1982 Yamaha Seca 750...me again...

For those of you that don't know.... my man is still trying to fix
this Yamaha Seca 750 that was sitting for 8 years or so. He has
rebuilt the carbs, replaced spark plugs, oil change, adjusted float
bowls a few times...
It has a hard time starting, and idling... once it's running, and take
it for a ride/warm it up, and shut it off, let it sit for 15-20
minutes it will start up with no problem. When it's totally cooled
off, it won't start up.
Found out that it's missing a battery sensor, could this possibly be
the cause of the problem...?
Any ideas??? Ideas please let me know asap... trying to get this bike
on the ROAD!!!

 

No.

 

Y'all DO know what a choke is, don't you? And when to use it ??

It sounds like the idle circuit is too lean. Could be the idle jets are
still not clean, a vacuum leak somewhere or the choke/enricher circuit not
working right.

 

The constant vacuum carburetors used on that bike do not have a real
choke like on a car. There is NO choke plate to restrict airflow and
make the
engine vacuum suck harder to draw fuel out of the float bowls.

Instead, there is a little valve built into the side of each
carburetor. When you move the "choke" lever or knob to the ON
position, you are actually opening four little valves and allowing
extra fuel to be sucked into the engine.

But the throttle butterflies in the carburetor have to be fully
closed, if the engine is going to suck up the gasoline.

Many new riders don't realize that they shouldn't twist the throttle
grip when starting their engine when it's cold.

All that is necessary is to put the choke lever or knob in the ON
position, turn the key on and push the starter button. No throttle
twisting is necessary.

If the engine is still hard to start when it's cold, turn the idle
speed adjuster knob clockwise until the end of the adjuster screw
doesn't touch the quadrant on the throttle shaft.

This will close the throttle butterflies all the way and that will
help the engine vacuum to suck up gasoline for starting. If the engine
starts and won't idle, you can manually twist the throttle grip to
keep the engine running, and then adjust the idle speed once the
engine is warm.

If the engine needs a lot of throttle just to idle, the idle jets,
idle passages and idle ports in the carburetors are dirty and you need
to run more carburetor cleaner through all the little holes.

No, it's just a battery water level sensor.

 

These old Yamahas are known for their weak ignition system. Combined
with a very strong (power hungry) starter, you will not get a good
enough spark unless the battery is in very good condition.

It might be worth trying a new battery.

You should also check the fuse box. After all that time, the fuse
holder may have gone brittle, and unable to make proper contact.

 

There's carb rebuilds and there's carb rebuilds. If the fuel was not run
out of the carbs when the bike was parked (the best way is to run
the bike until the gas tank is empty) then the carbs would have been good
and varnished up. If your man simply gave the carbs a quick once through
and
replaced all the rubber parts, he very likely didn't get all of the carb
passages cleaned out.

If the passages ARE passing fuel, then all that is really needed is to
ride the thing. All modern gasoline contains detergent (it's federally
mandated) and after 3 months of riding it every day any varnish in
the passages will have been cleaned out by the detergent in the
gas. You can also hasten the process with Berryman's B-12 chemtool
in the gasoline or Seafoam, or even Chevron Techron.

If, however, some of the small passages where solidly plugged,
then the only way is to soak the carbs, then blow out the passages
with compressed air, then soak them again, then blow out the
passages again, and repeat this process until the passages are
cleared out. If that does not help then you have to poke small
wires through the passages, but you really need to be very
familiar with the carb before doing that.

What was the condition of the inside of the gas tank? Was there
-any- visible rust?

Ted

 

Older low voltage ignition systems only put out 10KV or 12KV.

Therefore, the spark plugs had to be gapped around .025 to .028.

The spark gap was so small that random molecules of a lean mixture
couldn't find the spark as easily as it could find a larger gap of,
say .035
to .040...

So carburetor idle jets had to be bigger than optimum, and then, in
the
1980's, the EPA demanded very lean idle mixtures to pass exhaust
emissions tests.

So the cold starting problem became even worse.

If the engine starts easier with the spark plugs gapped around .022,
that tends to indicate lean idle mixture.

 

Seca carbs too lean

Im going through the same thing.
The battery sensor wont cause the problem you have.

Im guessing your bike wont run with the choke disengaged and
falters if the throttle is increased above idle.
Sounds like a lean carb or 4.
There isnt a mixture screw on that carb.