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Phantom RPM Fluctuation

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nastyf150
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Phantom RPM Fluctuation

Post #1 by nastyf150 » Mon Jan 18, 2010 5:12 pm

I took this video to give you some idea of the rpm fluctuation. What does this come from? Internals in the Edelbrock 500cfm? Intake leak, timing springs? I am at a loss.

I would be so pleased if I could get this sorted.

http://www.youtube.com/watch?v=28vUw0JohPs
1985 1/2 Ton Short Fleetside-300ci, 272/284Crane Cam, 500 CFM Edelbrock, C-class Offy, Headers, 360ci pistons, 1.6ex/1.9in Chebby stainless, DUI/HEI Dizzy, P/P head, Mazdog 5 speed
2002 Windstar LX-to haul the SEVEN (but preggers again) NEED 15passenger F350 STAT!

mutt
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Re: Phantom RPM Fluctuation

Post #2 by mutt » Mon Jan 18, 2010 5:24 pm

vac leak. sporadic misfire....can you gt it to idle down lower, say 750, then adj. the idle mix?

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nastyf150
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Re: Phantom RPM Fluctuation

Post #3 by nastyf150 » Mon Jan 18, 2010 5:30 pm

it is already idling at about 625 now. Look at the tach. It is just above the 500 mark and then just starts to claim on its on and settles back down to 625-650.

I don't know how a misfire would cause the rpms to rise. It seems I am getting leaned out but it is so periodic.
1985 1/2 Ton Short Fleetside-300ci, 272/284Crane Cam, 500 CFM Edelbrock, C-class Offy, Headers, 360ci pistons, 1.6ex/1.9in Chebby stainless, DUI/HEI Dizzy, P/P head, Mazdog 5 speed
2002 Windstar LX-to haul the SEVEN (but preggers again) NEED 15passenger F350 STAT!

mutt
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Re: Phantom RPM Fluctuation

Post #4 by mutt » Mon Jan 18, 2010 7:56 pm

my bad eyes....looked like it was idling above 1k.
does it resond to the idle mix screws smartly? I usually slowly tun them in on a THOROLY warmed up engine, til the engine falters, then back them out 1/4 turn.
a LOT of things can cause this. is your vcuum dizzy port hooked to tied or manifold vacuum, one.
is the cetrifugal advance sticky, or does it turn freely & snap back. Are the plug wires ancient? the plugs?
Give te manifold bolts a snug? Is it a rats nest of vacuum tubing, with a long chai of "emmissions" vcuum acessories?

The Plankster Prankster
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Re: Phantom RPM Fluctuation

Post #5 by The Plankster Prankster » Tue Jan 19, 2010 12:14 am

nastyf150 wrote:I don't know how a misfire would cause the rpms to rise. It seems I am getting leaned out but it is so periodic.

the only way a misfire would cause it to rise is if you set the idle speed with a cylinder not firing, then it does fire sometimes. but if it started doing this without recent work or tuning, i would rule out this option.

Mutt, your spelling is so good in that post that i'm wondering how many drinks you've had...does 5 sound about right? :beer:
83 F250 flatbed 300-6, NP435, 4X4, 5800lbs empty weight
87 E-350 6.9 diesel, c6, 3.55s, powertrax no-slip locker, onboard 120v power and compressed air, built out with toolboxes and toys

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Buddy Rawls
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Re: Phantom RPM Fluctuation

Post #6 by Buddy Rawls » Tue Jan 19, 2010 8:57 am

For a tach to be that reactive, I would contend that the source is in the ignition system. If this were a point style ignition, the tach response appears like burned, and to a lesser extent too narrow of gap, or possibly a sloppy distributor. I might would experiment with another distributor, if its convenient.
It doesn't run quick at the track, but it gets long stares wherever it goes.
Never in a million years did I think I would have an antique hot rod truck, much less with its own name emblazoned on the front fender and a freakin' six cylinder for power;
but it is a Ford, it is old, and it is definitely one of the funnest vehicles I have driven.

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nastyf150
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Re: Phantom RPM Fluctuation

Post #7 by nastyf150 » Tue Jan 19, 2010 1:33 pm

Buddy, I am supposed to have the industry leader in Dizzies, DUI/HEI. Not only that, I just had it re-maned at DUI in Memphis. I have it sorted I think.

You were right to think it was ignition oriented, though. My timing was too tight and it was causing the very lightly springed counterweights to engage a bit.

I think it is doing fine now with 12 degrees base. Still running like a spotted ape.
1985 1/2 Ton Short Fleetside-300ci, 272/284Crane Cam, 500 CFM Edelbrock, C-class Offy, Headers, 360ci pistons, 1.6ex/1.9in Chebby stainless, DUI/HEI Dizzy, P/P head, Mazdog 5 speed
2002 Windstar LX-to haul the SEVEN (but preggers again) NEED 15passenger F350 STAT!

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Re: Phantom RPM Fluctuation

Post #8 by mutt » Tue Jan 19, 2010 3:48 pm

no, no cocktail enhanced spelling prowess. REAL crappy keyboard- keeps dropping letters, and I didnt do my usual word by word check.
plus, even If i spell them RIGHT to my mind, I spell them wrong....
so- your centrifugal advance was binding??

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Re: Phantom RPM Fluctuation

Post #9 by The Plankster Prankster » Wed Jan 20, 2010 2:26 am

no worries mutt, i just know that when i drink and post, it comes out about as clear as your post did. but i've gotten in a little trouble for posting in that condition, so i try to take it easy. opening number 2 now.
83 F250 flatbed 300-6, NP435, 4X4, 5800lbs empty weight
87 E-350 6.9 diesel, c6, 3.55s, powertrax no-slip locker, onboard 120v power and compressed air, built out with toolboxes and toys

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