by the CLub Member Know as "I Like GMCs"
Indexing:
A performance trick that is a simple task is to index the plug, mark the spark-plug
insulator body with a felt-tip pen on the side where the ground electrode attaches
to the spark-plug body. Position the plug so that the gap is facing the center of the
cylinder toward the exhaust valve. This might seen trivial, but a spark plug facing
away from the valve reduces the plugs ability to burn the gas effiently. Moroso sells
indexing washers of different thicknesses so you don't need to go through boxes of
plugs to find 6 good ones. Don't double up on washers, though.
Gaps:
If you are running the very desirable GM HEI or Petronix system, you need to increase
the spark plug gap. The reason is the HEI and the other one produces more voltage and
amps. I would recommend starting out at .045 for the HEI and .040 for the other one.
You will notice a much smoother idle after ditching the points. Take it out and run it
through the RPM range. Lug the engine some and then wind it out to max RPM's. Does every
thing feel and sound ok? Increase the gap by .005" until the idle starts to suffer and
drop back .005". After that is all done, install another new set of plugs two heat ranges
colder and try expanding the gap once again. It is unlikely the plugs will carbon foul
which indicates either a rich mixture (too large of jets) or the plug range is too cold
to burn off deposits. The idea is to run the widest gap possible without misfires and a
smooth idle. The wider the gap, the smoother the idle quality and better high RPM power
will be. To top that off, milegae will increase, too. Those air/fuel adjustment screws
on the front of the car is for the idle ciruit only. Those screws do not lean out or
richen the mixture at all. What they do is increase or decrease the amount of air and
fuel equally to the idle circuit. To adjust: turn screws in or out to get the best and
highest idle, then back in to get a lean drop of 20 RPM's. A 50 RPM drop is ideal, but
your carb may not like it. This does nothing to hurt fuel ecomony or performance, but
does clean up idle emissions.
Voltage needs:
Higher engine loads increases the voltage needed to spark off the plugs. If the plugs
are worn or gapped to wide, misfire may occur under load. Also, misfire may not happen
hot rodding your truck empty compared to carrying or pulling a heavy load. A cold plug
(temperature, not heat range)requires more voltage to fire than a hot one. Always make
judgement calls on a fully warmed up engine.
Intermittent misfires with proper plug gaps can be caused by a variety of ignition, fuel or
mechanical problems. Lean misfire occurs when there’s too much air and not enough fuel,
so the engine should be checked for air or vacuum leaks, carburetion problems. If the
misfire appears to "jump around" from cylinder to cylinder, a manifold vacuum leak may
be the cause. Spray carb cleaner or even water in a spritzer bottle at both throttle plate
bushings and anywhere theres an intake gasket, including the where the carb mounts to the
manifold and all fittings. But if the misfire is isolated to a single cylinder, a worn,
fouled or damaged spark plug or a bad plug wire which is the most likely cause. Worn plugs
require more volt and amps to fire correctly. Worn plugs don't have to be replaced. You
can file the edges sharp again and as long as the plug is clean it will be function like
new, again.
There are also spark plugs today that are specially designed for truck engines. These plugs
have increased fouling resistance and oversized electrodes for longer service life. If
your engine has a lot of miles, this is something to think about when buying plugs.
I had a 1950 Chevy with a 235 in it and I installed an HEI disributor. I was very pleased
with the results. It started right up in sub zero winters and it was so nice not having
to mess with the dwell and points all the time. The plug gap I setteled on was .060". I
ran it up to .080! High RPM's and a WOT throttle caused a moderate misfire at .080". GM
used to call for a gap of .080", but later recalled that figure and recommened .060".
Since, I have touched on the HEI and you might gather I am not fond of the Petronix. Well,
you are right. Not that it is a bad unit. The HEI is just so much better. The HEI is very
a dependable unit. However, if I was offered a Petroix at a cheap price I would probably
use it. I would though, keep everthing to convert it back in case it failed me and left me
on the side of the road. More amps, more volts and parts are everywhere and reliability
are the HEI strong points. I must touch on the modules in the HEI. Not all of them are
equal. A GM made module is way better than the "el cheapo" units you get at Autozone or
other discount house. A GM module has built into it, the ability to change the dwell at
lower engine speeds. This will increase coil life by allowing it to run much cooler by not
getting so saturated with electric current when it isn't needed. The GM part numbers with
a "990" seem the best. It's a truck application. Always use the white dialectric grease,
not the clear. The white acts to fill the gap between the base plate of the distributor
and the module. Otherwise it will fail you sooner rather than later. The module requires
the base plate to rid itself of excess heat.Don't waste you money on some super duper module,
you won't gain a thing in a street truck. The stock coil is just fine, too. Always use the
vacuum advance connected to manifold vacuum and never run with it disconnected. Mileage will
suffer and the engine can run hotter than needed. The only thing you can do, now to hop up
the HEI is recurve the centrifical advance and vacuum advance. That can be another topic for
later if y'all are interested. One last word, the HEI REQUIRES a lot of juice to run, so
that wimpy wire to the stock coil will not power it above an idle. It requires an 8 or 10
gauage wire. You can use the stock coil wire to operate a Bosch relay. Look at Jolly's
6066 GMC Guy site for adding relays to the headlamps.
The pin-outs will be the same.
Here are few good sites on spark plugs:
Car Craft Technical Articles
Guide to Reading Spark Plug Performance
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Performance Upgrades For
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