|Q: What is a plug chop and how is one
A: A plug chop is taking the bike out
on the road, holding a certain speed and RPM for one to three minutes,
and without changing throttle position reaching over to the ignition or
kill switch and shutting off the engine, then pulling in the clutch as
quickly as possible and coasting to a stop. Get out the trusty plug wrench
and carefully pull at least one outside and one inside plug and look at
the color. Sometimes you might have to pull all four if you suspect there
are issues with the individual carbs.
Q: Why is a plug chop procedure necessary?
A: The reason it is done this way is while
an engine burns the fuel, the amount of fuel and how well it is burning
will color the the insulation (white stuff) at the tip of the electrode
(little jobbie inside the middle of the plug). Killing the engine at the
certain speed/RPM insures that the engine does not idle down and change
the color. If the engine idles down it uses different circuits in the carb
that are tuned separately. Even if only brieflly this can taint the results
of the plug chop and give you a false reading. Killing the engine before
letting the throttle go and pulling in the clutch keeps the engine from
burning any more fuel. Yes, there will be a moment where the engine pulls
thorugh and expels unburnt fuel due to no spark but this is minimal and
will not change the color of the plugs. The trick is pulling in the clutch
as pulling in the clutch as soon as possible after the engine is killed
to the RPMs drop quickly and the engine comes to full stop quickly.
Q: How do I do a plug chop for the upper
ranges. I can't go 100mph legally, right?
A: The best way is to find a stretch of
70mph interstate between exits that is not too busy that allows you several
miles of uninterrupted riding. Keep the bike at a steady speed, and keep
it in the upper RPM ranges, around 5000 to 6000 rpm. You might have to
run it in 4th gear to do this. It helps if the bike is under a little load,
so if you can pick a stretch of highway that is uphill, or if all else
fails, let the brakes drag a little, but of course not so much you smoke
them. Just enough to put a load on the engine for a minute or better two.
Start at one on ramp and right before you get to the next off ramp do your
chop as described above, coast up the ramp and pull over towards the top
and check your plug then. Keep an eye on any traffic coming up the ramp,
there are idiots out there.
Q: What color am I looking for on the tip
of the insulation?
A: In most cases you are shooting for
a light tan. Nevermind what the end of the threads looks like, this will
almost always be darker. If you do a lot of winter riding as well, or live
in a colder climate you probably want to see dark tan or light brown showing
a richer mixture for colder air. Some additives leave a greyish color instead,
so avoid additives if you can when doing chops, but if it has to be, light
grey to dark grey is good. White or no coloring at all is too lean, very
dark brown to black or sooty black is too rich.
Q: What do I need or preparations do I
make do a plug chop?
A: Other things affect combustion, so
you need to make sure these are right first.
Number one is timing. Have your points
gapped properly and timing set according to the shop manual. Bad timing
can affect plug chops. Good condensers. Clean points contacts. Spark on
all four plugs. You know the drill.
Make sure you alternator is putting out
the proper voltage, about 12.5 to 13 volts around idle, and about 14 or
a little better at higher RPMs. Low voltage causes weak or failed spark
and taints a plug chop's results.
Make sure youre valve lash is set properly.
A valve that is not completely closing affects compression and combustion
and will also taint plug chop results.
Vacuum sync the carbs. If one slider is
much higher or lower than the others the fuel to air ratio will be different
form one cylinder to the next and one plug may read differently than the
You will want a new set of plugs (or better,
two), pre gapped, and installed just before you do your chops. It is silly
to cheap out on a set of plugs that normally costs less than a six pack
of beer, so new plugs should not be an issue. It's spitting into the wind
doing chops on old worn out plugs. I normally use one set of new plugs
for lows and mids, and another new set for top end.
Avoid over use of the choke and it's best
to get the engine warmed up some before you put your new chop plugs in.
Um, don't forget to take the plug wrench
with you, and a pair of work gloves (hot engine, hot plugs).