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djpj
Getting up steam
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Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sat May 21, 2016 1:10 pm Reply with quote Back to top

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... Shocked )
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

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1981 CB750C

Last edited by djpj on Sat May 21, 2016 1:14 pm; edited 1 time in total 
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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sat May 21, 2016 1:13 pm Reply with quote Back to top

I have some mechanical knowledge but know little about carbs. I need to buy a carb for dummies book.

Any good suggestions for a carb kit to fix the needle valve and seat?
Also, where do I find some spark plug wires for the F3 coil swap?

I appreciate the help.

PJ

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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sat May 21, 2016 1:29 pm Reply with quote Back to top

http://imgur.com/a/bRVRx

More pictures.

PJ

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pd750
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Joined: Aug 01, 2007
Posts: 25067
Location: Waldron , Arkansas

PostPosted: Sat May 21, 2016 1:31 pm Reply with quote Back to top

Welcome to the site PJ ... Smile

You might want that fairing if you'll be traveling across state regularly . They make road time MUCH more pleasurable .

SeanG wrote an excellent guide for cleaning these carbs . Look in the Tech Docs section .

You may not need a carb kit . The seats can usually be polished with Brasso and Q tips . The jets and passages usually clean up nicely with brake cleaner and compressed air .

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Please put the year and model of your bike in the signature section on your profile / member account page and set \"Always attach signature \" to YES . Just below the Reply box , check the box in front of \"Attach signature (signatures can be changed in profile)\" .

1982 CB750C (Sold) Haven\'t found another , yet . 
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pd750
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Joined: Aug 01, 2007
Posts: 25067
Location: Waldron , Arkansas

PostPosted: Sat May 21, 2016 1:34 pm Reply with quote Back to top

One other thing : you would be wise to pull the oil pan and clean it and the pick up screen .
Even if they aren't dirty , it's the only way you can be sure .

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1982 CB750C (Sold) Haven\'t found another , yet . 
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Whozaa
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Joined: May 09, 2010
Posts: 6101
Location: St Louis, Missouri USA

PostPosted: Sat May 21, 2016 1:36 pm Reply with quote Back to top

Welcome

PD750 covered everything I was going to say. Except that you can use bulk 7mm spark wire and NGK caps for the coils. Or buy the full assys from Honda.

I'm right across the river in IL. Give me a holler if you need help with anything.

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79 CB750F
68 CT90

Hey new guy! Please read the New Members Thread behind this link. http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=7149
It contains the rules for forum posting and other valuable info you need to know. 
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Duke
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Joined: Sep 27, 2009
Posts: 13650
Location: ABQ, NM, USA

PostPosted: Sat May 21, 2016 2:10 pm Reply with quote Back to top

Welcome to the site djpj. That is a nice looking bike you have there.

All the manuals you most likely will need are available for download here like pd750 says.

Enjoy the site.

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Duke

1981 CB900 Custom

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seanathon
Feel the Power
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Joined: Aug 08, 2014
Posts: 549
Location: Bangor, Maine

PostPosted: Sat May 21, 2016 2:29 pm Reply with quote Back to top

Welcome! Nice looking bike - you'll love working on old stuff Laughing

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Seanathon
1981 CB750c
1963 BSA B40
1974 Triumph Trident T150v (future project)

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genesound
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Joined: Jul 24, 2007
Posts: 44787
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PostPosted: Sat May 21, 2016 5:04 pm Reply with quote Back to top

Welcome to here wave

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NMT - New Member Thread, a Must Read, click here!!! - Information For New and Veteran Members.
Contains SITE RULES, how to enter required bike information in your signature, thread and picture posting...
Also some important DOHC wrenching tips, and some other information, shop manuals.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
81 CB900C, 83 CB1000C. 
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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sat May 21, 2016 5:59 pm Reply with quote Back to top

Thanks everyone!

I would keep the fairing except the headlight sucks, the stereo sucks, I can't use the highway pegs because of it, and I don't like the additional weight over the front wheel. I might try and find a regular windscreen later.

I'll look into that carb cleaning. That sounds way easier. I forgot to mention the bike has 24,000 miles on it. And I will wait till next oil change to look into the pan. Does the exhaust need to be removed for that?

Any suggestions where to buy this bulk spark plug wire? I'm excited to build my own wires. Never tried that before.

Thanks again for all the suggestions and help. I have worked on a few old cars in my day and so far this bike seems way easier.

For swapping out the fork oil do you all recommend using syringes to get the right amount or what? I would prefer a little more resistance to what I'm experiencing now also. I know it's not like my typical sport bike but I don't want it to be a boat either.

Thanks again.

PJ

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Duke
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Joined: Sep 27, 2009
Posts: 13650
Location: ABQ, NM, USA

PostPosted: Sat May 21, 2016 6:42 pm Reply with quote Back to top

djpj wrote:


For swapping out the fork oil do you all recommend using syringes to get the right amount or what? I would prefer a little more resistance to what I'm experiencing now also. I know it's not like my typical sport bike but I don't want it to be a boat either.

Thanks again.

PJ


If your front fork springs are original it wouldn't hurt to replace them with Progressive springs as your originals are likely sacked and are contributing to poor suspension performance. Replacing the old with new will add a lot of life back into your suspension.

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Duke

1981 CB900 Custom

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flogger
Now you're talking
Now you're talking



Joined: Feb 14, 2016
Posts: 97
Location: Indiana

PostPosted: Sun May 22, 2016 1:07 am Reply with quote Back to top

Hello... Answered you on the other site... One note, if you are not a good mech on Carbs and Valves, seek out an expert to learn from... Most carbs problems are the POs mistakes (stupidity)... and a real mech fixes their mistakes... I am fortunate in that I have a great local shop that is trustworthy and welcomes older bikes AND is priced fairly. They DID tell me (and SHOWED me) I was missing some internal carb parts when mine were cleaned n rebuilt... Mine were done ONCE and they run like a dream... But carbs can run you 100 to 150 a barrel for good work...
 
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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sun May 22, 2016 10:31 am Reply with quote Back to top

Flogger,

Thanks. I think the carbs work alright for now besides the leak. I am going to find someone near me to look at them still.

PJ

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RDIII
On a roll
On a roll



Joined: Jul 26, 2011
Posts: 246
Location: Peru, IN

PostPosted: Mon May 23, 2016 7:52 pm Reply with quote Back to top

Welcome!

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1979 CB750 Limited (Lisa)
1981 CB900 Custom (Celine) 
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flogger
Now you're talking
Now you're talking



Joined: Feb 14, 2016
Posts: 97
Location: Indiana

PostPosted: Mon May 23, 2016 7:58 pm Reply with quote Back to top

djpj wrote:
Flogger,

Thanks. I think the carbs work alright for now besides the leak. I am going to find someone near me to look at them still.

PJ


Prob need rebuilt to stop the leak, see what the others here think...
 
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seanathon
Feel the Power
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Joined: Aug 08, 2014
Posts: 549
Location: Bangor, Maine

PostPosted: Mon May 23, 2016 8:23 pm Reply with quote Back to top

Quote:
Prob need rebuilt to stop the leak, see what the others here think...


If you're leaking, do a rebuild. Sean G has the software and hardware (allen heads) that are very reasonably priced, and his manual is a step-by-step guide (I've NEVER touched a carburetor before my rebuild, and even I couldn't screw it up too bad). Other than some pine-sol and your time, a re-build is well worth it. If you can get a manometer to sync up once you're done, you'll be golden for a few years.

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1981 CB750c
1963 BSA B40
1974 Triumph Trident T150v (future project)

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Whozaa
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Joined: May 09, 2010
Posts: 6101
Location: St Louis, Missouri USA

PostPosted: Tue May 24, 2016 7:59 am Reply with quote Back to top

The big question is where are they leaking from? If it's out of the drain nipples, you might get away with only polishing up the float valves.

I helped a guy out in Troy MO clean up the carbs on his 900C. His were royally messed up too. Mismatched parts from different models and what not.

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79 CB750F
68 CT90

Hey new guy! Please read the New Members Thread behind this link. http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=7149
It contains the rules for forum posting and other valuable info you need to know. 
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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Tue May 24, 2016 8:07 am Reply with quote Back to top

I'm going to take the carbs off soon and I'll get working on that rebuild. I downloaded that PDF on the carbs. I've never touched the inside of a carburetor either but I'm meticulous. Thanks for your ideas.

PJ

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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Tue May 24, 2016 8:07 am Reply with quote Back to top

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

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holysmokes
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Joined: Jul 25, 2007
Posts: 37017
Location: ridge road springfield nova scotia

PostPosted: Wed May 25, 2016 8:21 am Reply with quote Back to top

hello

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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Thu May 26, 2016 7:28 am Reply with quote Back to top

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

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seanathon
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Joined: Aug 08, 2014
Posts: 549
Location: Bangor, Maine

PostPosted: Thu May 26, 2016 7:55 am Reply with quote Back to top

You're doing great! These bikes are a ton of fun once you get them dialed in.

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1981 CB750c
1963 BSA B40
1974 Triumph Trident T150v (future project)

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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Fri May 27, 2016 7:50 am Reply with quote Back to top

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

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Whozaa
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Joined: May 09, 2010
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Location: St Louis, Missouri USA

PostPosted: Fri May 27, 2016 8:29 am Reply with quote Back to top

I haven't done the mod myself. But if it were me, I'd stick the relay close to the coils. Make yourself another little bracket and hang it off one of the coil mounting bolts.

If you mount it by the battery, you'll have to lengthen the trigger wire (black/white) and run it all the way back to the battery area.

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79 CB750F
68 CT90

Hey new guy! Please read the New Members Thread behind this link. http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=7149
It contains the rules for forum posting and other valuable info you need to know. 
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djpj
Getting up steam
Getting up steam



Joined: May 15, 2016
Posts: 34
Location: STL, MO

PostPosted: Sun May 29, 2016 4:59 pm Reply with quote Back to top

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

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