Topic: Swapping spark advancer's

[alimacdee]

Hi all,
Boyer bransdon Mk 3 ignition failed on my 1970 CB750, I fitted the Mk 3 unit, February 2003, Hoped it would last the lifetime of the bike! we fair wint value fir money here in the highlands of Scotland! so a bit disappointed! The ignition would cut out intermittently, and after reading all the fascinating posts on ignition and timing etc..!, decided to refit the original advance unit and points plate, but then remembered I'd ground the dogs off the end of the advancer's shaft, the ones the large slotted nut fits, this makes it awkward to rotate and line up the timing marks using the kickstart.
Luckily I had a later type advance unit and points plate, I'd purchased off ebay years ago and stashed, identical to the one in the post "My long-overdue Spark Advancer post" by Honda man.
I cleaned everything up and fitted the advancer and matching points plate, the points looked brand new, dwell at 49-50 on both sets, static timing fine, checked the advance, just spot on! thankyou posters.....
This forum has honestly been a godsend, so after many hours of returning the bike to a points setup, wanted to ask, are there any perceived problems, differences associated with fitting the later advancer unit, I read in one of the posts, the advance units are not interchangeable, but the springs are. So a little worried.
 

cheers
Alastair


http://www.youtube.com/user/alimacdee#p/a/u/2/2Fi18CaZj48

[HondaMan]

Hi all,
Boyer bransdon Mk 3 ignition failed on my 1970 CB750, I fitted the Mk 3 unit, February 2003, Hoped it would last the lifetime of the bike! we fair wint value fir money here in the highlands of Scotland! so a bit disappointed! The ignition would cut out intermittently, and after reading all the fascinating posts on ignition and timing etc..!, decided to refit the original advance unit and points plate, but then remembered I'd ground the dogs off the end of the advancer's shaft, the ones the large slotted nut fits, this makes it awkward to rotate and line up the timing marks using the kickstart.
Luckily I had a later type advance unit and points plate, I'd purchased off ebay years ago and stashed, identical to the one in the post "My long-overdue Spark Advancer post" by Honda man.
I cleaned everything up and fitted the advancer and matching points plate, the points looked brand new, dwell at 49-50 on both sets, static timing fine, checked the advance, just spot on! thankyou posters.....
This forum has honestly been a godsend, so after many hours of returning the bike to a points setup, wanted to ask, are there any perceived problems, differences associated with fitting the later advancer unit, I read in one of the posts, the advance units are not interchangeable, but the springs are. So a little worried.
 

cheers
Alastair


http://www.youtube.com/user/alimacdee#p/a/u/2/2Fi18CaZj48

They all interchange: the K0 advancers had more advance than the K2 and later units. At worst, you might be able to comfortably run Regular fuel instead of higher octane. It will lose a bit of top-end HP if it is a lesser-advance unit.

For reference: the K0 early sandcast advancers reached 41 degrees full advance, which became 40 degrees pretty quickly. The K1 was about 3 degrees less, and by the K4 it was more than 5 degrees less.

[alimacdee]

thankyou Hondaman, that's just what I wanted to hear! Having tested it on the road, the bike feels really good again, wish I'd stuck with the points now, though it did take this problem, and this forum! for me to learn how to set up the timing and points properly.  Initially I did think the electronic ignition was a good idea, in hindsight now I can see the points setup if properly sorted is just as good, and at least the points are serviceable if there's a problem!

now If I could just cure the plug fouling, with one of your transistorised ignitions!

[HondaMan]

Plug fouling was a mark of the 1969-1972 CB750s. It was due to 2 things:
1. Honda was scared to death of burned up engines in abusive American hands (for good reasons, as it turned out!).
2. Japanese gasolines don't carry the octanes like American fuels, making for some unusual tuning.

Things are quite different today, obviously...

To reduce the fouling on those bikes, take the first step and drop the mainjet a little. You'll find it to be either 120 (1970 tuning) or 115 (bikes made between about 9/70 and 5/71). The K0/K1 today can benefit from using the 110 mainjet and the modern versions of premium fuel, especially Shell/Phillips fuels.

If that's not quite enough to stop the fouling, check your needles: if they are in the center clip, then lower them one notch (raise the clip) and go back up a 5 step on the mainjet. By the time of the HM341 pipes (more restrictive than those original HM300 mufflers), the standard tuning became 105 mains with the 4th notch on the needles. This dropped some 5 HP at the rear wheel at high RPM, though.

One other thing: does yours still have the HM300 fiberglass-pack mufflers? If someone has substituted the HM341 pipes and left the original jetting in place, you won't be able to keep a plug clean for nuthin'...

One last thought: after the years passed, the (soft) seat for the little air screws on the carbs have often become slightly enlarged from owners tightening the screws too snugly when readjusting them for their 1 turn setting. This will make the 1 turn setting run rich, like it was at, say, 1-1/8 turns out. So, try setting the screws at 7/8 turn or so. Mine are at 15/16 on 3 cylinders, 3/4 turn on one of them, for foul-free riding.

If you decide to go Saturday-night street fighting, set the air screws back out to 1 turn or a hair more for the evening, for a temporarily better launch.