Topic: CB750 advance unit - sloppy springs- full advance constantly -Huge Progress!

[AJK]

I should have put in the conversion for 'F'. Here in Oz, we use 'C'. Sorry about that. Must remember next time.

[HondaMan]

I should have put in the conversion for 'F'. Here in Oz, we use 'C'. Sorry about that. Must remember next time.

Oh, that's cool! I didn't know that...but, you are correct: the temperatures at the points side of the engine can reach quite high. I once borrowed an aircraft-type IR measurement tool, used for monitoring air-cooled airplane engines during test runs, to measure the temps on my 750 (in 2006 while working on my book). I ran the bike hard (5000-7000 RPM for 2 miles on airport tarmac) and immediately stopped and measured the temps. It was a hot, almost 100-degree (F)    day, no wind. I had loosened the point cover screws beforehand so I could get quickly inside to measure those temps, as I was also designing the Transistor Ignition then and supposed I might put it down there, like a Dyna. To my surprise, I found temps VERY high inside that area, almost 250 degrees IIRC (have to go look it up again), which stopped me from making an in-cover version of it. This also pointed to why my own Dyna S and Dyna III failed, IMO. Since you must derate any semiconductor's current-carrying ability according to the temperature at which it will be heated, this left the power transistors with less than 1/2 amp of safe current load at 400v kickback potential from the Honda coils (Dyna coils are even higher). They may work for a while at high temps like this, but not for a long time. They age quickly "up there", and I wanted my design to last as long as these bikes can: about 200+ hours, at the least. In the end, my boxes are designed to over 10,000 hours expected life if used without cooling and bucking Dyna/Accel 3-ohm coils. That ought to do it...

[PeWe]

Thank you guys for all technical thoughts and ideas.
My idle is not that bad when looking at it with an ignition light, rather stable.

If the ignition is spot on F at idle, inside full advance marks at appr. 2600-2700rpm (K6 std 2500).

I checked rpm on the CB750 tacho which is tricky while holding the ignition light. I had help from a friend to read to verify what I had seen. It is sure before 3000rpm and earliest 2600 or later.

The rotor is moving freely with a distinct return without any mechanical scraping. I do not remember any lash when the springs hold it back.

The engine is far away from OEM
- 836 pistons with higher CR. (Probably NOS cast RC)
- Ported head with larger inlet valves
- CX-7 cam (like Megacycle 125-75, but 3 more degrees on both IN and EX.)
- Lightened alternator, will help to get a more sensitive idle.
- Mikuni VM29 carbs. The final jetting is still to be decided.

I'm currently testing different setups of pilots and mains. Still running a little bit rich on idle.

I have read the other thread where the slots were done due to the old type of advance unit with pins. My advance unit has no pins.

[AJK]

Hi Pewe,

This is not at all necessary, but a nice "side effect" since you have a pamco. Pick one of these laser rpm counters off ebay, they are fairly cheap. You can paint the edge of your pamco rotor with black paint (or sharpie), then add a small piece of reflective strip (comes with your counter). With it you can see how far your tacho is out and also do other things like help tune your air screw mixture settings and get idle rpm perfect. Again, none of this is necessary, but these things are cheap for what they are and i'm sure you will find other uses for it around the home if you play with such things.
  My tacho (clock) was reading 15% higher than what the digital unit was showing, so nice to know.

Hi Hondaman,
Couldn't agree more that "good engineering practice" means that all things should be kept away from the heat (as the temperature under that points cover is nothing to sneeze at after a hot soak). I can't touch it anyway. Nice that you have some measurements.
  The nice things like dyna & pamco is that they are fairly compact and all self contained in the points cavity, so this is convenient. The downside is that the electronics get hot and lifetime can suffer & your 100% on reduced lifetime. We have the 'Arrhenius Equation' which basically says the hotter things get, the worse these problems become.
  http://en.wikipedia.org/wiki/Arrhenius_equation

This is the part i wanted to point out as quoted from the link...

"the reaction rate doubles for every 10 degree Celsius increase in temperature."    So its an exponential effect.


Your approach of a separate (external) transistorized module away from the points cover wins hands down in this respect. No argument from me & also how i would do it (however with a micro - but thats my field).

Air port tarmac (now that sounds like fun) 

[PeWe]

I have had thoughts about a rev counter that I can buy cheap in the local car part shop here. ($50-60US)
An inductive one that I can check hands free while I use the ignition light. Multimeter with rpm counter.

I guess that the laser will be more reliable when it reads the spinning part, the actual rpm of the crank. The inductive reads the ignition pulse in the spark plug wire that is affected by electronic components.
-But both will show slightly wrong while the rotor moves by the advance unit, right?

[AJK]

You can get them from ebay
 http://www.ebay.com/itm/Digital-Photo-Laser-Non-Contact-Tachometer-Accurate-RPM-Measurement-DT-2234C-/111177652069

This ones around $17. They claim to be accurate to 0.1 RPM when reading under 1000 rpms, or accurate to 1RPM when reading over 1000rpms. Takes a 9v battery.

If you use it on your bike, close to +/- 50rpms will be good enough as you will find your idle won't be dead smooth & the readings will jump around. If your a hobbyist, you will find other uses for it as well, like lathes and other tooling work.

[PeWe]

No shipping to Sweden.
This one is shipped from UK where they usually have low shipping rates.  I might buy this one too when it is that cheap...

http://www.ebay.co.uk/itm/New-LCD-Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-Meter-/141081760470?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item20d920d2d6

NO extra fee to our government (VAT) +30% of total price with shipping as I have to pay for shipments from outside EU.

[Loctite77]

Resurrecting this old thread as well.... This was very helpful for me as I just installed Pamco ignition. First install noticed that the springs were loose and the advance arms were floppy. Timed at 1000rpm and it was nice and steady dead on the F mark for both sets of cylinders. However it wood not reach the advance marks, even at very high RpMs up around 5000 it would stop a few mm's before the advance marks.

So I read this thread and shortened the springs as mentioned. Reinstalled and timed to F..... Now it does not start to advance at all until about 2000 rpm and reaches full advance at closer to 3000, but not earlier than 2800. It's hard to work the throttle, timing light and read the tach at the same time but those numbers are pretty close.

I'm wondering if I made the springs too tight as it seems that may be a little high on the revs to reach full advance. If it is will this create any problems? I have not ridden it yet as I am still finishing some electrical but should get it out in a few days.

Thanks peewee hondaman and everyone else for all of the knowledge.


Yeah Son!

[Erny]

Resurecting this old thread of PeWe again...

I'd like to know your advice - on my 750 K7 (1977 model, US), all stock, ign set to F, points setup with Hondaman ign.
Full advance is comes about 2500 BUT with wtih more RPMs, it goes significantly beyond max advance (double marks). Is that normal?
I thought there is some mechanical block that will not allow advance going beyond these max marks.

Note: Honda service manual for K7 specifies full advance (25 degree) at 2500 RPMs and F (Fire) as 10 degree

[PeWe]

I have read about too much advance and need to fix advancer so arms will hit stops earlier. Either bend the stops or add material.
I'm sure Hondaman has described this somewhere. Look in the sticky Thoughts of Hondaman.
FAQ...

[Erny]

Thanks I'll have a look (also in Hondaman's book)

[HondaMan]

Resurecting this old thread of PeWe again...

I'd like to know your advice - on my 750 K7 (1977 model, US), all stock, ign set to F, points setup with Hondaman ign.
Full advance is comes about 2500 BUT with wtih more RPMs, it goes significantly beyond max advance (double marks). Is that normal?
I thought there is some mechanical block that will not allow advance going beyond these max marks.

Note: Honda service manual for K7 specifies full advance (25 degree) at 2500 RPMs and F (Fire) as 10 degree

In a word: no. It won't hurt to go further. The guys who were making the 12k, 14k and 16k RPM versions of these engines for their midget racers in the 1970s (near my shop) had, in some cases, just removed the stops altogether. The max advance angle on this engine has more to do with Honda's Warranty Department wishes than with engine performance, as the octane requirements become non-linear beyond 40 degrees of spark advance.

I had (for more than 25 years) a most remarkable Ford Galaxie LTD 2-door limited-production (mostly for Denver, CO and Salt Lake City, UT markets). It's 390 CID "H" engine (made only 2 years) had a special distributor, unlike the more common Police Interceptor distributor. This one had 2 different springs on the advancer, the first which caused 15 degrees of advance to begin at 1000 RPM and reach that point at 2000 RPM, right where it applies max torque and economy for in-town driving. The second, stronger, spring slowly let the advance reach 45 degrees by 3200 RPM, where I got nearly 20 MPH from that big-block engine, running on Regular grade gas. The distributor had unusual bushings inside and when it finally wore out at 200,000 miles the only one I could get (couldn't get the bushings, didn't have a lathe yet) was the Police Interceptor version. This one had 2 equal springs and 40 degrees total advance, and it never ran the same again, having less power and MPG all thru the range. I saved the old distributor until I got my lathe, but just then Obamacare took away my beloved Ford...