Surging when running cold

[Pete O]

So I recently rebuilt the Stromberg carb on my '51.  I adjusted the choke unloader  and fast idle cam , and did the preset for the idle speed and mixture adjustments per the manual .  When I first installed it and started her up, it ran smoothly at the fast idle while it was cold.  I let her warm up and set the idle speed and mixture per the manual.  I took her for a test drive, and found that it stumbled on accelerating from a stop.  I opened up the mixture adjusting needles some more, and that seemed to cure the stumble.  But now when I start the engine while cold, the RPMs surge and fall, surge and fall and it does that until the choke begins to open.  What could be causing this surge while cold?

[old-tank]

Try it with the choke blocked open.

[Pete O]

21 hours ago, old-tank said:

Try it with the choke blocked open.

So I tried that, and it ran worse.  It stalled.

[old-tank]

2 hours ago, Pete O said:

So I tried that, and it ran worse.  It stalled.

Ok, now that we know that it is not too much choke, try setting the choke a little less lean, and check for vacuum leaks.

[avgwarhawk]

What is odd is surging up and down. Now, if the Buick had a idle speed motor controlled by a ECM I could understand the surging.  This Buick has no such idle speed motor  The vacuum advance vacuum source is connected at the proper port?  The vacuum advance holds vacuum?  Check for vacuum leaks as Willie suggested. Another concern is the stumbling starting from a stop. Accelerator pump is squirting a healthy streams of fuel? 

Edited May 10, 2020 by avgwarhawk (see edit history)

[Pete O]

Vacuum line is hooked up to the only vacuum port on the carb, and the advance is holding a vacuum.  Accelerator pump gives a nice squirt.    The only thing that I can think of that might cause surging is the power circuit in this carb.  When vacuum in the throat is high, it holds a plunger open against a spring and that keeps the power jet closed giving a leaner mix.  But when vacuum drops as RPMs rise,  the vacuum can't overcome the spring pressure, and the plunger presses against a valve that opens the power jet, providing a richer mix.   Could it be that the fast idle speed is right at the spot where the power circuit begins to kick in, and it causes an over rich mix, which causes the speed to drop which closes the power  circuit, leaning the mix,  causing the speed to increase, etc., etc.?

[carbking]

How old is the fuel in your tank?

 

Six week old fuel can cause this.

 

Jon

[Pete O]

Fuel is new.    The car runs beautifully when it's warmed up.    I tried turning in the idle adjustment screws that I backed out in order to cure the stumble that I had at first.  I turned them in 1/4 turn, and this seems to have improved the surging on a cold start a little bit, and I still don't have the stumble on take off.  I think I'll try another 1/4 turn and see what impact that has.

[Pete O]

Well I seemed to have solved the problem.  What did the trick was a combination of turning the choke a little toward the "lean" direction, turning in (leaning)  the idle mixture adjusting screws, and turning in the idle speed adjustment screw just a bit increasing the idle speed.