Tune Changed Overnight

[Thumperbird]

445 Stroker dual quad.  So a couple days ago I spent an hour dialing eveything in from float level to timing, idle speed, idle mix, you name it, was running very well and pulling hard, could not keep traction.  Today I go for a ride and it seems rich, idles a couple hundred higher, bogs, and runs on when shut off.  What happened?  Temperature is maybe 10 degrees different (cooler) at most but engine seems to be running a little warmer than before as well, used to be failry solid 180 now 190.

[427LX]

What was the weather like when you tuned it and when the tune seemed to change?

I have seen that too when the weather gets a bit warmer and more humid.

[Drew Pojedinec]

Today I go for a ride and it seems rich, idles a couple hundred higher, bogs, and runs on when shut off.  What happened?

Do you have a choke?  Is the fast idle screw hanging up and keeping the primary open a tad?

[427LX]

What springs are you running in the Vac. Secondaries? Could be maybe the secondary throttle plates cracked open a little due to using the light spring. Check and see if there is any movement between secondary shaft and end link when throttle is closed.

[superduty]

Distributor roll pin?  Timing chain or gear?

[Chrisss31]

What are you using for a fuel pressure regulator?  I used to run a race car that had a single fuel line to one of those cheap diaphragm regulators.  During the day I'd have to crank it all the way in to get adequate pressure, if I left it alone when it cooled off at night it had enough pressure to overcome the inlet needle.  I ended up putting a fuel log with a return line.  This was a sprint car with a motorcycle engine so I'm not sure if it would apply to you, just thinking out loud.

[Thumperbird]

Temp was maybe 5 degrees cooler, humidity up a bit
No chokes or linkage
Mechanical distrib. just switched to blue bushing, ble/blue springs (MSD billet)
All new internals.
Timing is 16 initial
Fuel level good in bowls, not running regulator yet, gauges show 5.5psi
Quick Fuel slayers, vac. advance is screw adjust, out about 3.5 turns
I exercised the secondaries on several runs, maybe one of them hung up a little or something.
Primaries are closed off, secondaires used to set idle.
Thanks.

[Drew Pojedinec]

If the secondary plates are sticky at all, or hang up in the bore, running hard and opening them will leave the idle high since they may not shut all the way.
I had a QF do just that.  650rpm idle in gear, go for a spirited ride, and couldn't get idle under 1,000.

For the moment I bent the secondary link so it pushed the secondary plates closed.  Ultimately I had to reposition the secondary plates so they could close smoother.

[machyoung]

Black smoke out the tailpipe? Sure sounds like fuel? Leaking past the fuel inlet valves? I'm sure you double checked the timing?

[machoneman]

If rich, check that the o-rings on each needle/seat assembly haven't shrunken, allowing gas to pass, the gas going into the venturi wells.

Happens a lot now with 10%+ ethanol gas. Easy way to to tell: pull each assembly straight up. There should be drag due to the synthetic rubber o-ring rubbing on the interior wall of the float. Another way is to eye-ball how far the o.d. of the ring sticks out from the brass body compared to a new o-ring mounted on another assembly. 

Btw, the bad part here is that the o-ring doesn't fail quickly. It does so over time and can be hard to detect until a lot of gas starts creeping past the ring. 

[Thumperbird]

As usual a lot of great responses, thank you all.

Timing was checked, same.

Carbs are new, I looked them over before installing and must admit did not love the way the butterflies sat in the bore closed.  Thinking I am going to pull carbs and more meticulously go through the shafts, butterflies, etc..
I almost feel like a very slight rounding of the plates might be best to get a consistent seat in the bore.
Don't think it is needle and seat, fuel levels seem fine and like I said, new slayers.
Might deburr all linkage and sliding surfaces, seems factory finish is not the best on all of the little widgets and it makes for less than perfectly smooth mechanisms.

I'm finally getting what Drew and others always insist upon with duals, you can not be too careful and meticulous when setting them up, everything needs to be gone over and massaged so to speak to get dead on adjustments and consistency in operation.

Forcing me to be a carb guy I guess, I need to own them and make them do waht I want.

Thanks.

[Drew Pojedinec]

I almost feel like a very slight rounding of the plates might be best to get a consistent seat in the bore.

I'm finally getting what Drew and others always insist upon with duals, you can not be too careful and meticulous when setting them up, everything needs to be gone over and massaged so to speak to get dead on adjustments and consistency in operation.

The old ones had plates that were shaved perfectly to fit in the bores   The secondary plates on a C3AE-C carb is a fine and common example.  Please do not shave your own throttle plates   Way easy to get off base.

To the second point.... once you really and truly get these reigned in, they will be good forever.  Aside from pulling the carb off my truck in order to test other folk's stuff, I honestly haven't been into it or made an adjustment in years.

[Thumperbird]

Hoping that is the case before too long here, don't mind tinkering but don't like redo's.
As for cleaning up the mechanics, I am sure I could screw it up and maybe will but my day job has me dealing with fine fixtures, microns of material removal, polishing, all kinds of very high accuracy playing on equipment, hoping that keeps me out of too much trouble.  Leds is more, I get it and thanks for the warning and feedback, much appreciated sir.

[67GT500]

I would check for a leaking power valve.. I had this happen the other week.. they were fairly new valves, but all of a sudden i was having all sorts of issues.. pulled the carbs, broke them down and found the secondary carb had a wet power valve chamber.. I tested the valve and it held on a vacuum test, but i replaced it and we were all set again.running great..

[Barry_R]

Throttle plates "used to be" machined with a 7 degree taper on the O. D., and closed throttle airflow was set on an orifice box after assembly.  No idea what is done nowadays, but I would bet it ain't "better"....