for just a few months. The *tiny* idle mixture passages get all gummed
up, the *tiny* idle jets get gummed up, the float valves stick, the
petcock may be plugged up with rust, so you clean it all out with
Berryman's B-12 Chemtool Choke and CarburetorCleaner. It comes in
aerosol and liquid form for about $3.00 a can.

You can spritz out the jets and passages with the aerosol B-12, and you
can clean the whole fuel system by running 3 or 4 ounces of the liquid
B-12 through a full tank of gasoline.

Google up "EPA anti-tamper plugs" in this newsgroup and "transition
ports" and you'll see that I have described what the situation is
hundreds of times. The EPA doesn't want you to adjust the idle mixture,
they want it kept lean. So the engine is hard to start, it lacks
throttle response (is "cold-blooded") and takes a long time to warm up.

The other major thing that newbies don't understand is that the "choke"
is NOT a real choke, there's no separate flat plate to make the
carburetors suck gasoline out of the float bowl.

The carburetors have a vacuum operated slide that works like a choke,
in that the slide what makes the carburetors suck harder (as long as
the rider doesn't twist the throttle while trying to start the engine).
When you move the "choke" lever to the full ON position, you're
actually opening a bypass air passage *around* the throttle butterfly,
and it's like sucking gasoline through a soda straw, there's no venturi
in that little bypass passage.

Newbies think they have to operate the throttle handle to start the
engine. That's wrong. Just as soon as they twist the throttle, the
vacuum they need to suck gas through the starting enrichener goes away
and the enrichener can't suck gas out of the float bowl.

The engine should start and idle with just the "choke" lever in the
full ON position. The idle shouldn't run away, and the engine shouldn't
stall. You should be able to start the engine and stand by the
motorbike zipping up your jacket, putting your helmet on and putting
your gloves on while the engine warms up without you nursing the
throttle and fiddling with the "choke" lever.

When you think the engine is warm enough and you turn the "choke" OFF,
if the engine stalls, the idle jets and idle mixture passages are
plugged up.

The original owner may have noticed that his engine was stalling as the
idle jets gradually plugged up, so perhaps he adjusted the master idle
knob to make the engine idle faster. This screws everything up because
the throttle butterflies may be far enough open to uncover the three
transition ports just downstream of the butterflies.

When this happens the engine will idle too fast after the throttle is
opened and closed quickly. It will hang up around 5000 RPM and won't
idle down for 15 seconds or a minute when the engine is warm.

If you're having trouble starting your engine, one trick is to turn the
master idle knob all the way counterclockwise to close the throttle
butterflies all the way. This will increase the vacuum downstream of
the butterflies and help the "choke" suck gas out of the float bowls.

In this case, you will have to hold the throttle slightly open after
the engine starts. You can readjust the master idle knob after the
engine warms up.

The idle mixture passage cannot be seen from the outside of the carb
body. The passage goes four ways. One leg goes to the pilot jet to pick
up gas from the float bowl. One leg goes to the pilot air jet in the
carb inlet to get air to mix with the gasoline. One leg goes to the
single idle mixture outlet port that is controlled by the idle mixture
screw that's hidden under the EPA anti-tamper plug. And one leg goes to
the three transition ports downstream of the throttle butterfly.

When you spritz out the idle mixture passages, you can squirt the B-12
through the pilot air jet and it should come squirting freely out the
idle mixture port, the three transition ports and the idle jet. You can
put your fingers over the holes that the B-12 does come out of and keep
squirting until it squirts freely out of all the holes.

You can drill out the EPA anti-tamper plugs and remove the idle mixture
screws. That's been described a hundred times in this NG, so I won't
repeat it. I've also described removing the factory set idle mixture
screws, recording the setting and reinstalling them. So you can google
for that too.