stumble off idle - Team Camaro Tech
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post #1 of 17 (permalink) Old Apr 9th, 08, 02:28 PM Thread Starter
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stumble off idle

after engine is warmed up and i blip the throttle from idle i get a stumble.
everything is new so its gonna be a procces of elimination.
carb is set and i get adequate pump shot once throttle is moved just a slight amount.
im thinking i need to add more timing
i got a MSD digital E curve distributor
10 initial degrees all in at around 2700 to 3000.
total timing 29 degrees
no vacum advance hooked up
now I think the smallblock likes a bit more total around 32 to 34 degrees.
what are others running for total timing on a moderate cam?


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post #2 of 17 (permalink) Old Apr 9th, 08, 02:48 PM
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Re: stumble off idle

No matter what others are running, you don't have enough. I'd bump to 14 and go up from there.

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post #3 of 17 (permalink) Old Apr 9th, 08, 05:39 PM Thread Starter
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Re: stumble off idle

ok ill shoot for 34 total


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post #4 of 17 (permalink) Old Apr 10th, 08, 05:16 AM Thread Starter
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Re: stumble off idle

bump


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post #5 of 17 (permalink) Old Apr 10th, 08, 06:30 AM
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Re: stumble off idle

Well, total timing of 29 (or 34) is not gonna affect the stumble you mention. Base timing of 10* should not produce a stumble.

Not sure where you have idle speed set at, but if you have a high-performance cam and low idle speed and you blip the throttle, you'll probably get a stumble.

How much vacuum do you have at idle?

I would look carefully at basic items, like tune-up. Be sure plugs, wires, cap, rotor, fuel filter, etc are all new (or like new). Be sure valve adjustment (solid or hyd, u don't mention) is spot-on.

Also check all vacuum lines and fuel lines. Be sure fuel pump is making adequate pressure.

Many carbs have adjustable pump shot, maybe try adding a little to the squirt.

Is it making good power? Timing chain is installed correctly?

good luck

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post #6 of 17 (permalink) Old Apr 10th, 08, 07:46 AM Thread Starter
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Re: stumble off idle

im getting 16" at idle
750 rpm @idle
engine ran great on stand but used a different distributor
everything as far as carb-ignition is new
pump shot is set so that any slight movement ill get pump shot.
maybe thinking of increasing shooter size?


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post #7 of 17 (permalink) Old Apr 10th, 08, 09:03 AM
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Re: stumble off idle

16" at idle is pretty dang good - plenty.

That dizz offers lots of options, pretty easy to change around. I'd try using vacuum advance curve #1 or #2 with manifold vacuum source (vacuum at idle). You'll probably need to reduce idle speed screw a little. MSD says to use ported vacuum source (above carb plate), so perhaps try that if manifold vacuum doesn't work out.

These types of stumbles are usually a result of insufficient fuel shot, assuming all other tune items are in good shape. With a custom engine, it's usually a trial and error process to adjust the pump shot to get it perfect. At least, it always has been for me.......

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post #8 of 17 (permalink) Old Apr 10th, 08, 10:20 AM
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Re: stumble off idle

I once asked Earl Parker of Parker Carburetion about adjusting the idle mixture and timing and he sent me the following:

One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.
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post #9 of 17 (permalink) Old Apr 10th, 08, 11:48 AM Thread Starter
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Re: stumble off idle

wow nice write up.
just the info I needed thks!


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post #10 of 17 (permalink) Old Apr 12th, 08, 09:17 AM
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Re: stumble off idle

You should run at least 14 initial. I run 20 initial on an 11:1 400. Total should be around 34o to 36o. The sooner you can get the timing in the better. 2,500 is good as long as there is no detonation present. If so delay to 3,000.

I like the green pump cams. They give a lot of initial pump shot. Position 2 will give more initial than 1 on it. I always use a big squirter. Right now I am at 35s. You car might like 31 or 33. It is a matter of trial and error. A bigger squirter increases initial shot and decreases the time for full shot to release. A smaller squirter does the opposite.

Make sure the accel. pump arm is adjusted right. I bend the arm each time I put a different pump cam in. There is a big difference between them. You want about 1/8 to 3/16" of thread above the accel arm from the squirter arm screw. This allows the accel pump to compress okay and still have a little play. Bend the accel arm to get this. Don't crunch the spring down all the way to make up the clearance.
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post #11 of 17 (permalink) Old Apr 12th, 08, 02:36 PM Thread Starter
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Re: stumble off idle

reset timing and readjusted pump shot
all is well now


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post #12 of 17 (permalink) Old Apr 12th, 08, 04:43 PM
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Re: stumble off idle

Quote:
Originally Posted by lluciano77 View Post
You should run at least 14 initial. I run 20 initial on an 11:1 400. Total should be around 34o to 36o. The sooner you can get the timing in the better. 2,500 is good as long as there is no detonation present. If so delay to 3,000.
When you say 14 initial, is that with vac advance connected or disconnected?

alan
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post #13 of 17 (permalink) Old Apr 12th, 08, 07:24 PM
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Re: stumble off idle

Quote:
Originally Posted by alanrw View Post
When you say 14 initial, is that with vac advance connected or disconnected?

alan
Advance figures are almost always given with vac. advance disconnected. You will have no vacuum advance at WOT. Other than that the vacuum advance is too 3D to just give a what if number.
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post #14 of 17 (permalink) Old Apr 12th, 08, 07:58 PM
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Re: stumble off idle

My stumble disappeared with a $15 dollar carb spacer with 4 holes from A-zone.

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post #15 of 17 (permalink) Old Apr 12th, 08, 08:00 PM
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Re: stumble off idle

Quote:
Originally Posted by wiskeesour View Post
My stumble disappeared with a $15 dollar carb spacer with 4 holes from A-zone.

I would consider that dumb luck more than anything.

Most Holleys I have seen are not set up right for the street. Street cars like a lot of initial accel pump. You want to set the accel arm for no clearance meaning instant shot. The green pump cam and a .031" to .033" squirter should get rid of most bogs just off idle. It isn't the best thing to mask you transition, but it will work. If you want to fix the carb the right way get a wideband O2 sensor, a drill index, a calculator, and some extra gaskets.
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