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Author Topic: CB750 rat bike?  (Read 10600 times)

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #40 on: Sep 28, 2018, 22:17:40 »
Pressed in.

Today I was able to make a little more progress on the carbs.  One at a time pulled the float bowl off each (no ruined screws!) and sprayed the bowls with carb cleaner, pulled the drain valves and opened overflow and drain on each, cleaned the valves. Pulled secondary main jet on each (each one plugged solid with lacquer) and softened the lacquer with carb cleaner then was able to work a .008" guitar string through and finally shoot cleaner through them. Loosened the needles with carb cleaner and gently, padded with rag and tongue depressor, pried each piston up to free the needles fully. Pretty gummy, but still a LOT easier than working through the damage caused by ethanol gas.

Looks like the accelerator pump diaphragm will need replaced, however. Just so stiff that it hangs up when pressed. I'm sure all rubber will need replaced also, and already have bowl o-rings and rubber plugs on order. Should have the other o-rings in my supplies.

Probably at a holding point for now. Tomorrow we are showing the GL1000 at a nearby street fair, and next week we go to Barber for the museum and vintage fest.
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #41 on: Sep 30, 2018, 10:05:15 »
Afraid I'll need to replace the entire front end on this pig. Used a geometry calculator and see that trail with uncompressed forks is only ~3.1", and at full compression is ~2.3". No wonder a steering damper had been tacked on! This is all caused by the d@mned leading-axle forks. I'm sure that if I tighten the upper clamps back on 35mm forks they will crack, so I may look for a GL1000 front or original '79-'81 CB750F. Shazbot!
« Last Edit: Sep 30, 2018, 10:08:00 by pidjones »
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #42 on: Sep 30, 2018, 19:33:56 »
Remembered that I have a set of '78 GL1000 forks in the garage left from the Hunley project. The tubes are pitted in the upper area, but not in the seal area (well, a stone ding, but it will smooth out enough). They were too rough looking for a show bike, but should be just the ticket for the rat bike 750. I'll need to pick up a triple set and probably new head bearings (again - unless I can tap the new ones I had installed on the 750 stem back off). Then the fun of figuring out how to mount the instruments (which surprisingly seem to work Ok). I have a set of GL1000 headlight ears in a storage crate that I took out to the shed last month, so I'll have to bring that back in. They are real rusty, so should match up well!

All of this will have to wait until after Barber, however. After church the wife and I donned gear and I hooked up the Uni-go to the GL1800 for a test ride to one of our favorite restaurants. Loaded a box of books in the trailer to simulate luggage and it tracked beautifully over twisty roads there and interstate back. This was our first ride two-up with the single-wheel trailer and I think it will be great to haul to Birmingham. This is also kind of a test for us to make multi-week trips in the near future. I like that the Uni-go can be unhooked from the bike and rolled into hotel rooms as our luggage, convenient and eliminating the issue of security for an unhooked trailer left in the parking lot.
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #43 on: Oct 12, 2018, 21:13:04 »
Getting the forks together finally. Had to turn a cap for one because the original has disappeared. They are cleaned and assembled with new seals, waiting for new dust seals for filling and installation.

The carbs are now broken down individually and #1 is fully cleaned, new o-rings installed, and reassembled. The floats on all of them were way off - probably trying to fix leaky float valves - I'm relapping the seats with jewelers rouge.

The tank is flopping around all over with about 1.5 gallons of Evapo-rust in it. I think it will be fine, as this is all that my paint filter caught after draining a gallon of acetone that had been left in it for a couple weeks to get the varnish out.
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #44 on: Oct 13, 2018, 22:33:11 »
Fork dust seals came today as well as some nifty-looking red brake lines from China. Front forks are mounted with 200 cc each ATF. Now with them on, I see that I'll need a GL1000 front axle. Ebay!
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #45 on: Oct 15, 2018, 23:29:52 »
Finished cleaning, assembling, adjusting and testing through #3 carb tonight. Have to change out the acetone that I run the jets and such through in the US tank because it was getting too contaminated with varnish! Presently just pressure testing the float seats to see if they will hold ~2 psi for hours. Had to make a tool for lapping the seats as some were pretty dinged. Easy to pressure check while cleaning the next carb. I'll check them all with isopropyl before ganging them back together. Had to make the little washers on the pilot screw o-ring as they were all missing. Also bypassing all of the air cuts. Setting pilots 1/2 turn more open to accomodate. New accelerator pump is on order. This one is too stiff and often hangs up
"Love 'em all.... Let God sort 'em out!"

Offline trek97

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Re: CB750 rat bike?
« Reply #46 on: Oct 16, 2018, 10:04:40 »
How bout pics of this “pressure test” you speak of?

Offline pidjones

  • Posts: 996
Re: CB750 rat bike?
« Reply #47 on: Oct 16, 2018, 12:31:55 »
How bout pics of this “pressure test” you speak of?
Pretty simple "mountain machine shop" setup. the vacuum pump exhaust port puts out a small amount of pressure, and the compound gauge is just T'ed in with the fuel supply T from the rack. Have to plug the other port on #2 and 3. This gives me an initial idea if I have really bad problems. I expect it to leak down, but on these if it will hold 1.5 psi overnight, that's 41.5 inches of water or 59.3 inches of gasoline. For the GoldWing, I make sure it will hold at least 8 psi (GL1000 has a fuel pump).
These seats all had to be lapped - some quite vigorously with a steel mandrel - to get them to seat. The rubber tips all look great and were still pliable. There were tiny dings in some of the seats. I used a USB magnifying camera to inspect them.

#4 is pressure testing right now. It was the ugliest over-all. All had grunge (rust flakes) in the float bowls, but this one had pretty massive corrosion in the air cut area. Even though I am bypassing the air cuts, I cleaned it up and drilled out the plugged-solid port on the cap.
« Last Edit: Oct 16, 2018, 13:06:47 by pidjones »
"Love 'em all.... Let God sort 'em out!"

Offline trek97

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Re: CB750 rat bike?
« Reply #48 on: Oct 16, 2018, 14:29:10 »
So it’s pulling vacuum on the float valve and seat?   

As in, if it holds in one direction it should do the same in the other?

Offline jpmobius

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Re: CB750 rat bike?
« Reply #49 on: Oct 16, 2018, 15:10:18 »
It is positive pressure - 4 psi on the gauge.  You could install the bowl, plug the vent(s) and draw a vacuum on the bowl and achieve the same thing, though you would also be testing the bowl seal.  Looks like a good idea using the positive pressure though as you test just the seat assembly - it either seals or it doesn't so you can fuss with it while it is still all apart.  I have never thought to do this myself.  If the seat assembly didn't leak previously, I generally leave it alone.  If it is unknown (or of course leaks), I polish the seat with a Q-tip and rubbing compound - valve grinding paste if visibly bad first.  This works on assemblies with solid needles as well, and I absolutely never buy seat assemblies anymore unless there is mechanical damage or severe corrosion.
« Last Edit: Oct 16, 2018, 20:21:49 by jpmobius »
Mobius


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