1981 CB750K - Repair/Tune-up

[cdaiscool]

So, since I've got some work to do, and some time to do it, I figure I'd create this as a place to show what I'm doing, and to keep a log to myself of what I do to it.

Current plans:

-Is missing a mirror. And the stock ones are kind of ugly. So I have Chrome rectangular mirrors that should be in soon.

-Front brake is spongy; master reservoir has a screw that's snapped inside it (to hold the cover on). Bidding on a master cylinder right now, so I can steal the reservoir and cap off it. Will go on asap. If that's not it, then it's likely that it's the front brake pads. At which point I'll replace those before next week then.

-Need a cover yet. Integra fills garage, so I'll work on the bike in my driveway. Don't want this to get messed up by being rained on. Or isn't it a big deal?

-Have a Windjammer SS fairing, but the wrong bracket - 1972 CB750 bracket. Need to sell this to buy one for mine. Or find someone looking for a trade... It does have something on the bracket, I'll get the number off it tonight. Also need to make a wiring harness for it, and put a new cigarette adapter into it.

-Plan to do the valve adjustment asap. My dad's back from Florida on the 29th and I want this baby wrapped up by then.

-Going to pick up some chain lube tonight, and check the slack/adjust it, clean it, and lube it tomorrow. I remember seeing that this is best done when warm, after a ride, so I'm going to adjust it before riding, ride, then do the lube before calling it a night. Won't be warm, but it'll have ran a bit at least.

-Need to adjust something - Dealer said the chain is hitting the center stand. Is one or the other adjustable so it doesn't hit? I need to look at it yet to see what's going on precisely. I haven't heard it at all though.

I'm going to clean the carbs sometime, but not sure when. For now, I need to get on the thing and ride it! I'd say I'm excited to own a bike finally, but that's an understatement. Slight understatement. When I do clean 'em, I will probably use this kit:
http://webcache.googleusercontent.com/search?q=cache:y4X2OtBHQFgJ:www.randakks.com/DOHC%2520Four%2520Page.htm+&cd=2&hl=en&ct=clnk&gl=us

He's on vacation, so he took down his usual for-sale page. But that shows what I'm looking at. Since it was mentioned that my air pump may be bad, this guy includes the stuff to fix it. And a complete tear down/rebuild guide.

I also need to track down a headlight/turn signal switch. Mine's cracked straight through, and looks like crap. It does work, but would rather just swap it out if possible. Anyone have any leads? I've searched some, but haven't found anything.

I'll get video up tonight of the bike.

[TOOLS1]

Here is a link to a master cylinder. http://slingshot-cycles.com/index.php?main_page=product_info&cPath=53&products_id=91 Don't waste your time on an old crapy piece of junk. Also on the centerstand, the stock muffler had a piece on it to hold the centerstand from hitting the chain. Most guys just take a piece of metal, and bolt it to the passenger peg to hold it off the chain.
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[cdaiscool]

How does that work? I would think that would cause it to not go down if it's bolted down. Do you have to unbolt it first?

Here's a link to the video I made an hour ago. Comments are not only appreciated, they're desired!

Okay, I can't get embedding to work. Direct Link: http://www.youtube.com/watch?v=5yDmrUTRHuI

[cdaiscool]

That will work on the stock bars? I think that's what I've got, at least. And I like how they feel.

[Hellbilly429]

Those are not stock mufflers on that bike which will set off the carbs on these bikes as they are CV carbs, which control the throttle by vacuum, so basically since that exhaust allows more airflow it may be causing it to run and idle rough because you have reduced the vacuum, but also you have made the bike run lean, which keeping the choke on richens up the mixture and if it runs ok with the choke on then its probably running lean. If I was you I would start by running some seafoam through the tank, then see if you can get the carbs synced, maybe professionally would be better if you don't have the tools or expertise like I don't.  Bike looks real clean though, wish my bike was that clean when I got it, ha.

[cdaiscool]

Carbs were sync'd by the local Yamaha dealer which also works on these bikes. Here's the full write-up from them on the bike's issues:

Recommendations:
1. The compression is low.
2. The front brake pads are worn out.
3. The front brake fluid is low. (WHY THE HELL DIDN'T THEY FILL IT??? Derp.)
4. The drive chain is rubbing on the center stand.
5. The right hand mirror is missing.
6. Wires are routed wrong under the seat and may short.
7. The left hand side cover is broken at its upper rear mount.
8. The right hand side cover is broken near the bottom mount and the piece is wedged in place.
9. The carburetors may need to be cleaned.
10. The compression is low and head gasket is leaking, the engine needs top end repairs.

Resolution:
Performed carburetor synchronization, found the vacuum to be very low at 3 in/hg and it is supposed to be 7 in/hg. Hd to turn the idle speed adjustment in about 2 turns to obtain proper idle speed. Performed compression testing and found with dry testing it was #1=75 #2=65 psi #3=70 #4=70psi. Wet testing results were #1=120 #2=95 #3=105 #4=100psi. Test results indicate the rings are at fault for sure and there may be other contributing problems with the low vacuum also such as restricted low speed jets in the carbs.



So... I've got some fun ahead of me. I've got mirrors coming in tomorrow, ordered up the brake master cylinder posted earlier, going to get brakes soon (was going to wait, but I'm pretty sure they are shot - I can just hear 'em grinding) and need to figure out the center stand issue. Then it's do the wiring and I'm good. Except for a carb cleaning and top end rebuild.

[TOOLS1]

The centerstand just bumps up against it when it is up. vintagecb750.com even sells a pre bent, and drilled piece, for this.
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[shinyribs]

[cdaiscool]

Ahh, okay. I have to admit, that's still metal on metal contact. I think they have it wrong - vintagecb750 says they have a rubber piece that goes with theirs. I'm betting the center stand is supposed to bump against the metal piece, and that is what keeps the chain free and clear of stuff.

Something I'll pick up next week once the bike's off the center stand.

BTW, that was hard to get up on there! It took myself and my roommate yanking at it. But, alas, I am a weakling.

Anyways, I think I need to get a caliper rebuild kit. I picked up new pads at the dealership today ($20 or so for EBC pads - heck yes!) and took off the caliper. My god was it easy compared to the pick-up truck! But the piston is rusty on the inside, dirty and well extended on the outside, and apparently it's not retracting as it's supposed to. The outer pad was nearly new, and the inside pad had metal that was rubbing away!

I'm a nice guy, and grabbed you guys some pictures. And I want a 2nd/3rd/10th opinion.











You can't see it, but this thing is yellow! I bet it's a minimum of 10, 15 years old.


I've also ordered my valve cover gasket from Honda ($37) and the cover gasket for the motor where you turn the crank ($6). And the spanner wrench, because I gotta turn up my shocks. They're bottoming out - previous owner lowered 'em all the way.

Now I just need to find myself a caliper rebuild kit and a valve adjustment tool, and I'm good!

[TOOLS1]

I would definitely rebuild those calipers.
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[cdaiscool]

Would you recommend a new piston as well?

I have access to a sand blaster at an old job of mine. Going to have the caliper body blasted anyways. Or should I just get a piston?

I'm thinking this set-up:

http://www.vintagecb750.com/products/2/chassis-brakes/42/brake-caliper-rebuild-kits-brake-caliper-piston

Piston and caliper rebuild kit.

Or do I need to pull out the piston to find out?

Also, I can get the tool from there then too. http://www.vintagecb750.com/search/?q=valve+adjustment Part #35-1041

Or isn't that the right tool I need?


What about line? The only ones I found are $50 for OEM rubber, $100 for stainless. I haven't a clue for how to tell if it's any good or not...

[shinyribs]

Im sure you know this,but stuff some rags in the caliper when your blasting it.Dont mar that polished surface the piston rides on.Like I said,Im sure you know that...just dont forget!!!

They are kinda hard to get on the center stand at first.When I first got mine I thought it was impossible to do and wrote off the center stand as some useless option threw on there to up the price tag. Once you get the hang of it you can jump it right up on there. Just crank the bars to the left and lean back while you kick the center stand away. You'll get it. Dont jerk it and risk dropping your baby.Just lean back.

[cdaiscool]

Ooh, good point. I'll just blast the outside then. I'll plug up all holes and the piston chamber.

Should I just purchase a new piston, to have all that stuff new? Or just blast the inside of it, and leave the outside alone so it's nice and polished only on the outside, to mate with the caliper?

I'm going to take it apart tomorrow, and take it to my friend's place Monday to get blasted. Pick it up Tuesday maybe.



I knew I should have pulled it apart tonight, so I could have brought it tomorrow and gotten it back Monday. To put it all together Tuesday.

[TOOLS1]

I would take the piston out, and inspect it before buying a new one. From what I can see of the piston it does not look too bad. Also I would not blast the caliper. If, you want the black finish off of it, just use a wire wheel. It will leave the surface smooth. Blasting it will pit the hell out of it.
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[cdaiscool]

Okay. I'm going to buy the caliper rebuild kit and valve adjustment tool anyways. Need them regardless. I want to get 'em next week, as I want to go riding Friday with my dad. And I can only work on the bike between around 5 - 8 for 3 days next week, given that I have stuff happening Monday and Wednesday, and can't work after it gets dark. Dang Integra being in the way. :/


Maybe I'll put the wheels back on it after I get the brakes buttoned up and roll it outside. And put the bike in the garage to work on it. There's an idea....

[shinyribs]

Glass bead or walnut shells will leave a nice smooth surface. Walnut shells take foreeeever on certain coatings. Soda blasting is harmless,too.Glass is a bit more aggresive,but still smooth. It's what we use to clean the carbon build up out of the piston ring grooves on our race engines. Glass bead then back in the hole.Its that smooth. It all depends on what you got available. Now regular blasting media like Tools is talking about will leave pits,but most paints normally can cover them. Especially brake caliper paints. They are fairly thick paints designed to go on calipers which are normally cast pieces,therefore rough textured to begin with. You caliper is aluminum,right?Polish it man

[TOOLS1]

Is your living/bed room too small, for it to fit in. However the kitchen is a great place to work on bikes. A dish washer is great, for cleaning parts.
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[shinyribs]

Ok,I'm putting this out there for a laugh. Don't hold this against me guys! Some things do not belong in the dishwasher...a buddy of mine got busted by his old lady when she found his...um.....pocket "friend" in their dishwasher He was telling me looking for sympathy and I laughed so hard I couldn't breathe!

[cdaiscool]

NO. NOT HAPPENING. I'M NOT GETTING CAUGHT UP IN POLISHING AS YOU DID.

And I like my security deposit on my place. I'm good, no need to mess up the carpets. Though the bike WOULD fit through the french doors into my living room. It'd look so perrty in front of the flatscreen....


I don't have a dishwasher, so that's out. :/


As for my blasting equipment, I'm not sure. It's my old work, they have 3 machines for blasting the turbine side of a turbocharger. Those bitches get RUSTY. These clean 'em up awesome. I think they're all sand, no glass. Guaranteed no walnut.

I'll probably get some paint stripper, a bunch of paper towels, and a few hours. And then some water to rinse it, and finally primer and caliper paint for it.

The body looks badass, need the caliper to match!

[TOOLS1]

Paint stripper will not do anything to the factory black finish. Break fluid will not even touch it. Also the caliper paint with ceramic really SUCKS!!!!! The only ways to make the break parts look good is to leave them alone, polish them, or carefully take them off the mounts after bleeding them. Then mask, and paint them.
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