Topic: CB750K4 what did I get myself into?

[newday777]

What are those rubber damper things? I didn't see any in my Comstars when I pulled the bearings. Do the Comstars get them?

I looked up a parts diagram of the comstar rear wheel and it doesn't look like you have the rubber rear wheel dampers. But not certain since I haven't had that style of wheel before.

The comstars do have the rubber dampness. #6 (on a 78 CB750 F diagram)

Oh, great! Another thing to do Looks like they're in the hub? Now how do I get the cap off for that hub!

Pull on the sprocket while feet on the rim.

[BrockCB750]

More progress tonight. Go the rear wheel together...

[SOHC4 Cafe Racer Fan]

Unmistakable MR head work!

[BrockCB750]

Made a lot more progress.

[BrockCB750]

I should be getting the engine and head back within the next two weeks. I am not sure how much more mocking up I need to do at this point to feel comfortable with cutting, prepping and painting the frame. I am going with paint because it feels like there is 100% chance I will scratch the finish on the frame and want the ability to touch it up.

My next steps as far as I can tell:
1) Pull everything apart.
2) Cut (tabs, center kickstand, etc), sand the frame
3) Fit the seathoop and seat.
4) Cut and drill an aluminum pan for under the hook for electronics
5) Get someone to weld the seat hoop on
6) Prep the frame and swingarm for paint.
7) Paint the frame and the swingarm.
Decide if I want to paint the head and valve cover.
9) Assemble the engine.
10) Get the engine into the frame.

[Galactica]

Great thread.  I’m now starting to go through and read some more threads than the one I started. I’ve a few comments.

In the ultrasonic cleaner, I use a 4:1 solution of water:Simple Green.  You do have to be careful because it will blacken the aluminum carb bodies.

Acetone is good carb cleaner.  Aerosol carb cleaner is just acetone and propellant (probably propane).

I’ve cleaned stock jets in acetone for reuse with no issues.

Engine stumble or stalling at throttle tip in, can be caused by an initial lean-out condition.  All else being well, tiny passages in the carb bodies can still be restricted.  When gasoline has been allowed to sit in the carbs over time to evaporate, the resulting varnish can be extremely hard to remove.  Takes lots of ultrasonic cleaning, maybe immersed and soaked in acetone.  The lean-out condition typical of older carbs was addressed later by manufacturers by going to a pumper carb.  These tiny pumper passages in the carb bodies (I have experience with a ‘78CB750) can be difficult to clear of evaporated gasoline varnish.

[HondaMan]

Great thread.  I’m now starting to go through and read some more threads than the one I started. I’ve a few comments.

In the ultrasonic cleaner, I use a 4:1 solution of water:Simple Green.  You do have to be careful because it will blacken the aluminum carb bodies.

Acetone is good carb cleaner.  Aerosol carb cleaner is just acetone and propellant (probably propane).

I’ve cleaned stock jets in acetone for reuse with no issues.

Engine stumble or stalling at throttle tip in, can be caused by an initial lean-out condition.  All else being well, tiny passages in the carb bodies can still be restricted.  When gasoline has been allowed to sit in the carbs over time to evaporate, the resulting varnish can be extremely hard to remove.  Takes lots of ultrasonic cleaning, maybe immersed and soaked in acetone.  The lean-out condition typical of older carbs was addressed later by manufacturers by going to a pumper carb.  These tiny pumper passages in the carb bodies (I have experience with a ‘78CB750) can be difficult to clear of evaporated gasoline varnish.

Some good advice, here! I use acetone, too, and/or lacquer thinner.
In the carb bodies of the K0-K6 and F0 bikes with the 086a carbs, there is a long passage between the bell area at the back of the carb and the mainjet. This shows up as a little brass port in the back side of the carb. It's passages goes straight forward a few mm, then turns about 30 degrees downward, then turns striaght toward the top of the mainjet's emulsifier tube, entering that site at an angle near the little brass jet in the throat of the carb. This passage often plugs up with a white powder (zinc corrosion) if the carbs were exposed to the years of MTBE gasolines, then parked and left to dry out. When this passage plugs up fully or partly, the cylinder will foul plugs in less than 10 miles (usually in 4 or 5 miles). Clean it manually by removing the needle jet from the carb's body (I use an old screwdriver that I ground the tip to fit nicely into the needle jet to press or tap it out) and then run a soft wire thru from the little brass hole in the bell area of the carb, all the way to the mainjet emulsifier's passage. It will come into the passage at about a 40 degree angle. Go gently, feel your way around the 'corners' at the back as it starts in, and push it back-and-forth to push out any of the white powder you might find.

This didn't accumulate in the bikes that were run thru the MTBE years and then kept running, nor those that were parked before that nightmare was imposed on us all: it occurs in those that were parked with gas in them during those years.

[newday777]

Great thread.  I’m now starting to go through and read some more threads than the one I started. I’ve a few comments.

In the ultrasonic cleaner, I use a 4:1 solution of water:Simple Green.  You do have to be careful because it will blacken the aluminum carb bodies.

Acetone is good carb cleaner.  Aerosol carb cleaner is just acetone and propellant (probably propane).

I’ve cleaned stock jets in acetone for reuse with no issues.

Engine stumble or stalling at throttle tip in, can be caused by an initial lean-out condition.  All else being well, tiny passages in the carb bodies can still be restricted.  When gasoline has been allowed to sit in the carbs over time to evaporate, the resulting varnish can be extremely hard to remove.  Takes lots of ultrasonic cleaning, maybe immersed and soaked in acetone.  The lean-out condition typical of older carbs was addressed later by manufacturers by going to a pumper carb.  These tiny pumper passages in the carb bodies (I have experience with a ‘78CB750) can be difficult to clear of evaporated gasoline varnish.

Some good advice, here! I use acetone, too, and/or lacquer thinner.
In the carb bodies of the K0-K6 and F0 bikes with the 086a carbs, there is a long passage between the bell area at the back of the carb and the mainjet. This shows up as a little brass port in the back side of the carb. It's passages goes straight forward a few mm, then turns about 30 degrees downward, then turns striaght toward the top of the mainjet's emulsifier tube, entering that site at an angle near the little brass jet in the throat of the carb. This passage often plugs up with a white powder (zinc corrosion) if the carbs were exposed to the years of MTBE gasolines, then parked and left to dry out. When this passage plugs up fully or partly, the cylinder will foul plugs in less than 10 miles (usually in 4 or 5 miles). Clean it manually by removing the needle jet from the carb's body (I use an old screwdriver that I ground the tip to fit nicely into the needle jet to press or tap it out) and then run a soft wire thru from the little brass hole in the bell area of the carb, all the way to the mainjet emulsifier's passage. It will come into the passage at about a 40 degree angle. Go gently, feel your way around the 'corners' at the back as it starts in, and push it back-and-forth to push out any of the white powder you might find.

This didn't accumulate in the bikes that were run thru the MTBE years and then kept running, nor those that were parked before that nightmare was imposed on us all: it occurs in those that were parked with gas in them during those years.

This is where pictures sure would come in handy to show the spots pointed at in the carb and the screwdriver ground to fit and going in. Do you have pictures of this? Did you put them in the 750 book?

Edit: I did some net searching for the time-line of MTBE use, found it started in 1979 in many big air polluted cities especially in the winter months as late as May known as winter blend gasoline and mostly stopped by 2006 when ethanol replaced it. MTBE still is being made and exported to other countries especially to Mexico.
More disturbing than even the damage it did to our carbs is the health effects it is still producing from the ground water pollution it caused through leaking in ground fuel storage tanks as also airborne particles of it while being pumped, inhaled and and airborne spread to the ground into the ground water after dropping to the ground........
I have a friend who tests wells and municipal water that says it will be with us a long long time to come, it's not even close to being fully disclosed as to the damage it is doing to people and animals.

Some reading of it's exposed results from the National Library of Medicine
It says that MTBE in water is absorbed through skin pores and breathing taking showers.....

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6957274/

[MauiK3]

MTBE is one of the forever pollutants I think

[newday777]

MTBE is one of the forever pollutants I think

Yes that is what I've learned in this past year. It is hydoscopic so it mixes with the ground water and travels polluting more and more as it goes.
Thankfully there are filtration systems available, with state aid to clean well water at the house and off of municipality water supplies (yes they have high levels also) not much is said about it unfortunately because of the vast areas that have been contaminated by leaking gas and huge international companies that go in to an area and do their thing with the chemicals that they use and then run out of country when caught to avoid prosecution. There was a French company local @ 15 miles from me that did just that. Left the employees hanging out of jobs and the cities and towns surrounding areas from airborne particles polluting the ground water.

[BrockCB750]

Two quick questions. Having a hell of a time figuring out what these two mounting points are for.

1) Underside of the swingarm:


2) These on the frame:


Going to keep digging but hit a wall with these two.

Thanks!

[newday777]

Rear brake stay mounts to the front under the swingarm.

[newday777]

The second picture is the top of the air box mounting bolt.

[BrockCB750]

Great! Thanks newday777. Much appreciated!

[BrockCB750]

Before and after on the engine!

[Stev-o]

Wow...that engine looks great!

BTW - I'm a big supporter of powdercoat on frames, it is so much more durable. Have not had an issue of chipping it when installing a motor on a couple of my projects. Just need to protect the frame same as if it was painted.

[Floshenbarnical]

Before and after on the engine!

What a transformation! Doing god's work, resurrecting these tired old girls!

[BrockCB750]

Got a local-ish guy (MotoRelic) to help with frame hoop, battery, seat pan. electronics tray...





At this point I decided to go with powdercoating the frame and swingarm instead of paint.

[calj737]

Be mindful of the rear tire hitting that hoop pan under full compression. The stock “U” bend between the rails allowed suspension travel and avoided conflict with the seat.

[BrockCB750]

I didn't really consider that. Crap.

What are my options to mitigate that problem? The bike will be significantly lighter it will also never have a passenger. Can I just stiffen up the shocks a little above normal?

Thanks!

[Mark1976]

Wow...that engine looks great!

BTW - I'm a big supporter of powdercoat on frames, it is so much more durable. Have not had an issue of chipping it when installing a motor on a couple of my projects. Just need to protect the frame same as if it was painted.

   Without a doubt the best way to go, I refuse to paint frames these day, why do it when you can pay someone else to do it for ya, it's better looking and more durable and just easier.

[Floshenbarnical]

Wow...that engine looks great!

BTW - I'm a big supporter of powdercoat on frames, it is so much more durable. Have not had an issue of chipping it when installing a motor on a couple of my projects. Just need to protect the frame same as if it was painted.

   Without a doubt the best way to go, I refuse to paint frames these day, why do it when you can pay someone else to do it for ya, it's better looking and more durable and just easier.

But a can of Duplicolor cost $10 and it looks almost as good.

[BrockCB750]

Can someone confirm that my oil jets are installed correctly??

It feels weird that they sit in there loose like that:

[PeWe]

CB750?
NO, the oil orifice should sit flush with the head surface cam holders sit on, not raised.

O-ring look too thick.
Stock size needed, 5.8x,1.9mm
I have tested viton 6.0x2.0mm, too thick, o-ring cracked with risk of blocking the oil.
O-ring need space sideways when compressed.

[BrockCB750]

PeWe thank you! The jets needed a little nudge to seat correctly. Does this look better? Also swapped the oring.