Exhaust popping during fast idle when cold.

[Tyler73Deville]

73 Cadillac 472 with stock cam and near stock calibrated quadrajet #7043234. Only changes made were made to the jets (2 sizes larger to .070, and long green power piston spring) runs non-ethanol 91 octane. Pulls around 19" vacuum at idle and easily starts even in sub 20 degree weather for it's winter start ups.
Deleted egr and has dual exhaust. Runs great, idles well and has awesome throttle response otherwise (absolutely smokes the rr tire lol)
Has msd 6al running 16 initial, 37 total by 3000 ish

What gets me is running vacuum advance to manifold gets rid of said popping during fast idle on cold starts... likes lots of initial...it's 30 degrees with it connected at idle...maybe she's a bit lean on idle/ transfer slots perhaps?  No nozzle drip and goes away a bit when completely warmed up.
Also did the tip in method at higher rpm with the choke and seems cruise metering is spot on.
 
Thank you.

[Tyler73Deville]

Went from the much thicker insulating base gasket on the left, back to the correct one on the right, fixed the issue.. always the small things... thought I'd share...

[Shiny]

What effect do you think the gasket had?  Glad it went away but I'm curious why...

[Cliff Ruggles]

The thickness of the gasket had nothing to do with the improvement.

You are allowing one side of the intake to see the other side.  I've found better results going to the first gasket you had in place as far and engine performance was concerned.

As far as the skip or miss in the idle you should be using vacuum advance, but may have to reduce how much it is adding.  Most vacuum advance cans add WAY too much timing when we start advancing initial timing.

Is it a factory distributor are an MSD unit?

How much timing does the VA add and are you using it?

[Shiny]

Still curious...so the effect of the gasket change was to somehow decrease vacuum signal to the advance unit?

[Tyler73Deville]

Should have specified.. it was just one of those cheap black gaskets that just sticks to everything when you take it off, didn't notice it and reused it when I removed the carb to bring home to change the the secondary wind-up spring and cam  (car is in a storage unit and didn't wanna drop anything) had to do a lot of scraping...
as far as vacuum advance goes it adds a total of 14° advance at the crank according to the timing light,  it's the original can that comes with the msd billet unit.

[Tyler73Deville]

@shiny The gasket had nothing to do with the vacuum advance, I did state though, that connecting it to manifold (originally it's ported) completely made it go away when fast idling cold (popping starts when the choke opens a bit)

 I suspected a possible vacuum leak so that's what made me go change the gasket. Stopped it. I did however keep it connected to manifold as the car seems to like it better lol.

BUT it's about 30° idle timing at the crank with it connected, so I thought the car was possibly just a bit lean (suspected vacuum leak, and it was cold outside, never seemed to do it in the summer ) to seemingly want all that much timing to make the popping go away.  Hope that kinda cleared it up a bit..

[Tyler73Deville]

Should have specified.. it was just one of those cheap black gaskets that just sticks to everything when you take it off (was from a cheap rockauto rebuild kit the very first time I did the carb, don't cheap out on the gaskets) didn't notice it and reused it when I removed the carb to bring home to change the the secondary wind-up spring and cam  (car is in a storage unit and didn't wanna drop anything) had to do a lot of scraping...
as far as vacuum advance goes it adds a total of 14° advance at the crank according to the timing light,  it's the original can that comes with the msd billet unit.

[Cliff Ruggles]

Just curious but did you try adjusting the unloaded angle of the choke flap on a cold start?  IF it likes a butt-load of timing cold at initial start up the flap angle may not be quite ideal which would cause the popping and require a LOT of timing to make it happy.

On those early divorced choke carburetors they are not easy to adjust as you have bend things and must work very quickly after it fires  up.

It may actually take two or three attempts from dead cold to find the ideal angle for the choke flap, but once you find it most likely you'll never have to adjust it again.

As far as the black gaskets being sold that go between the main casting and baseplate they are dubbed by the folds selling them as "skinned" and "non-stick".  They do just the OPPOSITE and stick like super-glue to everything.  I like to have the hours I've spent scraping them off on carbs that have been sent here because everyone is selling those cheap gaskets and folks tend to cheap out when buying rebuild kits.

My kits come with far superior main body to baseplate gaskets and the main body to airhorn gasket for those early carburetors is .015" thicker and made of a better material as well......https://cliffshighperformance.com/product/quadrajet-rebuild-kit-sr

[Tyler73Deville]

Probably didn't do it right..lol
but using parts carb for picture examples, so I experimented with and straightened the choke rod and found that the best setting is when it touches the top of the oblong hole rather than the bottom, when the divorced choke coil is fully open.
Gave me a much better angle to work with when first started.

Doesn't idle like an old diesel anymore after a 50 year slumber for a couple minutes..but instead revved to life (had to turn down the fast idle screw as rpms were much higher than before).