Ross’ 1975 CB750

[Steve G.]

https://www.charlies-place.com/product-tag/cb750/

[Galactica]

It’s been a few weeks since I really even looked at the CB750. 

The last couple of times I had a pretty hard time getting it started.  Seemed ok once warm.  Haven’t had a chance yet to take it on the street.  Couple things I was lazy with when I did my initial checks.  The previous owner assured me it had been running when parked.  Ok, I guess. 

I had checked compression, acceptable.  I had checked the valve clearances, pretty much spot on.  I had looked at the points, they looked ok so I left it at that.  Today I thought I’d actually check the timing and points gaps.  Using a test light to check static timing, the timing on both sets of points was a couple degrees advanced so I put them spot on.  The points gaps were a bit tight, .010” vs .014” (.012”-.016” spec)so I adjusted them as well.  In order to make it easier to turn the motor with a wrench while making those adjustments, I had removed the spark plugs.  They were all black and sooty, so I cleaned them up before reinstalling them.  So she’s definitely running rich.

After our conversation at club coffee on Sunday I reckoned I’d do the Wired George coil mod to ensure 12v at the coils as power goes through the kill switch to get to the coils.  But I checked voltage at the coils first.  Whoa, 12.3 volts there, so no need to mess about with a modification. 

Gonna hold off on anything else until we get some riding weather, see if I can get the rich condition sorted. Well… maybe if I get bored I might attack the front brake again with braided lines.  We’ll see.

[Steve G.]

  An original 45+ year old coil I would automatically have skepticism.

  Regarding the desired color of choice, how many times have you flipped your lucky coin??

[hardrockminer]

The coil is easy to check.  Measure ohms on the primary and secondary sides.  I don't know Hondas but for Kawasaki the numbers should be around 4 ohm primary and 15,000 ohm secondary. 

For the carbs, I would check choke plungers if the engine is running rich and float valves are correct.

[Galactica]

Ya, not at all concerned about the coils.  Have good spark.  I was just bored not having done anything on the bike for a while.  I’d done the Wired George thing on all of my Kawasakis just because.  Never really noticed a difference. 

[Galactica]

And the carb choke is actually a choke not a fuel enrichment.

[hardrockminer]

Yeah, the WG coil voltage improvement isn't something you will immediately notice.  I've done it on my KZ1000, but not my Z1B.  Both bikes run just fine.  I converted the KZ1000 over to 4 ohm Dyna coils with a Dyna pickup.  The Z1B is the same but coils are original.

[Galactica]

  An original 45+ year old coil I would automatically have skepticism.

  Regarding the desired color of choice, how many times have you flipped your lucky coin??

I suppose time can degrade the coils, but I have really strong spark.  There’s not even 23k miles on this bike. 

Colour choice?  Flipped a dozen times.  I was really stoked when I got this bike for sentimental reasons I guess.  As I said, I had one in Planet Blue when I met my wife in 1977.  I thought (hoped) she’d be enthused as well…not so much.  I asked her opinion about colour choose if/when I have it painted.  Planet Blue or Apricot Sparkle Red?  Well…she couldn’t give a sh*t. 

Gotta get her running well first.  The bike that is.  My wife works just fine.  I’m still leaning towards blue paint.  For the bike that is.  My wife’s colour is just fine.

[Galactica]

As to the rich burning condition, I wonder if the float levels are a bit high.  I set them using the dry setting method as per Honda.  I wish I’d kept the clear tube tool for my Kawasakis.  I reckon it would fit the Honda  bowl drains as well. 

[Twisted_Twin]

The coil mod may help more just on startup than anything.

The starter will pull a lot of power out of the whole system and if you've got any way to reduce the voltage drop to the coils while the starter is engaged it will probably make a difference.

[Bucko]

After our conversation at club coffee on Sunday I reckoned I’d do the Wired George coil mod to ensure 12v at the coils as power goes through the kill switch to get to the coils.  But I checked voltage at the coils first.  Whoa, 12.3 volts there, so no need to mess about with a modification.  

That mod is very popular with the 750/900/1100F crowd.  I'm not a big fan as it just adds more points of failure and if there is a voltage/current problem, just cleaning the various contact points in the circuits fixes it.  Having said that, measuring more than 12V at the coil is a very good sign, but the current in the coils can still be reduced by dirty contacts which may not show up in a static voltage test.  As a test, you could just run a temporary jumper directly from the battery to the coil and see if the starting behaviour improves (if you wanted to get technical, you could measure the current at the coil with and without the jumper and if they're roughly the same, you're good, and if not, cleaning the various contacts will fix it).

[Galactica]

I’m pretty confident the starting issue is carburetors.  With clean plugs it starts with like half a crank.  I might just pull the carbs off again and lower the floats a couple of mm.

I agree really with everything you’ve said about the coil power mod.  I was thinking about doing it just because.  But when I saw 12.3v at the coils, what’s the point?

[hardrockminer]

As to the rich burning condition, I wonder if the float levels are a bit high.  I set them using the dry setting method as per Honda.  I wish I’d kept the clear tube tool for my Kawasakis.  I reckon it would fit the Honda  bowl drains as well. 

I don't use the clear tube method any longer.  I just pull the carbs off the bike, drop the bowls and set the carbs on four glasses with the floats moving freely.  Then turn the fuel on and let the glasses fill until the float valve closes.  Then measure the distance from the bottom of the miniscus to the top of the glass.  I actually do it with two carbs at a time but you can do all four that way as well.

[Galactica]

What the heck is a miniscus when it’s at home?

[Runningdog]

Meniscus is the curve in the upper surface of a liquid close to the surface of a container.   Most liquids have this tendancy, including water, and gasoline.