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Author Topic: 69Z hard start, backfires on acceleration, stalls when breaking, related issues?  (Read 3766 times)
doomer
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« on: November 20, 2013, 06:11:29 PM »

After a recent rebuild, I find myself chasing down gremlins here and there. One thing is bothering me, and since I am not much of a mechanic at all, I am hoping someone can spot the problem.

1. The weather has gotten cool, and I have an issue with difficult starts. Once started, she idles well while I am waiting for it to warm up.
2. Once running, if I accelerate more than a tiny bit, I have backfiring and sputtering. If I accelerate very slowly, it's fine. This goes away if I drive it for a long while, and lessons as the day warms up, but never goes away completely.
3. One longer cruises, sometimes when I roll into a turn while breaking, it will stall. Sometimes it will stall when coming to a stop even on a straight away.

Frustrating!  Cry

'69 Z/28
DZ302, recent rebuild.
Holly Carb (4053)
New break booster/Master Cylinder
Orig. M21 tranny
Pretty much all stock otherwise

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cook_dw
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« Reply #1 on: November 20, 2013, 06:30:48 PM »

First things first.  Make sure the float level is good.  Backfiring through the carb is a lean condition.  It could jetting, timing or both.  More than likely it is just carb tuning that needs to be done.  I would make sure the timing is in line with what it should be..  Also carb jets should be in the range of 68-72's on the primary and 72-76's on the secondaries..  But those are just stock settings and a lot of the time need to be dialed in to the elevation, etc. 

But all in all I would bet you either have trash in the carb, too low of a float level or jets too small.. 
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Darrell Cook

1967 LeMans Blue SS/RS L35 clone
1968 Rallye Green SS L78 - unrestored original
1968 Matador Red Z28
janobyte
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« Reply #2 on: November 20, 2013, 06:49:28 PM »

Also check for vacuum leaks , like Darrell said ,you are presenting a lean condition ; not good ,higher internal temps. Check your plugs ,post pics of ALL 8---we can read them, also check for moisture/cracks in your cap.   To check floats: trickle of fuel out side plugs at idle , I'm assuming 780 vacuum secondary's ?   I'm stopping.Could be something as simple as a fouled plug--pull them and read them.
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BULLITT65
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« Reply #3 on: November 21, 2013, 12:05:14 AM »

I had the same problem with Mustang stalling out when braking. I assume you have power brakes. I had a vacuum leak at the carb base, and a couple of old carb gaskets. An easy way to check is with some starter fluid. Get the car running and idling on its own and then spray starter fluid around the out side base of the carb, if the rpm picks up you have a leak.
Spittering and sputtering: once you solve your vacuum leak, then new plugs or (check the gap) , points (check the gap), condenser, cap and rotor, check your vacuum on the distributor (they are adjustable), new plug wires. (pictures of the plugs will tell you if its to lean (white colored), or to rich (very black, and full of carbon).
Check your timing.
If you still have the issue Clean out the carb, I assume its a holley which if any little bit of debris is in there it will not run smooth. blow out the carb and make sure all passage ways are clear and free flowing. Check (or replace) your power valve while you have apart, and as Cook mentions check to make sure you have the right jets, set your floats.
If it still runs a bit rough post up what you took care of, and we'll go from there...
Good  Luck!  Smiley
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
JKZ27
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« Reply #4 on: November 21, 2013, 07:04:42 AM »

All of the above... BUT, be careful spraying flamables on a running or hot engine.
And, make sure the choke is working right.
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69 RS/SS Cortez Silver, L48 MC1
68 RS Ash/Ivy Gold 327EFI M20
79 Z28 Brown LM1 M21
04 SRT 10
69pace
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« Reply #5 on: November 21, 2013, 05:06:29 PM »

All of the above... BUT, be careful spraying flammables on a running or hot engine.
And, make sure the choke is working right.



X 2 on the working choke and double check its adjustments. I chased my tail this month with a working choke, but poorly adjusted resulted in flooding and very rough idles until it burned off some fuel.

Also watch the paint job starter and carb cleaners eat paint off metal surfaces so use them as leak detectors with car to your new painted intake. An unlighted propane torch is also a good way to test for leaks.
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1969 Z-11 350/300 with 4 Speed
doomer
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« Reply #6 on: November 22, 2013, 07:07:47 PM »

Thanks for all the great advice! As for the choke, I am pretty much an idiot. I may have just been a little too aggressive when trying to start up on a cold morning and flooded it. The weather has only been cold a few mornings so far, and I think I just pumped it too hard. The weather has warmed back up and I have to wait until another cold morning comes around to verify things.

Now that the weekend is here I hope to have time to go through a basic tune up, and if I can find a decent  reference for this old 4053 I will try and ensure that it is set correctly.
« Last Edit: November 22, 2013, 08:20:29 PM by doomer » Logged
doomer
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« Reply #7 on: November 23, 2013, 01:45:44 PM »

Here are my plugs.

http://s829.photobucket.com/user/vetteran81/slideshow/1969%20Camaro%20Z28/Plugs

Obvious tuning issue. #5 worried me a little.
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BULLITT65
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« Reply #8 on: November 23, 2013, 08:46:33 PM »

If your choke wasn't set right, yes your going to see some carbon from that. Most of your other plugs look ok. I would go ahead clean up the plugs make sure the gap is ok, re-install, and your probably good to go!
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
cook_dw
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« Reply #9 on: November 23, 2013, 08:56:57 PM »

Just replace the plugs..  You can not get an accurate reading on reused plugs plus new plugs aren't that expensive..  With choke working correctly also check the resistance on the #5 plug wire  & reference it to the resistance to another wire of similar length..  Once that is don't install new plugs & drive it for 10-15 miles & look at plugs again.  Making sure #5 isn't fouling..  Just my $0.02
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Darrell Cook

1967 LeMans Blue SS/RS L35 clone
1968 Rallye Green SS L78 - unrestored original
1968 Matador Red Z28
BULLITT65
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« Reply #10 on: November 23, 2013, 09:18:09 PM »

If they are old plugs not a bad idea. I would give it whirl with the ones you had if they are fairly new. Hell even if they aren't it only takes a minute to swap them out. I assumed your spark plug wires are in good shape, but I would follow Cooks advice if number 5 continues to be an issue.
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
Ramjett54
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« Reply #11 on: November 23, 2013, 09:53:53 PM »

Also check the brake booster diaphragm
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doomer
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« Reply #12 on: November 24, 2013, 09:26:05 AM »

I replaced the plugs, and checked the gaps on the originals. The old plugs were not gapped to 35 as prescribed. Looks much larger (60?) However, my original points dizzy is out of the car waiting on a rebuild. I have a Pertronix Ignitor III
 distributor setup in it now, and a Pertronix flamethrower coil. This basically means much more spark, so I am not sure the larger gap made a difference.

After gapping and installing the new plugs, 2-pumps and she starts right up. It was 40 degrees out this morning, and no problem at all with starting. I'll take her for a short spin today before I proceed further. Then I hope to tackle the carb & vacuum leak checks.

How does one check/adjust the choke? I understand the need for the gap check on the choke plate when closed. It is about 9/32". Are there other adjustments to check? This is the original style choke, not electric.

Shane

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janobyte
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« Reply #13 on: November 24, 2013, 09:51:20 AM »

Appears some variation in gap---your wider gaped plugs appear to be the culprits. If your running a stock ignition it doesn't have the power to "jump" the gap---spark weakens. Try a new set ( as suggested ) of Autolites, set per manual. I'm not going to suggest a change in heat range due to a few look good. You can check your wires with an ohm meter--also watch it running in the dark to see if any wires are arching. One step at a time, work from your plus back in your trouble shooting ,isolate systems. Wow, like I posted could be as simple as a fouled plug Wink
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janobyte
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« Reply #14 on: November 24, 2013, 10:01:44 AM »

You posted as I was typing. .060 is a large gap ,more than we run on the race car. Sorry ,those early chokes I do not know how to adjust the speed in opening. Later models were easy +/-  2 hold down screws. I do not run a choke, no cold driving.

I'm sure dozens do on this site.

Get on Holley's website ,tech ,you will learn everything you need.
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