Distributor Cap Movement
Race Director
Joined: Jul 2001
Posts: 15,892
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Hey MikeM- look! He bought the story! Bwahahahahahaha. Ooops, is this thing on? Sh*t. 
Mark,
It's not unusual for timing to fluctuate a few degrees especially on a relatively high mileage engine. Unless it's affecting performance I wouldn't worry about it.
Mark,
It's not unusual for timing to fluctuate a few degrees especially on a relatively high mileage engine. Unless it's affecting performance I wouldn't worry about it.
Last edited by Mike Ward; Oct 21, 2009 at 10:26 PM.
Tech Contributor
Joined: May 2005
Posts: 15,576
Likes: 118
From: Conroe Texas
I've been told that my experience and conclusion has no basis in fact, so your mileage may vary.
Race Director
Joined: Nov 2000
Posts: 10,346
Likes: 672
From: Beverly Hills (Pine Ridge) Florida
Most of the time it is a function of a worn shaft bushings (especially the upper bushing) and the dist. shaft. As the shaft wobbles in the bushing, the points gap / dwell changes as well, shifting the timing.
I would not expect end play to have any effect, other than shifting the worn shaft downward to a "better" (slightly less worn) part of it's shaft. The dist shaft is always biased upwards by the helix on the cam and dist. gears (while running). It cannot come downward (while running) due to the load of the oil pump and dist. shaft on those helix gears.
Plasticman
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Jeff,
Most of the time it is a function of a worn shaft bushings (especially the upper bushing) and the dist. shaft. As the shaft wobbles in the bushing, the points gap / dwell changes as well, shifting the timing.
I would not expect end play to have any effect, other than shifting the worn shaft downward to a "better" (slightly less worn) part of it's shaft. The dist shaft is always biased upwards by the helix on the cam and dist. gears (while running). It cannot come downward (while running) due to the load of the oil pump and dist. shaft on those helix gears.
Plasticman
Most of the time it is a function of a worn shaft bushings (especially the upper bushing) and the dist. shaft. As the shaft wobbles in the bushing, the points gap / dwell changes as well, shifting the timing.
I would not expect end play to have any effect, other than shifting the worn shaft downward to a "better" (slightly less worn) part of it's shaft. The dist shaft is always biased upwards by the helix on the cam and dist. gears (while running). It cannot come downward (while running) due to the load of the oil pump and dist. shaft on those helix gears.
Plasticman
Be careful with shimming the gear too tight to the distributor. In some cases you can bottom the distributor shaft into the oil pump shaft. Then you raise the question as to why your distributor moves after you've tightened it down with the clamp.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,401
From: At my Bar drinking and wrenching in Lafayette Colorado
1. Movement of the distributor cap has no effect at all on timing. If there's misalignment, the spark jumps the gap. If the gap is so excessive that the spark can't jump, the spark will find another path to ground and the cylinder will misfire.
2. "Timing scatter" is caused by a few things. The most common is that the hole in the breaker plate that connects the plate to the rod on the vacuum advance is worn, and this allows the breaker plate to "rattle" back and forth several crankshaft degrees. This can be easily fixed by removing the breaker plate and "smacking" the raised area at the hole with a hammer. This will reduce the size of the attach hole and eliminate the play. This greatly improves timing accuracy. It is possible for dwell (and therefore timing) to vary if the distributor's upper bushing is worn. However, since there is no load on the upper bushing, it is very rare to see a worn out upper bushing - most wear occurs in the lower bushing. A poor quality set of points will also cause timing scatter, as will worn advance weights and a sticky advance system.
Lars
2. "Timing scatter" is caused by a few things. The most common is that the hole in the breaker plate that connects the plate to the rod on the vacuum advance is worn, and this allows the breaker plate to "rattle" back and forth several crankshaft degrees. This can be easily fixed by removing the breaker plate and "smacking" the raised area at the hole with a hammer. This will reduce the size of the attach hole and eliminate the play. This greatly improves timing accuracy. It is possible for dwell (and therefore timing) to vary if the distributor's upper bushing is worn. However, since there is no load on the upper bushing, it is very rare to see a worn out upper bushing - most wear occurs in the lower bushing. A poor quality set of points will also cause timing scatter, as will worn advance weights and a sticky advance system.
Lars
