470's - Interesting Timing Revelation

[blkzoe]

All:

First, my 470's (1986, 170hp) are running well. I know, I'm just experiencing the afterglow after a successful rebuild. But they both idle without shaking at about 600rpm. Kinda' creepy..........

Any way, I'm timing one of the engines w/ an induction timing light and got it right on (6 degrees BTDC; I'll explain later). Zero timing fluctuation on #1 cylinder. Then I thought about #3 cylinder as long as everything was hooked up. As I have an induction timing light, I simply moved the clamp over to #3 spark plug wire. Recall that #1 and #3 cylinders are images of each other; i.e., #1 and #3 are in identical stroke positions at TDC, but performing functions that are 180 degrees out-of-phase. I thought the timing mark would be in the same position. Silly me.

When #3 fired, its timing was at about 13 degrees BTDC. I moved the clamp back to #1 cylinder; right where I left it at 6 degrees BDTC. I can't come up with an explaination for this. Any ideas? Again, the engine purrs; no firing issues at any RPMs, no overheating. Power output is smooth. What am I missing?

 

[achris]

Re: 470's - Interesting Timing Revelation

The timing should be the same on all cylinders. Only explanation I can come up with is a worn/bent distributor shaft or a worn points cam lobe... or a bent breaker plate...

Ok... A 'successful re-build'... Did you use the right tool to align the oil pump/distributor?

Chris......

 

[blkzoe]

Re: 470's - Interesting Timing Revelation

The timing should be the same on all cylinders. Only explanation I can come up with is a worn/bent distributor shaft or a worn points cam lobe... or a bent breaker plate...

Ok... A 'successful re-build'... Did you use the right tool to align the oil pump/distributor?

Chris......

Yep, sure did use the tool for oil pump alignment. We are thinking along the same lines, however. How about a worn distributor bushing causing a concentric wobble? My counter to that is the fact that the timing mark(s) are stable. One would think a worn bushing would produce wandering timing marks.

 

[achris]

Re: 470's - Interesting Timing Revelation

Yep, sure did use the tool for oil pump alignment. We are thinking along the same lines, however. How about a worn distributor bushing causing a concentric wobble? My counter to that is the fact that the timing mark(s) are stable. One would think a worn bushing would produce wandering timing marks.

Agreed. That's why I would be looking at lobes and the breaker plate... A bent shaft would cause a variation, but not the fluctuation...