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I have had so many anomalies with tuning, I am getting frustrated. Here is my set-up. 351c, 2000ish stall, edelbrock performer intake, carter 625 carb, msd pro-billet dist, 270H comp cam, roller rockers.
I have the timing set so it runs good, idles at about 800 rpm in park, and drops to around 600 in gear. At a stop, the idle lopes and it surges up and down.
Is that normal? It doesnt do it all the time, its wierd.
Plus the motor seems to me it should run harder. It has good power, But it should have alot more. I wonder if my carb, intake and cam are holding it back.
What should I try??? FEEX sent me a holley 750 to try, I will be trying that out soon.
Thanks......Kevin
 

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You need more carb in my opinion. That 750 should make that motor come to life, vacuum secondaries would be nice.
 

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I got that same surge at idle due to a faulty PCV valve. I'm not sure if you have one or not, but if you do when the engine is surging, squeeze the PCV hose with some pliers to shut it off and see if that effects the surge.

Phil
 

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1. initial timing too high

2. carburetor idle screw so far out that ported vacuum attached to vacuum advance alters timing....any slight fluctuation of idle vacuum will create a larger oscillation of engine speed.

3. lean idle mix and a power enrichment setup that occasionally opens up at idle.

4. Any combination of these....many other factors influence these.....


I just ridded myself of slight idle surge by adjusting the holley 600 on my cleveland so theres no ported vacuum (I drilled a .050' hole in each primary throttle blade).....it was aggravated by my initial timing of 18BTDC (open chambered heads and a decent sized cam).
 

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A vacuum leak can sometimes cause the symptoms you describe. The 625 cfm carb should be more than adequate for your engine unless you routinely run over 6000 RPM.
 

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I agree with DailyDriver on both counts. 600-625 cfm should be perfectly adequate for a 351 up to 6000 rpm. In any case, the max flow rate should not affect the idle at all, IMO.

Like DailyDriver said, vacumn leaks can definitely cause surging.

Also - You didn't say in your post, but if this only happens on warm days at operating temperature (for the engine), fuel percolation could be the culprit. Do a search on "percolation" and you'll find plenty has been written on that topic here.

A phenolic carb spacer is often all that is needed to fix it, if percolation is the problem.
 
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