Waking up the ZZ

[pdawg]

Hey experts! Looks like spring is here so I need some advice on a problem I put my '72 Custom Deluxe to bed with last fall. My son & I completed an engine swap for the aforementioned GM ZZ383, but I haven't been able to get her running to its potential yet. After over carbing it I put a Holley 670 Street Avenger on it. I'm not a mechanic but I have played around a bit over the years. I completed it with a Mallory HEI distributor. It has no vacuum advance - per GM so I've timed it as per spec at 10 degrees, with the distributor bringing in the rest. When the engine is hot ( which is barely into the normal range) - it starts to run rough & if I put it in park it revs up considerably & will diesel if I shut it off. One would think vacuum leak but I chased the usual suspects ( carb base, intake manifold) with an unlit propane torch looking for an increase - to no avail. I am using an Edelbrock Airgap manifold. I've replaced all the vacuum lines including the brake booster line, but still no joy. Any tips would be greatly appreciated. Also - once I do get this solved I've been thinking of changing to a posi rear end & 700R4. Thinking 343 gears should wake it up a bit but still not rev too much on the hi way. Any suggestions re: brand of posi, parts required, or tips regarding this would be appreciated. Thanks in advance!!
Perry

[Hollow65]

What octane fuel are you using? You say you over carbed it originally? How many hours on this engine? What's your fuel pressure at, should be around 7psi max. Sometimes carbon buildup will cause this.

[pdawg]

I have been running the best octane we can get here in Canada, which is a 94 octane. Not sure what my fuel pressure is but I purchased a higher volume Holley mechanical pump that met spec. The plugs are nice & clean & I have about 2000 miles on it now, so I'm hoping it's not carboned up. I ran an edelbrock 800 for a very short time but it was too much & wouldn't run right.
Thanks for getting back to me.

[Hollow65]

Have you checked the timing at rpm? Maybe an issue with the idle speed circuit in the carb. Did it diesel with the 800?

[pdawg]

I bought a timing light with an electronic tac & it seems to be advancing properly when you rev it & it comes back down, but it may be worth a look so I'll take the cap off & make sure all is moving smoothly. Don't know if I'll get to it today as I'm putting a cold air kit on our Avalanche today, but I'll post when I do. Thanks!

[pdawg]

The tach on my timing light shows it's advancing & returning but it's worth a look under the cap. Thanks!

[Hollow65]

Take care, I know in Cali the engines like more inital timing like 12* is a good starting point here. I would focus on the idle circuit. I bet that cures it. Maybe a carb spacer also? Good luck.

[geezer#99]

You're on the right track Hollow!
Timing is his problem.
12* isn't even close. More like 20 to 22 initial is needed. Adjust the dizzy mechanical to 12 and add 10 degrees from the vac pot.
With that much timing he'll be able to turn his idle down so it idles better. Right now it's idling on the power circuit.
I had a similar built sbc when I lived at his elevation (3500 feet) 10 years ago. Lived about 100 miles north of the cow town (Calgary).

[Hollow65]

I wasn't sure about the Canada thing. I know different places like different settings. It's just a 9-9.5:1 383 right?

[pdawg]

So Geezer you're suggesting retarding the mechanical timing & connecting the vacuum advance back up? I know the Mallory distributor came with an advance limiter so I could install that & try. She idles nicely enough until you take it out for a run. Then back in the driveway or wherever you go the revs go up when you put it in park. It's pretty embarrassing when you shut it down & it sputters & coughs 😳!

[pdawg]

Yes Hollow it's 9.1-1 compression ratio.

[geezer#99]

It's all about total timing.
If you run at the 10 degrees initial that the gm guys suggest, then 22 degrees in the dizzy is needed to arrive at the recommended 32 degrees all in by 3000 rpm.
Those specs are specified for warranty purposes. You won't hurt your motor and gm has no warranty issues to contend with.
You can run it like that, but it'll be rich running at idle, have a high idle in park and deisel when you shut it off.
The reality is you need way more initial. Likely 20 degrees. Then you adjust your mechanical timing to 12 degrees and add 10 more from your vac pot.
Your total's the same but it idles nice on the idle circuit, hauls butt when you stomp on it and doesn't idle high in park or deisel.
It's your choice.
Worry about the warranty and have a foul smelling, sputtering, diesiling embaressment at the local cruise- in or tune it right and enjoy all the benefits of a well tuned power plant.

[pdawg]

Thanks for the advice. I'll give it a try & post the results. May be a while before I get to it but I'll let you know.The vacuum advance may even give me a bit better mileage heh? Couldn't be worse but that's not why I put it in anyway ��!

[crazy longhorn]

Originally Posted by geezer#99

It's all about total timing.
If you run at the 10 degrees initial that the gm guys suggest, then 22 degrees in the dizzy is needed to arrive at the recommended 32 degrees all in by 3000 rpm.
Those specs are specified for warranty purposes. You won't hurt your motor and gm has no warranty issues to contend with.
You can run it like that, but it'll be rich running at idle, have a high idle in park and deisel when you shut it off.
The reality is you need way more initial. Likely 20 degrees. Then you adjust your mechanical timing to 12 degrees and add 10 more from your vac pot.
Your total's the same but it idles nice on the idle circuit, hauls butt when you stomp on it and doesn't idle high in park or deisel.
It's your choice.
Worry about the warranty and have a foul smelling, sputtering, diesiling embaressment at the local cruise- in or tune it right and enjoy all the benefits of a well tuned power plant.

Well said ,old man The vac can does not hurt a thing....not needed on a race car, but on the street it gives the mileage & throttle response in the lower rpm ranges. You have a slower "burn" in the cruise range, & when you open the throttle, the vac can drops out of the mix....you are @ 32 degrees WOT. It wont hurt the motor Longhorn

[Hollow65]

With vac advance I've done the timing with the vac advance disconnected and set the total 32* all in around 2800rpm. Then I hook it up and mess with the idle timing and shoot for the highest vacuum reading at idle.

It's a lot quieter this way.

[pdawg]

Thanks again for all the input guys! Looking forward to more driving & less tinkering !

[crazy longhorn]

Originally Posted by Hollow65

With vac advance I've the timing with the vac advance disconnected and set the total 32* all in around 2800rpm. Then I hook it up and mess with the idle timing and shoot for the highest vacuum reading at idle.

I have to also agree with you Hollow65....the TOTAL timing is more important than where the initial sets....bet the other "geezer' will agree also Generally 20 -22 @ idle speed (intitail+ vacumn advance), will give you the spot to close the throttle a tad & get the transfer slots about right. Then she don't run pig rich @ idle speed.... Longhorn

[cableguy0]

Get your total timing to 36-38 and it will run a bunch better. The higher initial timing will help too. Don't touch the carb until timing is correct. Most carburetor issues are actually timing issues. Once timing is correct then adjust the carb. The 670 should work great on that engine.

[dwcsr]

Have you check the actual vacuum at idle and revving. Vacuum gauges tell you a lot about an engine and why its not running right. I'd also look at the valve adjustment.

[pdawg]

I had a vacuum gauge hooked up to it when I was setting the mixture on the carb & it seemed normal at idle & when you rev'd it. I will hook up the vacuum advance & reset the timing, then I'll hook up the vacuum gauge again to dial the mixture & see what happens. Thanks for the input!

[geezer#99]

Originally Posted by pdawg

I will hook up the vacuum advance & reset the timing

You realize that's backwards!
Set your timing with the vac advance plugged off, then hook the vac back up.

[pdawg]

Hi Geezer. Yes - I realize that you have to set the timing with the vacuum advance disconnected & plugged ( thanks for double checking ). I meant that I will have to re-check the timing once I hook up the vacuum advance to ensure I'm not getting more than the 32 degree max, as I also have to install the vacuum limiter that came with the distributor. Hopefully this weekend if spring sticks around! My son snapped this yesterday - I think Blue wants to awaken from her slumber!

[crazy longhorn]

pdawg, set your total @ 32,(2800 -3000 rpms) with the can unhooked & plugged (as you know). Then hook up the vac can & read what you have....if you added 12 degrees on the can, you will read @ 44 degrees @ 3000 , with no load in the engine. This is ok , as the vac drops when you open the throttle & you are only running on initial+ mechanical @ WOT The vac advance is for cruse& economy on a 'slow burn" (part throttle). The vac can is NOT considered as part of the total timing....just a load compensator longhorn

[pdawg]

Thanks for the clarification Longhorn. So you don't think I'll need to install the limiter in the distributor? I had heard the total safe range for advance was 36 * for a sbc. Is that correct? My old mechanic (RIP) used to tell me to load up the engine & listen for valve chatter to make sure you weren't advanced too much, as he would recommend more advance than factory spec.

[Hollow65]

36-38 is fine.