Hi, Nolan
I can see from your response that you have a lot of experience with
carb synchronization. I do think you misread Randall's response to you
though. He was trying to say that the SU tool does NOT block the air flow,
it rides in the top of the piston and measures, not the depression, but the
actual RESPONSE to the depression. This is actually (in my opinion) more
accurate than a manometer since a manometer only measures the vacuum, not
the piston/needle response to it. It is true that these should be
equivalent, but of course they do not have to be especially with older
equipment, and especially since SU's use close tolerances to produce the
lift, not a seal like in Strombergs.
Regards,
John
Re: Date: Wed, 6 Dec 2006 13:32:54 -0500
From: "Nolan" <foxtrapper@ispwest.com>
Subject: Re: [TR] Carburetor Synchronization tool
Anything you stick in the front of a carburetor screws up airflow into that
carburetor. That's how it gets a reading in fact. If you were to place an
identical snail in front of each carburetor at the same time, that would
work.
It would screw up the air flow uniformly. But doing it one at a time
creates imbalances across the carburetors, producing false readings and
therefore isn't as accurate. There's also the matter of the inherent
accuracy and stability of the tool itself.
As for how critical is synchronization and where do I come down on it, that
varies a bit. With these engines, the multiple cylinders per carburetor,
and their very large balance tubes, you can be pretty sloppy and have things
running quite well. With dedicated carburetors and runners to each
cylinder, like on a motorcycle or multiple DCOE's, it doesn't take much
imbalance to have an engine that surges and splutters under light load.
Personally, I'll balance as accurately as I can with the equipment I'm
using. It only takes about half a second to go from "pretty close" to "dead
on" with a synch stick.
Remember too that synchronization is irrelevant from about 1/4 throttle up.
The values are too gross. It's at those close throttle positions that it
matters. The closer the throttle positions are to closed, the more it
matters and the more sensitive things are to changes. Floored at 5000 rpm,
there's no such thing as synchronization of the carburetors. But coast down
a hill at 40 mph with just a little throttle to maintain speed, and lack of
synchronization gives you a lurching vehicle.
And Randall, I said throttle blade, not piston. There is a difference you
know. Most carburetors have a specific throttle blade clearance listed for
initial installation. This value is also usually included in the
documentation that comes with a rebuild kit. While it sounds super
accurate, in reality it doesn't work that way for many reasons.
And a manometer is not a synch stick. You need multiple manometers to make
a synch stick. The name synch stick is old and well known to those in the
field. As the mercury units were replaced with multiple gauge banks like I
linked to, the name carried over to them as well. My bank of gauges is
frequently called a synch stick by the mechanics in the motorcycle shops.
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