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Author Topic: CB750 rat bike?  (Read 5645 times)

Offline trek97

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Re: CB750 rat bike?
« Reply #50 on: Oct 16, 2018, 18:25:49 »
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.

Offline pidjones

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Re: CB750 rat bike?
« Reply #51 on: Oct 16, 2018, 18:59:49 »
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.
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

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Re: CB750 rat bike?
« Reply #52 on: Oct 17, 2018, 19:49:33 »
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!
"Love 'em all.... Let God sort 'em out!"

Offline cxman

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Re: CB750 rat bike?
« Reply #53 on: Oct 17, 2018, 20:42:41 »
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
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline The Jimbonaut

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Re: CB750 rat bike?
« Reply #54 on: Oct 18, 2018, 11:05:45 »
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'm telling you Donnie, nuthin' but nuthin' but right"

Offline pidjones

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Re: CB750 rat bike?
« Reply #55 on: Oct 18, 2018, 20:37:11 »
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.
"Love 'em all.... Let God sort 'em out!"

Offline trek97

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Re: CB750 rat bike?
« Reply #56 on: Oct 18, 2018, 21:37:08 »
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.

Offline pidjones

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Re: CB750 rat bike?
« Reply #57 on: Oct 22, 2018, 19:27:19 »
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).
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

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Re: CB750 rat bike?
« Reply #58 on: Oct 22, 2018, 23:25:45 »
Bondo I pulled off the tank. Has anyone tried the trick with part of an inner tube poked in the tank to pop out dents? Youtube farce or valuable tool? I don't want it perfect, but this is taking up fuel space.

« Last Edit: Oct 25, 2018, 09:29:09 by pidjones »
"Love 'em all.... Let God sort 'em out!"

Offline pidjones

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Re: CB750 rat bike?
« Reply #59 on: Oct 25, 2018, 09:27:55 »
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.
"Love 'em all.... Let God sort 'em out!"