<VV> Synchronizing carbs

[Kenneth Schifftner]

SM:

Smitty is right...to rotate the carburetor throttle shaft force must be applied.  The left pushes down and the right pulls up.  The slack at the connections and cross shaft supports should be taken up off idle.  The only time the linkage slack may not be taken up is at idle when the idle speed screws land on the throttle pivots.  The throttle cross shaft does nothing at idle...just sits there.

At off idle, if the cross shaft "bearings" are worn however, pressing the accelerator may push the cross shaft up rather than cleanly rotating the shaft given the slop. By checking the draft off idle when the throttle linkages is "loaded" one can find out if the right carburetor is still slightly favored.  Usually it is because if it starts being balanced slightly to the right, it stays that way until higher rpms are achieved because at least in theory both carburetor throttle plates etc rotate the same.  At higher rpms the balance tube seems to smooth things out.

I agree that neither GM nor Rochester said the right should be favored at anything other than idle.  Indeed the balance tube quickly comes into play to balance the draft.  In my post, I simply relayed the experience I've had that slightly favoring the right seems to help the drivability and checking it can negate the sloppiness of the cross shaft.

Ken Schifftner



> On Jul 30, 2015, at 6:22 PM, S M <shaun_mcgarvey at shaw.ca> wrote:
> 
> ... this paragraph directly contradicts what Smitty's point was, and is exactly how the carbs need to be synched with the linkage under tension, whether it be at 1000rpm or 1500 or 2000.... usually it's only a turn or two anyway...
> 
> "Certainly the slop in the linkage is not good. If the effect of sloppy linkage results in the left carb having a higher draft than the right carb at speeds just above idle (I usually use about 1000 rpm) then that slop should be removed regardless of how many extra turns it may take to reconnect the left carb linkage at idle balance."
> 
> From: "Kenneth Schifftner via VirtualVairs" <virtualvairs at corvair.org>
> To: VirtualVairs at corvair.org
> Sent: Thursday, July 30, 2015 4:07:45 PM
> Subject: <VV> Synchronizing carbs
> 
> Gang:
> 
> What Smitty says is correct from my experience using a Dwyer zero center gauge. Indeed the balance is set at idle when the speed screws are in contact with the levers on the carburetors.  Once the linkage is moved to increase the engine speed the idle speed screws do nothing but sit there.
> 
> Why the engine is revved further is not to maintain balance it is to make certain that the right carburetor is favored (higher draft than the left) that helps actuate the vacuum advance, etc..  GM only allowed for one complete turn of the link on the left carb so it is easy to be one turn off. I made an adjuster for the left carb and wrote an article about it in the NJACE newsletter.  With that homemade adjuster, the right carb can be favored more accurately.  It seems like at higher revs the balance tube has greater effect.  Really, drivability is what matters. 
> 
> Certainly the slop in the linkage is not good. If the effect of sloppy linkage results in the left carb having a higher draft than the right carb at speeds just above idle (I usually use about 1000 rpm) then that slop should be removed regardless of how many extra turns it may take to reconnect the left carb linkage at idle balance.
> 
> If the demonstration that was mentioned was the one at Flagstaff, that car did not have the standard linkage.  The car was a last minute substitute.  It had the bolted in swivel type linkage and the adjusting rod was not left/right threaded to act like a turnbuckle.  The swivels had to be removed for each adjustment. The difficulty was with the linkage not with trying to get balanced draft at both idle and higher engine speeds.
> 
> I've balanced dozens of carburetors this way with good effect.
> 
> Ken Schifftner
> 
> 
> 
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