Brief Rough Idle After Start - Team Camaro Tech
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post #1 of 28 (permalink) Old May 27th, 14, 04:17 PM Thread Starter
Richard
 
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Brief Rough Idle After Start

I recently posted an issue with the fuel gauge on my 1969 Camaro and got the problem solved very quickly by a couple of great board members. Thanks Vette-Freak and 69-Pace! I’m back again with another puzzler—at least for me—that I’ve been battling for a couple of months now. As usual, thanks in advance for your thoughts.

The problem:
After I drive the car normally and then let it sit for a few hours, I can start the engine but then really struggle to keep it running. For about 20 seconds, it stumbles like it’s running on only half of the cylinders and then smooths out and is fine. Of course, this only happens when everyone’s watching. It’s a real crowd pleaser among anti-Camaro crowd at the local drive-in on Saturday night. Supremely embarrassing.

In many other situations, especially ones that don’t involve Camaro haters, like in the morning after sitting in the garage, after stopping at the drug store for a few minutes, etc., the car starts and runs fine.

Also, there is a raw fuel smell very often when driving. Prior to the installation of the phenolic spacer (see below), I noticed the fuel boiling off in the primary and secondary bowls.


The general setup:
  • 1969 Camaro purchased about 4 months ago (We’re still in the wooing phase and getting to know each other.)
  • 427cid, stock heads, cam
  • Road Demon 750 (vacuum secondary), electric choke
  • Stock cowl induction air cleaner assembly
  • HEI ignition
  • Mechanical fuel pump, no return line
  • 92 octane Chevron with “up to 10% ethanol” (With my luck, it’s right at 10%.)
What I’ve done so far:
  • Double-, triple-, quadruple-checked for fuel leaks; none detected
  • Checked that choke linkage was not obstructed by air cleaner assembly
  • Inspected distributor cap, spark plug wires; look new
  • Installed new spark plugs
  • I thoroughly disassembled, cleaned and rebuilt the carburetor with the usual new bits
  • Bowl floats set so fuel is near the bottom of the sight glasses
  • Installed ľ” phenolic plastic spacer between carb & intake
  • Measured spark timing, 12 (initial), 38 (all-in)
  • Set idle mix screws as per Demon, then adjusted for maximum manifold vacuum at a constant rpm
  • Set primary fast idle when warm to approx. 850 rpm
What I HAVE NOT done:
  • Replaced plug wires, distributor cap or coil—because it’s all looks fairly new, but I don’t know for sure.
  • Measured fuel pressure—because I wouldn’t expect pressure above 7 psi from this mech. pump, but I don’t know for sure.
  • Constantly monitored for fuel boiling in the bowl after each drive--because I think it's not boiling off now and I don't know where to go from here regarding heat soak
My gut feeling is that the problem is carb-related. Unfortunately, I don’t have another carb laying around, and can’t justify (yet) purchasing another one simply for testing. If this persists, I might just break down and get one. (Any suggestions which one?)

Also, I noticed that there was still pressure in the fuel bowls after the car sat for a while and when I loosened the float set screws and fuel spurted out. This is normal, right?

The main question:
What might be happening immediately after starting after sitting for a few hours? Do fouled plugs cause the rough running? If so, do they stay fouled? How do I check for this condition prior to starting?

I really don’t want to have to start parking in the dark back corner of the drive-in parking lot so no one notices me. Thanks for your input.

1969 Camaro 427/425HP M22 - American Muscle
2003 BMW M5 - German Muscle
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post #2 of 28 (permalink) Old May 28th, 14, 09:47 AM
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Kurt
 
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Re: Brief Rough Idle After Start

I think you are correct in thinking it's a heat soak/boiling problem. You could try setting the floats as low as possible without effecting top end performance. If I had them low enough to solve the starting problem it would run lean at top end on the highway (well above legal speeds ). I just kind of learned to live with it last summer. I pretty much knew when it was going to be a rich start and would open the throttle wide open (as slow as possible trying to reduce the pump shot) before trying to start it and hold it there for a bit. This allowed some fumes to escape the manifold and equalize the air/fuel ratio. I can relate to the embarrassment you are feeling.

I did buy a new carb over the winter and things seem better but it really hasn't gotten super hot up here in MO yet to know if my problem is gone yet.
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post #3 of 28 (permalink) Old May 28th, 14, 10:19 AM
DjD
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Re: Brief Rough Idle After Start

Wow! I've been going to cruise in's and car shows with my '69 Camaro for 15 years or so and don't think I ever met a "Camaro hater"... I had my starter fail and have to be pushed out of the way at a cruise that got grid locked for a while and I shut it down but everyone cheered when I got it fired back up! I know that it's embarrassing when a car acts up in public but the folks into the car scene here are not haters...

That said, what is your starting routine? Every engine is a little different and you may be pumping the throttle too many times causing the engine to load up a little. I can tell you my Camaro will fire up with 2 quick pumps if it's been less than 2 - 4 days since the last time I drove it. If it's been 2 weeks I may have to repeat that and then it fires up. Once I've driven it and it's really warmed up it will typically start up without touching the gas. If it's sat a few hours one pump and turn the key typically does it. It's a routine and every old carb'd engine has one, you just have to figure it out. I'm only leaning towards this because you say it starts and runs fine and once you clear it up a bit I assume it runs fine.

On the raw fuel smell, make sure your gas cap is installed properly and functioning properly. The only way you would smell raw fuel coming from under the hood is if it were leaking or the carb was acting up and then the engine would not be running well as you went down the road.

...Dennis

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post #4 of 28 (permalink) Old May 28th, 14, 10:34 AM
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Re: Brief Rough Idle After Start

Choke might be out of adjustment. It's a fine art to tune one that works good both cold and hot. Does it start and run good cold? What would you say the temperature of it is when you're having the restart problem?

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Dart 400-AFR 195-224/224 HR-Powerjection III TB with F.A.S.T. Sportsman XFI
TKO 600-Moser 3.42-Detroit Truetrac
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post #5 of 28 (permalink) Old May 28th, 14, 11:18 AM
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Re: Brief Rough Idle After Start

All good ideas to pin point your issue. The raw gas is a dead give away that someplace, you have an exchange of fuel to air. If you are sure it is not at the carb / intake then it is someplace for sure.

The Fuel tank gas cap as suggested is one good source for trouble, but I would inspect all your fuel lines fromthe tank top forward to the carb - rubber and steel for issues. The lines to and from the fuel pump may be under certain situations and loads could be sucking air in and it is causing momentary fuel bowl starvation on crank and idle just off starts.


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post #6 of 28 (permalink) Old May 28th, 14, 11:40 AM Thread Starter
Richard
 
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Re: Brief Rough Idle After Start

Thanks for your comments. Well, the "camaro hater" thing was a bit overstated to add a bit of levity to a frustrating situation, but there is certainly lots of eye rolling (like "this guy looks like a dope"), especially since the body is perfect (and black!) That said, there has never been any cheering once it started up . . . maybe just a tough crowd on those nights!

Follow up from above:

What is your starting routine?

I pump it once to the floor until I hear the choke plate close, then once again before I turn the key. Since the car is already sitting for quite some time when the problem happens (see below), I depress the pedal fully to close the choke. I will try starting without touching the throttle next time.

Also, the gas cap should be vented, right?


Does it start and run good cold? What would you say the temperature of it is when you're having the restart problem?

Now that you ask, I would say the the problem might exist even in certain cold start situations, but much more subtly. In fact, the last time the real problem happened, the next morning I started it cold and it ran very slightly rough for the first 30 seconds or so, then RPMs increased and idle smoothed out.

In the drive-in scenario, I drive it for about 20 minutes then park it for 3-4 hours with the hood open. I get to full operating temperature between 185-195. It's been actually relatively cool here in Texas in the evenings, around 80 degrees. I guess that the temp at restart is in the 140-160 range.

Regarding the choke out of adjustment:

I've checked the adjustment per the Demon instructions. What in particular about the choke would cause this behavior? Maybe the choke being closed again causes an overly rich situation? I'm particularly interested in details about the cold vs. hot tuning.


Can anyone provide any ideas on not necessarily the root cause of the problem, but possible conditions that might actually be occuring when it's startup-stumbling? It is flooded? Fuel-fouled plugs? Simply too rich? If I could start there and work backwards, then maybe I can determine exactly how I get to that condition.

Thanks for bearing with me on this. My first car was a '69 Firebird 400 with a 650 cfm Holley that I never touched after the day I installed it. The Demon is a different beast altogether, and has quickly made me feel like a newbie. Furthermore, it doesn't help that my other fast car is the complete polar opposite and under total electronic control (it even has fly-by-wire individual throttles for each cylinder).

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post #7 of 28 (permalink) Old May 28th, 14, 11:52 AM
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Re: Brief Rough Idle After Start

If your choke is operating at 70 deg or more air temp then I'm betting that's the problem... You shouldn't need the choke unless it's getting into the 50's or colder.

...Dennis

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post #8 of 28 (permalink) Old May 28th, 14, 12:35 PM Thread Starter
Richard
 
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Re: Brief Rough Idle After Start

Thanks for your patience, guys. It seems I don't fully understand the electric choke operating scenarios. I though it was supposed to close fully at any ambient air temperature upon opening the throttle. As ambient temps approach 70 degrees, should the choke plate not close as far?

I guess what I really need to know is how the electric choke should properly work (i) for ambient air temps below 70 as well as above 70 and (ii) when the engine is below operating temp as well as at operating temp. It's been in the 60s here at night and the 80s during the day, so I could easily test all scenarios.

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post #9 of 28 (permalink) Old May 28th, 14, 02:39 PM
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Steps
 
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Re: Brief Rough Idle After Start

I get the impression that it didnt do it and now it does ???
Yes could be choke and stuff as suggested above, and certainly sounds like flooding.

OK when it happens, put your foot SLOWLY down on the accelerator pedal, so as not to engage the accelerator pump on the carb.. HOLD your foot down turn the engine over till it fires up....what u are doing is 'pumping the excess fuel out of the engine till an ignitable mixture is present..

The problem ...I would be looking at your float levels and chances are in the float valves, where u adjust float levels u have a VERY small bit of crap jammed in there....
enough to allow any fuel expansion/pressure from heat in the lines to over fill the bowls, and flood the engine.
And when the engine is turned off for any longer length of time, this excess fuel get evaporated away.

The bit of crap will be so small that when u pull the valve out chances are it will be dropped unseen.

My Spelling is not incorrect...it is creative

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post #10 of 28 (permalink) Old May 28th, 14, 02:42 PM
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Re: Brief Rough Idle After Start

In very simple and partially incorrect terms, the choke adjustment basically adjusts the tension of the spring that holds the choke plate closed and also pulls the fast idle cam into position to hold the throttle open. Then as the electric element heats up, the spring looses tension and the venturi vacuum of the carb essentially pulls the choke plate open. Tapping the gas then lets the fast idle cam snap back so your throttle comes off it.

You can watch the choke plate with the air cleaner removed. With the engine cold, activate the throttle with your hand so the plate shuts. Poke it with your finger and you'll feel the tension from the spring. If it feels real loose, you may need to adjust the choke spring more towards the "+" side. If your car doesn't want to run right up to fast idle in the morning when you start it, I'd try going towards "+" first. This puts more tension on the spring so the venturi vacuum can't pull it open so easy, and it has to heat up longer to loose tension.

'69 Camaro
Dart 400-AFR 195-224/224 HR-Powerjection III TB with F.A.S.T. Sportsman XFI
TKO 600-Moser 3.42-Detroit Truetrac
500hp/538lbft

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post #11 of 28 (permalink) Old May 28th, 14, 03:12 PM
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Re: Brief Rough Idle After Start

Quote:
Originally Posted by rak530 View Post
I recently posted an issue with the fuel gauge on my 1969 Camaro and got the problem solved very quickly by a couple of great board members. Thanks Vette-Freak and 69-Pace! Iím back again with another puzzleróat least for meóthat Iíve been battling for a couple of months now. As usual, thanks in advance for your thoughts.

The problem:
After I drive the car normally and then let it sit for a few hours, I can start the engine but then really struggle to keep it running. For about 20 seconds, it stumbles like itís running on only half of the cylinders and then smooths out and is fine. Of course, this only happens when everyoneís watching. Itís a real crowd pleaser among anti-Camaro crowd at the local drive-in on Saturday night. Supremely embarrassing.

In many other situations, especially ones that donít involve Camaro haters, like in the morning after sitting in the garage, after stopping at the drug store for a few minutes, etc., the car starts and runs fine.

Also, there is a raw fuel smell very often when driving. Prior to the installation of the phenolic spacer (see below), I noticed the fuel boiling off in the primary and secondary bowls.


The general setup:
  • 1969 Camaro purchased about 4 months ago (Weíre still in the wooing phase and getting to know each other.)
  • 427cid, stock heads, cam
  • Road Demon 750 (vacuum secondary), electric choke
  • Stock cowl induction air cleaner assembly
  • HEI ignition
  • Mechanical fuel pump, no return line
  • 92 octane Chevron with ďup to 10% ethanolĒ (With my luck, itís right at 10%.)
What Iíve done so far:
  • Double-, triple-, quadruple-checked for fuel leaks; none detected
  • Checked that choke linkage was not obstructed by air cleaner assembly
  • Inspected distributor cap, spark plug wires; look new
  • Installed new spark plugs
  • I thoroughly disassembled, cleaned and rebuilt the carburetor with the usual new bits
  • Bowl floats set so fuel is near the bottom of the sight glasses
  • Installed ľĒ phenolic plastic spacer between carb & intake
  • Measured spark timing, 12 (initial), 38 (all-in)
  • Set idle mix screws as per Demon, then adjusted for maximum manifold vacuum at a constant rpm
  • Set primary fast idle when warm to approx. 850 rpm
What I HAVE NOT done:
  • Replaced plug wires, distributor cap or coilóbecause itís all looks fairly new, but I donít know for sure.
  • Measured fuel pressureóbecause I wouldnít expect pressure above 7 psi from this mech. pump, but I donít know for sure.
  • Constantly monitored for fuel boiling in the bowl after each drive--because I think it's not boiling off now and I don't know where to go from here regarding heat soak
My gut feeling is that the problem is carb-related. Unfortunately, I donít have another carb laying around, and canít justify (yet) purchasing another one simply for testing. If this persists, I might just break down and get one. (Any suggestions which one?)

Also, I noticed that there was still pressure in the fuel bowls after the car sat for a while and when I loosened the float set screws and fuel spurted out. This is normal, right?

The main question:
What might be happening immediately after starting after sitting for a few hours? Do fouled plugs cause the rough running? If so, do they stay fouled? How do I check for this condition prior to starting?

I really donít want to have to start parking in the dark back corner of the drive-in parking lot so no one notices me. Thanks for your input.
Richard,
Sounds like you may have a problem compounded by the phenolic spacer and the possible use of a Holley gasket. Demon carbs will not function properly using Holley intake gaskets because they inadvertently cover the 4 hole idle circuit and cause stumbling. Check to make sure this is not happening!

Karl / VetteFreak
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post #12 of 28 (permalink) Old May 28th, 14, 03:18 PM Thread Starter
Richard
 
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Re: Brief Rough Idle After Start

This is all great stuff. Thanks again. Tonight I'll do the following per all of your responses above and report back. (I know some of these are unrelated; all worth checking.)
  • check fuel lines thoroughly front to back
  • check gas cap (again, it should be vented, right?)
  • pull needle/seat valves and reinstall
  • observe the fuel levels in P and S bowls
  • observe the choke operation when cold and hot
  • drive the car a bit, park it, check for percolation in fuel bowls at various times afterwards
  • A few hours later, I'll restart it and see what happens

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post #13 of 28 (permalink) Old May 28th, 14, 03:25 PM Thread Starter
Richard
 
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Re: Brief Rough Idle After Start

. . . and I'll check that the spacer and/or gasket aren't covering the idle transfer holes. Thanks for your additional input, Karl!

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post #14 of 28 (permalink) Old May 28th, 14, 05:46 PM
DjD
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Re: Brief Rough Idle After Start

Quote:
Originally Posted by VetteFreak View Post
Demon carbs will not function properly using Holley intake gaskets because they inadvertently cover the 4 hole idle circuit and cause stumbling.
Is that part of the "idle-ease" where you have an adjustment under the air cleaner stud?

...Dennis

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post #15 of 28 (permalink) Old May 28th, 14, 07:56 PM
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Re: Brief Rough Idle After Start

Quote:
Originally Posted by DjD View Post
Is that part of the "idle-ease" where you have an adjustment under the air cleaner stud?
Dennis...I was referring to possible unintended interference to the 4 corner idle feed restrictors in the primary and secondary metering blocks using the phenolic spacer. The restrictors control the amount of fuel that enters the idle circuit from the main well. The orifice controls the amount of fuel available for the idle circuit. Since these are factory drilled for the engine performance desired and you change any parameter which could result in a lean or rich idle circuit, an adjustment here may be necessary. Therefore, if you close off the blown idle circuit that diverts air directly under the mixture screws by using an incorrect gasket, you're changing the mixture intended for proper burn at idle. Might be a wild goose chase, but, he should check to make sure that he is using the proper Demon gasket and that the phenolic spacer is not inhibiting the butterflies. Just another suggestion if the problem persists.

Karl / VetteFreak
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