I have to agree with most of these guys. Until I understood the basics
of each carb circuit, I ended up banging my head over the poor throttle
response, bucking, bogging, etc.
It has been about ten years that I have been practicing these new
techniques, since having professional training. I wish I knew this stuff
14 years ago...it would have saved me a few headaches.
As the others have mentioned, a properly tuned carburetor is like night
and day for engine performance. If even one circuit is out of balance,
it can really mess up your opinion of whether you have selected the
I have been guilty of this starting out, and I have seen numerous cases
when setting up carburetors for customers in their street/strip cars:
- Air bleeds not matched for application
- Idle screw (throttle blades) opened up too much, idle mix too fat
- Accelerator pump circuit not tuned properly (bog off the line)
- Incorrect power valve
9/10 I have found that once a bigger camshaft has been installed, people
forget to change the power valve, and/or the air bleed sizes. Dropping
manifold vacuum by 8 or more points (in./hg.) requires some attention
to carb tuning.
The first instinct is to open the primary throttle blades beyond the transfer
slot. This pretty much renders the idle circuit useless as the pressure
across the transfer slot is too high to draw fuel.
In return, the next logical step is to back out the idle mix screws to
compensate. At this point, the engine may idle, but it's likely surging
and excessively rich.
When you hammer the throttle from a dead stop, the engine tends to
bog/sputter because the transition from idle to main circuit is lost.
To complicate matters, the stock power valve is opening too soon, and
the accelerator pump is not timing the fuel shot/duration properly.
If I may offer some tips:
- Leave the idle speed screw alone for now. About 0.040" of transfer slot
should be in view.
- Install a power valve that is about 2.0 in./hg. less than the idle manifold
- With a bigger camshaft, installing smaller idle air bleeds helps to tune the
idle circuit fuel curve to flow sooner.
- The idle mix screws can start at about 1.5 turns out (or best vacuum)
As long as timing is set well, and the carburetor is not grossly oversized
for the RPM/Engine CID, the engine should idle smoothly.
If the engine bogs/sputters from a dead stop when accelerating hard,
my money is on the pump shot timing/duration.
Do yourself a favour and buy a tuning kit! Try different pump cams and
squirter sizes. You will be amazed at how little change in pump cam
can be the difference between a falling flat on your face, or lighting up the
rubber (or hooking hard
It may take you a few tries to get it right, so be patient. You can even
try to disable the secondary throttle blades and tune the primary side first,
then mix in the secondary side.
Pump cam > when the fuel is pumped
Squiter size > duration of fuel shot. Smaller hole = longer spray
Thanks to everyone else in this thread offering tuning help. I really have
learned from you!