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Old 06-27-2009, 11:51 AM
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What can cause a surge/low speed miss?

My sbc 400 has a low speed miss/surging under 2000 rpm. You can feel it driving down the road at a steady speed or when driving slow thru town. It's not there when you mash the gas down.
I'm just wondering where to start. What can cause a surge? Is it always a lean condition and not rich? Can the timing cause it to surge or is it a air/fuel problem? What about the vacuum advance?
I've checked the float level in my carb and checked for vacuum leaks so far. I haven't found anything that looks suspicious.

Thanks for your time.
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Old 06-27-2009, 12:17 PM
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Quote:
Originally Posted by 4speed57
What can cause a surge?
The FIRST thing I'd check is timing. Over advanced timing will do as you've described.

Easiest way to tell is (w/ or w/o timing light- just to test) to simply turn the distributor CW a small amount and test drive it. Might take two times unless you turn it a fair amount- or use a timing light- before you retard it enough to get the surge to go away.
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Old 06-27-2009, 12:56 PM
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Bad plug wire can cause the miss. Two tests: 1. Verify all plug wires have similar resitance while wiggling the wires. 2. look at all plug wires at night to see if any sparks are visible.

The miss may occur at higher RPM but you can't feel it.

Fouled plug. Pull all plugs. They should all be white or have only a slight brown/gray color.

Yes, a lean carb can cause the miss. For now I would tune cruise to produce plug with slight brown/gray color. Later I would lean cruise until plug is white to improve MPG.

You should tune the carb to produce following plug colors:

Idle=white(verify idle is smooth)
cruise=white (verify good driveability)
2bbl acceleration=slight brown/grey
4bbl aceleration=slight brown/grey

Verify timing changes as you disconnect/connect vac adv. Blow into vac adv to test for busted vac adv diaphram.

Does it miss when engine is hot and cold?

Last edited by 001mustang; 06-27-2009 at 01:08 PM.
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Old 06-28-2009, 01:20 AM
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Thanks for the replies and help. I now have an idea of where to start. I'm going to check my timing and vacuum advance tomorrow.

The engine miss/surge is present went the engine is cold and hot. I checked all of my plugs and they have white tips. They are firing good, I may need to go a size colder than what I have.

I changed the weights and springs in my distributor so my timing would come in quicker. Could this be the culprit? Maybe I slowed down the timing instead of speeding it up? Could the wrong weights and springs cause this?

Thanks again for your help and time,

Joe
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Old 06-28-2009, 10:33 AM
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Two issues with the vacuum advance can cause a pesky serge condition at cruise.

1. Intermident distributor megnetic pick up coil wires. These little wires that connect the mag pick coil to the module move with the rotation of the pick up coil assembly as the vacuum advance comes in and out. Often are intermident creating miss fire with a vac advance connected that disappears if vac advance is disabled.

2. Rotor/ cap "phasing" missaligned distributor cap/rotor/ distributor mag pick up coil orientation- alignment causeing the rotor and distributor cap posts to be excessively out of alignment when the plug fires.
The action of the vacuum advance on the magnetinc pickup assembly effects the rotor/cap phasing as vac advance goes in and out.
If the alignment phasing of these moving parts are too far off, the distributor will "cross fire" at full vac advance.
Inaccurate aftermarket advance curves kits can aggravate this .
Check your distributor rotor/cap phase alignment with a old "Windowed" distributor cap and a timing light while the engine is running. and the advance systems working.

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Old 06-28-2009, 01:49 PM
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Hold on.

You may have a lean condition.

I would richen your carb cruise mode to produce slightly off white plugs for now.
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Old 06-28-2009, 10:10 PM
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Quote:
Originally Posted by 001mustang
Hold on.

You may have a lean condition.

I would richen your carb cruise mode to produce slightly off white plugs for now.
I've read your repeated recommendations to "run the plugs white".

Do you know how to read plugs? There is a hell of a lot more to it than the color of the porcelain, for cripes sake. And that's NOT what indicates fuel/air mixture ratio, anyway.

How- with your recommendation to run them white- is a newbie supposed to know that he's reached terminal leanness, and the next thing he hears is the silence where his engine once was.

I hope no one actually tries to tune an engine like this.
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Old 06-28-2009, 11:35 PM
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Quote:
Originally Posted by cobalt327
I've read your repeated recommendations to "run the plugs white".

Do you know how to read plugs? There is a hell of a lot more to it than the color of the porcelain, for cripes sake. And that's NOT what indicates fuel/air mixture ratio, anyway.

How- with your recommendation to run them white- is a newbie supposed to know that he's reached terminal leanness, and the next thing he hears is the silence where his engine once was.

I hope no one actually tries to tune an engine like this.
Well, anyway here is how I set the plugs:

WOT : Plugs are slightly gray. If plugs are white post WOT then engine damage can result. As a minimum you should at least verify plugs are not white post WOT; if you plan on keeping your engine for any time.

2BBL acceleration: Plugs are gray. If plugs are white post 2BBL acceleration then engine damage can occur.

Cruise: Plugs are white. First I find jets to produce gray plugs then decrease jets till plugs are white. If any cruise drive ability issues then I go up a jet size. Engine damage is not an issue by running slightly leaner during cruise.

Idle: Plugs are white. I tune idle by adjusting mix screw lean till engine starts to idle rough then I richen up idle just enough to smooth out idle. The vac and RPM techniques (which I used to use) are not reliable for my engine. Engine damage is not an issue for a leaner idle.

Not a perfect technique but at least it will get you in the ball park by reading plugs by the naked eye. You can actually tune a car pretty accurately by reading the plugs with a plug reader using the proper technique.
I know countless people just installing new carbs without checking the jets at all. I have seen too many cases where the idle is tuned so rich the plugs are black and on the verge of fouling. My edelbrock 1405 came from the factory 4 stages lean for 2BBL acceleration, 3 stages lean for WOT, Cruise was OK. If I ran that carb without checking jets I could have damaged my engine. Now my engine is 10 years old and runs perfect.

If you don't believe in plug readings then you may like tuning WOT using a G-Force meter; but that won't help the problem addressed in this post.

By the way plug color is an indicator of air fuel ratio and if your plugs are white post a WOT run then you probably have reached terminal leanness.

Last edited by 001mustang; 06-29-2009 at 12:19 AM.
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Old 06-29-2009, 12:00 AM
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Thanks for the replies and help everyone. I was running a set of Autolite 24's and they kept fouling out. I went with a set of Bosch plugs that are equivalent to a set of Autolite 26's, which are two sizes hotter. They aren't fouled at all now. Maybe I should try the equivalent of a 25 and see if my plugs go back to a light brown/tan color.

My distributor is a HEI and I'm sure it's probably worn. I could have some wire movement issues like F-BIRD'88 spoke of. I may throw another distributor into and see what happens. i think I may have another laying around. If it did improve with the distributor swap I would then buy a new one.

Thanks again for all of your help everyone.
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