New member from the STL. 1981 CB750C

[djpj]

New bike so I'm posting to a few forums to gather the most knowledge possible.

Hello everyone. Just bought a new to me 1981 CB750C. It's not my first carb bike (I mostly ride newer sport bikes) but this is my first old carb bike. I found the bike on craigslist and after a thorough inspection and test ride I brought it home.

Problems at pick up:
Carbs leak out the overflow (Needle and seat probably)
Front tire is cracked (cracked a bunch more on the way home... )
Runs a little rough
Horn barely works
Lever brake light switch not working
And a huge front fairing!

So far I've completed a compression check. Dry 125, 120, 125, 140. Wet 160, 150, 170, 170. After soaking the cylinders in Marvel's Mystery Oil overnight and then running the engine back up to temperature I got some better numbers. Dry 155, 150, 160, 150. I think some previous owner did a valve check because it has dried gasket maker leaking out the valve cover. I believe that's some good numbers so I'll stick with it for now.

I've also done an oil and filter change with the Shell Rotella after adding a bunch of Seafoam and MMO to the crankcase oil and running it for a good fifteen minutes, spark plug swap (looked dark and dry), added some Seafoam and MMO to the gas tank, and swapped the air filter.

My main goal is to remain nearly stock-ish looking and still be able to ride two-up next year. My wife is currently preggo with our first so no riding this year for her. It'll also be a good transportation for me when I go off to military duty all the time on the other side of the state.

To do list:
Brake fluid change
Replace front tire and put balance beads in all the tires. I love this in my sport bike
Rebuild the carbs
Running new lighting after removing the fairing
Swap the cracked coils for some F3 coils and wires
Fix brake lever switch
Reseal the gas tank
Clean up the chain and adjust
Change out the fork oil and adjust the overall height as needed.

Here she is: http://imgur.com/RZoheqX

PJ

 

[djpj]

http://imgur.com/a/bRVRx

More pictures.

PJ

 

[Flogger]

Duplicate post

 

[Flogger]

Sounds a lot like mine did back in January when I got her... Looks a lot similar too, lol... But I didn't have the fairing. Mechanically all I really needed was a good carb cleaning and rebuild. That plus a quality valve job... Did all the other stuff like plugs, oil, tune up, etc... I replaced all the cables n tires since she was 35... here's a before and an after... (she did get a new paint job) so you can see what a mostly stock one looks like... BTW, military duty? Guard or Reserve? Retired AF myself... that's why/how I have the time to play with motorcycles, lol... but I am 57 SOOOO no kids nor expecting, all done there...

 

[djpj]

Flogger,

That's a good looking bike. One of my side covers has two broken tabs so I'll need to replace them or come up with something different.

Army National Guard. Yet, I've probably spent more time active than reserve status.

 

[Flogger]

JB weld works great for repairing side panels on the inside... keep those in good repair, they are expensive to replace...

 

[djpj]

It's missing all the tabs that fit into the rubber bushings so I have nothing to JB weld together. Might be too late for saving.

PJ

 

[Flogger]

Too bad... buy nice ones on Ebay, prices vary and get the best buy ya can... or buy sort of dingy cheap ones and make sure they have the parts you need. Then combine them using JB weld to make a good one...

 

[djpj]

Update:

Lever brake light switch fixed. Checked it with a multimeter and it showed no connectivity. Threw it on the ground, rechecked and now it's working. I went ahead and filled it up through the two small holes with some contact cleaner and then some dielectric grease since it's 35 years old and been in a bunch of weather. All lights work now.

Inspected the horn and I believe it had to be removed to put the front fairing braces on, so I started to wonder if they connected it back correctly. I took the horn off the bike and connected it directly to the battery but it was still quiet. I decided to flip the connection and now I have a loud horn and some ringing in my ears..... It's a go for inspection now at least.

I have ordered the F3 coils, plug wires, relay, and inline fuse. Waiting on delivery and with the bad weather this week I think I'll find time to complete that.

PJ

 

[djpj]

Update:

New front tire - Dunlop Metric Cruiser
http://imgur.com/yfa5JXa
Balance beads in both tires. I made a tool to help the beads into the valve stem. Pretty ingenious.
http://imgur.com/BU12IyY

So the front is all torqued up and greased. Now I'm waiting on the F3 coils and I should get those today and completed before the weekend.

Next week: Carburetors


PJ

 

[Flogger]

Take pix of your work!

 

[amc49]

Man I NEVER balance bike tires, never had the need to. 125 mph and faster and no issues.

 

[djpj]

My rear tire had 3/4 ounce weights on it already so I went ahead and pryed them off and added the beads. You just have to make sure there isn't a pile of lube left in the tire after it gets mounted or the beads will gum up together and not work. I've heard of many bike tires not needing balanced, this is just cheap insurance.

 

[djpj]

Update:

Coil swap completed. Bought some 1997 CBR600 F3 coils from FleaBay and a coil kit from Z1 Enterprises: https://www.z1enterprises.com/product/DYDW600

Ten layers of wrap and tape keeps coils safe I guess. Here they are after unwrapping: http://imgur.com/LiZS7um

Measured the different length between the coils and it came out to be 15/16" shorter. So I grabbed the old license plate thaat came with the bike, cut out two strips, folded them in half, drilled out some holes, and Robert's your mother's brother.
http://imgur.com/mhHGGlt
http://imgur.com/PoGbSA8

Then I plugged them up to the bike and tested it out. It started very quick for having sat for a week and didn't require a lot of throttle or choke to keep running.
http://imgur.com/RZtlpNY
http://imgur.com/hMsptKc

After that test it was time to cut my own spark plugs. I've never tried it before so I figured it was going to be complicated. Turns out it was easier than I thought.
I took the coil cap off the red plugs to use for the new wires. Measured the old black wires and added 4 inches to the new black wires (in case I muff it up). Then I cut the new wire and put on the old black wire heat shielding and wire labels. I don't know if these wire labels are stock but they are SUPER helpful. If you don't have some, get some. The new plug wires came with a cap to put on the spark plugs so the wires attach properly, so I did that. Then I put the coil caps from the red plugs on the new wires. That had three pieces: the big screw-on cap, small plastic washer, and the grommet. How I imagine it works is the grommet gets squished onto the wire when you tighten down the screw-on cap with the plastic washer inbetween them. I should have taken pictures of these things, I might have the old ones still sitting in my trash. Then I took a small nail and inserted it into the coil end of the new black wires about an 1/8" to split the wiring evenly. then I add some grease to the outer sleeve and slid it into the coil. The inside of the coil has a sharp point inside that pushes into the wire. then I tightened up the screw-on cap. I did all four like this with the coil off the bike. After they were all measured, cut, and assembled on the coil I reinstalled the coil on the bike. Then connect the wires to the plugs with some more grease.
http://imgur.com/gP9w4gm
http://imgur.com/j7FeqGd
http://imgur.com/GayIVhQ
If I thought it ran good before this swap, then it ran great after putting on the new coils. And then after adding the new wires it's still even better. Smooth idle, instant start on cranking, quick acceleration in the revs.

I think the extra length in the wires is too much but I liked having the ability to move the coils under the frame easier. Plus they tuck up nicely. I think the black wires keep the older look.

Wires: 17+ shipping = $26.15
Coils: 34 + Free shipping
Use the screw-on coil connectors, grommets, and washers. Don't forget some grease. Easy upgrade.

I'm going out of town this weekend so the relay will have to wait. In the mean time, I need to find a suitable place to mount it.
Location options:
Under the tank with the coils - hot, high voltage, gross engine fluids
Under the seat - no room, and too jammed with other wires
Side cover - no mounting spot, tight fitting, but could be organized well
Above the airbox - no mounting spot, could be in the way later on

I just want a clean spot that I don't have to unplug the relay everytime I want to do simple maintenance. Is under the tank, next to the coils really the worst idea? I'm up for suggestions.

Here's all the pictures I took so far: http://imgur.com/a/bRVRx
I'll try and get the pictures of the grommets and screw-on caps.
Let me know if you can think of the best place for this relay.

PJ

 

[djpj]

Here is the order of the screw-on caps with the wiring.
http://imgur.com/Gkyvh4H

Here is the nail I used to start a pilot hole in the wires.
http://imgur.com/6We5GkL

And here are the spikes that are inside the coils themselves that the wires get pressed onto.
http://imgur.com/MZjgokG
http://imgur.com/jfsQR56
http://imgur.com/GFTBVhC

I hope these pictures help someone else who decides to do this coil swap.

PJ

 

[djpj]

Update:

Wiring all made up for the coil relay. I determined that the best position is going to be by the battery. Under the tank would let the relay get too hot and it might short out. I found the operating temp range for these things is around 185*F so above the air-cooled engine is a no-go.

Wires:
Black = negative from relay, 10 gauge
Red with fuse = power to relay, 10 gauge
Brown = From one of the blk/whte trigger wires on the coil, 14 gauge
Green = From relay to coils, split to go to both coils, 14 gauge
http://imgur.com/8iC2o5F
http://imgur.com/lejepCS
http://imgur.com/w0Rpn3o

Ran the wires alongside the original wiring harness and shrink wrapped like a doomsday prepper. (Excessive? I'm the one riding the bike so I'll be the one stuck somewhere if it's not perfect)

http://imgur.com/u6RoKlH
http://imgur.com/LqPKNyD
http://imgur.com/sLecgfh
http://imgur.com/9MmcxLH
http://imgur.com/rYaLqNF
http://imgur.com/RvkOeYz

Grounded at the airbox to frame mount. Received a strong ground there so I'm happy with it. I can still loosen it a little and move the airbox to get at the carbs.

Next I connected all the wiring to my relay and the battery.. POOF. Blew the inline fuse. Then, I checked and rechecked all the connections but I was still getting a current running through the inline fuse without the bike running. I pulled the relay and tested the ohms from the power to the ground without any other wires connected. Got a positive connection reading so I guess my Amazon relay is trash. I then went up to the local auto store and got another, tested it, and we are good to go again.

http://imgur.com/ZqeII2u
http://imgur.com/9JQaKYy

Test fire was a great success. It fired up quicker than my sport bike. Tested all the plugs and they are firing evenly. Then I proceeded to shrink wrap every connection and grease up any spots that could be susceptible to some water. Once again three layers of varying sizes of shrink wrap will hold it together until Skynet comes for us all. I might even add some more to the green wires connected to the coils.

Out of focus: http://imgur.com/zez0Pas

Test fire again and it's all good. I like to test after every little stage so I don't get too far along and realize that after 5 steps my bike won't run. Now it's onto the carbs:

http://imgur.com/iKCozGW
http://imgur.com/6uKMB3T
http://imgur.com/cVN00nr

Labeled every connection and took some pictures. I broke one vent hose and I'll replace both when I reassemble. Next step is to clean those dirty dirty carbs. I'll also use this time to get some degreaser on that engine and rinse it off, after I close up all the holes of course.

PJ

 

[Flogger]

Very nice work, great following your progress and results...

 

[djpj]

Update

It's been a weekend of failures.

I tried the lemon juice boiling trick and either I didn't get the real lemon juice (said 0% daily amount of vitamin C) or I didn't cook it long enough (30 mins and moving it every 5 min) but it barely did a thing.

So, I polished the valve seat for carbs 1 & 2 with some metal polish and it came out like a mirror. Then I clean up all the float bowls with some carb cleaner. Put it all back together and then the number 1 carb wasn't getting any fuel into the cylinder so I opened the drain and nothing came out. And I noticed 3 & 4 were now overflowing.... I know, I know, I should have polished those seat valves while I had it out.

So I took the carbs back out of the bike and opened up number 1 and it had no fuel and the float was way higher than the other. I guess I got a bunch of carb cleaner on the float needle and the rubber part swelled up so it was blocking the passage. And the passage plug got soaked and just fell apart. I think this is a case of "too much of a good thing". Then the bowl gaskets fell apart.

So I ordered 4 new gaskets, a pack of passage plugs, and a new float needle from vintagecb750.com. Hopefully it gets here soon. Otherwise they are pretty clean, maybe that lemon juice did something after all.

http://imgur.com/OhD72LD
I added those tubes to the overflow to help me recognize problems more accurately and drain them more cleanly as needed.

While I am waiting on that I decided to flush out the front brake lines. The fluid was black as night. I proceeded to open the bleed valve and squeezed lever and I got a little bit out but right before the master cylinder was drained the lever went rock solid. I poured the remainder of the fluid out of the cylinder and the bottom was filled with sediment. Couldn't even make out the holes in the bottom. Spent 20 minutes working on it and it looked like this
http://imgur.com/Z8ez801

I pulled the lever and cylinder off the bike and cleaned it all and got some fluid flowing through it but the main brake line is clogged solid. I can't even get brake cleaner through the line and I'm not about to ram a wire through it all.

So I'm left without front brakes and no carbs... Better to find out now than on the road I guess......

PJ

 

[Flogger]

You're learning and you're trying, you gotta feel good... they are fun to work on... Did you get complete carb rebuild kits? I'd recommend that...

 

[djpj]

Flogger: I did buy a carb kit this time.

And now I've heard of using brake cleaner to clean out the carbs so you don't ruin the rubber parts. Brilliant. I had no idea.

I am going to go find some place to make me some new brake lines for the front. I can't believe they are clogged. I figured the pressure from squeezing the lever would push anything through. I'm going to test the master cylinder on another brake hose also.
Any advice on how to get the master cylinder apart to clean it better?

PJ