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spark scatter

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jvandyke

jvandyke

Luke Skywalker
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Whenever checking timing with my lights, I notice quite a bit of "scatter" probably a good 5 degrees of "float" according to the light. Is this known as spark scatter and a symptom of worn distributor bearings? Is there a way to check your dizzy for this? I looked at Vizard a bit and he has an inset where they machine a distributor to accept a roller bearing. I don't really know if it's effecting performance much but I don't have much of a baseline to go by.
I should mention I'm running a Crane Cams XR700 (Allison) pointless system although I noticed it too before installing that.
 
Yes and no, while this can contribute to this symtom, it's by no means the definate cause. A loose advance plate, either in the center or at the vac advance will do the same thing, a broken vac advance too. A bad balancer will make it appear timing is changing also. ( I don't know if you have a rubber mounted balancer on this or not, as I have NO excperiance w/ 1098's.)

A worn mechanical advace mech or springs will too, AND I've seen a bad carb do this as well. Cars with a rough idle that may or may not surge can cause the mechanial advance to shake and advance or retard timing. Timing chain stretch and dizzy gear clearance also. While the carb itself won't change timing, it can make other defects in other systems more prevalent.

Can you wiggle your shaft?

Anything seem loose?
 
I'll have to play with it some. Once again, I'm happy with how she runs and I'm not looking for trouble, just an observance during timing and wondering if "what's good" can't be made better....as always.

Maybe I'll pull the timing cover and see what the chain and gears look like. I don't think that's a big deal.
 
If chain is not noisy, don't mess with it.
 
It is noisy though, or something in that neighborhood is anyway. I'll have to look at the procedure for accessing that stuff, if it isn't too invasive, may as well have a look.
 
IMHO spark scatter doesn't mean much to a 'street' engine, but it has a substantial effect on power and other stuff in a race engine. Try setting the timing not by holding the engine at a fixed rpm, but by checking the advance with the timing light as you rev the engine through the specified rpm (a number I do not recall). Sort of a static vs dynamic deal.

That said, the only thing that will eliminate scatter is a crank-triggered ignition.
 
What brought this on was my checking of the timing. Not because I suspected it was fishy but because I have a new light with an advance knob and wanted, just out of curiosity, to see what the vacuum canister did and what my total advance was. I pulled and plugged the vacuum line to the dist. advance module, idled at about 700 rpm and timing was right near 8 BTDC (but with scatter) reconnected the vac line. Timing at 700 rpm did not change (I still don't know what this ported vac source from an HS2 is supposed to do....) Then I revved to about 3k and held it there, used the knob reset to 8 and it showed about 22 degrees, so I guessed I was getting about 30 total. Still, at 3K there was scatter. Sounds like, since it runs fine, I should just move on to other things I can break....er uh, I mean "fix".
 
If the cam wasn't shimmed properly on the last rebuild it will do exactly as you describe. If you pull the timing cover check the fore-to-aft play of the cam.
BillM
 
There was a similar post on another board several weeks ago about similar problems and it turned out to be a flakey timing light, not a bad distributor. That's the first thing I'd try, a different light. With a pointless electronic system movement in the distributor shaft shouldn't affect the timing to any extent, quite different with points where the movement in the shaft affects the opening and closing of the points.
 
I blamed it on my cheap light too. Ironically a second after I stood reading the label on the pickup clamp "fragile Do Not Drop", I dropped it and sure enough, that pickup clamp disintegrated. Used it as an excuse to get another. This one with an advance knob. Same scatter still though, although, this light is Harbor Freight quality too so highly suspect as well. And with the pointless ignition, well, it's probably not a "problem" then.
 
Good point on the light as I've had problems with mine in the past picking up neighboring wires. I don't have a cheap light. If the pickup is near the alt it'll cause mine to read funny sometimes. Good call, sometimes it's the simplest things.

I've had problems with cheap/bad wires crossing up or blocking signals with my light too.
 
You can pull the valve cover and find a rocker partly open. Than you can rotate the engine both ways looking for the first movement of the rocker and determine roughly the amount of looseness in the timing chain.
 
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