GNJ

Rebuilt SBC motor with above average compression running too lean. (Headers, mild port work, rollercam- about .500lift, 266 degree duration, and aluminum intake manifold.) ~ 400hp
Compression test showed all cylinders around 175 - 180#'s each. Timing is set correctly and advance seems to be working.
All the plugs look white and the electrode is grayish ash looking, so it looks to me, it is too lean. (Carburator was off a stock 210hp motor.)
I would like to know what to do to the Q-jet to add more fuel to the motor.
Also, does anyone know what octane I should run with this compression?

Badtoon

Trash Can - Then Call Nickerson

Iggy

Pick up a copy of HP Books "Rochester Carburetors", ISBN 0-89586-301-4. Retail price is 16.95.
In it you'll find all kinds of info on rebuilding and performance tuning.

Whiteknuckle

The primary side of a q-jet can only be richened up a slight amount, but usually the primary is not the problem. You have to change the secondary metering rods. The smaller the rod size in diameter the more fuel flows through the secondaries. In Dennis Moore's book on Big Block Marine engines he has a chart on the secondary rods available. You may also look in an Edelbrock catalog for a q-jet metering rod assortment pack. They are easy to change, and can actually can be done while the engine is running. You may also change accelerator pump size also, and you should also raise the float level a little. They are not a bad carb!

OLDRAT

GNJ,

Whiteknuckle hit it on the head. Secondary fuel flow will get
that larger engine what it needs when you are running
with the back barrels open. The primary side is adjustable
by jetting and rods, but the good primary rods are getting
harder to find (jets can still be purchased). It must be noted
that the ratio of the jet size to the rod size is important in
the primaries to keep the engine crisp. Don't get too worried
about plug color as long as it is not real lean, blistering or
transferring metallics to the electrode, the gas today has a
lot of additives and solvents that make plug readings a challenge.

Secondary rods come in many configurations. Remove the
screw and lift the hangar to see the rods. The rods come
in different sizes and are designated by letters stamped on
them which relate to diameters in thousandths. Marine
rods usually have a fairly progressive and smooth taper
(smaller, the richer the mixture). Just find some that are
slightly smaller and try them, it is a trial and error basis just
like tuning any carburetor.

Other areas to check for proper function are the float
level since QJ'S are very sensitive to the setting, and the
power piston spring. Larger cams with lower vacuum
may need a softer spring under the piston to keep the
rods in longer to lean the idle and off-idle transition.
In a pinch, the spring can be trimmed by one or two
coils to assist with a low vacuum idle. The setting
of the dashpot air break and spring that controls the opening
of the secondary air valves can be taylored to take out any
bog that may occur when transitioning from the primarys
to the secondaries.
Even though it will show up as a rich condition, be sure
to check the metering well plugs under the base-plate
to ensure they are not leaking. Epoxy can cure these
if they leak.

I have had the book Iggy spoke of for many years along
with other literature. These carbs got a bad rap due to how
they work, parts not being readily available, and just not
understanding them. They are very adaptable and work
well in many applications on a wide variety of engines.

I, like a lot of folks had to learn by trial and error when
racing NHRA Super Stock in the 70'S since many classes
back then mandated this carburetor. I also love Holleys,
Demons and Predators, you just need to work with them
to suit your needs.

My .02. Sorry for the long disertation.

OR

GNJ

Thanks - I appreciate the response. What is blistering though? All the plugs lappeared like a little grayish color on one side of the insulator of the plug, while the other side was completely white. The finger part of the plug that 90 degree angles to the electrode had the look like metal after a torch would heat it red hot and then it cooled down. Kinda of ash in color. Therefore, it didn't look right to me. Also, there was a huge hesitation when getting into the secondaries and once the secondaries came on, it felt like it was starving for fueland spuddered a little as well.

Also, how can you tell if the metering well plugs are leaking?
(Carb does idle well.)

Anyways, I will try another set of metering rods and look into the other areas of the carb as well.

One last thing, what about octane rating? Is there a way to tell about what compression is from compression readings?
Should I add octane booster?

Whiteknuckle

You may have several issues. Make sure the engine is not running to much timing advance. I know nothing about your engine, but it's always best on a rebuild or new motor to be conservative on your timing at first and adjust it from there as required. Also get more fuel into that engine. Keep riching up the secondard rods until no more performance improvement is noticed, let that be a starting point. Also, make sure you are not running to hot a plug. A hot plug allows less heat to be dissapated out of the cylinder. I suggest getting these issues solves then worry about octane.
This is why I am an advocate on running the engine on a dyno before it goes into the boat. To a least get it in the ball park on these kind of issues. Just my .02cents

OLDRAT

The large "bog" is probably a excessive lean condition in the
transition when you go into secondary mode. If it didn't
backfire through the carb then richening the back rods like
Whiteknuckle stated should start showing an improvement.
You can also go up a couple of sizes in the primaries if you
have idle problems or surging when at slow speeds.
Also richen up your idle mixture screws to assist with the
bog. Note: some QJ's required up to four turns out of
the screws to get the mixture correct (usually later model
ones).

Small hesitations or "bog" can be corrected by tighening the
air break spring which is adjacent to the rod running from
the vacuum dashpot to the secondary air break arm. There
is a small screw that faces outward on the boss where all
this stuff meets up and it is retained usually by a hex head
locking screw (underneath the boss) to hold it firm.
Loosen the lock screw while holding the slotted screw and
tighten it a little to increase the tension on the air break
flaps. The more resistance these flaps have the smoother
and slower the secondaries will react. Even though the
secondary blades open with the throttle, air cannot enter
the back of the carburetor until those top flaps open and
start the fuel siphon to the jet wells. This is why this
carb works on all types and sizes of engines, it is a variable
venturi set-up.

The metering well plugs (two soldered/lead plugged wells)
that you can see when you turn the carb over can leak
over time due to degredation of the lead solder or from
high heat (usually more prevalent in automotive applications).
You can take the base plate off and put some epoxy on
the tips of these wells (let it cure before putting back on
engine) and it will stop the puddling or rich conditions which
usually show up at cold starts or hot re-starts.
These wells are in the casting to allow machining of some
internal passages during the manufacturing process and are
sealed by Rochester when the machining is complete.

As far as the octane, it won't be a problem with 91 or 93
on your engine. Get a book on tuning or carburetors and
most will have a pictoral section showing spark plugs with
different problems. A light grey or straw brown color is
a good place to start. What I meant by transferring metallics
is when a engine is really lean or getting ready to harm
themselves, metal can disapate from the aluminum pistons
or off valves. This would be a worse case seniero. Ever see
a top fueler go way lean? Sparks and metallics will start
coming out of the zoomies like sparklers right before it
blows up.

Good luck.