Yamaha FZ600 runs like crap when warmed up

 I have an 86 Yamaha FZ600 that runs great until I ride for a 1/2 hour
then is  dies at stops and acts like it is missing...I have checked
for an intake leak and all seems fine...any ideas of what to it could
be would be helpful..thanks

 
My street creds are low in this group (as they should be, 'cause I'm a
mechanical idiot), but here is my advice ...

The fact that the bike runs well until warm probably rules out the major
mechanical components as causes.  I doubt that a compression test, for
example, would reveal anything.

It seems you're down to mixture and spark.

When it is running rough, try using the choke (if you can on that bike).  If
you can enrich the mixture and it runs well, that gives information that
spark may not be involved.

Put some B-12 Chemtool and/or Seafoam through it and see if there is any
improvement.  (This is cheap).

Change the plugs, and see what happens.

Perform a spark test while the engine is hot.

If you feel that spark is implicated, and given that it may be a thermal
problem, it might be simpler to swap out every ignition component.  Swap out
the coil, plug wires, and whatever drives the coil (points?).

Double-check and clean all grounds, especially those that are associated
with the coil.

Datesfat

 
It might be tight valve clearances that keep one or more valves from
seating when the engine gets hot, so check the clearances and adjust
if necessary.

When dealing with shimmed valves, I won't remove a camshaft to change
a shim if the clearance is only 1/1000th of an inch too tight. That's
too much work for not much change in clearance.

Another possibility is lean mixture or low fuel level in the float
bowls.

You can check out the running fuel level by installing a piece of
clear aquarium tubing on the spigot that's on the bottom of each float
bowl and then opening the drain screw.

Holding the open end of the tubing up next to the top of the float
bowl.

The fuel level will be about 1.0 ~ 1.5 mm lower than the gasket
surface.

If the fuel level is correct, open the idle mixture screws about half
a turn and ride the bike to see what happens when the engine is hot
and you roll off the throttle.

If the exhaust sound goes piffle-piffle-SNAP!, that indicates lean
idle mixture.

If the exhaust sound goes fart-BANG! the mixture is way too rich.

 

First, you don't have to pull cams on many bikes.

Secondly, if you do, it's hardly a lot of work. You don't have to bother
with re-timing the engine etc if you know what you're doing.

Thirdly, some clearances are actually quite tight - if you have a two
thou clearance (not uncommon - one of the bikes in my sig has it) then
at one thou too tight, you're running half the recommended clearance.

Fourthly, if a valve clearance *is* too tight, it will only get tighter
and so risk buring out the valve sooner.

In short, if the valves aren't at the recommended clearances, adjust
them until they are. It's called "maintenance".



--
BMW K1100LT  Ducati 750SS  Honda CB400F & XBR500  Triumph Street Triple
Suzuki TS250ER
If you don't know what you're doing, don't do it. And RTFM.
chateau dot murray at idnet dot com

 

At first I thought stuck float, but I'm going with
bad coil... temperature sensative.  When it's
missing, start pulling plug wires... find the
one that doesn't change the miss, that's the
missing one.  Got an inductive timing light?
Those work great for finding the miss, if
it's secondary ignition.

Good luck.. I don't like 4carb, 4coil bikes!
FI for me!
__
Steve
.