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Discussion Starter · #1 ·
Can't quite seem to diagnose this issue. Eng is a factory 1995 ZZ5 400hp 385tq 350 with under 5000 miles on it. Original carb. I don't want to over detail here so I'll state what I've done. Replaced the entire HEI unit and rebuilt the original Holley 770 Street Avenger. New factory AC Delco MR43LTS plugs gapped at 38-40. Static timing at 14. Revs happy in neutral, won't make it down the street. After I rebuilt the carb (it was really gunked up) the issue went away. Then I changed plugs, set timing at 14 (it was at 12) and the issue is back. Really stumped. Any suggestions are appreciated
 

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Can't quite seem to diagnose this issue. Eng is a factory 1995 ZZ5 400hp 385tq 350 with under 5000 miles on it. Original carb. I don't want to over detail here so I'll state what I've done. Replaced the entire HEI unit and rebuilt the original Holley 770 Street Avenger. New factory AC Delco MR43LTS plugs gapped at 38-40. Static timing at 14. Revs happy in neutral, won't make it down the street. After I rebuilt the carb (it was really gunked up) the issue went away. Then I changed plugs, set timing at 14 (it was at 12) and the issue is back. Really stumped. Any suggestions are appreciated
Define “Won’t make it down the street”. Are the plugs and timing what is supposed to be in the engine?
You rebuilt the carburetor and the problem went away. Then you changed the plugs and timing and the problem is back.
 

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Discussion Starter · #5 ·
Define “Won’t make it down the street”. Are the plugs and timing what is supposed to be in the engine?
You rebuilt the carburetor and the problem went away. Then you changed the plugs and timing and the problem is back.
Well it will load up on fuel when you step on it and will stall if you stay in it. I throw it in neutral and rev it it cleans out the raw unburned fuel and will billow dark smoke out the tail pipes. Then as long as you keep it in neutral it will rev fine until you drop it back in gear. Yes I replaced plugs with the exact same brand and gap and advanced the timing 2° for an initial setting of 14° which this engine should accept all day long. I think possibly it's stressing the ignition system somehow and maybe throwing a weak spark? I had a similar issue with my 65 302 in the Mustang and it was the coil going bad. It would rev fine in N but when warm and you take it out it would sputter. Just like now
 

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Well it will load up on fuel when you step on it and will stall if you stay in it. I throw it in neutral and rev it it cleans out the raw unburned fuel and will billow dark smoke out the tail pipes. Then as long as you keep it in neutral it will rev fine until you drop it back in gear. Yes I replaced plugs with the exact same brand and gap and advanced the timing 2° for an initial setting of 14° which this engine should accept all day long. I think possibly it's stressing the ignition system somehow and maybe throwing a weak spark? I had a similar issue with my 65 302 in the Mustang and it was the coil going bad. It would rev fine in N but when warm and you take it out it would sputter. Just like now
Heres my two for openers the plugs are way too cold switch to MR45LTS check you float levels check your HEI unit something there could be defective . Just because something is new don't mean its good and see where that gets you. Alex
 

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Yes it could be the floats, but I had a very similar problem on my 69 years ago. Is it a manual fuel pump or electronic? Mine was manual. My problem was it was starving for gas when I got on it. I put a 6 lb. fuel pump on it instead of 4 lb. and instantly screamin.
 

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Discussion Starter · #8 ·
Heres my two for openers the plugs are way too cold switch to MR45LTS check you float levels check your HEI unit something there could be defective . Just because something is new don't mean its good and see where that gets you. Alex
We think very similar. I checked float levels and they are right at the bottom of the peep hole threads. I completely agree on new parts. The new HEI unit i got is a cheap Chinese product from eBay/Amazon. I hadn't thought of the plugs being to cold but that definitely could b a factor. I was just going with what the OEM had on the crate engine from Chevrolet. I also question the size of the carb itself. It's a 770 which seems kinda large to me with its size 70 primary jets but who am I to question Chevrolet engineering?
 

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Discussion Starter · #12 ·
How did it run before you changed the parts? Seems the carb is dumping way too much fuel and/or the accelerator pump isn’t working. Spitballing here.
Thank you for the reply. Well in order to keep my initial post from being a book I left some info out. When I purchased the car about a year ago the kid told me for some reason once in a while the distributor stops working 🤔. So sure enough a few months later the car just wouldn't start. I tested and had no spark. So I replaced the ignition module and coil. Still no start. So I then just replaced the entire thing with a cheap Amazon seller Volstom HEI. Boom. Started right up. But all I did was dive car around the block and put in my shop. Then a few weeks later I go to drive it and that's when the loading up/bogging issue happened. So I figured it's gotta be the carb as it's original to this 1995 crate motor and has probably never been rebuilt. I was correct. It looked pretty bad inside and one of the primary jets was finger tight and backed out half a turn. I thought sure that was my issue as it was just sucking gas all around the threads as well as thru the center jet hole. So after I put it back together it ran much better. No more stumbling, loading up. So a few weeks later I thought I'd check the plugs as I'm sure they were original as well. They looked horrible. Most likely as much a result of age as constant flooding. So I replaced with OEM to that crate motor, the gap was factory set (i checked) and thought I'd bump up the timing from 10° to 14°. Took it for a drive and bam. Back to square one. Wouldn't even make it down the street without the carb loading up and bogging. Throw it in neutral to blow out all the unburned fuel, then it would feel fine. Drop back in gear, give it some throttle and BAM. It would immediately load up and bog down again.
 

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Discussion Starter · #14 ·
When you rebuilt the carb did you replace the power valve and PV gasket? I assume you're using a Holley. Another thought, is the choke fully open?
Yes. I replaced with a 6.5 which is perfect as the eng is pulling 13" of vacuum at idle in gear. And yes, new gasket as well. Now, the choke i haven't looked at but since you mentioned it it stopped working as it should. It's a electronic unit and no longer fast idles at start up like it used to. And the + wire is getting power at ign on. I will check to make sure the flap is full open when attempting to drive.
 

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Discussion Starter · #17 ·
Yes it could be the floats, but I had a very similar problem on my 69 years ago. Is it a manual fuel pump or electronic? Mine was manual. My problem was it was starving for gas when I got on it. I put a 6 lb. fuel pump on it instead of 4 lb. and instantly screamin.
It is a manual pump. Factory installed. It was part of the ZZ5 crate eng package. It's high volume. I don't believe it's being starved for fuel as it's loading up as in too much fuel not enough air and or spark. I have had a few of my cars go lean as I was romping them and would usually result in a backfire and or massive pinging
 

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It is a manual pump. Factory installed. It was part of the ZZ5 crate eng package. It's high volume. I don't believe it's being starved for fuel as it's loading up as in too much fuel not enough air and or spark. I have had a few of my cars go lean as I was romping them and would usually result in a backfire and or massive pinging
It is a manual pump. Factory installed. It was part of the ZZ5 crate eng package. It's high volume. I don't believe it's being starved for fuel as it's loading up as in too much fuel not enough air and or spark. I have had a few of my cars go lean as I was romping them and would usually result in a backfire and or massive pinging
Does it have a MAP sensor? EGR? The only other things I can think of are it's running TOO rich or there is a vacuum leak somewhere. I'm leaning toward vacuum leak because it only does it under load.
 

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Timing curve may be causing your issue. Coming all in at too low rpm?

Back it down to 4-6* and see if that helps. If so the centrifugal springs are too light. Or maybe the vacuum advance cannister is advancing at low vacuum, not retarding enough when throttle opens under load. From idle to load timing has to be pulled. You could also just disconnect the VA and plug the vacuum for a test.
 
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