Stock 78 Z28 Running Poorly. Help!

[my6nova]

Hello,

I'm new here to the forums. I am working on my uncle's 78 Z and am really struggling with getting a really bad stumble out of it. Anything more than an 1/4 throttle when driving it, the car starts missing really bad and doesn't want to pick up RPM. Here is the info on the car:

Engine- 100% original, no modifications. 128K miles
Exhaust- factory manifolds, 2.5" pypes system with crossflow muffler.
Fuel system- new fuel tank, new sending unit & filter, new stock style fuel pump. All new rubber fuel lines. Hardline from pump to carb replaced. New fuel filter. I went ahead and replaced all of the vacuum lines as well.
current carb setup- APT- 3 turns out, idle air mixture screws, 3 turns out. peak vacuum observed, 18". The APT was 4.5 turns out when I took the carb apart originally, I've tried this setting, 4 turns, 3.5, 3, 2.5 and 2 turns and it doesn't seem to make much of a difference.
timing- adjusted timing by watching vacuum gauge. was able to get another 2" of vacuum when adjusting the timing for a final reading of 20"
ignition- new wires, cap, rotor, ignition module(did use grease when installing) and plugs(R45TS gapped at .045")

I purchased a rebuild kit for the Q-Jet from Cliff Ruggles last year, went through the carb, reinstalled it with a new manifold to carb gasket and fired it up. The car idles well, revs fine, but runs absolutely terrible when I get it out on the road. I have not put a timing light on it yet, but the main reason for that is I can't find a timing tab to check it. Shouldn't it have a timing tab on the drivers side of the of the engine at about the 2-3 o'clock position of the balancer?

Could I be chasing my tail with the carb and it be something as simple as an ignition coil starting to go out?

Don't mess with a quadragunk, get a holley.

 

[Adolfo rivera]

As previously stated. Check the easy stuff first. My nephew recently had a similar problem. Assure all plug wires are fully seated unto spark plugs and rotor cap.
I would think you have HEI so there shouldn't be any ignition coils. Make sure there is no corrosion going on inside the cap. Humidity tends to make them do that just from seating. Look at the engine running in the dark and check for jumping sparks. Try monitoring the fuel pressure to make sure its stable. On mechanical pumps should be 5-6 PSI. Good luck

 

[tom3]

Seems like you have hit all the bases on the normal stuff. I'd pull the valve covers and check really close the valve springs and watch the valve/rocker arm operation with the engine idling. I'd guess you'll find the problem under there. Is there still a cat. converter on the exhaust system? Those can get clogged and really screw things up.

 

[tom3]

Also that APT adjustment controls the fuel air mixture at high vacuum cruising at steady speed mostly, once on the throttle the metering rods are off this setting.

 

[78Z28350]

That's the normal timing tab position for a 305.
(I thought all 78 Z28s got a 350.)

What's the 5th digit of the VIN?

I will check at lunch today. My uncle bought the car in late 77 as a dealer demo with 7,000 miles on it. He has never had the engine out of the car since he's owned it. The pad on the block under the alternator should also have a stamp that indicates what the engine is, correct?

 

[78Z28350]

Was the pole piece in the distributor changed? If it has a 305, with that kind of mileage you can expect rounded cam lobes.

Are you referring to this? If so, no, it has not been changed.

 

[78Z28350]

Well i would go after the low hanging fruit first and check the timing. Is this thing backfiring or just missing?

It seems like it's missing. It behaves similar a rev limiter when I try to go past 1/2 throttle. I assume 10-12 degrees would be good for the initial base timing and target 32-34 degrees for total timing? I run 36 degrees in my 383 and 406 engines but they are both pretty hot combinations. I wouldn't think a stock 350 would need that much total timing.

 

[ob355]

what oil are you running in this engine? did this problem start after you tuned it up?

 

[mallard]

That's the part. The wires on those can fray or break and cause a misfire when the distributor advances. If the broken wire separates it can kill the engine on acceleration and then restart once the engine stops running.

 

[Doc Russ]

Did you replace the fuel filter(s)?