'78 Pontiac L78 carb - stumble at idle and mid-range

[KeithinMI]

Having a few small issues with my Pontiac 400 L78 QJET. (Note the L78 is the lower-compression (7.7:1) version as compared to the 1978 W72 at a whopping 220HP). I rebuilt it recently, including a thorough cleaning of everything on the carb and made sure to blow out all the passages, air tubes, etc. with filtered compressed air and kept everything spotless.

This is (was) a factory-original carb off a 34,000 mile 1978 L78 Trans Am. Has 72 primary jets, "DB" secondary rods and an "L" secondary hangar. 45K primary metering rods. Automatic transmission with stock torque converter.

I have replumbed the entire vacuum system for the car, and verified that there are no vacuum leaks anywhere. Car is plumbed with original emissions intact, including EGR, EFE, PCV, heat door, and the spark advance. Static timing advance is set to factory L78 specs. Car has new plugs, new plug wires, new cap & rotor, MSD HEI ignition module and coil (same as I'm using in my '78 W72 trans am). Voltage to +12 of distributor is correct as as it should be. Distributor vacuum advance is working properly.

Rebuild included new accelerator pump, new needle and seat, new fuel filter, new gaskets all around. Accelerator pump still looked good even when I removed it. Seals were not degraded.

1. Hard to start after it sits. I did not see any evidence of the well plugs leaking. prior to putting the air horn back on as a final step, I filled the float bowl, and let the carb sit on a piece of cardboard, and saw no fuel on the cardboard. yet it takes a bunch of cranks to get it started cold after it's been sitting a few days.

2. Has a slight off-idle and midrange stumble. I verified the butterflies are closed all the way so I am reasonably sure the car is idling off the idle circuit and the transfer slots. Note that I did not do anything to the secondary flap spring tension. Car generally idles pretty smooth once warmed up and will even idle as low as 500 RPM without stalling.

3. I set idle screws initial at about 3.25 turns which is what they were from factory (before I removed the limited caps and did the rebuild). Prior to me taking a look at it again this weekend, I was getting a lot of drop-throttle popping through the exhaust when taking my foot off the throttle at higher rpms. I know that's caused by the ultra-lean condition of the engine having high rpm's and the butterflies being closed. I set the mixture about 3/4 of a turn more rich and that doesn't happen anymore.

**However** I don't seem to get a lot of response from the idle mixture screws. My only comparison is a Holley 4555 on a '70 Vette and the Holley seems to be pretty sensitive (receptive) to a change in the idle mixture screws. On this car, the vacuum at start was about 20" Hg and even with that 3/4 turn on each side, it only went up to close to 21" Hg. Idle RPM went from 670 to around 700 so not a tremendous difference. Opening them farther didn't seem to have any further effect either +/-. (On that Holley, going further than optimal immediately causes a drop in vacuum and idle).

Do I need to tighten the secondary flap spring? Does that require removal of the air horn on the '78 Pontiac QJ?

any suggestions would be greatly appreciated.

[KeithinMI]

This is the stock L78 cam which I think is the pontiac 527471, which **should** have the following specs:
 Intake: 264 .364 gross lift, dur@.050: 189 adv 264
 Exhst : 264 .364 gross lift  dur@.050: 189 adv 264

I am not sure of these specs as Butler Performance lists the int/exh a@ 274 with .374 lift. don't know specs on lobe sep.

[KeithinMI]

I neglected to mention. Carb is a QJET 17058266, if that helps. 49 state, Pontiac 400, Automatic transmission.

[steve s.]

Just so you know. The 17058266 carb is for the W-72 automatic engine.

[KeithinMI]

My mistake. The 8266 is the one I want to buy to replace it. It’s a factory stock L78 qj so 17058???

[Cliff Ruggles]

Did you use one of our rebuild kits?

If so it would have had a new secondary cam/spring in it.

Did you remove or at least clean the idle tubes during the rebuild?

The only way you are doing to detect a small leak at the bottom plugs is to pressure test them.  This procedure is described in my book......Cliff

[KeithinMI]

Cliff,
Bought your book long ago. Carry the .pdf of it all the time, since I bought the digital copy.

Unfortunately I did not get the kit from you. It was a kit I purchased prior to my first carb I sent to you a bit over a year ago (which you did a great job on, btw!)

Did not get a secondary cam or spring with the kit that I had, but it did have the accel pump with the blue gasket and garter spring.

I did clean everything in the carb including the brass tubes, and the air transfer passages. Made sure I blew out everything with compressed air after rinsing everything with brake cleaner after the soak in carb cleaner bucket.

[Cliff Ruggles]

Did you remove the idle tubes from the main casting and manually clean them? Blowing them out will NOT remove any of the "varnish" inside them from many drying up cycles.

"Blue" seals mean nothing, every single one we tested no matter where we sourced them out failed in this new fuel, sooner or later.

Your kit will not have had the correct high flow .135" N/S assembly in it either, which effects the fuel level at any given pressure.  This also effects fuel delivery at all levels as well......Cliff

[KeithinMI]

couldn't edit the post above for some reason:

Did not remove idle tubes. Looks like she will be coming apart again. Do I remove the tubes from the top or do I need to remove the throttle plate to get them out? Will they come out with the crimped on collar?

I'll need to order a proper accelerator pump, secondary cam, and spring from you, as well as a .135 needle/seat

I'll get the proper carb number tonight.

[Cliff Ruggles]

The idle tube removal procedure is described in our book.  Basically you will need to drive the tubes down slightly and pull the collars out of the castings.....Cliff

[KeithinMI]

Ok, so followed your procedure and got the tubes out. I must have tapped one in slightly too far because it was a pita to extract and it was bent.

i did use a very fine wire followed by brake cleaner to make sure those passages were clear (and verified the other air tubes and passages were clean too.

So, I placed an order online today Cliff. Ordered the .135 needle and seat, the HP accel pump, the secondary cam and spring, and air tubes. The carb is a 17058278 which is the carb on the L78 Pontiac 400 'standard' trans am engine with a whopping 180 hp.

You'll give me the correct parts to get the most performance I can out of that QJ, right?

The only mods I have done to the car are install a low-restriction Magnaflow cat converter and a Dynaflow crossflow muffler. Still using the single intermediate pipe from the cat to the muffler which has dual outlets. Also put in a MSD digital heat hei module (to get a rev limiter on that engine) as well as an MSD HEI coil. Rest of engine is stock.

Car is automatic with TH350. Not modified. Stock torque converter as well

thanks!

[Cliff Ruggles]

I don't pre-drill the idle tubes unless the order is placed by phone and we're asked to do so.  Everything else will be correct for what you are doing.....Cliff