Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Warm idle fluctuation with slight backfire [VIDEO]

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turboguard:
So I just rebuilt my quadrajet using cliff's HP kit, and while it definitely improved my throttle response/performance (no more bogging from a takeoff) and gas mileage (this is the first time I've seen 200 miles between fill ups!!!), I've developed an idle fluctuation when warmed up:

https://www.youtube.com/watch?v=nQJdGQ06ES4&feature=youtu.be

There is a low thumping sound from the exhaust when the idle drops which I can only assume is a slight backfire?

Also, if I try a warm start after letting the car sit for a while, it will sometimes stumble and die if I don't give it a little gas first.  Hasn't died in traffic yet, but it almost acts like it wants to.  Picks right up with no stumble or bogging when I floor it from a stop.

I also installed a brand new TPS and MCS as well.  The only other thing I changed were my spark plugs since they had fouled up pretty bad due to the carb running fairly rich before and also some oil leakage (replaced with NGK V-Power plugs)

What would be the most likely cause of this idle fluctuation? My carb didn't have it before I rebuilt it.

The other thing that is odd about my quadrajet is my airhorn doesn't have a rich stop screw (as in there isn't even a hole there where it should go). My carb number is 17085204, which according to my resources SHOULD have the rich stop screw, so I'm thinking the carb is kind of a mish-mash from the PO.

I still tried to set things the way they should be, but I don't have a dwell meter to exactly dial in my air bleed valve.  It seems to idle the best at under 2 turns out from seated, which seems low?  I have the idle screws turned out 5 turns each, but past 4-5 there doesn't seem to be any change in rpm/vacuum (vacuum is about 21 hg).  If I fully seat both screws the engine stumbles and dies.  Choke is fully open, and if I cover the primaries the engine does start to die as it should.

Another thing I just noticed was my Idle Stop Solenoid seems to be engaged all the time when the engine is on (I actually have it turned down so it doesn't interfere with normal driving)...don't know if that is a separate issue or related.

So basically, running like a champ except for this idle (which is embarrassing when stopped in traffic :-[).  Any advice as to what I should start checking first?  Maybe something to do with the fact it was running extremely rich before is why this problem just started?

Car info:
1985 Trans Am LG4
Stock except for AIR and Cat delete
Edelbrock 14"/3" open air element
MSD cap and rotor, Accell super coil and, plug wires

turboguard:
Decided to make another video tonight...there is definitely a loud hissing coming directly from the carb, don't know if this is normal or not.  Theres also a loud whistling when I open the throttle plates which I assume is caused by maybe an imperfect fit, don't know if it has any major effects or not.

Please let me know if anything in particular jumps out as a major issue to anyone, thanks!!

https://youtu.be/D-TqLcbpNHw

Cliff Ruggles:
Did you adjust the new TPS via the factory Service Manual?

Did you adjust the upper idle airbleed screw in the airhorn per the Service Manual?

Is the float set to factory specifications?

Might want to set the idle speed screw right against the linkage so see if it stabilizes idle speed instead of letting the computer control it.......Cliff

turboguard:
I set the float as precisely as I could to 11/32 per the instructions

I haven't messed with the TPS adjustment (mainly due to not having the right tool to turn the little hex head), but when I tried to check the voltage by backprobing I was getting weird readings from my multimeter (the voltage would fluctuate slowly up and then down and sometimes show negative??), on both my old TPS and my new one...not sure if I was doing it wrong or my multimeter won't work right.  I actually found a multimeter that apparently reads Dwell that I'm going to pick up today:

http://www.oreillyauto.com/site/c/detail/BOST/FIX7677/N1001.oap?ck=Search_multimeters+%26+analyzers_-1_1430&keyword=multimeters+%26+analyzers

Would this be sufficient to set pretty much everything to what it needs to be? 

Jeffs68:
Sounds pretty normal to me for an 85 T/A. The only thing I would do is get rid of those "novelty" NGK plugs and put a set of  A/C Delcos in.

Double check your timing as well.

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