Thread: Problem with new ZZ4 350 Motor

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Hello
Looking for some help.
Just got my new ZZ4 350 motor with 750 Holley carb running in my 1940 Ford Coupe and after it warms up and if you rev it up fast in park by hitting the gas and letting off the engine will backfire out the carb and the exhaust with the RPM is up around the 3000to 3500 range. I checked the timing and it is at 10 degrees bftdc. Plug wires are on the right plugs and order.

But I am getting cunfussed when you rev it up and have the timing light running it shows the timing will go up to 50 degrees advanced with the vacuum advance on the distributor hooked up to the timed vacuum port.Is this right? If I hook it to the full Vacuum port it will go up to 45 degrees advanced with it hooked up but still backfires.

Also I have heard that there is two ways to hook up the Vacuum advance on the distributor but I would like to find out the right way. Do you hook it to the timed vacuum port or the full vacuum port on the carb.

Any help would be greatly appreciated

Carnut

Denny,

I know we have differing ideas on vacuum advance; however, these are the instructions from a ZZ4 spec sheet, verbatum:

Set spark timing at 32 deg. before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32 deg. of total advance at wide open throttle (WOT). The HEI vacuum advance canister should be connected to a ported vacuum source (no vacuum at idle). The HEI distributor supplied with the ZZ4 engine has mechanical centrifugal and vacuum spark advance with the following curves:

Mechanical Advance:
1100 RPM 0.0
1600 RPM 12.0
2400 RPM 16.0
4600 RPM 22.0

Vacuum Advance:
2.00" 0.0
7.50" 20.0

Based on the 32 deg. at 4,000 RPM, and 22 deg. in the mechanical advance, the engine should drop back to about 10 deg. with no vacuum advance.

Now, that being said, from searching the ZZ4 discussion boards, I have found some indications that the GM HEI shipped with these crate engines doesn't have a really good curve. Too much vacuum and mechanical coming in too late. I found this thread particularly interesting:

http://www.sallee-chevrolet.com/Disc.../653/1607.html

You may convert me yet . . .

Here's another few threads that may be of value. It gives a GM vacuum canister part number better suited for the ZZ4.

http://www.sallee-chevrolet.com/Disc.../653/2608.html

http://www.sallee-chevrolet.com/Disc.../653/2297.html

The tech guy at Sallee Chevrolet recommends NO VACUUM ADVANCE - just disconnect. Or, if you want vacuum advance for a little more mileage, get part #1973681 and replace the stock vacuum canister. This cannister puts in 10 deg of vacuum instead of 20. and has a stiffer spring. If this one is used, ported (timed) vacuum is recommended. The part is available from http://www.gmpartsdirect.com for $22.81, including shipping. I'm going to give it a try.

This question set me off on a research binge, because I have a Fast Burn 385 with basically the same specs - just a little more HP.

Ok
I did what you guys said to do by setting the base timing at 10 degrees and with the distributor vacuum disconnected I rev the engine up to 4000 rpm I got 32 degrees of advancement. This is ok.
Then if I hook up the vacuum advance and rev the engine upto 4000 rpm I will get 50 degrees of advancement and the bobing will start.
Is this still ok?
I think something is wrong with the Vacuum advance on the distributor.
Please help I am pulling my hair out with this one.

Keith,

I thought those threads I posted answered the questions . . . but here goes.

1. Yes, you can get the 50 degrees you talk about, and no, it is not good. That's why your engine is acting up.

2. The HEI that is shipped with the ZZ4 has a crappy vacuum canister on it. It's not broken, but Chevy made a poor choice by using that part. It puts in too much vacuum too fast - as you have found out.

Here are three courses of action:

1. Disconnect the vacuum advance and plug it. Set your timing at 36 deg. at 4000. Run it that way. If it pings, drop the timing a degree at at time. You may get less gas mileage than with a vacuum advance.

2. Get GM part #1973681 and replace the stock vacuum canister. This canister puts in 10 deg of vacuum instead of 20 and has a stiffer spring. Use ported vacuum. Set initial at 12 deg. See how it runs. If it pings, drop it a degree at a time.

The part is available from http://www.gmpartsdirect.com for $22.81, including shipping.

3. Take your distributor to a good shop, tell them your application, and have them recurve the distributor.

No matter which route you take, you need to play around with your timing, moving it several degrees both ways and noting any change in power or pinging.

Hope that helps.

I am hearing now that an adjustable vac advance is better for the ZZ4. Mine has a Edelbrock 1406 carb. Iknow Mr. Gasket has one. Does anyone know of the others that are available?

http://cranecams.com

this vac. adv . thing has been discussed to death with no one agreeing on nothing, but I've always said do what ever the cam manf. and dist. manf. recommends and you would be ok. the part I don't agree with is I've read on here about how GM came from the factory mostly with ported vac. this is not true and has never been that I know of. all of the smog cars had manifold vac. and was reg. with switches and sensors at dif. times to adv. and retard the timing. didn't need ported vac. here is a diagram of a basic smog 350 motor. they had vac. at the dist. at all times while in hi gear. most people by-passed these switches and run the manifold vac. straight to the dist.

I think I'd install the 1973681 that Jack recommended and then do some testing with ported vacuum and then manifold vacuum, first on acceleration characteristics and then on a gas mileage basis. Then you'll know for sure what works best.