CB750 rat bike?

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.

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!

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.

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.

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!

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

How bout pics of this “pressure test” you speak of?

trek97 said:

How bout pics of this “pressure test” you speak of?

Click to expand...

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.

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?

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.

Ok I’m sorry but still unclear.

What is stopping it from just blowing the air past the valves needle? It wouldn’t take much to push the weight of needle and float.

Quite so. That's why I use this low pressure compond gauge. On the 'Wing, the fuel pump (as do most on carburated systems) puts out around 4.5 psi. Gravity-fed carbs on this bike should be able to handle a couple feet of head (~.5 psi). This isn't my final test, but I'm doing it before joining the carbs so I can find major problems. Then I do it once they are joined before testing with alcohol.

One thing the pressure test doesn't tell you is if your overflow tubes are cracked! Dooo. #2 and #4 both cracked. So, solder? JBWeld? How about - is heat shrink tubing gasoline proof? Let's find out! At least all of my float valves are seating well. But, I had to make a drain screw for #4 because it was too far corroded to clean up. Made one out of a bolt. Didn't worry about a screwdriver slot or an o-ring groove. This IS a rat bike!

when it come to screwed up drain screws these are the best replacements i have found i keep some around all the time

https://www.siriusconinc.com/pro-detail.php?pid=&product_id=1371

Cool thread man, thanks for the link - I'll be following along with interest as I'm working on a later model ('82) right now. How did you get on with cleaning out the tank? I ended up using MEK (Methyl Ethyl Ketone) as acetone just wasn't leaving a dent in all the varnish - MEK is hardcore in extreme. Knocked out the rust with some loose chain and then used Metal Rescue, job's a good'un.

I got some work to do on my carbs too, good luck mate with yours! Actually I'm trying to source some decent quality float bowl gaskets that won't cost me my shirt - any ideas?

I think you can find the gaskets on the same site with the bowl drain screws. My tank still has Evapo-rust in it. There was a place right on top of the spine that I couldn't get the liquid onto with rolling, so I covered it with a shop rag and keep re-soaking it with a turkey-baster type setup whenever I go by it. Busy day today and tomorrow, but hope to empty out the Evapo-rust on Saturday, hit it with the pressure washer, then hook up the shop vac to suck it dry (sucking vs blowing should dry it faster in theory). Finishing with a generous spray of rust preventative spray.

I tried soldering one. And its definitely possible to be a good fix. However mine wasn't good enough. I think if you use needle nose to squeeze the crack closed and then solder.

I drilled one out and replaced the tube...failed

I tried regular heat shrink...nope. You CAN get fuel resistant heat shrink...may work.

I coated them w Caswell tank sealer...it worked great for a year but eventually failed on one out of two.

Then, cxman (Murrys Carbs) sent me a perfect pair. fixed.

The heat shrink hot glue liner had already softened in the remaining bit of isopropyl in the carbs, so I cut it off with a razor blade, wiped the glue off with carb cleaner, scraped the crack with an Xacto blade, fluxed and soldered it up. With the powerful butane iron I have, was able to get a good flow until down near the bottom where the aluminum sucked the heat out. Hold the isopropyl well, so the carbs are finished except for the accelerator pump rebuild kit that is on a slow boat from Taiwan.

Chain is off and in the trash - too rusted to consider. Front wheel bearings are rusted solid, but I have replacements. Made a tool to remove the bearing nut and it worked well.

Tank is now de-rusted, flushed with pressure washer, dried, and sprayed with fogging oil. Pulled a ~1/2" thick chunk of Bondo off of it and think I'll try the inner tube trick to pop some of that big dent back out (will give more fuel space if nothing else).

Bondo I pulled off the tank. <b>Has anyone tried the trick with part of an inner tube poked in the tank to pop out dents?</b> Youtube farce or valuable tool? I don't want it perfect, but this is taking up fuel space.

Yesterday's task was to figure out tach/speedo/idiot light setup. Prefered would be reuse the original set. Bolt spacing is a bit wider than the GL1000 mounting holes, though. Considered moving the CB750 units to the GoldWing bracket, or even using the GoldWing set that I have (in very poor shape, however). Then I noticed a couple pieces of ~1/4" x 1" x 1.25" steel laying on the benvh left over from a bracket that I had made for one of our garden tractors. Already had two holes in each - perfect! A bolt through each to attach them to the upper tree and spread them out a little, drop the CB750 meter bracket bolts through them - magic! The CB750 bracket bolts are way too long, and I didn't want to cut and re-thread them (plus they are turned down a bit where threaded, and that might be a problem). So, mountain engineering provided a (possible temporary?) spacer via two 8 mm nuts stacked on each side as spacers. Now, I could fairly quickly spin out a couple nice spacers out of bar stock. But, this does look very Rat Rod. BTW, the little steel pieces have sharp, rough ends (cut from a longer strap by hack saw). Might just leave them, too. Wanted to take the CB750 unit apart and check things, but one of the screws snapped just above the brass insert inside. Tried to drill it out, but the bit walked over into the plastic so I ended up pulling the insert out, redrilling the hole, and super glueing in a 1/4" standoff tapped for 4 mm screw.

Today the task is to pull the front wheel and forks so I can mount the GL1000 headlight ears. They are pretty rusty, but mainly on the inner surfaces. They won't be cleaned up, and I'll have to mountain engineer the upper rubber mounts because they are misding from this set.

I'm limiting myself this week to a simple task per day as Monday morning I woke up with a real sore back which starts screaming at me after working for about four hours.