Thread: ZZ4 350 stutters on acceleration...cold weather?

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Well, I spent the end of the summer breaking in the new ZZ4 crate motor. Its been running great, until I took it out this weekend to discover the engine began stuttering/surging between approx 700 and 2000 rpm on acceleration, after that at higher rpm's it runs and pulls great. It almost feels like its misfiring but then goes away at higher rpms. Of course the car was well warmed up, choke opened all the way, ran it for about 45 minutes, so not a cold engine issue. Thinking it was a fouled plug or bad wire, I changed the plugs and wires today. Didnt solve problem (wasted money as usual!). But then I thought perhaps the colder temps outside (50 degrees this weekend) was the culprit, the car has only seen 80 degree days until this weekend. Could it be the carb does not like colder weather and needs to be tuned again? Anyone else ever have this issue?

I have a Holley Street Avenger 4bl Carb, timing set at 14 degrees at 800 rpm 36 degrees total.

Any thoughts on what the problem can be? Its making me nuts.

Thanks...

I am surprised no one has repsonded to you yet. Normally they jump all over questions like this.

1. Did you make ANY changes just prior to the first occurrence? (always go where the hands were last)
2. What kind of ignition system do you have?
3. Does it run crappy when you try to increase throttle or does it idle crappy too?
4. Have you inspected the plugs? How do they look? (rather, how did they look when you removed them?)

Sounds to me like maybe you arent getting any vac advance or maybe your carb went out of tune for one reason or another. I think reading your plugs will tell you alot and I dont believe that it is weather related. I am far from an expert on these matters though. Sorry I dont have your answer but maybe my comments will spark something for you. Let us know how it works out.

btw: I also have a street avenger, brand new, havent fired the engine yet. Hope that is not your problem.

Hey 65cayne, thanks for the reply.

Yikes ...I have to think where my hands were last under the hood, Im always doing something in there...I do remember moving the HEI wire to another location the day before, Ill have to go back and double check to make sure thats ok. I know for sure its getting 12V, I tested the new spot right out of the fusebox with my trusty voltmeter.

Anyway I pulled the plugs and changed them all. They looked a nice tan color except one plug was rather black, but not too bad, and I thought for sure that was the problem (thinking maybe it was carbon fouled). Put in all new plugs, but problem persisted. Perhaps a bad plug wire is to blame? But those are new too...Ill double check for strong spark tomorrow.

Car idles great at 700 rpm, no problem there. Runs great after 2,000 rpm. Also runs fine if I ease slowly into the throttle from idle. But stutters/surges alot if I slam on the gas from idle. This occurs from 700 to about 2000 rpm, then pulls like a scalded ape.

When you say not getting vac advance, what exacly do you mean? I have the vac advance can on the distributor disconnected and plugged as per advice of many posts ive read here about the ZZ4. with it attached I was getting something like 50 degrees total and couldnt get the darn thing to idle right.

Thanks for your input, gives me more to think about. Maybe someone else has more ideas, cant wait to spend my weekend under the hood...again.

Good luck with the holley, it seems to be a good carb.

Originally Posted by 63BoxNova

Hey 65cayne, thanks for the reply.

Yikes ...I have to think where my hands were last under the hood, Im always doing something in there...I do remember moving the HEI wire to another location the day before, Ill have to go back and double check to make sure thats ok. I know for sure its getting 12V, I tested the new spot right out of the fusebox with my trusty voltmeter.

Anyway I pulled the plugs and changed them all. They looked a nice tan color except one plug was rather black, but not too bad, and I thought for sure that was the problem (thinking maybe it was carbon fouled). Put in all new plugs, but problem persisted. Perhaps a bad plug wire is to blame? But those are new too...Ill double check for strong spark tomorrow.

Car idles great at 700 rpm, no problem there. Runs great after 2,000 rpm. Also runs fine if I ease slowly into the throttle from idle. But stutters/surges alot if I slam on the gas from idle. This occurs from 700 to about 2000 rpm, then pulls like a scalded ape.

When you say not getting vac advance, what exacly do you mean? I have the vac advance can on the distributor disconnected and plugged as per advice of many posts ive read here about the ZZ4. with it attached I was getting something like 50 degrees total and couldnt get the darn thing to idle right.

Thanks for your input, gives me more to think about. Maybe someone else has more ideas, cant wait to spend my weekend under the hood...again.

Good luck with the holley, it seems to be a good carb.

An H.E.I. really needs 12 or more volts to operate correctly.

Originally Posted by 63BoxNova

Hey 65cayne, thanks for the reply.
But stutters/surges ....Good luck with the holley, it seems to be a good carb.

I would look at the carb first. You might want to change the power valve. See what's in it, probably a 6.5 if so, and if it is good ( check by sucking on it, place your tongue on it and see if it lets go or if it holds) go for a 8.5. If it doesn't hold vacuum, replace it with the same one that was in there. When cold weather hits, you need more fuel. Also, does the intake have exhaust travelling through it to warm it?

What brand of plug wires are you running an have you messed with the carb?

Originally Posted by TyphoonZR

I would look at the carb first. You might want to change the power valve. See what's in it, probably a 6.5 if so, and if it is good ( check by sucking on it, place your tongue on it and see if it lets go or if it holds) go for a 8.5. If it doesn't hold vacuum, replace it with the same one that was in there. When cold weather hits, you need more fuel. Also, does the intake have exhaust travelling through it to warm it?

He say's the engine idles great.
If it where a blown power valve it would idle up and down.

Id like to check out the power valve, but hate like heck to start taking apart my new carb, never done it before and dont feel like breaking more stuff...is it easy to remove? I hope so. If it was a blown valve wouldnt it run rich? Would a rich or lean mixture cause this problem?

Do you think I should tweak the idle mixture screws to see if that helps? Already set the floats to the bottom of sight window as per holleys spec.

Using AC Delco R44LTS...engine came with MR43LTS which is a marine version and impossible to find. The guy at parts counter said the R44LTS is the closest replacement.

Thanks guys, I wish the idiot at my local auto part shop was this helpful.

Any more suggestions are appreciated...its gonna be a busy weekend under the hood...

Just saw your post Erik...If your right I guess that saves me from taking my carb apart. It does indeed idle very nicely.

If the power valve was blown, which I doubt, it would still idle but bog upon acceleration, been there done that. If the wires aren't crapping out, I'd look real close at the accelerator pump.

Just came across this thread and everybody has very good suggestions,but something that you might be over looking is the fact that the ZZ4 has no exhaust ports in the heads to heat the bottom of the intake and that is fine for a race car or a EFI injected car, But on a carbureted street car in cooler weather it really needs that heated crossover to vaporise the fuel. I can't begin to tell you how many vehicles I cured cold run problems on back in the carb days just by pulling the intake and chiseling the carbon outta the exhaust ports.
You might try dropping the jet size a couple of numbers and see if it improves.

Didnt know having exhaust ports on the heads could make such a difference, why would GM not include that? Anyway its going to be almost 70 degrees here today, first "warm" day since the problem started, so Im going to test it out to see if there is any improvement. If not Im going to test everyones suggestions this weekend.
I Knew I should have gotten the RamJet!

Well I rushed home from work to take the car for a spin while it was still warm outside. The stutter was still there but less than the other day. So I decided to richen the mixture a bit by turning out the idle mixture screws. It helped quite a bit, but still not 100% (i tried turning them in first and the car immediately idled like crap). I guess its true that the car needs more fuel in cold weather? And I guess what HWORREL said about having exhaust warming the intake would make a big difference since it makes the car "immune" to the cold air outside.

Are the idle mixture screws only for low rpm as the name implies, or does it have some effect on higher rpm as well?

Thanks for all your input guys

Not exactly right, it needs to vaporize the fuel thats there instead of entering the combustion chambers as a liquid instead of a gas. Those screws affect the idle mixture only..

If your idle screws and throttle plates aren't adjust properly it can cause what is called tip-in hesitation. You can get the idle rpm's right with the throttle adjustment screw but idle mixture (that some say doesn't effect any other circuit) won't be right. If the throttle plates are open to much, at idle, you can get the bog you are talking about.