nate99

Searching around a bit for a idea of how to start trouble shooting my problem, I cam across the issue of timing "scatter".

I'm not sure if that is what I am experiencing, but I thought I'd throw it out to see if anyone might have an idea of a good next step.

Basics of setup:

New 290 HP GMPP crate 350
New non-CCC HEI distributor
Rebuilt non-CCC Q-jet

I installed the dist. using Lars write-up, and have that to thank for getting as far as I have. Without that, I'm sure I'd be searching for a new starter having burnt at least one up.

I am now to the point where the engine will start if you pump the throttle a bit and it will stay running (rough) if you keep pumping/feathering it. If you give it gas it with smooth out a bit and keep running at a higher RPM. I was hoping that I could dial in the timing to smooth it out a bit more and then try to get the idle sorted, but here's where the "scatter" issue comes in.

When I use the timing light, there is no distinct flash line that I can see on the damper (dial back light) it's more of a general blob flash that is kind of all over the place. It is entirely possible that I am doing something wrong with the timing light as this is my first attempt at using one. It is also possible that I am expecting something that isn't going to happen. I read about "scatter" and the possible need for shims, but I am not sure if that is what I am dealing with or not.

I'd have more info if I had some help, as I'd be able to move the dizzy to smooth things out a bit, but I'm on my own on this one, so I have one hand playing with the throttle keeping the car from dying, while trying to use the timing light with the other.

Any ideas?

I'm tempted to throw out a pizza and beer offer to someone local to come help me out, but my opportunities to work on the car are infrequent, irregular and unpredictable.

Mike Ward

'Cranking' means turning it over on the starter motor, not the engine actually starting or running.

How much scatter are you seeing? A rough running engine will naturally exhibit scatter as the RPM raises and falls. A new smooth running engine should show less than two degrees or so.

Try using the carb idle screw to keep the rpm up while making other adjustments or enlist a friend to hold the light while you hold the rpm and adjust the timing.

Put a line of white paint on the damper at the TDC mark and another at the desired mark on the timing tab. Set the timing light at zero setback and slowly increase it until the lines meet up. Try to achieve 'all in' by around 2500 to 3000 rpm.

Make sure the vacuum advance line is removed from the dist. and plugged.

08vycpe

Assuming you have done all the right things to check timing properly think about this:

Multiple spark HEI systems such as the MSD ignition control boxes don't work well with dial back timing lights. The MSD box for example provides multiple spark over 20 deg of crank rotation. This helps with easy starts, smooth idle and better throttle response. Over 3,000 rpm there just isn't enough time for the multiple spark so you only get one. For this reason I get a blur of flashes just as you are describing. It's ok with a non dial back light.

Try checking your timing with a NON dial back light as MSD recommends. For about $7.00 you can get a timing tape for your balancer so you can check timing all the way past the 38 deg mark. You shouldn't need to go past that although the tape goes way beyond that.

nate99

I changed "crank" to "start" in my earlier post. I was being a bit sloppy with the wording there. I am not sure if I am seeing scatter or what, but I'm not able to see precisely where the indicator is lining up with the marks on the damper, it seems to be jumping within a 10-15* range, but next time I try it, I will pay closer attention.

My damper has marks on it to 60* if I remember correctly, so the "dial back" is not that critical. I did confirm that the zero mark was at or extremely close to TDC.

I am not running anything other than a GMPP HEI distributor, not sure if that causes any of the issues described with the MSD box. I can see the spark advance as revs build, but only relatively, there is just no way to get any kind of precise reading as it is now. I'm thinking I have the vacuum advance on the distributor open, but the carb/manifold side was capped.

MelWff

Try the light on another car to see if it is defective.

nate99

Worth a shot. My DD Civic doesn't have spark plug wires though (coil on plug, in the head). Maybe my wife's Tahoe, they have a short wire between the coil and plug.

Would it make a difference where I clamped the alligator clip for the timing light? I used the hot lug on the starter, since it would not reach all the way back to the battery.

08vycpe

There is probably an arrow on the timing light spark plug clip. Make sure it faces the plug when you connect it.

nate99

I did not notice this, so it is entirely possible that I had it on backwards.

Richard Cooper

Nate99,
You have a PM.

nate99

Thanks Richard.

7T1vette

The timing light DOES just make a 'blob' of light. You are not looking for some line of light that the timing light makes. You are looking for the TDC line grooved into the outer surface of the harmonic balancer (damper). The light just flashes for a instant each time the cylinder where you have it connected fires. You should have other lighting turned down or off, so that you don't bleach out the timing light's output. It's basically a "strobe" light. When you are connected to #1 cylinder (left front of engine), with the distributor advance can hose removed (and plugged so that it doesn't leak vacuum), you should see the TDC mark stabilize on the balancer (while engine is running).

Note: this is with the dial back set to "0" offset.

If any of this is a 'surprise' to you, go back and read the instructions on the timing light you bought. Good Luck.

nate99

It is difficult to describe what I am seeing when I use it. Perhaps it would be better with more of the ambient light cut out, but the key point is that the strobing was no where near precise enough to peg any particular mark on the damper to the zero mark on the indicator. I could tell that it was advancing with revs, but at any given RPM, there was just no consistency to the flash.

I thought I remembered seeing videos with fairly clear marks from the flash, but I may be thinking of something that wasn't there.

Alan 71

Hi Nate,
You asked about where to clip the power lead for the timing light.
An easy place is the the large wire coming off the alternator. You just need to slip the large, black, rubber boot off the connector.
Regards,
Alan

7T1vette

If you are saying that the damper mark is 'jumping around' under your timing light...but the engine is running fine....then your harmonic balancer may be 'toast'. When the synthetic rubber joint in that damper fails, the outer ring (which has the timing mark on it) wiggles around due to 'free motion' since it is not firmly attached to the inner portion of the part.

When the engine is stopped, see if you can grab onto the outer portion and actually get it to move while the inner portion is stopped. If so, buy a new balancer.

nate99

Yep, should have thought of that too, thanks.


The damper is brand new, though I guess it could be faulty. I'll give it a check, but I really think that it is the strobe not firing at a consistent time, or maybe just me trying to do too many things at once to get a really good look at it.

In any case if I can get the idle worked out, then I ought to have a hand free to fine tune a bit.

7T1vette

If you have worn distributor shaft bushings or 'sticky' mechanical advance components in the distributor, you can get an erratic timing indication, also.

nate99

Distributor is brand new, so the bushings should be fine, but the mechanical advance might be hanging up. I've probably missed something basic though.

Thanks again.

GS977

mine was doing the same thing. i have a snap-on digital timing light and my mark on my balancer would jump around. i could tell where it was within 4-6 degrees. then i came across a post on another forum about distributor end play. it should be @.015 for an hei. mine was .060. so i pulled my hei out and shimed the endplay to .015. that was all i changed. i put it back in and now the mark is steady. no more jumping around. try and pull up and push down on your rotor and see how much it comes up and down.
hope this helps

nate99

I'll read up on end play, thanks.