High RPM Misfire

The Zephyr has developed a misfire at about 8K RPM, at full throttle.

If you back off momentarily when it starts to sputter, then "go again", it
will stammer a little more then jump to the 10K redline.

I have changed the plugs (as they were due anyway), but am thinking that it
may be a fuel issue. Perhaps a clogged main jet?

Before I start pulling 4 carbs apart, does anyone have any suggestions on
where to look first?

 

yep sounds like a jetting issue

about that RPM that you go from the pilot to main jet IIRC

check air filter and would also check mixture screws are set at
what they are supposed to be as mix screws plays a part right there

 

As it wasn't doing it until a few days ago (co-incidentally it was the day
that I filled the bike with fuel from a different servo), I suspect that the
issue is dirt rather than incorrect jetting sizes or mixture screw
adjustment.

I'll run a few tanks of fuel with some carb/injector cleaner through first,
and pray for a micracle cure.

 

One day Rod Bacon got dressed and committed to text

Hmmm, there I was thinking the mixture screw only adjusted the ratio of fuel
to air when it was idling. Ahh these modern things are a worry.

 

You are correct. In this case, the pilot screws are almost completely
unreachable when the carbs are on the bike anyway.

The mixtures are largely ok I feel, as the bike was running well until a few
days back. Inspection of the plugs also revealed a healthy, uniform tan
colour.

 

I'd be spewing 'cause my rev limiter kicked in two grand too late!

 

On the way to work this morning I did some more testing. The problem is
definitely related to throttle position/movement, not engine RPM. If I
deliberately grab a gear or two higher than necessary, and crack the
throttle open, the "hesitation" happens at lower RPM. In other words,
it seems to happen at about 90-95% throttle, no matter where in the rev
range I am. I am also noticing some popping on decelration. It's always
been there, but seems to be worse with certain fuels. My limited
knowledge is starting to lead me towards suspecting an overly-lean mix.
If it is indeed across the whole rev range, am I correct that I'll need
to potentially need to (i) adjust mixture screws (for idle mix), (ii)
move needle clips (for mid) and (iii) change main jet (for top end)?
The plugs don't look heat stressed, which I assume would be the normal
indicator of a lean mix?

 

To add to my uncertainty (I had almost decided that it was a
fuel/mixture problem), a learned colleague of mine ccame up with the
following:

If the plugs don't look too hot, then remember that when you grab a
handful of throttle you're putting up the pressure in the cylinder to
quite a high figure, and the spark needs more voltage to fire through
the higher pressure. If the stagger occurs nearly every time you grab
a handful at whatever motor revs., then it's unlikely to be mixture
related. Unless, of course the float levels have all dropped, and the
bike is lean all the time - but that should show on the plugs.

Sigh... back to caps, leads, coils and stator.

 

Working on the KISS principle, first change the plugs.

Theo

 

what like reading the thread first?

 

Hehe. Yup.

Theo

 

No thanks Theo. You're not my type.

 

What, simple and stupid?

 

Errr?

Theo

 

you have to read your original reply about KISS
Was having a go at mr bacon

 

Yes, yes, and yes ... a clogged main will show up earlier in my
experience ... I'd also be considering the valve clearances
and valve springs, because that stuff becomes critical near
redline.

-------sharks

 

Ignore previous post re: valves and fix carb diaphragm(s)

-----sharks

 

Try checking if you have an air leak on the inlet. spray airostart around
the inlets. if revs go up you have an air leak.

 

I never read my own mail.

I knew that. My 'Errr' was in any kind of comparison with pig-man,
kind or otherwise.

Theo

 

Funny you should mention that...