Timing Jumping

[Ken A.]

Re: Timing Jumping

Guys,

I appreciate all the replys, they are all great suggestions but I have covered these is great detail. I have tried a set of non-shielded ignition wires and there is no difference. I started with my old Sun timing light and then bought a newer Digital timing light, no difference. I am using original style OEM plugs and did put in a new set. When I sent the distributor out my guy did repair the slop in the point plate. Polarity of the coil is good too. Everything is tight in the distributor. There was no change when I did change out the points, condensor, cap, rotor and plugs. I definitely will post the solution when I figure it out. I am also open to any other suggestions. Thanks to all for your support!!

Have your guy put it on a Sun machine. I suspect a bent shaft or a worn cam. Have him specifically check for spark scatter. If he doesn't know what you're talking about, then you've got the wrong guy.

[Jim T.]

Re: Timing Jumping

After ready the posts and what you have done to verify where the 20 degress difference is coming from I have a couple recommendations.
With the possibliity of the harmonic balancer slipping, in your check of this did you take a piece of chalk or white out and draw a line on the inner and outer pieces of the balancer and then check with your timing light that the line does not separate?
I know you said your distributor was checked and worked on. Was the actual clearance of the assembled distributor gear and the housing verified to be within specifications? Without looking at my 63-64 service manuals, I believe the maximum is about .007.
I acutally experienced a problem with my 70's engine acutally dying but it only did it coming off the interstate after many miles of driving to a stop at the end of the off-ramps stop sign. Have the turbo 400, so no clutch depression. Car would start right up though. Checked a lot of things, everything OK. Pulled the distributor and discovered to much clearance in the distributor gear and housing. Bought the GM shims and adjusted the clearance. This fixed the problem.

[Michael H.]

Re: Timing Jumping

Tried to check number 6 and the coil wire as well same reaction. Just popped in a old nova distributor out of a small block. Saw some improvments not as much jumping around. More flashes on timing tab and more flashes in the advance area (around 35 degrees BTDC). only a few after. I am going to take the weights out of the picture but have to run off to work. Will try this afternoon. I am thinking of biting the bullet and swithing back to TI. Still dealing with sticker shock!

Stuart, I have tried two timing lights and the most recent one is new. I have connected to the alternator and directly to the battery, no difference. Thanks!!

I want to thank everyone who has offer suggestion and help this is try a great board. I never give up so will find out what is going on and post.

Thanks!
Steve

Ok, from what we've learned so far, here are some items that can probably be eliminated as a cause of the problem.

The timing has been checked at the flywheel with the same results so that completely eliminates the loose balancer/inertia ring theory.

The distributor has been rebuilt, with no improvement, and finally replaced, with a possible slight improvement. Assuming the replacement distributor has the normal excessive amount of end play, and the rebuilt unit didn't, that means the end play really has no effect at low RPM. (and it doesn't)

The vacuum advance unit and where it's connected never was in this discussion.

A cam timing chain would have to be VERY loose to allow a change of 20* to 30* so I don't think that could be an issue. Also, I think you mentioned that you replaced these items. If the new chain fit properly without excessive slack, that would eliminate the possibility of the crankshaft centerline being raised from line boring.

Most of the other items in question have been replaced (timing light, points/cond etc) so they can be eliminated.

How about the cam gear that drives the distributor gear? Don't laugh, I've seen these things severely worn on cams that came out of high perf/race engines.
With the distributor installed, is there a lot of rotational movement in the main shaft? I doubt there could be that much lash at these gears but I suppose it's possible.

Does the timing mark location somewhat stabilize at higher RPM?

[Duke W.]

Re: Timing Jumping

Have your guy put it on a Sun machine. I suspect a bent shaft or a worn cam. Have him specifically check for spark scatter. If he doesn't know what you're talking about, then you've got the wrong guy.

This is not a conclusive test. It does show whether there are any problems inside the distributor, but does not address the end play issue.

In a dist. machine the gear is clamped by a chuck, so the shaft has zero end play. On the engine it is free to move up and down through the end clearance.

If you check end play on the engine, notice that the rotor rotates as you move it up and down. This changes the timing, which is spark scatter that can be seen with a timing light.

If the end play is shimmed down to the two to seven thou spec the mark should be dead steady. With the typical as built end play, several degrees of spark scatter is usually observed.

Duke

[Ken A.]

Re: Timing Jumping

Duke, I disagree with your definition of spark scatter. If the shaft is true and the cam is true, then the Sun machine will show perfect firing every 45 degrees of rotation. If the shaft is off .001, then spark scatter will show up on the Sun dial, with lobes firing early or late from the ideal. The original Delco shafts were precision ground within 10 thousandth of an inch and the bushings were reamed to give very tight clearance. If a distributor has ever been dropped on a concrete floor, that is usually enough to do the damage. Otherwise I've seen a stock distributor still dead accurate after 100K miles. End play does not really change or affect the cam position relative to firing.

[Steve K.]

Re: Timing Jumping

The end play is dead nuts. I also had very little end play in the other distirbutor I tried this morning. I am going to try fixing the weights to see if this will steady out and check for vacuum leaks. I do not know what shape the cam is in since the guy I bought the car from said they rebuilt it but for all I know they put back in the original cam like they did with the timing chain. I have checked the distributor for slop by grabbing the rotor and trying to rotate back an forth but there is no play very tight.

I will seek out a friend with an analyzer to see if we can find anything else as well.

Thanks!!

[Duke W.]

Re: Timing Jumping

Duke, End play does not really change or affect the cam position relative to firing.

Like I said, watch the rotor rotate as you move the shaft through its endplay installed on the engine. This definitely affects spark timing because the dist. mainshaft angular indexing to the cam changes with where the shaft is within the endplay limits, and this alters the spark timing.

Test it on a dist. machine and it's spot on because the chuck keeps the shaft from bouncing up and down. Install it on the engine and watch the timing notch jump around.

Or take a typical slopy end play OE distributor, observe the scatter on the engine, shim up the play, retest and voila!... no scatter.

Been there, done that... many times...

Duke

[Joe L.]

Re: Timing Jumping

All-----


The "end play" issue is, in my opinion, way over-rated and claimed to be attributable to just about every distributor problem there is. The GM spec for the end play on most of these distributors was 0.060". I have several, NOS, in-the-box GM tach drive distributors. Every one has an end play of right at 0.060".

Might less end play be better? Yes, it MIGHT be. However, that's NOT how they were built and they worked very well when they were new.

[Duke W.]

Re: Timing Jumping

All-----


The "end play" issue is, in my opinion, way over-rated and claimed to be attributable to just about every distributor problem there is. The GM spec for the end play on most of these distributors was 0.060". I have several, NOS, in-the-box GM tach drive distributors. Every one has an end play of right at 0.060".

Might less end play be better? Yes, it MIGHT be. However, that's NOT how they were built and they worked very well when they were new.

The sloppy end play single point distributor certainly didn't work very well when new on my 327/340, and most high revving mechanical lifter engines had similar problems with spark breakup at high revs except FI distributors, which were assembled with minimum end play. If you never had a mechanical lifter carbureted SB with a single point, you've probably never experieced the problem.

On a 6000-7000 rev engine the single point distributor has to be "right-on", including minimum end play, or the ignition will break up at high revs. It can start as low as 4500-5000 in worst cases, so base engines often don't have the problem.

Been there, done that countless times until I figured out how to make a single point dist. work on a high revving engine, and being within the 2 to 7 thou end play spec called out in the 1963 Corvette Shop Manual is critical to achieving reliable ignition at high revs, and the elimination of spark scatter yields smoother engine operation due to less cyclic variation in individual cylinder torque.

Duke

[Dick W.]

Re: Timing Jumping

I always found that a good set of Echlin high performance ignition points cured most of the high rpm spark scatter. The OEM points (1931988 Delco) would start exhibiting point bounce above 5,500 rpm's and by 6,000 you could not distinguish individual lobes on a Sun Distributor tester. The Echlins would turn over 7,500 before you started to see any scatter or bounce

[Clem Z.]

Re: Timing Jumping

if it is point bounce just wedge some foam rubber between the movable point arm and the fixed part of the points

[Duke W.]

Re: Timing Jumping

I have checked the distributor for slop by grabbing the rotor and trying to rotate back an forth but there is no play very tight.

Something is wrong with your statement or you have a distributor problem. The rotor should rotate through about 10-15 degrees, which is the centrifugal advance, but it should have little noticeable up and down play.

Duke

[Duke W.]

Re: Timing Jumping

I always found that a good set of Echlin high performance ignition points cured most of the high rpm spark scatter. The OEM points (1931988 Delco) would start exhibiting point bounce above 5,500 rpm's and by 6,000 you could not distinguish individual lobes on a Sun Distributor tester. The Echlins would turn over 7,500 before you started to see any scatter or bounce

Echlin and all the other brands offer several different point sets. In fact, nowadays, they are all the same sets and manufactured by Standard Motor Products regardless of the name on the box. Some have the 19-23 oz. breaker arm tension. There are two or three different part numbers - uniset, screw-on or spring clip terminal. Then there is the 28-32 oz. set.

Saying "Echlin" means nothing.

I recommend the 28-32 oz. set for all engines with redlines over 5500, and have listed all the part numbers before.

Duke

[Michael H.]

Re: Timing Jumping

All-----


The "end play" issue is, in my opinion, way over-rated and claimed to be attributable to just about every distributor problem there is. The GM spec for the end play on most of these distributors was 0.060". I have several, NOS, in-the-box GM tach drive distributors. Every one has an end play of right at 0.060".

Might less end play be better? Yes, it MIGHT be. However, that's NOT how they were built and they worked very well when they were new.

I agree. The clearance in the distributor shaft has very little to do with it, especially at low RPM.
The distributor gear/shaft also turns the oil pump shaft and pump so there's always a great deal of resistance that automatically forces the distributor gear up, which automatically eliminates the clearance. This is by design and a result of the cam gear and distributor gear tooth angle.

Now, at a certain RPM, there IS some issue with cam chain "whip" and lash, that will cause the timing to change by several degrees but that normally occures at a much higher RPM and the actual change in timing is no where near the 30* variation that was mentioned.

Years ago, I rebuilt the original 093 distributor in my 66 425 HP car. It had a main shaft clearance somewhere near the numbers that Joe mentioned.
I did NOT reshim the main shaft and, 25 years later, the timing mark is steady with zero variation.
I don't think there's anything wrong with setting the shaft clearance tighter but I just don't think it makes much difference in most passenger car applications and it's highly unlikely it would have any effect at idle speed.

I do believe that if distributor shaft clearance was as much of an issue as the hot rod magazines make it out to be, GM/Delco wouldn't have allowed that clearance to be as great is it is.

[Steve K.]

Re: Timing Jumping

First off I want to thank everyone for their help and knowledge. Through this process I have fixed everthing that we thought was wrong but were only marginally off. None the less the engine appreciates it.

I found the problem and it was two things. I waited for it to get dark and turned the lights off in my garage. I noticed a slight spark escaping from the coil wire. I never switched this wire out when I switched out the spark plug wires because it was not shielded but what I also noticed was a bit of spark from the neg distributor wire to the coil. It was jumping across to the base of the distributor. I switched both out and the problem is gone. The engine appeared to run fine since the idle was turned up. Once this was taken care of I could set the timing and idle and went for an awesome ride!!

Thanks again, this board is great!!