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I am restoring a 66 coupe, 289 4bbl, c4 tranny. I have an annoying rough idle. I recently replaced the head and intake manifold gaskets, replaced the old autolite carb with a holley 600cfm, replaced all wires, plugs, and distributer. The timing is right on, point and plug gap checks out. Compression is right around 150 all around. I've checked for vacuum leaks everywhere! Even at the modulator. Vacuum reading pulses from about 7" to 25" below 800 rpm, after that it settles out to 25" What do you guys think it is? Can a leaking modulator cause the problem? Any suggestions would be helpful. Also I have the following things left to do: (1) Bodywork/paint, (2)interior, (3)alignment and rear springs. Which order do you think is best?
 

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I'd go alignment, bodywork, then interior.

As far as your problem goes, what's your idle speed? If its low, it'll be rough. What is the cam grind like? Lots of intake lift will result in a rough idle. Also, how's the PCV?
 

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25" sounds wonky...check vacuum guage with known vacuum source...

Disconnect and plug all manifold vacuum ports, including PCV...test engine.

Check for vacuum leaks at carb base and carb spacer (if applicable) in addition to manifold ports.

Intake manifold make and model please? Carb spacer? PCV connection arrangement?

With engine idling, rap throttle open then release quickly...noting how fast engine returns to idle...post your impressions.

What rpm is engine currently idling at?

Can you kill the engine by turning the carb mixture screws CW? How many turns in is the idle speed screw from where the throttle plate is bottomed? Are all carb vacuum ports plugged?

Describe spark plug readings after comparing to this chart

How is engine performance otherwise (at part and WOT)?

Get back to us...
 
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Thanks for the suggestion. I have the idle set at 500 rpm, but honestly it does seem low. I briefly set the idle at 700 (closer to that of a manual transmission), and it ran much nicer. Who knows maybe my tach is BD.
 
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- Wonky? Do you mean too high? I'll check a known source and get back to you.

- Disconnect and plug... I'll do it tomorrow

- Check for vacuum leaks... I did it by spraying carb cleaner around the mating surfaces - is there a better way?

- Intake manifold is the original stock 289 straight from the factory

- When the throttle is rapped open the engine sometimes coughs (threatens to backfire) then catches strongly. The engine returns to normal fairly quickly.

- Current rpm 500 rpm

- I can kill the engine with CW turns of the idle speed screw. At present they are turned out 1 1/2 turns, all ports are plugged

- Spark plugs are new and look good

- Engine runs great at WOT no misses, skipping, or backfires. What is "Part"?

You don't think much of my modulator theory eh? By the way the Holley is a remanufactured carb.

Thanks buddy... I'll get back to you on the other items.
 

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Thanks for the update...

Part means part throttle...sometimes my brain gets ahead of my fingers...*G*

Comparing engine operation between the two helps me identify carb and ignition problems and isolate them...
The coughing might be not enough accelerator pump squirt...check pump lever clearance at full open; should be about .015" between pump arm and actuator lever when pump arm is fully depressed at WOT...

Make WOT run, return with minimal idling, read plugs...we're trying to isolate the problem to fuel or ignition...If engine runs strong under load at WOT, I would discount ignition. Skip this step if you've already done it...

A VMF'er came up with a better, less dangerous way of checking for vacuum leaks and for the life of me, I can't remember it...I do it the same way you do..*G*

Was the modulator theory meaning vacuum leak from modulator? I can't recall...long day..*G*

If so, you would find oil in modulator vacuum line or the line would be cracked or otherwise damaged. I'll assume you plugged line at manifold for tests.

Do Holleys fit OK on stock Ford manifolds without a spacer (you didn't note one)?

Oh, yeah, wonky means weird...25" is huge vacuum...likely the needle isn't damped and you're getting pulsation rebounds...

It's possible that there's a blockage in the carb's idle circuit...did you squirt carb cleaner down the air bleeds? Also, if you turn mixture screws CCW, what happens?

Can't remember camshaft...did I work this problem earlier in the week with you or was it somebody else? *G* Sorry 'bout that.

Hey, have a good weekend...hope it's cooler where you are!
 
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