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hiebertm

how jumpy should timing marks be?

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I'm trying to sort out a plug carbon-fouling issue on my 72tii. Searching through the archives, it looks like thedebugging order is valves, timing, then fuel delivery... I've done my first valve adjustment recently, and I figured I'd make sure timing is right next(never played w/ a timing light before this or w/ the ignition on my 02 in general). Has a stock mech-adv dizzy and Petronix.

I've found that damn ball, and w/ the advance on my light I've roughly observed the advance curve from idle through 3500rpm.

My concern is, when I'm looking at any timing mark, I see the mark quickly jumping about +/- 3 degrees (tach/engine speed is also jumpy). I'm guessing timing marks should be rock solid under my Craftsman timing light, and +/-3 degrees means that something is wonky with the spark delivery through the Petronix or coil, right?

I checked the connections to the coil (cleaned wire connectors and re-seated the wires, re-seated the connector from coil to distributor, and I didn't feel any unreasonable play in the distributor shaft). I replaced the plug on cylinder 1 (which is where my light is triggered) 'cause I figured that the carbon fouling could cause erratic spark delivery. None of this yielded any improvement in erratic timing marks. Are there petronix problems that can cause this type of erratic timing issue? Or should I be focusing on the coil(i.e. replace the coil)?

Mark

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The erratic engine RPM is showing as the jitter in your timing mark. Its telling you that there is a misfire. The pertronix and the coil are probably doing the best they can given the state of the engine. Diagnose the misfire and run a fresh tank of fuel. When the misfire disappears the timing should steady out.

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Peter is right on.

You could check to see how worn the distributer is. I don't know how a worn distributer shaft and bushings affects the Pertronics but I do know they don't affect the opticle Crane units. Reading what I have about people that have gone to EDIS with a Megasquirt always comment on how rock solid the timing is.

I can't wait to get my motor running someday....

John

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OK,

I'm trying to understand how misfiring can cause erratic timing marks. Since my RPMs are varying, I guess that the mechanical advance will be varying and this will make timing jump around. Is that what's happening here?

If it wasn't for advance action in the distributor changing at different RPMs, would a good coil and distributor produce spark at the same time of the engine cycle regardless of whether there was misfires?

Mark

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The erratic engine RPM is showing as the jitter in your timing mark.

Respectfully, I'd have to disagree. Misfire for me (for electrical reasons)

shows up as a stutter in the strobing of the timing light. It's quite obvious

when you clamp the coil wire and shoot a white wall- you can see it better

than you can hear it. If you see stutter in the plug wire but not coil wire,

that helps to narrow it down to the cap, rotor, wires or plug...

+- 3 degrees sounds like a worn distributor. The street car with an old dizzy

looks about like that. Wear in the shaft is the usual culprit, but

vertical play can do it too (helical gears) or a sloppy mechanical advance

in the base. The race car started off rock solid 5 years ago, and as things

have worn, it now has a little variation. Time for Megajolt!

My take on it,

t

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Mark: you're overthinking it....really. Don't let the words get in the way.

The small perturbations (ugh) in the combustion process cause the same spark, sent at the plug the same time each pulse to ignite sooner or later (very small time differences). If the motor has to wait for a cylinder to fire even a minute amount, the RPM drops momentarily. This brings the timing mark into the beam of bright light slightly later. The blinking light is on time because it is triggered by the points or its modern day replacement (pertronix, optisensor, hall effect device, etc. ), but the flywheel mark stumbles and arrives late. When the lamp is on, it illuminates the mark but the flywheel mark is in a different position. The jitter in the 'apparent' position of the timing ball is a clear index of (or lack of) ignition at the proper time. That's called a misfire.

Review C.D.'s prescription for baseline tuning of your engine for clarification on the elements of 'the misfire.' It can be ignition based but somehow I think yours is 'aspiration based.' You know like one of your injectors if dirty, or something like that. Or. what he said, your dizzy is getting worn. Something like that.

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Dont forget your dizzy is driven off the cam which is chain driven off the crank. Worn chain & gears will all affect a rock solid timing mark. Your car is a 72 - ever rebuilt the motor? replaced the chain, cam, dizzy? These are all reasons why your mark is not steady. I also seriously doubt that a brand new engine would be rock steady. Just accept it.

Its time to look elsewhere for your plug fouling - tried a hotter plug just in that one cylinder? or is it multiple cylinders?

Beaner7102

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So I've swapped out the plugs, and played w/ timing until I get a smoother engine speed and my timing marks are a lot less jumpy under those conditions... so my newbie confusion on jumpy timing marks is probably a waste of time, and I need to look elsewhere to sort out my mixture/misfire/run-on issues. Thanks for the timing education everyone.

Yeah, wrt plug fouling, its all cylinders. I've just moved to the NGK BP5ES plugs this morning, 1 code hotter than the last set.

Plus this morning, I notice that I have a leaky connection in my fuel supply line so now I have to tackle that before anything else. I replaced the electronic fuel pump cause the old one died, and suddenly I must be getting more pressure in my lines. The tii gods are testing me.

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