Hi everyone-
I would definitely suggest Roe's Rochester Carbs book for anyone seeking
more information-
Regarding jetting for modern fuels and special circumstances, from doing a
fair amount of this work on a variety of vehicles the following seems to be
the trends-
Some places like California use 'oxygenated' fuels containing MTBE or
Ethanol etc. in thier gas. These additives reduce the thermal content of the
gas somewhat, as they displace actual gasoline, and the same VOLUME of fuel
contains slightly less (~2-5%) less 'power'. This means ~102% of the
original amount of gas must be used to get identical performance to the old
fuels more or less. Many places use such fuels, at least part of the year.
The net effect is that under light loads and cruise, the engines run a
trifle lean. This means in normal driving slightly more throttle needs to be
applied to maintain speed or accelerate and this can occasionally require
activation of the power enrichment system (the metering rods lifting up from
the jets) and resultant poor economy.
Generally speaking, drivability is not affected much, but fuel economy can
suffer slightly, to the tune of perhaps 10-15%. Much depends upon a persons
driving style also of course.
I find jetting richer about 3-4 sizes than the factory did seems to
compensate adequately for the typical Quadrajet equipped engine at sea
level. At higher altitudes, no compensation may be needed, say 2500 feet two
larger, and 3200+ feet no changes. GM originally calibrated the engines
extremely lean, and most will run better with a little more gas availible.
Also, as engines age annd wear a bit, vacuum levels diminish, and they need
a bit more gas just to maintain the original ratio of mixtures both at idle
and cruise.
I hasten to add that a larger jet does not reduce fuel economy neccessarily,
as having correct mixtures allows the engine to run with maximum vacuum,
minimal use of the ultra rich power enrichment system and smaller throttle
openings needed to climb slight grades and keep up with traffic, so you may
find your economy is materially improved by a significant amount if your new
jetting suits the engine better than the old jets did.
Areas that bear special attention in a Quadrajet in the GMC especially are
the idle circuits and small modifications to idle air bleeds that permit the
carb to close when you life your foot without drawing large amounts of very
rich fuel thru the idle circuits under high vaccum, say engine braking on
long grade descents.
I would not modify the factory metering rods or hangers however, as they are
a quite good match and very few people will be able to materially improve on
the factory profile.
On vehicles equipped with altitude compensating bellows type Q-jets(some
~1975, not sure if its used in any GMC) backing it out one full turn helps
adjust for modern fuels also.
So to sum, if you live or usually operate your GMC at lower altitudes, I
feel there is no reason not to try a jet slightly richer during a carb
overhaul. I would start with one three to four sizes larger (higher number)
than originally supplied. If you want even greater refinement and
drivability, get Roe's book and see what else can be done. Much can, and
results are gratifying !
Good luck!
Brent Covey
Vancouver BC
I would definitely suggest Roe's Rochester Carbs book for anyone seeking
more information-
Regarding jetting for modern fuels and special circumstances, from doing a
fair amount of this work on a variety of vehicles the following seems to be
the trends-
Some places like California use 'oxygenated' fuels containing MTBE or
Ethanol etc. in thier gas. These additives reduce the thermal content of the
gas somewhat, as they displace actual gasoline, and the same VOLUME of fuel
contains slightly less (~2-5%) less 'power'. This means ~102% of the
original amount of gas must be used to get identical performance to the old
fuels more or less. Many places use such fuels, at least part of the year.
The net effect is that under light loads and cruise, the engines run a
trifle lean. This means in normal driving slightly more throttle needs to be
applied to maintain speed or accelerate and this can occasionally require
activation of the power enrichment system (the metering rods lifting up from
the jets) and resultant poor economy.
Generally speaking, drivability is not affected much, but fuel economy can
suffer slightly, to the tune of perhaps 10-15%. Much depends upon a persons
driving style also of course.
I find jetting richer about 3-4 sizes than the factory did seems to
compensate adequately for the typical Quadrajet equipped engine at sea
level. At higher altitudes, no compensation may be needed, say 2500 feet two
larger, and 3200+ feet no changes. GM originally calibrated the engines
extremely lean, and most will run better with a little more gas availible.
Also, as engines age annd wear a bit, vacuum levels diminish, and they need
a bit more gas just to maintain the original ratio of mixtures both at idle
and cruise.
I hasten to add that a larger jet does not reduce fuel economy neccessarily,
as having correct mixtures allows the engine to run with maximum vacuum,
minimal use of the ultra rich power enrichment system and smaller throttle
openings needed to climb slight grades and keep up with traffic, so you may
find your economy is materially improved by a significant amount if your new
jetting suits the engine better than the old jets did.
Areas that bear special attention in a Quadrajet in the GMC especially are
the idle circuits and small modifications to idle air bleeds that permit the
carb to close when you life your foot without drawing large amounts of very
rich fuel thru the idle circuits under high vaccum, say engine braking on
long grade descents.
I would not modify the factory metering rods or hangers however, as they are
a quite good match and very few people will be able to materially improve on
the factory profile.
On vehicles equipped with altitude compensating bellows type Q-jets(some
~1975, not sure if its used in any GMC) backing it out one full turn helps
adjust for modern fuels also.
So to sum, if you live or usually operate your GMC at lower altitudes, I
feel there is no reason not to try a jet slightly richer during a carb
overhaul. I would start with one three to four sizes larger (higher number)
than originally supplied. If you want even greater refinement and
drivability, get Roe's book and see what else can be done. Much can, and
results are gratifying !
Good luck!
Brent Covey
Vancouver BC
