surging

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1973dusterkid

1973dusterkid

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I was setting at a red light and the car started surging from 1000 to 1200 and back did this over and over but slowly what is causing this.

750 holley vac sec ele choke
 
Surging is often a symptom of a lean condition. Read your plugs & let us know what you find.
 
What's your normal idle rpm? I'd check everything associated with vacuum external of the carb 1st. Power or standard brakes?
 
Has it ever backfired throught the carb? A Blown out power valve can cause the same symptom. Holleys are notorious for it. (even the so-called new improved design ones do it too.) Also check your ignition timing & make sure the mechanical advance isn't sticking.
 
Read the plugs there good all the carb bolts are good no leaks that i can find.I changed the power valve and jets out 3 months ago and it ran fine now starting to do this.
 
Agree with all above. Idle speed is controlled by air flow. If your throttle plate holds position (most likely) a varying vacuum leak could make it surge. As mentioned, a lean idle makes it erratic. Also, the spark controls can cause a "positive feedback" situation where as the engine speeds up, it gets more advance which makes it go faster, ... I think weak springs on the weights can cause this. Also, since you are idling at 1000 rpm, I suspect you have trouble idling - like a big cam and big carb. That could make it run better faster, causing the same positive feedback condition.
 
What's your normal idle rpm?
If at 1000 maybe back off to 800 just to see if the surge quits or reduces frequency.
Reset the float level front AND rear.
Retest vacuum at manifold and check power valve specs for that figure.

[ame]http://www.holley.com/data/TechService/Technical/power_valves.pdf[/ame]

And Bill has a good point. Check to make sure you didn't loose an advance spring or something came loose.
 
Are the idle mixture screws responding to adjustment? If not,pull the air /fuel screws out.You don't have to pull the metering block to do this. Get yourself a rubber tipped blow gun,hooked to a air compressor. Shove it in the adjustment passage each side,hit that port with compressed air a couple of times.Reassemble & adjust accordingly. Sounds like you got some crud in the idle circuit,or some water in the gas.Did this just start happening on 1 tank of gas? Did you change where you get fuel?.
 
Could be The advance springs in the distributor are to light and the timing is moving. It could also be the timing chain loose. Just something to check. Its easy to check the chain. Take the dist.cap off and turn the crankshaft back and forth. you will see how far the crank moves before moving the Dist. It should be zero.
 
The idle screws respond i have not filled up on gas so it could be bad gas.as far as the timing chain i will have to check
 
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