Topic: CB750 K1 Resurrection

[grcamna2]

The spark is meant to fire at the precise moment the points just begin to open.

[ZTatZAU]

The spark is meant to fire at the precise moment the points just begin to open.

Ahhh... I understand that grcamna2!  Unfortunately, you must have misunderstood my questions.

1)  I'm wondering about the timing marks on my advance mechanism and why they look different than those shown in the SM and OM.  You'll have to look closely at my previous post to pick up on the difference.  Have you (or anyone else) ever seen timing marks like the timing marks in my first picture?

2)  Should my timing setup using the setup in my last picture using a continuity meter work as I think it should?  I.E.  Shouldn't there be no continuity as soon as the points begin to open?

TIA for answers to these questions!  ZT

[HondaMan]

Those timing marks: it was common for the Honda Shop Manuals to have diagrams and images from prototype bikes in their articles. It was so bad with the CL72/77 manuals that the bikes were downright hard to recognize when compared to the images in their shop manuals!

Your timing marks are the correct ones as found on the normal 750. The only time I have seen a picture of an actual CB750 with timing marks as shown in your manual was from one of the first 5 prototype 750 bikes Honda made, which were run in Nevada for a week at max RPM after the bike shop in Las Vegas. Their idle timing was only 5 degrees (ours are either 7 degrees or 10 degrees, depending on which engine] and max angle was said to be a little over 44 degrees (ours are 35 or 39 degrees), probably to match the [different from ours] cams those 5 bikes had. They were literally hand-built engines, though.

[ZTatZAU]

Those timing marks: it was common for the Honda Shop Manuals to have diagrams and images from prototype bikes in their articles. It was so bad with the CL72/77 manuals that the bikes were downright hard to recognize when compared to the images in their shop manuals!

Your timing marks are the correct ones as found on the normal 750. The only time I have seen a picture of an actual CB750 with timing marks as shown in your manual was from one of the first 5 prototype 750 bikes Honda made, which were run in Nevada for a week at max RPM after the bike shop in Las Vegas. Their idle timing was only 5 degrees (ours are either 7 degrees or 10 degrees, depending on which engine] and max angle was said to be a little over 44 degrees (ours are 35 or 39 degrees), probably to match the [different from ours] cams those 5 bikes had. They were literally hand-built engines, though.

Thanks HondaMan!  That's great info and one less thing for me to worry about.

Can you (or anyone else) explain why my continuity setup to set the timing isn't working as intended?  If the insulated fiber washers on the active (cam driven) point connection are functioning correctly, where is the continuity path to the fixed point and the breaker plate after the points are opened?

Bottom line is... can I use the continuity function on my MM to assist in determining the moment the points begin to open?  If so, where should the MM leads be connected; if not as shown in my setup picture?

TIA!  ZT

[newday777]

I was taught the practical purpose as I wasn't into theory memorization so I won't try to answer your why....

Rather than using a MM, I learned back in the 70s to make up a test light with alligator clips to set static timing. I used an available bulb socket I had on hand with positive and negative leads and attached the alligator clips. (1156 bulb and socket) I carried it with me on all my trips cross country to use whenever my points needed cleaning.
Positive to the points bolt as you did and negative to the engine case. When the bulb just lights up, you have your set and lock down the screws.
Points gap plays into the equation too. If too small points gap, that effects the fire timing location. So rather than .012 try .016 points gap if you are having a problem setting the static timing.

Here's an old school bloke using a test light made from a turn signal.

[ZTatZAU]

I was taught the practical purpose as I wasn't into theory memorization so I won't try to answer your why...

Rather than using a MM, ... I used an available bulb socket I had on hand with positive and negative leads and attached the alligator clips... Positive to the points bolt as you did and negative to the engine case. When the bulb just lights up, you have your set and lock down the screws.

Points gap plays into the equation too. If too small points gap, that effects the fire timing location. So rather than .012 try .016 points gap if you are having a problem setting the static timing.

Here's an old school bloke using a test light made from a turn signal. ...

Thanks Stu, for the "Old Bloke's" video link which provided a key piece to my puzzle.  I've used a friends "buzz box" on other machines to set magneto timing.  The idea is the same, a lamp lights and a buzzer sounds when the points open but the buzz box contains its own power source.  I have a 12v bulb with soldered alligator leads around here somewhere, that I've used to test for power in 12v circuits, but never thought I could use it here.  The missing piece to the puzzle, for me, was where would the power to light a simple bulb come from.  The answer of course, (duhhh!), is from the bike after switching the ignition on.  I'll hunt up my little 12v test light and finish up.

I understand that point gap can affect timing which is why I set the gaps first.  I used the midpoint setting of .014" which is about the same as the "Old Bloke's" first attempt of 35mm.  So if I wind up at the extreme of the available adjustment, like he did, I'll bump the point gaps up to 0.016".

Thanks again for your reply!
ZT