Strange Idle Issue - Team Camaro Tech
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post #1 of 14 (permalink) Old Apr 17th, 19, 02:22 AM Thread Starter
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Tony
 
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Strange Idle Issue

Morning, Guys;

I'm experiencing a strange idle issue when hot, and I am hoping for some ideas on what to check next. '69 Z/28 with original non-matching numbers 302. Motor was recently rebuilt and runs strong. Jerry just red-did my Z/28 intake (looks beautiful), with DZ Holley carb; brand new reproduction (original is in a box getting ready to go to Jerry. Idle's fine when cold. When the motor gets warm, I have an idle dither at stops, and will stall without goosing the gas.

Weird; I took her for a drive up the highway last Saturday morning with my daughter. Drove 20 or so miles to a gas station that sells 110 racing fuel, picked-up 5 gallons and drove back. Ran great - coming off the off-ramp, i put her in neutral and let her idle for 1,000 feet or so as i rolled to the exit; idled dead-nuts fine at 1,000 rpm. Came to the stop light and she started to dither... Also; never fails, she dithers as i pull into my driveway every time... I've checked floats and idle screws, timing is good and she runs well otherwise.

Also of note; i'm running pertronix distributor, but it's at least 10 years old with low miles. Brand new plug wires and plugs. I'm thinking maybe i need to replace the coil?

Any ideas on what to dig into next would be great. Thanks for your help!

Finally, I'd love to post some photos. i use ios iphone and can't seem to figure out how to make them smaller so i can post here.

Tony

Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #2 of 14 (permalink) Old Apr 17th, 19, 03:08 AM
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Re: Strange Idle Issue

I'm assuming "dither" means it idles up/down (surge)?

If so, check for unmetered air leaks. On a warmed engine, if there is a small air leak getting into the intake manifold (bypassing the carb), an engine will surge at idle. Check the carb's base gasket, and any vacuum lines for cracks/leaks.

Check the intake manifold gaskets to heads by the intake runners for potential air leaks. A small amount of starting fluid sprayed at the intake runners where they mount to the heads or base of the carb would indicate a momentary jump in the idle if there is an air leak at that point. This air leak is "unmetered" air; the carb is not seeing this air as it is designed to mix fuel with the air that comes into the top intake of the carb only.

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post #3 of 14 (permalink) Old Apr 17th, 19, 04:48 AM Thread Starter
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Tony
 
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Re: Strange Idle Issue

Thanks, Kevin - sorry, I should have been more clear. I think of "dither" as the motor idling down, or losing rpm to a point of stalling. Just installed the intake last year with all new gaskets, etc. I sprayed and checked everything for leaks after install; for a 302, i have fairly decent vacuum; around 10-12. I will recheck everything for vacuum leaks for sure.

Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #4 of 14 (permalink) Old Apr 17th, 19, 05:37 AM
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Re: Strange Idle Issue

I'd suggest you change to full manifold vacuum if you've not already done so. Read the article at the link posted below. I made these changes to my 302 and the car is much more driveable, better throttle response, less prone to stalling at idle. If you do change the vacuum source to the carb be sure to reset your timing.


http://www.camaros.org/pdf/timing101.pdf

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post #5 of 14 (permalink) Old Apr 17th, 19, 05:52 AM Thread Starter
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Tony
 
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Re: Strange Idle Issue

Thanks Chris - i am running full manifold vacuum, and also using the recommended vacuum can for 302 with lower pull rate for full advance.

Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #6 of 14 (permalink) Old Apr 17th, 19, 06:58 AM
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Re: Strange Idle Issue

If the carb to intake gasket isn't the cause of an air leak, and your lines are all tight, no dry rot etc then I'm intrigued by your statement Tony that without fail it does so into your driveway.

So tell us more about the angle of your driveway in relationship to the road, etc. Was the off ramp when you experienced the same starvation for fuel the same type of angle of decent? Just a thought - Maybe your issue is not at the carb float or bowl but in the tank and your intake "sock" is either carrying some debris or the fuel sender assembly might have some issues, at one angle or another or maybe there is a crimping rubber fuel line between metal lengths where the tank meets the lines?

Brian


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post #7 of 14 (permalink) Old Apr 17th, 19, 08:17 AM
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Mine never liked to idle less than 1000 rpms. That was just the nature of the beast. You can't get a fine tune with the 780 like you can a good 4 corner carb. I welded a o2 bung in to get my mixture close.

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1969 x-77 Z/28 orange/ white stripes, named by my daughter "Clementine ", M-22 close ratio, 4:10 12 bolt, CE 302. Hugger orange with white stripes. White standard interior. Restoration complete May 2018
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post #8 of 14 (permalink) Old Apr 17th, 19, 08:22 AM
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Re: Strange Idle Issue

I was also thinking about the statement regarding the engine dither/stall when pulling into the driveway... or coming to a stop. Some fuel may be coming out of the carb's vent tubes and going into the throttle bores (temporarily flooding your engine) due to the sloshing of fuel in the fuel bowls during these situations.

Do you know if your Holly carb has the fuel bowl vent baffles (whistles) installed?
https://www.summitracing.com/parts/aed-5897?rrec=true

If you do not have the vent baffles, here is a quick test to see if this is the problem. Remove your carb's air cleaner lid + filter and find a piece of tubing that would fit tightly onto the carb's vent tubes. Cut the length so that the tube attaches to both front and rear vent tubes > forming an upside-down "U" shape. At the top of the bend on the tube, cut a small hole/slot into the top portion only (do not go all the way thru the tube) to vent air into the fuel bowls. Now take your car for a drive and try to duplicate the same conditions that causes your engine to stall. If the engine no longer stalls, you may want to consider installing some vent baffles onto your carb.

Last edited by oldschoolchevy; Apr 17th, 19 at 08:43 AM.
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post #9 of 14 (permalink) Old Apr 17th, 19, 09:08 AM
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Re: Strange Idle Issue

If power brakes, unplug and plug the manifold nipple feeding the booster. Booster could be damaged.
My 302DZ had a 3310-1 and GM's -140 camshaft, and she had 12-13 inches at 950 RPM, barely.
Run the vent hoses inside air cleaner, much easier for testing.

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post #10 of 14 (permalink) Old Apr 17th, 19, 09:24 AM
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Set idle at a grand, and enjoy the drive. That engine will never run/ idle like a stock v8. The 302 is more of a race engine. Don't expect it to like to idle low. You can play with your timing, and play with the screws, but at the end of the day it It a solid lift hardly standard cam engine with 11:1 ratio pistons,a 780 holley . great engine, But the run of the mill tune will never work.
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1969 x-77 Z/28 orange/ white stripes, named by my daughter "Clementine ", M-22 close ratio, 4:10 12 bolt, CE 302. Hugger orange with white stripes. White standard interior. Restoration complete May 2018
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post #11 of 14 (permalink) Old Apr 17th, 19, 10:11 AM Thread Starter
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Tony
 
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Re: Strange Idle Issue

Wow! These are all great responses guys. I'll take this one at a time. This issue has been haunting me for quite awhile but i've always had other "pressing" stuff to focus on. Now the Z is just about done, so i can focus on some of the "little" things...

So; flat driveway, but a hard left turn coming in. Never fails; i get a stall just as I pull into driveway and I can't get into the garage unless i goose it a little to keep 'er running. New stainless steel fuel line within the past year and it's done this with 2 different (mechanical) fuel pumps. I also checked all hoses when i replaced the fuel line, to check for crimps, etc. Never did any work on the gas tank, however. Off ramp was flat but it'll do this basically at any stop once the motor is warm. She does not run hot; new radiator in the past year and high flow chevy water pump.

I will definitely try the vent baffles as I know these aren't currently there.

Aware of the idle issue with the 302, but of course before this cruise; tried to idle it down just a bit - i'll bring her back up to 1,000.

New brake booster, master and brakes throughout within the past 2 years also. I'll check the booster, but I think she's air tight there also.

Thank you ALL for your contributions - i'll check all of this out and report back.

Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #12 of 14 (permalink) Old Apr 18th, 19, 08:47 AM Thread Starter
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Tony
 
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Re: Strange Idle Issue

Guys;

Not sure if you can see this, but I took my coil off the intake and noticed a small crack in the plastic insulator - could this be contributing to my issue? I've ordered a new coil either way.
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Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #13 of 14 (permalink) Old Apr 18th, 19, 08:54 AM Thread Starter
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Tony
 
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Re: Strange Idle Issue

Retrying pic.

https://www.dropbox.com/s/81h7la4gr6..._0137.jpg?dl=0

Tony
'69 RS Z/28
Glacier Blue with Black Stripes
Non-matching numbers 302
M22 4 Speed
4.11 rear end
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post #14 of 14 (permalink) Old Apr 18th, 19, 05:05 PM
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Re: Strange Idle Issue

Doesn't hurt in trying another coil.
Yes, 302 wasn't designed for idle. Set at 1000, and go for it.
Use vacuum gauge to tune and adjust for best reading, including advancing timing , then back off 2 degrees until it hot starts easily, since you have full vacuum on vacuum advance.

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