'69 Z28 Tuning Issues - Team Camaro Tech
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post #1 of 6 (permalink) Old Sep 23rd, 04, 10:38 AM Thread Starter
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Hello,

just finished my '69 Z28 - stock DZ302 w/Holley DZ Carb. Installed an MSD ready to run distributor, AC Delco plugs, gapped at .35. Timing is set at 8% TDC. Factory intake, Hooker Headers.

Car idles fine but when I "stomp" on the gas, it backfires through the carb. I can gradually up the gas and it runs great but tends to stumble, hesitate and backfire at times when I punch the gas. Any thoughts? Runs great at cruising speeds
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post #2 of 6 (permalink) Old Sep 24th, 04, 06:44 AM
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Mark
 
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It is running lean when you punch it. That is what causes the backfire. Check the accelerator pump arm adjustment. There must not be any looseness at idle. The arm must move as soon as the trottle is cracked. If you need instructions on the proper setting, just ask.

-Mark.
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post #3 of 6 (permalink) Old Sep 24th, 04, 07:31 AM Thread Starter
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Mark,

if you can, please send me the instructions. Someone suggested I change the weights in the distributor. Thanks
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post #4 of 6 (permalink) Old Sep 24th, 04, 01:12 PM
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Give it some more timing...like 15 initial and 35 total. Be careful to listen for detonation, but mine did the same thing. Messed with the carb for a while and a buddy at my work told me to double check my timing. Sure enoungh it was a 8 initial. Bumper her up and she ran great. Quick check before working on the carb....

Craig

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post #5 of 6 (permalink) Old Sep 25th, 04, 05:45 AM
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Mark
 
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Danny,
The first thing to check before you do anything to a carb is the float settings. The sight hole should have fuel just barely trickling out at idle if it is set right. Get this right and tune from there.

You need to set the curb idle as this will have an effect on off-idle transition. Bad idle settings can cause a lean stumble as you take off. Screw them in very lightly then back out 1.5 turns. Go in or out to get the best idle. Use a vacuum gauge to make it a little easier if you need it. Set the RPM to the speed you need and recheck the mixture screws again until you have this where you want it.

The accelerator pump adjustment is the bolt with the spring that is on the accelerator pump arm. You want to put a .015" feeler gauge between the pump arm and the pump lever and take it to wide open throttle. This will verify the .015" minimum clearance at WOT. You do not want it to bottom out or pinch the feeler when you floor it. Take the feeler gauge out and let the throttle return then verify that there is absolutlely zero slack between the pump arm and the pump lever while you are at idle. You need zero slack at idle and minimum .015" additional travel available at wide open throttle.

Once this has been set right you can check other things. 90% of the time, the first stuff above was the problem. If this did not fix it, you may need to increase the primary jet size. Check to see what jets you have there now. I ran a stock 302 with big tube Hookers and needed to jet up. I run cast iron manifolds now and have it jetted back down a bit. I need to do some more tuning some day but it runs very well now so the Camaro will have to wait for now. I have another car I am doing right now that takes up my time. Maybe a wideband O2 sensor will be in my christmas stocking.

-Mark.
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post #6 of 6 (permalink) Old Sep 27th, 04, 04:26 AM Thread Starter
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All - thanks again for all your help! [img]graemlins/thumbsup.gif[/img]
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