Author Topic: 1978 Pontiac trans am L78 Qj stumble off idle  (Read 528 times)

Offline KeithinMI

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1978 Pontiac trans am L78 Qj stumble off idle
« on: September 24, 2019, 07:08:22 AM »
17958266 stock carb engine stock exhaust stock distrib 7.7:1, factory cam
Timing set to factory spec. Vacuum around 20

Offline KeithinMI

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Re: 1978 Pontiac trans am L78 Qj stumble off idle
« Reply #1 on: September 24, 2019, 07:39:11 AM »
17958266 stock carb engine stock exhaust stock distrib 7.7:1, factory cam
Timing set to factory spec. Vacuum around 20
Half my post disappeared:
17958266 stock carb engine stock exhaust stock distrib 7.7:1, factory cam
Timing set to factory spec. Vacuum around 20
Half my post disappeared:
Auto transmission too.
Rebuilt with cliffs kit - new air idle tubes and accel pump. Secondary pull offs working. Choke and fast idle step working. Primary and secondary flaps closed all the way. Spring tension on secondaries is sufficient. I completely cleaned carb so it is spotless. Obv new fuel filter too.
Accel pump is cliffs and new. Needle and seat new - cliffs as well.
Idle screws out about 5-5.5 turns. Idle is still lean- if I put my hand over primary air horn engine speeds up. If I open a vacuum line engine slows down. Can
« Last Edit: September 24, 2019, 07:42:52 AM by KeithinMI »

Offline Kenth

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Re: 1978 Pontiac trans am L78 Qj stumble off idle
« Reply #2 on: September 24, 2019, 01:23:23 PM »
I would set the APT 4.5 turns up from seated and drill for .052"-.055" idle down channels.
Also, 13/32" float setting and acc pump rod in inner hole on arm will help some too.

Offline KeithinMI

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Re: 1978 Pontiac trans am L78 Qj stumble off idle
« Reply #3 on: September 24, 2019, 06:08:39 PM »
not sure why my post only got halfway posted from my phone but in any event let me continue:

stumble problem disappears when engine is fully warmed up although performance is somewhat anemic - probably expected on a 185 hp 1978 smog pontiac 7.7:1 400.
I get a good accel pump shot when i move the throttle - no slack between throttle and accel lever.
accel pump is in outer lever as per stock.
have not played with the apt since it's a 1978 carb and the apt is under the air horn with no access from the top - I have not drilled out the hole for the apt.
i am pretty sure i originally set float to 13/32 when i rebuilt carb but i will obv check that if I have to pull the air horn for the apt adjustment as well.

am i correct in assuming that my idle screws stop working because once i've backed them out enough, the idle tubes are limiting the flow of fuel into the bores?

car is completely stock however I do have the vacuum plugged to the egr so it does not open and I have the vacuum plugged to the EFE on the exhaust manifold and the hot air door on the intake so neither will activate.

timing is stock and set to factory specs which i am pretty sure is 14 btdc. with the vacuum to the distrib I am getting idle timing of 24 or 26 btdc. I have the idle stop screw backed out as far as I can to give me about 630 rpm in D at idle, AC off. 

Offline KeithinMI

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Re: 1978 Pontiac trans am L78 Qj stumble off idle
« Reply #4 on: September 26, 2019, 10:46:48 AM »
that should read a 17058266 for the QJ number. sorry for the fat-finger typing.
note to self: do these posts on my computer not my cell phone.....

Offline Cliff Ruggles

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Re: 1978 Pontiac trans am L78 Qj stumble off idle
« Reply #5 on: September 27, 2019, 07:39:59 AM »
No drilling out any hole for APT, just remove the plug and tap the casting for a 3/8"-16 set screw about 3/8" long.

Make sure the power piston hanger arms are level and EXACTLY even.  Adjust the APT to bring in some more fuel and opening up the idle system just a tad will correct the issues you are seeing.

A slight jet/metering rod change may be required as this new fuel requires about 5-7 percent more of it in most cases to make things happy, even with bone stock set-ups......Cliff