dwwebste

1984 Z28, HO 5.0, Edelbrock carb and non-computer distributor. Engine makes a rattle/knock sound when under load. Not just when revving the engine, but when accelerating up a hill. I have a video but it won't let me attach it (MP4). Thoughts?

sofakingdom

iTrader: (1)

Yerbasic spark knock. Pretty commonplace.

Back the ignition timing off until it quits. If it takes more than a few degrees, post back.

dwwebste

Since this is only under load, could it also have something to do with the vacuum or mechanical advance? I know mine has vacuum, but I assume (please do correct me) that there would be mechanical springs/weights as well?
Unfortunately there is no timing indicator that I can see on the car, so I've never actually set or adjusted the timing.

Thank you for your quick reply! I've been learning a lot about these cars over the past three months of ownership.

David

sofakingdom

iTrader: (1)

Yes it should have both forms of advance.

Don't worry about "indicator" or "set" or any of that. Just back it off until the noise quits.

mr6x

You could try 93 octane fuel first, see if that helps. If it doesn't pull the timing back as sofakingdom said.

Vader

SFKDM knows what is happening with your configuration, and has given you one solution. Your learning curve is about to get a bit more of a slope, since you are about to see some of the potential reasons, and not just one part of the fix.

Without a timing indicator, initial timing can be set "by ear" but the lack of a reference will make any tweaking/fine adjustments a circus.

A non-computer controlled ignition system has no way of reducing the preignition, so other steps will need to be taken to prevent engine damage. A factory-style ECM and EST would use the knock sensor and ECM to retard timing before you could even hear the knock, and resolve the problem.

An aftermarket carburetor, made to be a generic fit to a multitude of engines, had no chance of managing fuel as well as the original system. That has several implications, including uncontrolled chamber temperature from incorrect mixture, and a general lack of ability to compensate for temperature and barometric changes. That incorrect mixture can also create carbon deposits in the chambers, effectively turning your gasoline engine into a diesel.

The last good carb capable of that kind of control was a Holley 1685, and it had integral mixture control and EGR addition. It was also larger than a Prius engine. Not to bash Edelbrock, but there was NEVER an Edelbrock carb on a production vehicle that had to meet emissions and mileage standards, and had to meet warranty requirements.

There are more things which can be done to reduce the preignition, but more details about your configuration are needed.

• Is the original EGR system installed and intact?
• What type distributor is installed?
• What fuel is being used?
• What is the normal cooling system operating temperature?
• What cylinder heads are installed, and/or how have they been modified?
• What camshaft is installed (profile)?

That information can help clarify the conditions, and aim us toward solutions.

sofakingdom

iTrader: (1)

Just back the timing off until it quits. Don't overthink it. You don't have a stock motor; even if you had a completely functional "indicator" system, you have NOT THE FIRST CLUE what the optimum setting would be. Listen to the engine. Let the engine tell you what's best. Just back the timing off until the noise quits and then go from there.

mikeceli

iTrader: (2)

dwwebste

Let me back up and give aa very brief history. This was a non-running 84 Camaro that I bought for my son (who is much more mechanically inclined than me) to get running again. We have worked with fuel injected vehicles (Civic, Silvia, Silverado) but never a carb vehicle. We were told it needed a new carb, some gaskets, etc. So we took off the Q-jet and intake, and replaced it with an Edelbrock Performer intake and carb. I have been told more times than I can count that this was a mistake, but alas, I'm not buying another carb. That, and an ACDelco non-computer, vacuum advance distributor. I replaced the plugs and wires at the same time. So please forgive my ignorance as I am just learning all these little details, and getting to spend what little time I get to with my kids.

• Is the original EGR system installed and intact? Honestly, I have no idea.
• What type distributor is installed? See above
• What fuel is being used? Regular 87 octane from the local Exxon
• What is the normal cooling system operating temperature? Coolant temp sensor is not accurate, but after driving around, an infra-red thermometer reads 195-205 depending on what I'm aiming at
• What cylinder heads are installed, and/or how have they been modified? Stock heads as far as I know.
• What camshaft is installed (profile)? Stock cam as far as I know

Which way do I turn the distributor, as we want to advance the timing, correct?
Thank you all!

OrangeBird

iTrader: (1)

You want to retard (delay) , not advance (make earlier) the timing, the pinging is telling you your already too advanced (early) for your engine's present conditions.

To retard (delay) the timing, you will slightly turn the distributor in the direction the rotor rotates. If you were needing to advance the timing (you don't) you'd turn the distributor body the opposite direction of the rotor's rotation.

Use tiny little bits of distributor movement, like moving it 1/4 of an inch at a time, lock it down, and then drive it. Punch it going up a few hills during these drives, and if you hear pinging turn it yet another 1/4". Rinse & repeat till you can fully punch it going uphill without hearing any pinging, and then for good measure back it off another quarter of an inch.

dwwebste

And in the 305 SBC, the rotor turns clockwise, correct? So I will turn it 1/4" clockwise, and then go have fun.

I have to admit, I really enjoy driving this car. It has an interior and gauges that still need a ton of work, but the speedo, oil pressure, and fuel gauges are correctish, and that is all I need to drive around some.

Vader

If the intake was replaced with the carb, it's an even bet the EGR was not installed. You'll have to try to compensate with less timing advance (as advised above), overcooling the engine, running higher octane fuels, and altering the mechanical advance of the distributor to keep it all under the point of preignition. If the original CCC QuadraJet is not installed, and the distributor was exchanged with a non-EST version, the ECM is also probably not doing much. You should be able to get away with a much cooler thermostat, probably down to 160° without causing additional issues. That can cool the heads enough to reduce the tendency for preignition. This is also a place where some alcohol in the fuel can help due to its tendency to absorb heat in the intake and chambers.

It would probably help to install an air intake system which draws air from outside the engine compartment, like the factory air cleaner and snorkel did, but with possibly larger ducts than the factory used. The "chrome hat" air cleaners are dismal for providing cool, external air, but for some reason they keep moving off the store shelves.

sofakingdom

iTrader: (1)

​​​​​​​Yes.

dwwebste

Great news! I was able to turn the distributor clockwise, about an inch back, and the noise under load stopped. The car feels better, too.

Thank you for your advice and help
Still have so much to do, but this has been a great step forward

"The "chrome hat" air cleaners are dismal for providing cool, external air, but for some reason they keep moving off the store shelves."
Dyno runs (Engine Power TV show) have shown that the more open air cleaners gave more power than the stock cleaners did. Plus they just look cool...

David

sofakingdom

iTrader: (1)

Than SOME stock ones did. Not ALL.

And of course, NOWHERE NEAR as much as a GOOD non-stock one.

The typical open-element jobs are designed to be SOLD, not for "most power". Typical buyers are the people who get into those sq-in-of-krome contests in parking lots, and people who ... simply don't know any better, or are just lazy; and among that latter group, those whose butt dyno is recalibrated by the extra SOUND coming from them.

One of the biggest problems they have, besides the hot underhood air, is that the engine tilts back, meaning things that are at the front of the engine, get tilted up at the front; but hoods slope THE OPPOSITE way, such that as the hood slopes downward toward the front, the air cleaner tilts UPWARD. The result is, a "drop base" is required. Butt then, the TOP of the air cleaner ALSO gets "dropped" (that being the point of the whole drop-base exercise in the first place), with the result that the air cleaner lid literally sits on top of the carb air horn. The incoming air has to sqwwwweeeeeeeeezzzze through a mail-slot and then make an immediate 90°. Pathetic. These cars, particularly the Firebirds with their "sleeker" hoods, are among the worst in this respect.

A GOOD air cleaner (cold-air, unrestricted, giving as straight, vertical, and open a shot into the top of the carb as possible) will beat an open-element, EVERY TIME.

dwwebste

I have a feeling most folks buy those air cleaners based on looks and not on Dyno numbers. Have dynos proven that CAI on these 3rd gens make a difference? If so, any certain brands/styles recommended?

Tom 400 CFI

I agree with this. And...
...I'd point out that the test you're referring to was likely done on an engine dyno? PowerNation? On an engine dyno, typically the dyno room is "room temp", (probably between ~60° and 80° depending on the season) and thus, that is the inlet temp that the open air filter is "seeing". The filter is pulling from an room of coolish or cool air.

^That is very different than what that same open intake will "see" under a closed hood, especially sitting stationary at, say, the starting line at the drag track. Autocross. NOW, that air intake isn't pulling 60-80° air, it's pulling 250-300° air off the engine, rad and headers/exhaust. Thus, the benefits that they saw on the engine dyno are not real or attainable in a real car, driving on the road.

IDK who's tested CAI's vs. HAI's on 3rd gen's specifically, but we know that:
1. Cold air is denser = more power potential, and
2. HOT air, will limit timing advance potential and thus power. Car may run great when cold or in cold weather, but climbing big hills/summer time, you get spark knock, retard, power potential slips away and you're not making what you think or should.
2.1. Plus the air path that Kingdom was talking about.

Food for thought.

dwwebste

Yes, you are 100% correct that this was in a shop on an engine dyno in a very controlled environment. I wish I could find the dual cool air intake that my 5.0 HO engine came with, but when I bought the car there was not an air cleaner at all on it. That set-up looked nice and unique. What other CAI options are out there?

Right now I am dealing with a missing belt that was replaced in April, so too young to break. But the belt is already showing some wear on the edge. Losing this belt caused the car to overheat. So I'm planning on replacing the water pump, pulley, and thermostat (180). My coolant temp sensor is hit or miss so I'm replacing that as well. I was surprised my fan didn't come on, so I'm hopeful that with the new coolant temp sensor that will come back on...