Topic: Advance Spring Solution for our SOHC750's ??

[cougar]

Hey folks; Since there seems to be no availablity of advance return springs for our beloved SOHC 750's, here's a thought. If I had the cash (unemployed) I'ld try this. How about the distributor advance kits (the ones with springs) they sell for cars? 2 examples, one cheap and one more (but a lotta springs), cheap http://www.summitracing.com/parts/MRG-928G/ , and expensive http://www.summitracing.com/parts/MAA-29015/ .  If 1 of us would be willing to get a set then take the time to try all the springs in the kit. They could find out which springs from which kit would work for a stock replacement on our bikes as well as find out which springs would bring our timing in at different RPM's. Hell I'm between jobs right now (so I've got the time  ) so if someone wanted to get the kit and send it to me I've got a sweet running 76F motored bike to try 'em out in  . This is just a thought !    ...cougar...

[chewbacca5000]

You might be on to something I read over at Hondachopper about double springing.  When I get more time I am going to hook up some HEI coils and HEI units to my CB750 and see how it goes.  You can find details over at Hondacb650.com.

[fastbroshi]

Do you have a spring or are you just missing yours?  I thing Hondaman said a good solution to get a bit more advance would be to snip off a coil off of the existing one and reattach.

[cougar]

Hey fastbroshi; No, my advance is good ! Snaps back, moves freely and advances like it should. I've just noticed on this forum and other Honda cb750 forums that I belong to that it seems a lot of folks have problems with stretched springs causing their advance mechanisms to not operate properly. Since I'm 'ol school and like the points system since it's so easily and cheaply dealt with I just thought that something like this would help folks to restore their systems to original working order. Also something like this would also allow a person to be able to change his or her advance to come on quicker or later depending on their personal preference with out having to modify a set of 30 odd year old springs to maybe get close to what their looking for. Just throwing out an idea that might help fellow Hondachopperheads in their improving of these great bikes without spending BIG bucks on electronic items that may last forever or may leave them stranded. But don't get me wrong, I love the thought of the "Hondaman Ignition" system and from Everything I've read about it, it sounds like a Rock Solid system and when I have enough cash I plan on adding at least one to my bikes.       ...cougar...

[fastbroshi]

Sounds good Cougar, it's great that you have such a positive attitude.  I've actually got a Hondaman ignition on my bike and I have no complaints.   So maybe his idea about cutting a coil would work for you?

[Tintop]

I'll add my .02.  The issue is the springs weaken over time.  This results in the ignition starting advance too soon, and reaching full advance at to low an rpm.  Cutting a coil will stiffen the spring, and delay the start of advance.  It will also change the 'rate' of advance. Without testing the result, you have no way of knowing what you actually have done.

There are also spring kits available for Lucas & Bosch units, and these may be more suitable for our application.  Having rebuilt distributors (I used a SUN 504 distributor tester) for MG's etc., reworking the advance curve is a little more complicated than just changing springs.  Not only are there the springs, but you also have the weights themselves to work with.  The 1st step is knowing what the length, & spring rate of a NOS spring is.  This can be done using a simple spring tester gauge.  The problem there is finding a NOS spring to use as a baseline.  You then need to know how much the spring was preloaded.  This  measurement comes from measuring the distance from post to post, vs the spring free length, and then measuring the rate of the spring at this 'installed length'.  This 'preload' determines the start of advance for a given set of weights.  The spring rate in combination with the weights determine the speed the ignition reaches full advance.  Changing the spring rate, &/or the weights themselves allows this 'curve' to be tailored.

At one point I thought about offering an 'advance rebuild' service, doing this testing/setup on the 504.  However I haven't yet come up with a way of mounting an SOHC4 ignition on it.

[Sam Green Racing]

A good idea in the 1st place and well answered by TT but this is not in the correct forum, I'm sure a lot of members that run points would like to know the ins and outs of manual A/R units, but most on here run electronic.

Sam.

[Tintop]

A good idea in the 1st place and well answered by TT but this is not in the correct forum, I'm sure a lot of members that run points would like to know the ins and outs of manual A/R units, but most on here run electronic.

Sam.

Agreed Sam on the forum, this should be in SOHC.  One note though, only systems like the Dyna 2000 do away with the advance mechanisim.  The Dyna S, and Pamco units still use it, so it is relevent to some electronic units as well.

[Sam Green Racing]

Yes, the CR in my avatar had a similar Boyer system that retained the manual A/R but finding the correct spring is only replacing what was there stock in the first place, not exactly a performance mod.

Sam.

[Tintop]

Agreed that if all you do is rebuild the advance mechanism to stock spec, and then install on a stock motor - wrong forum.  That's a basic 'how to' for the advance mechanism.  The answer to the thread 'subject' is simple - find a spring with the same free length, and rate as the original.  It's the doing part that is difficult.

However, being able to change the start / end / & rate of the advance is part of basic high performance engine tuning.  It is why the Dyna 2000 is popular, you can try different curves by simply moving a switch.  If that isn't an option, then knowing the stock spring spec's is valuable tuning knowledge.  The spring kits for the Lucas used in MG's, Healey's ect. come with x3 sets, stock / & x2 slightly stiffer.

The SUN isn't restricted to testing just points distributors.  I have an adapter for use with ones converted to electronic pickups (boosts signal so tester can read it).  Your Boyer, a Dyna S, or Pamco would fit into that category.  Question as we're in the High Performance forum -  the Samauto used a Dyna 2000, then was converted to MDS, does the MDS use the stock advance unit?

[Sam Green Racing]

Not a clue on the MSD TT but I can see where you are coming from with the springs. Another option would be to pick up a few spare bob weights and play about removing or adding weights.

Sam.

[Tintop]

Not a clue on the MSD TT but I can see where you are coming from with the springs. Another option would be to pick up a few spare bob weights and play about removing or adding weights.

Sam.

I use to buy up as many shot distributors (bad bearings) @ swap meets as possible, just for the weights, springs, and cams.   The weights were weighted, and bagged in sets.  You can also lighten them by drilling, like balancing a crank shaft.  Just really small amounts.  All that stuff is long gone now.  I still have the tester, and its attachments though.  Functionally there is no difference between an MG advance, and an SOHC/4's.  There is a big difference in parts availability.  It would be interesting to know if 500 / 550 / 650 / 750 weights & springs were different, and how interchangeable.  Somewhere in my storage boxes there should be 500/550/750 advances, but it will be awhile before I can dig them out and have a look.

[cougar]

Sam; Since you are a moderator would you please move this thread to the correct section. Or do you want me to copy & paste it myself. If so, is the correct place "SOHC/4 BIKES" ? Thank you for your time.   ...cougar...

[Tintop]

It kind of belongs in both cougar.  If your question had been - 'Where can I get replacement springs for a CB750?', then SOHC4 would be appropriate.  When the thread gets into modifying advance curves by changing springs & weights, High Performance would seem to me to be correct.  However as Sam mentioned, most here are running full electronic, so actual interest may be small.

Maybe start a 'Where can I get replacement springs for a CB750?'  thread in SOHC.  I'll join that one too, as I would like to know the answer.

[scottly]

Tintop, I think maybe you could adapt an auto distributor to mount the Honda advance mechanism? I have used a Sun machine in the past to dial in VW distributors. Nice machines!

[Tintop]

I was thinking of something along the lines of the SUN flat head Ford adapter scotty.  Were you could have access to the springs / weights; without having to take the whole d*^m thing apart each time.

Those VW Bosch distributors were bullet proof.

[scottly]

I looked at a Ford 302, as well as a couple of Mopar distributors today. They all appeared to have about a 5/16" shaft that the point cam fit over. Maybe it could be turned down to 6mm? Or removed and the end of the main shaft drilled and tapped to take the Honda stud?

[Sam Green Racing]

We can leave it here for a while, It's kinda turned from a quick repair fix to an old fashioned ignition tuning mod. so if you can't afford a modern ignition system and are stuck with points and manual A/R, this is your tuning manual.

Sam.