Ross’ 1975 CB750

[Galactica]

a couple of puzzling things.

The brake light doesn’t come on until the front brake lever is pulled hard almost to the bar.  Rear is fine. 
The rear park lights don’t work (neither of them), signals work fine. 

[Runningdog]

a couple of puzzling things.

The brake light doesn’t come on until the front brake lever is pulled hard almost to the bar.  Rear is fine. 
The rear park lights don’t work (neither of them), signals work fine. 

In both cases, could be a dirty switch......trak them apart and give them a good spray with conract cleaner and four ought steel wool, or something similar.....nail file?

[Steve G.]

   Is the brake light switch on the front a pressure type switch?

[Galactica]

   Is the brake light switch on the front a pressure type switch?

Yup.  It’s a pressure switch.  Anybody have a spare one kicking around that they don’t need?

Pretty sure the real park lighting problem is a wiring issue I need to track down. 

[kilowop]

Glad things are perking up.  BUT, don't worry about a parking light.  You are SUPPOSED to be riding the bloody thing, NOT parking it.

[Galactica]

Heh heh.  Ya, and if you ride fast enough, everyone else is too far behind to care about park/running lights.  Myself, I like to be as visible as possible.  It’s not urgent, or essential, but I will take the time to track it down and get the lights working.

I’ll try giving Bent Bike a call to se if they have a pressure switch and tach cable.  Then I have to try and track down some good quality thin wall 5mm (3/16”) fuel line and clips.

[Bucko]

Yup.  It’s a pressure switch.  Anybody have a spare one kicking around that they don’t need?

Here's one for about US$11 including free shipping to US.

https://www.cb750supply.com/products/5/honda-electrical/46/brake-switches

[Galactica]

I called Bent Bike.  He said he’d look around and see if he has a switch and tach cable and get back to me today or tomorrow.  They also sell the nice flexible thin was fuel line and spring clips. 

[Galactica]

After getting some extra eyes to look at the wiring diagram, we determined that the rear signal are just that.  Not park/ running lights.  The front signals are dual.

Started it again this afternoon.  Checked again to see if everything was working.  Had to replace the high beam indicator bulb in the dashboard.  Now everything works.  Except the front brake light pressure switch.  It works but only at high pressure.  Ordered a new one along with a tach cable. 

[Galactica]

I was hoping the weather would clear enough this afternoon to run the bike up and down the lane a couple of times.  No luck.  Here are a couple of pictures of how she’ll look in the mid term.  Long term hopefully will see paint, new fork ears, exhaust collars, mirrors, etc, etc.

[magpie]

What a great thread! Thanks for posting all your hard work. I'm just at the start of another CB750K resto. I've started with a pretty good K6 but as usual the further I get into it the better I want to make it. Cheers, Cliff.

[Galactica]

Thanks Cliff. 

So, after running the bike on the lift for a bit yesterday, I switched it off, stuffed all the wires back into the headlight bucket and reinstalled the headlight.  I got some appropriate 5mm fuel line today and hooked it all up.  The weather is heavy overcast but dry.  I backed it off the lift into the lane for a test.  Fuel tap on, choke on, key on, clutch lever in, press starter button.   Damn.  Motor turns but won’t fire!   What the hell?   I just had it running.  Damn.  Back on the lift.

Check it has fuel.  Yup.  Check it has spark.  Nope.  Tank off, check coil connections, okay.  Remove headlight.  Pull out the wires.  Yup, a black wire from the right switch block had come adrift.  Plug it back in.  Hook up temporary tank.  Fuel on, choke on, key on, pull in clutch lever, hit the start button.  VROOM. 

So I put it all back together, back it off the lift.  Run it up and down the lane once.  Front brake is not working well at all.  I have a master cylinder rebuild kit, brake light pressure switch, and tach cable ordered.  Might be here end of next week.

But hey, it runs. 

One note of concern though.  At idle, in neutral with the clutch lever out, the transmission rattles quite noticeably.  The noise mostly goes away when you pull in the clutch lever.  Is that normal for a CB 750?  Likely causes?  Should I be concerned? 

[Steve G.]

  Ah yes, the dreaded “Hy-Vo primary chain rattle , a well known character trait of sohc 750’s. Two massive chains without tensioners connecting engine to gearbox.
 The noise of the chains can be dramatically reduced by proper carb synchronization, and slightly higher idle.
  The fix is a case split, but will often return.

[Galactica]

Ya, I reckoned it was the primary chain.  I’ve read there is a hydraulic tensioner, but over time requires a bit if rpm to get the pressure up.  I also read that an engine imbalanced by compression differential can also be a factor.

I’ve not yet done a vacuum synch, only on the bench. 

[Bucko]

One note of concern though.  At idle, in neutral with the clutch lever out, the transmission rattles quite noticeably.  The noise mostly goes away when you pull in the clutch lever.  Is that normal for a CB 750?  Likely causes?  Should I be concerned?  

Certainly normal for later model CB750/900/1100Fs.  Telltale sign is that the rattle goes away when you pull in the clutch.  The rubber dampers in the clutch basket get hard and shrink, leading to the basket rattling.  The rubber dampers are not supposed to be replaceable, but they can in fact be replaced and kits are available from various sources.  Not sure if all that applies to your particular model though.  

This is what the basket looks like with the back cover removed, revealing the rubber dampers (which in this case appear to be new).