Topic: Jittery timing

[Johnny5]

I was checking the timing of the 750K7 with a electronic timing light and 1-4 were spot on, checked 2-3 and it was right on the mark, but a little jittery. Im talking it was only fluctuating within a mm. Seemed weird that 1-4 wasnt doing this. Cam chain, valves adjusted.

Curious what can cause this?

[HondaMan]

Slightly bent timing shaft does it.
If I can find the post (archived by now), I showed how to straighten this up with a dial indicator and a few judicious taps of a small hammer.
Description:

You must remove the timing plate and spark advancer, leaving you with a long, thin shaft that sticks out of the end of the crankshaft. Mount a dial indicator to the engine block and measure the out-of-round run of the shaft, about 1/4" down from the threads. Tap with a small hammer to recenter-ize it (is that a word?). You should be able to get it within .002" with patience, and this patience pays off later. I think Honda's spec is a generous .010", but I've seen this still leave jitters behind.

This shaft has become non-concentric over the years, probably from someone yanking left-right-left-right on the big crank nut while setting the timing with a light bulb, which was (is) a popular method. The spark advancer has just one locating dowel in the end of the crank, so it loads this shaft sideways when you do that, then bends it a little. I never remember these being bent until the owners read these articles in Cycle World magazine in the early 1970s, and then they would bring the bikes into our shop, obsessing about the timing jitter (which generally adds a hint of vibration at highway speeds, but no other problems. Back then, I used a bent coat hanger, clamped to the frame and pointing to the end of this shaft, to align it: I was too broke (in school then) to afford a dial indicator, like the Honda bulletin said...    So, I just got it as perfect as I could, and it usually worked out fine. 

[Johnny5]

Ah, I do have a hint of vibration after 50 mph. I thought it was a wheel out of balance, but after checking that and everything was okay, I just figured it was the drag bars.

Question though, if this shaft is a tad misaligned, wouldnt it also give the timing for 1-4 the jitters?

[HondaMan]

Ah, I do have a hint of vibration after 50 mph. I thought it was a wheel out of balance, but after checking that and everything was okay, I just figured it was the drag bars.

Question though, if this shaft is a tad misaligned, wouldnt it also give the timing for 1-4 the jitters?

Interestingly, no. It has to do with the position of the dowel on the back of the spark advancer: it always bends the shaft away from the 2-3 side (actually, the center of the bend is a little below the 2-3 opening point). I think in all the Fours I've worked on, I only saw one where it was bent enough to have both the 1-4 and 2-3 jittery. It was way off, and the shaft was curved, hinting at a larger accident. I imagined someone having the engine out and it falling over on the points side, with the points plate removed, to cause the bend I was seeing. It was pretty easily straightened, though, in the end: it's not a hard shaft.

[technojock]

Fluttering timing marks when viewed with a strobe timing light are caused by bad condensers.  Go to your local auto parts store and ask for a couple Chevy V8 condensers and specify that you want the ones with a slip on bracket not the soldered on one.   Replace the stock condensers with these and the problem should go away (for a lot less money than new Honda condensers too...)

Tony

[Johnny5]

Thanks for the explanation, Mark. I'll check that shaft with a dial and see what I find.


Techno,

New points and condenser last fall.