The Revival, a crusty 1980 750K with 32K miles on the clock "Purple Dragon"

RSS83

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Hello All,

New member here, just got myself a '80 CB750K for a winter project, goal is to make it my daily next year. (just moved to Wisconsin and it seems like EVERYBODY rides up here) My other ride is a 78 Yamaha XS400, it did great around town for college, but it is pretty taxed on the highway. It can do highway, but the parallel twin vibes are not fun for more than 15-20 min on the highway at 5500-6000 rpm.

Here is the Dragon as found.
20171018_180335.jpg


Since I have purchased the "purple dragon" ,as I'm calling it, (probably should be called the "purple money pit") I have focused my attention on the engine, since I'm pretty sure the P.O. never checked the valves, adjusted the cam chains or any of that, and with 32K miles I'm hoping no valves are burnt.

First order of business was a complete carburetor disassembly. They were "rebuilt" when I got it. I had to remove the several shims under the slide needles and block off the aircut valves. It was running VERY rich when I got it, and just barely running at that. I can't believe I was able to ride it across town (1 mile, :wink2:) to get it home. All the aircut valve diaphragms had holes, so that's the reason for blocking them.

Next, I balanced the carburetors on the bike, with a nice home-made manometer. It does pretty well. (I did it the first time w/o the airbox, will do them again later with the airbox, and after valve lash adjustment)

20171022_142246.jpg

And now I have been working on the top end of the engine, with the carbs decently cleaned/balanced, new plugs and plug caps (the original plugs were so fouled that it wasn't idling on all cylinders) I warmed up the bike and took some compression readings. From 1 to 4 I got: 150, 150, 155, 150. I thought this was pretty surprising for the bike's mileage and the valve clearances I found next.

The valve clearances were next, I measured using the service manual's instructions
Exhaust were 1 to 4: .004, .003, .0025, .003, .004, .003, .0025, .002 (inches)
Intakes were 1 to 4: .004, .004, .003, .003, .004, .005, .003, .003

Cylinder 4 had some firing issues at idle and from the exhaust valves I can see maybe why, hopefully not burnt, fingers crossed there.

A shim kit and shim change tool is now on the way...... at least one was at .005 :laugh:

Hopefully once the valve clearances are set, I'll be able to get my idle smoothed out and further set the carburetors. I haven't been able to set the idle screws currently, they don't move the idle speed unless I take the screws several turns in or out. I get some primary chain noise at 1000RPM idle, goes away and smooths out at 1500 RPM, so it's fairly close currently. (I can feel the cylinder "hits" in the clutch lever at 1000, but not at 1500)

Other things I have noticed in my adventures, the cam chains are adjusted fine and do not make noise, surprising. From what I can see of the tensioners, they look to be decent.

20171023_161448.jpg

While checking valves, I noticed that, on the valves with low clearance, the cam lobes looked like they had taken some wear. So I took my China slide caliper and took some rough measurements. I only had a few at the service limit for the cams but they were within the "accuracy" of my china slider. So I'm not too worried about them yet, if need be I'll get some CB900 cams.

Other than the engine, here are some interesting things I found on the bike:

The "custom" rear lowering option from the PO :banghead: :wtf: :eek: :yikes::yikes: :doh:

Needless to say, new shock/spring assemblies are on the way....

20171019_160302.jpg

Other than the springs, a front fork rebuild with new seals is in order.

For the bike, the chrome is fubared, a local place specializes in powdercoating small jobs so I'm currently thinking of going that route. Disassembling the wheels, powdercoating the rims and spokes. Also powdercoating the fenders. As for the body, hell maybe just re-paint it with plum crazy purple!


Anyways, :cheers: and let me know what you think of my new mess!
 

amc49

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Shoot for .005" on the valves, .004" is OK. They burn at .002", the numbers you get are not real and you toss .002" of whatever you get to be closer. So, a .004" reading is actually closer to a .002" and why the .002" burns valves. BTDT.
 

RSS83

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Shoot for .005" on the valves, .004" is OK. They burn at .002", the numbers you get are not real and you toss .002" of whatever you get to be closer. So, a .004" reading is actually closer to a .002" and why the .002" burns valves. BTDT.
When checking the valves, do you follow the service manuals procedure for cam position, or do you think it works better to point the lobes 180 away from the lifter buckets, or no difference? Just curious.

Sent from my SM-G950U using Tapatalk
 

RSS83

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Update: Started to tear-down the bike some more. Dropped the oil pan and cleaned the sump screen, nothing out of the ordinary has been found. I also looked at the engine serial# and noticed that it is not the original engine to the frame, only problem I have with that is I now don't truly know how many miles are on it, doesn't particularly matter but it is nice to know anyway. Also found some more things to purchase for the money pit.

Carb boots/insulators are brittle and fell apart, I had noticed one was causing a vacuum leak so they need to be replaced.

20171028_135123.jpg

20171028_120259.jpg

One nice thing about having the pan off is I was able to sneak a peak at the gears and primary chain. Gears look great, nothing unusual.
 

amc49

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I point the lobes 180 away from the shim contact point myself, it guarantees you are away from any clearance ramp and you are truly on the base circle then.

Honda changed the marks up once from where they started out at, maybe in an effort to deal with the clearance issue but they mainly do the marks to speed up service as you have to turn the engine around less, that loses some accuracy though.

FYI, when the clearance disappears then the cam lobe begins to wipe all oil off of it and why the lobes then wear faster, it causes huge fits on the bigger motors with more lobe lift and Honda did nothing to improve the heat treat on those over the 750 parts. Why it's hard to find truly good cams on 900 and almost impossible on the 1100. I consider .010" of wear on any single lobe as a dead cam, once that much the heat treat is gone and they wear like lightning past that.
 

RSS83

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Update, Did some part cleaning while waiting for other things to arrive. The Trip meter reset was sticking so I disassembled the cluster and cleaned everything. Polished the plastic too. I didn't have fine enough sandpaper to wet sand it to remove the scratches, but it does look better with a nice polish anyway. I also found that the PO had replaced the back-lights in the gauges with 6V bulbs, they slightly melted the holders and popped of course. So this was a nice time to try some LED bulbs! Anytime you can reduce power consumption on these bikes its a good thing, although these bulbs don't amount for much of anything in power use. either way the originals are 3.4W each and the LEDs I got are 1 watt, so a nice 1/3 of the power.


Before some Polishing
20171028_173223.jpg

After some polishing
20171028_173343.jpg

LED vs. the Bulb
20171031_174235.jpg

Looks good for the backlights! I also like the white light over the bulb's color. Too bad it shows the dirt inside of the gauges, guess that will be something else to do, un-bend the metal ring and clean under the glass. Can re-paint the needles and lubricate while in there too.
20171031_175453.jpg

They were a bit too bright for the turn and other indicators though, the backlights had the advantage of not shining directly on the gauge face
20171031_174515.jpg

The bulbs were just 10 for 10 bucks too, so a pretty cheap upgrade for the gauges!
 

RSS83

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Update

Got some more time to work on the mess. Was working on getting the carburetors figured out, found out that the previously installed cheap-o rebuild kits were causing some problems. The springs on the needles were not all the same rate and the float levels varied by 8mm! one was constantly flooding. New needles on the way to fix that. I wish the previous owner hadn't put in the cheap-o floats as well.....

20171114_165547.jpg


I also spent some time (a lot of time) polishing the engine's side covers. They were oxidized all over and a couple had some concrete rashes from the bike being dropped. I didn't do anything near a show-quality polish job, but from a couple feet away they look great! I started with 120 grit in the rough areas, moved to 220, then 320, then 600, then 800, then 1500, then 2000, finally 3000 and some 0000 steel wool. Buffed with white rouge. Each cover took between 2-5 hours start to finish. After polishing I took some enamel paint and filled in the logos with a syringe and blunt needle. After the enamel was dry I used some high temp gloss engine clear coat to prevent oxidation from starting again. The clear is noticeable but this will be a daily rider and I don't have time to continually polish aluminum, so its good enough.

20171111_152928.jpg

Lastly I have begun to work on the seat. I'm short so I decided to narrow and lower the seat. I also wanted to narrow the seat so it would match the frame tubes at the top. I'm not going to use the original side covers as mine are cracked. To replace the side covers I'm using Aluminum triangles on each side and probably some Dzeus fasteners to affix them. I'll weld on the Dzeus fastener tabs in place of the original grommet tabs.

I took about 3" off the width of the seat, as far as I could go using the original seat pan and its mounts.
20171119_165530.jpg

Re-shaping the foam:
20171119_172504.jpg

I'm using a grinder and flap wheel to rough-shape the foam. To finish I think I will try some rough sandpaper and a block, will see how that goes.

For a new seat cover, I have some marine vinyl and 1/2" foam to make pleats. That will be another fun experience, glad my wife is good at that part of the project.

In the picture below, the idea for aluminum side panels is shown by the cardboard moch-up:
20171119_172829.jpg

Hopefully the bike won't look goofy, I'll have to get the foam done, then get it outside to look at from a distance. Trying to keep the original shape mostly intact, just smaller/shorter. Also don't want some shitty cafe seat with no foam. I would like to ride it for more than 20 minutes at a time. I'm not going for any particular look, just whatever works with what I need and what I have on hand. Probably going to strip and paint the frame myself to save cash. Would like to get Progressive fork springs, cartridge emulators, and rear shocks/springs. Might also do a hydraulic clutch conversion, my cable has been improperly routed and bent so its pretty stiff and the clutch perch has bad mirror threads so I figure why not get a matching set of master cylinders. The front brake master cylinder on the bike would be costly to rebuild, and I never seem to have good luck with them still having a good enough bore not to leak after rebuild. So its gonna go.
 
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