Author Topic: Dorothy III - CB750  (Read 50541 times)

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Offline gnarlycharlie4u

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Re: CB750 made in March, 1970
« Reply #25 on: June 18, 2011, 02:38:52 pm »
# 4 carburator will not go over -15 of vacuum.  The other three I was able to tune to somewhat same reading - 22, but this one just will not go up.

I wonder why, and also the needle movement was excessive at least - maybe it has not stock cam, not sure how to find out.

Checked voltage and with revs up it is out of the chart - like 17V for 5 000 or so.  Will run it with headlight on from now on, looks like it is time for Oregon motorcycle parts.

are you sure your #4 cylinder is good and the vacuum connect to your carb is good?

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #26 on: June 20, 2011, 07:12:29 am »
Good idea, will test compression, I dont expect it to be low, there is no smoke, nothing, but you never know.

I will take carbs of again and reclean, the idle was rough again yesterady.  Took off the tank and dropped it - dent in tank, I would kick myself  >:(

Offline gnarlycharlie4u

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Re: CB750 made in March, 1970
« Reply #27 on: June 20, 2011, 08:15:27 am »
Good idea, will test compression, I dont expect it to be low, there is no smoke, nothing, but you never know.

I will take carbs of again and reclean, the idle was rough again yesterady.  Took off the tank and dropped it - dent in tank, I would kick myself  >:(

urgh... that sucks. I'm still meeting you wednesday right? You want to bring me the carbs and I can throw them in my ultrasonic cleaner?

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #28 on: June 20, 2011, 08:32:08 am »
Yeah, Wednesday holds, but carbs are ok, they may need some touch up, but nothing serious.  Thank you!

Where do you want to meet on Wednesday?

Offline gnarlycharlie4u

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Re: CB750 made in March, 1970
« Reply #29 on: June 20, 2011, 08:58:49 am »
Yeah, Wednesday holds, but carbs are ok, they may need some touch up, but nothing serious.  Thank you!

Where do you want to meet on Wednesday?

Does Manassas somewhere work for you? Or if you're not too far out I can just swing by your place.

As for the ultrasonic cleaner, that's mainly what it's for; touch ups. You do the majority of cleaning by hand but it gets stuff out of the little passages that it is impossible to clean yourself.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #30 on: June 21, 2011, 05:58:54 am »
Manassas sounds good, I have some errands to run on that day - Dicks or the plaza where Manassas Honda/Kawa is?

Offline gnarlycharlie4u

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Re: CB750 made in March, 1970
« Reply #31 on: June 21, 2011, 06:06:45 am »
Manassas sounds good, I have some errands to run on that day - Dicks or the plaza where Manassas Honda/Kawa is?

MHKS sounds good to me

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #32 on: June 21, 2011, 12:44:49 pm »
Cool, what time?

Offline gnarlycharlie4u

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Re: CB750 made in March, 1970
« Reply #33 on: June 21, 2011, 12:48:32 pm »
Cool, what time?

I get off around 6 ish? I'm like 5 mins from there so 6 works for me. I might be there a couple minutes early to pick up some gaskets for my goldwing.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #34 on: June 21, 2011, 12:52:58 pm »
OK, 6 it is.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #35 on: June 30, 2011, 05:53:15 pm »
Made an adapter to finally check compression.  Cold engine is 35 psi 1 through #4, hot engine is 45 on #1 and 45 psi on #4 - since I don't like to burn myself on purpose, I skipped 2 and 3, it is most likely the same.

I guess the top stands on the last leg and rebuild next winter makes sense.  Oh well:(

On the other hand I turn an adapter for oil pressure gauge from some 6061 and oil pressure @ 1000 rpm and hot engine is 20psi, looks like the bottom is in excellent shape.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #36 on: July 09, 2011, 03:39:55 pm »
Adjusted tappets for the first time - I think last time I did that was some http://en.wikipedia.org/wiki/GAZ-69 owned by my friends over in Czech:)

Checked the pressure on hot engine again and with the home made adapter it gave me 90 psi.  I call it good and giving up on making new adapter, it runs well enough.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #37 on: July 11, 2011, 05:30:40 pm »
My home made aluminum oil pressure adapter, too big, but will do for now.  Seals just fine. 

Those two holes are for tightening, I have only lathe and hex head would be too complicated even in 6061.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #38 on: July 29, 2011, 02:58:07 pm »
I got a mercury vacuum synchro tool from a friend and attempted to synchro carbs.  It just will not hold idle.   Decided to repack carbs with a kit from Vintage750,   bike is parked for now in front of the shop with the gas tank on a ladder.  Hope the kit helps.  It runs ok, but  when I gun it, it does not drop down to the idle, stays typically over 2500.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #39 on: August 02, 2011, 04:29:47 pm »
Replaced slow jets with new ones from Vintage CB.  Boy, what a difference in idle!  Actually rekindled my interest in the bike.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #40 on: August 04, 2011, 06:08:50 pm »
Busy like one legged jockey, aiming for VA safety inspection next week.

- adjusted regulator
- greased rear fork
- adjusted rear brake
- #3 carb keeps peeing  from the bowl
- adjusted clutch
- adjusted tappets
- swapped main jets for old ones - 140

gotta change the oil this weekend.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #41 on: August 16, 2011, 03:23:21 pm »
My printed manual, at least the way I understood it, said to loosen up the adjusting bolt and tighten it again.  There was nowhere anything about valve cam position or TDCC.  HondaMan post says this:

First question above: "What is TDCC?" That's "Top Dead Center, Compression" stroke. The camchain should be set when cylinder #1 is 15 degrees past TDCC or #4 is passing TDC WHILE THE INTAKE VALVE IS STARTING TO OPEN, stopping in that forward direction at the "corner" of the spark advancer: that's the 15 degrees point. This geometry sets all the tension to the front side of the chain at the highest mechanical load point on the chain. Don't let the crank back off from here: release the tensioner's bolt, make sure the plunger moved inward (you can give it a tap with a small screwdriver and hammer to help), then tighten the little bolt and nut.

(http://forums.sohc4.net/index.php?topic=27593.0)

Apparently the cam has a different tension in different positions.  I took the tension adjuster out, cleaned it and polished the shaft with scotch pad (gray) and  put it back in.  Tomorrow I will find TDCC and adjust it.

After adjusting the chain and cleaning carbs - again - it idles really nice.  I may dare to take it to safety inspection this week.

Offline MCRider

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Re: CB750 made in March, 1970
« Reply #42 on: August 16, 2011, 05:45:32 pm »
"Apparently the cam has a different tension in different positions.  I took the tension adjuster out, cleaned it and polished the shaft with scotch pad (gray) and  put it back in.  Tomorrow I will find TDCC and adjust it."

You could say that. What is happeneing is that as the engine turns slowly, the valve springs, thru their rocker arms, will conspire to push the cam forward, or backwards, putting the slack on the front of the chain or the back. You want to adjust when the slack is on the back. This alternates 2 to 4 times per revolution. At least 2 maybe no more. I never really tried to test it.

At speed, this still happens, but at a microscopic, or better, high speed, level. Momentum trying to roll the cam forward. The valve springs putting up resistance and trying to turn the cam backwards against the pull of the chain, then forwards in sympathy with the pull of the chain.  This sets up a whipping dynamic which is very hard on the chain. Hence carb synching and ignition timing become crucial to smooth running and low sound.
Ride Safe:
Ron
1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #43 on: August 16, 2011, 06:18:55 pm »
Makes sense, thanks for the explanation!

Offline lucky

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Re: CB750 made in March, 1970
« Reply #44 on: August 16, 2011, 09:16:43 pm »
DANGER DANGER!  On the top triple clamp: there are spacers that go in the gap of the clamp on each side. They keep that clamp from being drawn too tight and breaking like your original one. If you don't have those spacers, STOP, and loosen the clamps and get the spacers. That triple tree is no longer available and routiinely sells for more than $150, like $300+. You were only LUCKY that you found one for less. The spacers may not be available any more either, they can be made from head nut washers. Standard hardware washers are too thin and the triple will still crack.

Edit: Since you don't have the stock instrument mounts, you can get away with a later model top clamp that would be cheaper than the original, and very rare "Unicorn" style that your bike should have.

As to the oil tank, it appears stock. The early tanks were slab sided, no emblem dimples.

That triple clamp should be saved, NOT thrown away.IT can be TIG welded.Not kidding.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #45 on: August 19, 2011, 04:44:44 pm »
Not planning to, I have it in my storage.  I am mech eng by trade and worked in aluminum quite few years,  yes it can be welded back together.

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #46 on: August 20, 2011, 02:49:24 am »
Accomplishment!

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #47 on: August 20, 2011, 10:55:11 am »
Oil change had it's own challenges, the oil drain in the pan was welded shut, proly somebody knocked the bolt off with a piece of the oil pan.

Took the pan off, drilled and tapped the welded spot for 1/4"-20 and used flange bolt and copper washer made from piece of   copper plumbing tube.  Holds well enough.

Also to get the filter on, I had to use a C clamp on #2 exhaust to move it out of the way.
« Last Edit: August 20, 2011, 10:56:54 am by 70CB750 »

Offline MCRider

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Re: CB750 made in March, 1970
« Reply #48 on: August 20, 2011, 06:56:29 pm »
DANGER DANGER!  On the top triple clamp: there are spacers that go in the gap of the clamp on each side. They keep that clamp from being drawn too tight and breaking like your original one. If you don't have those spacers, STOP, and loosen the clamps and get the spacers. That triple tree is no longer available and routiinely sells for more than $150, like $300+. You were only LUCKY that you found one for less. The spacers may not be available any more either, they can be made from head nut washers. Standard hardware washers are too thin and the triple will still crack.

Edit: Since you don't have the stock instrument mounts, you can get away with a later model top clamp that would be cheaper than the original, and very rare "Unicorn" style that your bike should have.

As to the oil tank, it appears stock. The early tanks were slab sided, no emblem dimples.

That triple clamp should be saved, NOT thrown away.IT can be TIG welded.Not kidding.
Many have tried. Have never seen a successful repair. By the time you make the hole perfectly round to accept the fork tube, it just doesn't pan out. Seems to break again.  Can't make it cosmetically successful either. It will always look like a repair.

And the ability of the clamp to clamp after being welded just isn't there. You don't have any continuity to the flexibility of the metal.

If someone can do it, more power to them.
Ride Safe:
Ron
1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline 70CB750

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Re: CB750 made in March, 1970
« Reply #49 on: August 25, 2011, 04:45:37 am »
Darn thing will not start again.  Well, carbs out  and clean.

 

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