Topic: 1973 CB750 Rescue

[j343my]

It got over 10° C here today, so it seemed like the perfect opportunity for a first startup attempt.


It took a little playing with the choke and the throttle, but I managed to get her fired up...  on three cylinders.  The culprit turned out to be a stuck float needle on the second carb.  After that was fixed and shaking off the cobwebs for a bit, she ran quite well for just a bench sync. 

There were a few issues.  It turns out the insulation on the positive battery cable had worn through, and at one point it grounded out on the frame and started sparking like crazy.  Next the tach seems to be a bit lazy.  I'm not really sure what to do about that, so I'll have to dig into that a bit.  The last one was it sounded like the clutch was rattling a bit.  It was kind of intermittent, and pulling in the lever quieted it down.

Also the started safety relay somehow fixed itself.  I can now start the bike without pulling in the clutch lever.

An interesting thing I noticed also is the headlight has a plastic bubble over it.  Would that be some sort of aftermarket add on?


So now that I have a runner, I'm going to start working my way towards a fully functional motorcycle.  I'm thinking the game plan is to start at the back and move towards the front.

[newday777]

Congratulations on getting it fired up.
The clutch rattle would be the carbs out of sync.  Dial the carbs in with vacuum gauges and the rattle will/should be gone.
That bubble on the headlight is aftermarket.

[j343my]

Congratulations on getting it fired up.
The clutch rattle would be the carbs out of sync.  Dial the carbs in with vacuum gauges and the rattle will/should be gone.
That bubble on the headlight is aftermarket.

Thanks!  I wasn't aware carbs being out of sync could cause the clutch to rattle.  Doing a proper sync is on the to-do list, so hopefully that fixes that issue.

[RAFster122s]

The bubbles don’t hold up to UV for a long time of parked outside, they can keep you from breaking a headlamp, but a set of them for car use years ago was about 1/2 the cost of a discounted pair of headlamp bulbs for the car… bike headlamps aren’t nearly as cheap or as readily available.
The H4 replaceable bulb headlamps Cibie makes are far superior optically in my opinion to most out there. Valeo makes them.

[HondaMan]

Also check the ignition timing under the strobe light: see if the timing marks are "jittering" back-and-forth. This will also make for clutch rattle.

The cure for this is to remove the spark advancer, straighten the shaft on the end of the crankshaft to within 0.004" or less, and tighten up the springs on the spark advancer while it is out: they are likely loose. Also check to see how soon the timing marks reach full advance: it must not happen before 2500 RPM (3000 RPM is better with today's gas, and mine is more like 3500+ RPM) - this can be adjusted by shortening the spring(s) by about 1 coil on one spring, or 1/2 coil on each, after the slack is taken out.

[j343my]

Also check the ignition timing under the strobe light: see if the timing marks are "jittering" back-and-forth. This will also make for clutch rattle.

The cure for this is to remove the spark advancer, straighten the shaft on the end of the crankshaft to within 0.004" or less, and tighten up the springs on the spark advancer while it is out: they are likely loose. Also check to see how soon the timing marks reach full advance: it must not happen before 2500 RPM (3000 RPM is better with today's gas, and mine is more like 3500+ RPM) - this can be adjusted by shortening the spring(s) by about 1 coil on one spring, or 1/2 coil on each, after the slack is taken out.

I did check the timing with a strobe light.  it did seem to be jittering a bit, but I didn't know if that was because the idle was a little erratic.  I will look into that a bit more.  What's the best way to straighten the shaft if needed?

[BenelliSEI]

Congratulations on getting it fired up.
The clutch rattle would be the carbs out of sync.  Dial the carbs in with vacuum gauges and the rattle will/should be gone.
That bubble on the headlight is aftermarket.

+1 to doing a carb sync first. A smooth idle makes for a much quieter clutch basket....

[j343my]

Continuing to pick away at this bike over the past couple weeks.

Rear end teardown:


After unbolting the shocks from the swingarm, I could feel there was some slop in the bushings.  So I'm going to swap out the old ones for bronze bushings.

I was mentally prepared for a battle in getting the old ones out, but they actually came out quite easily.  I did the washer trick I think is saw on here somewhere.  Grind the edged down of a thick washer so it slips into the swingarm and use it to pound out the bushings.  I gave the swingarm/old bushings a little blowtorch action first and they came right out.


The wheel bearings on the other hand, were more of an ordeal than I was expecting mainly because of the retainers.  But the 50 year old bearings have been removed:


Hub cleaned up and new bearings installed:


My swingarm bushing and rear tire should be arriving in the week, so that's next on the to-do list.

[BenelliSEI]

That’s one of the crustiest wheel bearings I’ve seen. Good to look!

[HondaMan]

That’s one of the crustiest wheel bearings I’ve seen. Good to look!

+1 !
The only worse one than that I have seen came from a CB500-4 that sat in the floodwaters from the Missouri River, circa 2008, I think it was: it was in 24" of water for a week, then sat for 2 years before someone put some love to it again.

[j343my]

Started working on the front brake this week.

Reservoir full of something that was probably brake fluid at some point:


The piston was stuck, but it was no match for the grease gun trick:


The master cylinder was also stuck.  I think a rebuild kit is probably a good idea:

[HondaMan]

That brake-fluid crust came from someone mixing synthetic brake fluid with the old DOT3 (or J1703) fluid. They probably didn't know that you can't do that...
Use DOT3 fluid after the rebuild. It is harder to find, but chances are you'll only need to find it once!

[newday777]

I was working on a friend's bike a couple years ago and found a cup of brown sugar in the master cylinder.....

[grcamna2]

I was working on a friend's bike a couple years ago and found a cup of brown sugar in the master cylinder.....

I hope you didn't taste it to make sure   

[newday777]

I was working on a friend's bike a couple years ago and found a cup of brown sugar in the master cylinder.....

I hope you didn't taste it to make sure   

😆 🤣 LOL

[Johnie]

Make sure you get the 2 holes in the bottom of the master cylinder cup clear. One is bigger than the other. For the smaller one I use 1 strand from my wire wheel to gently clear it. Once cleared you can see it come through the body of the unit. Not sure what the inside of the MC looks like, but before I put the guts into it I usually give it a slight cleaning with the hone to be sure it is smooth. You don't want to wear out the rubber seal on bad edges inside. One last thing did you check to be sure you have the "D" washers at the top of the triple tree fork clamps. Without them you can over tighten those clamps and break the tree.

[j343my]

Make sure you get the 2 holes in the bottom of the master cylinder cup clear. One is bigger than the other. For the smaller one I use 1 strand from my wire wheel to gently clear it. Once cleared you can see it come through the body of the unit. Not sure what the inside of the MC looks like, but before I put the guts into it I usually give it a slight cleaning with the hone to be sure it is smooth. You don't want to wear out the rubber seal on bad edges inside. One last thing did you check to be sure you have the "D" washers at the top of the triple tree fork clamps. Without them you can over tighten those clamps and break the tree.

Thanks.  I dealt with the same thing on the master cylinder of one of my XS650s last year, so I already have the perfect hole poking wire.  The inside looked about the same as the parts that came out of it, but its cleaning up nicely so far.

The "D" washers are all accounted for.

[j343my]

And the back end of the bike is done.


Ended up changing the wheel bearings, swing arm bushings and upper shock bushings.  The brake shoes and drum looked good, so they just got cleaned up and adjusted.  The sprockets looked good also, but I did swap on a nearly new chain from one of my XS650s.  That bikes needs a gold chain anyway.  For the rear tire I went with a Dunlop K70 which I know is a controversial tire on here, but it looks the part and is cost effective.

While everything was torn down I Evaporusted and cleaned up the shocks.  They turned out quite well.  Here's a comparison between finished and unfinished:


Now onto the front end.

 

[denward17]

j343my,
Did you make that portable gas tank?

Those shocks turned out really nice...

[grcamna2]

j343my,
Did you make that portable gas tank?

Those shocks turned out really nice...

Yes,the difference is literally Night to Day on how beautiful the shock turned out 

That portable test tank hanging from the ceiling,a Motion Pro ?

[newday777]

And the back end of the bike is done.


Ended up changing the wheel bearings, swing arm bushings and upper shock bushings.  The brake shoes and drum looked good, so they just got cleaned up and adjusted.  The sprockets looked good also, but I did swap on a nearly new chain from one of my XS650s.  That bikes needs a gold chain anyway.  For the rear tire I went with a Dunlop K70 which I know is a controversial tire on here, but it looks the part and is cost effective.

While everything was torn down I Evaporusted and cleaned up the shocks.  They turned out quite well.  Here's a comparison between finished and unfinished:


Now onto the front end.

Nice. Did you put the shocks in liquid bath of Evaporust?

[RAFster122s]

Terry in Australia uses a phosphoric acid solution to soak the exhaust systems in to clean them up and restore the shine to the chrome. I believe he uses a heavy trash can liner in a 30gal trash can and puts stuff in between the bin and liner to take up space for odd shaped items like exhausts, etc.

I do not recall if he does anything to remove any soot and oil inside the exhaust first.

Milk stone cleaner at the farm and fleet used for milk tanks is probably going to be the cheapest source for phosphoric acid in larger quantities.

Thought I would share that hint for cleaning up the exhausts for now…

[j343my]

j343my,
Did you make that portable gas tank?

Those shocks turned out really nice...

Yes,the difference is literally Night to Day on how beautiful the shock turned out 

That portable test tank hanging from the ceiling,a Motion Pro ?

The fuel tank is from vintagecb750.com

Nice. Did you put the shocks in liquid bath of Evaporust?

Yes, I disassembled them and put them in a bucket of Evaporust for about a week.

Terry in Australia uses a phosphoric acid solution to soak the exhaust systems in to clean them up and restore the shine to the chrome. I believe he uses a heavy trash can liner in a 30gal trash can and puts stuff in between the bin and liner to take up space for odd shaped items like exhausts, etc.

I do not recall if he does anything to remove any soot and oil inside the exhaust first.

Milk stone cleaner at the farm and fleet used for milk tanks is probably going to be the cheapest source for phosphoric acid in larger quantities.

Thought I would share that hint for cleaning up the exhausts for now…

Thanks.  That's something I've never heard of before.  I'm still unsure what I'm going to do with the exhaust.  The headers are in quite good shape, but the collector/mufflers have quite a bit of surface rust.  I was thinking of stripping the collectors back and painting that section maybe.

[newday777]

Nice. Did you put the shocks in liquid bath of Evaporust?

Yes, I disassembled them and put them in a bucket of Evaporust for about a week.

Thanks I'll give it a try as I have a 5 gallon bucket of it for cleaning gas tanks.

[Johnie]

I have that same portable gas tank which works good. Once at the dismay of my neighbor I hooked that portable to my helmet to give the bike a quick test drive. My tank wasn't ready yet, but just had to know if the mechanical was good to go...LOL. Those shocks turned out really great. Looking good!