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WWW.CB750C.COM :: View topic - New from Portland with a custom 1981 CB750C

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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Thu Aug 04, 2016 9:50 pm Reply with quote Back to top

Hello all. Despite the fact that this is my first post, this form has already been a help.

With essentially no research other than 'I think a Honda CB750 will make a cool retro bike', I bought a starting-but-not-perfectly-running 1981 CB750C. It had already been modified slightly towards a 'bobber', but the PO quit on it due to too many projects (he had Mitsubishi 3000 parts and spare engines and spare cars covering his driveway and garage).

This is how I got it home:

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I began with mechanical work. No sense making a beautiful bike that does not run well. Change oil, valve adjustment, then pull the carbs. Someone at some point may have attempted to clean these, or SeaFoam them, but the slow speed jet under the plug was very dirty. It had been forgotten, so I cleaned it up.

ImageImage

Now after getting them back on (hair dryer to warm the boots), the bike still didn't run right. I did more research and found via this excellent guide that I likely had blocked slow speed passage ports. I modified his carb spray with tape method slightly, and used a guitar string until I had all 4 passages clear.
http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf

Back on the bike again, and it runs, just decently (or perhaps marginally).

Once running, I removed the fuel petcock I received with the bike (it was from a different model) and replaced it with a non-vacuum one. The SOS had already been removed. The one I removed was too wide and the tank would not install without the petcock hitting the valve cover or carbs.

As it is now, I have a brat seat I'm getting ready to install, but I would like to get the bike running a little more smoothly before I do any more cosmetic work.

Is this the ideal thread for questions about hesitation/idle/etc, or would that be more appropriate in it's own thread?

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1981 CB750C - in progress (stock airbox)
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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Thu Aug 04, 2016 9:58 pm Reply with quote Back to top

Just to note:

I have the FSM
I will be putting new tires on as soon as it's ready to go further than around the block
Oil & filter changed as mentioned above
Valves adjusted as mentioned above
Compression, unfortunately, untested as yet
NGK Spark Plugs
I currently have the in-tank filter on the petcock pickup. I have additional inline fuel filters on the way. Are both recommended?
Have not had any electrical issues, so fuses remain.
Carbs were cleaned and bench synced
SOS is removed
I have an aftermarket battery that's quite good. It has let me start and test and start and test soooo many times. I will confirm it's charging well next time the bike is running.
I'm using the original airbox
My coils seem to be working fine
My brakes need attention, will address when engine runs like I want
The starter clutch did have the screws backing out. I opened it, the internals are good, so I reassembled with a bit of loctite, which did the trick.
Given that I have the CB750C forks, which have a more 'lazy' trail, I'd like to change the forks on the bike.
I may change the rear springs to slightly longer ones, depending on what I do with the front.

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1981 CB750C - in progress (stock airbox)
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seanathon
Feel the Power
Feel the Power



Joined: Aug 08, 2014
Posts: 518
Location: Rochester, NY

PostPosted: Fri Aug 05, 2016 8:22 am Reply with quote Back to top

Welcome to the forum wave

Seems like you're on the right track. Thanks for reading the FSM and sizing your pics appropriately Wink

Couple questions:

(1) Stock paper filter in the airbox?

(2) Are you valves adjusted to FSM spec, or to "forum spec" The FSM spec is too tight - they need to be between .004" and .006" (the FSM calls for .002" - too tight)

(3) When you broke the carbs down, did you clean them all the way through the manual, or just the slow speed passages? They could probably use a full on clean/rebuild. Not too much cost other than time. Sean G. sells the o-rings as a kit, and Randakk (outside the forum) sells the bowl gaskets.

(4) Are the jets stock sizes, and are they in the right place? I believe you should have 68 primary jets, and 102 secondary. Make sure they didn't get mixed up

(5) Are the insulators (boots) stock? Might be worth replacing while they're still available.

(6) When you say it's running marginally, what exactly do you mean? Hard starting? Hesitating at certain revs? Won't rev past a certain RPM?

(7) As far as what to do on the forum, go ahead and start a new thread in the "Technical Help" section. You can reference other threads with links, but don't commander someone else's thread to answer your questions. Helps the mods stay organized Wink

Again, welcome! You'll like it here Laughing

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

New members, read the NMT (New Member's Thread) HERE
Find the FSM (Factory Service Manual) and other technical documents HERE
"They're not clean until they're 'Sean G.' Clean" - Sean's (not mine - better Sean) Carb Cleaning Manual HERE
USE THE SEARCH FEATURE FIRST before asking questions

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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Fri Aug 05, 2016 11:20 pm Reply with quote Back to top

I am using what I believe is a stock type filter, but it is fairly clean; I believe it's been replaced.

I shimmed the valves to the manual spec. I put them all between .07mm and .1mm

When I cleaned the carbs, I had all the jets out, the bowls and floats and the tops and everything off. I cleaned everything I could see or find.

I am using stock jets, and they're in the right spot (I believe). 102 in the main, and 68 in the slow.

I'll start a thread in tech help so it can be titled appropriately. Thanks!

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1981 CB750C - in progress (stock airbox)
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genesound
administrator



Joined: Jul 24, 2007
Posts: 41231
Location: Studio City, CA

PostPosted: Sat Aug 06, 2016 1:41 am Reply with quote Back to top

Welcome here wave

Be sure to read the NMT (NMT = New Member's Thread). It has the site rules, and some good info about these bikes we like everyone to be familiar with:

The NMT wrote:
_____________________
Valves

Proper valve clearance is probably the most important maintenance issue to keep these motors running next to frequent oil changes and cam chain tension adjustments. Valve clearances need to be checked every 4000 miles or so. Most mechanics and long term owners of these bikes agree that the ideal valve clearance for these DOHC motors is .005" or .13 mm, regardless of what the book says. So, somewhere between .004" to .006" or .10 mm to .15 mm is acceptable. The book says .002" to .005" or .06 mm to .13 mm and those low numbers are just too tight and can lead to burnt valves.

The 25 mm diameter shims these motors use are commonly made from 2.3 mm to 3.5 mm in steps of .05 mm. On real Honda OEM shims, the decimal is left out, so what Honda calls a 280 shim is actually 2.8 mm thick. The in-between sizes with sizes that end in 2s and 8s like 272 and 278 were never sold over the parts counter and were only ever available in new Honda motors from the factory.

NEVER turn over the motor without shims in ALL the buckets! The cam lobe will bend up the bucket and the valve will stick open, and possibly hit the opposing valve and then you can have bent valves. Lobe damage can also occur.

_____________________

_________________
NMT - New Member's 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.
Kindly read this linked thread with the rules before posting
pictures or asking lots of questions that are already answered in it.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
For Honda Shop Manuals (FSM), Click Here.
Click here to go fiche at Parts Pit Stop.
Click here to go fiche at Partzilla.
Sean's Bike Maintenance Checklist Click!
MacGregor Carb Rebuilding Manual, Click Here.
⊳⊳⊳⊳Learn to use the Search Engine Click Here.⊲⊲⊲⊲
Be sure to use Copper Anti-Seize on your Spark Plug Threads!
To Fix and Better Understand Your DOHC Honda Charging System, Click Here.

ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
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___ImageCB1000C __ Image ___ CB1100FImage__
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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Sat Aug 06, 2016 1:25 pm Reply with quote Back to top

genesound wrote:
Welcome here wave

Be sure to read the NMT (NMT = New Member's Thread). It has the site rules, and some good info about these bikes we like everyone to be familiar with:

The NMT wrote:
_____________________
Valves

Proper valve clearance is probably the most important maintenance issue to keep these motors running next to frequent oil changes and cam chain tension adjustments. Valve clearances need to be checked every 4000 miles or so. Most mechanics and long term owners of these bikes agree that the ideal valve clearance for these DOHC motors is .005" or .13 mm, regardless of what the book says. So, somewhere between .004" to .006" or .10 mm to .15 mm is acceptable. The book says .002" to .005" or .06 mm to .13 mm and those low numbers are just too tight and can lead to burnt valves.

The 25 mm diameter shims these motors use are commonly made from 2.3 mm to 3.5 mm in steps of .05 mm. On real Honda OEM shims, the decimal is left out, so what Honda calls a 280 shim is actually 2.8 mm thick. The in-between sizes with sizes that end in 2s and 8s like 272 and 278 were never sold over the parts counter and were only ever available in new Honda motors from the factory.

NEVER turn over the motor without shims in ALL the buckets! The cam lobe will bend up the bucket and the valve will stick open, and possibly hit the opposing valve and then you can have bent valves. Lobe damage can also occur.

_____________________


Thanks! Yes, I've been through the NMT a few times, but obviously not close enough to catch that! (And, to be honest, hadn't read the NMT at the time I was doing the valves).

I will likely be redoing those if it's going to have an effect on operation and longevity. Do you think I need to reshim the valves before addressing the problem described here:

http://cb750c.com/modules.php?name=Forums&file=viewtopic&t=29437

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1981 CB750C - in progress (stock airbox)
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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Tue Feb 21, 2017 4:28 pm Reply with quote Back to top

Progress has been made. I took it out for my first ride last weekend. Here's how I got there.

I got the engine running ok. It it a little hesitant below 5.5k. To start it I turn up the idle screw, pull the choke, and as it warms up I back off the choke then turn the idle screw back out. Works every time.

Once I determined the engine was going to be ok, I started the external stuff. Brake caliper rebuild, new stainless lines, radial master cylinder.

Brakes off the forks. Ordered a rebuild kit. They were pretty tight, but I got them out using an air compressor with the nozzle jammed in the brake line hole. They came off at high velocity, but I was prepared.

Image
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Got the forks off. Cleaned everything while I was at it. On the fork bodies, you can tell which one was on the right side of the bike, because it was more exposed when the bike was on the kickstand.

Image

I got new Sonic Springs, and replaced the oil with 15wt fork oil. It seems to be great so far. I have all my replacement parts ordered and ready to go in.

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Slide the fork seal 'stack' onto the fork tube, then put the fork body in place. I found the only way I could get the copper/teflon seals in was to cinch them with a zip tie so they were compacted, then tap them down into place. Once they were in their slot, I was able to nick the zip tie with a sharp knife/exacto, and remove it.

I made my own fork seal driver with a cut out bit of PVC. This is a pvc pipe straight joint that i cut a slot out of, and removed the rib in the center with an angle grinder. I used this to drive the copper/teflon seals with the metal spacer over them, then used it to drive the main fork seal with the spacer over it.

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I was ordering a new chain, so I ordered a new sprocket too. The old one looked a little worn. Probably had life left, but now I have a matching set. Put a new front sprocket on as well.

Image
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Now it's back together with new tires, clean wheels, refreshed brakes, refreshed forks, etc.

Image

As for the hesitation, my guess is carb insulators. They're fairly worn. I was hoping they were sealing, but the behavior seems like they're letting air in, so the CV slides don't quite behave until 5.5k rpm, at which point the vacuum is enough to overcome the leaks. So with this in mind, replacing the carb insulators is next on my list.

I have a couple things left: I need side covers. I'm wondering if I should just cut some leather, waterproof it, and put ties through grommets on it to manually tie them in place. It'd look old school.

I already ordered a brake/light turn signal combo for the back. I need to install that. I need front turn signals. I'll probably get small ones and mount them where the OEM ones were.

I have already ordered a different handlebar. The one on it is not stock, and it's so far forward I can't reach it all the way with the bars turned too far. I'll move the controls, and when I do so I'll clean up the ignition wiring and remount the choke cable.

The neutral sensor and oil pressure light on the cluster are not hooked up to anything, so they're on constantly. They're so bright they cast really bad glare off my visor at night. I'll need to either enable or permanently disable.

And that's it, I think.

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1981 CB750C - in progress (stock airbox)
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Whozaa
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Joined: May 09, 2010
Posts: 5699
Location: St Louis, Missouri USA

PostPosted: Wed Feb 22, 2017 9:14 am Reply with quote Back to top

Quote:
I got the engine running ok. It it a little hesitant below 5.5k. To start it I turn up the idle screw, pull the choke, and as it warms up I back off the choke then turn the idle screw back out. Works every time.

The choke is supposed to work as a fast idle too. Perhaps yours isn't adjusted right. Check out SeanG's carb manual for the details.

Quote:
The neutral sensor and oil pressure light on the cluster are not hooked up to anything, so they're on constantly. They're so bright they cast really bad glare off my visor at night. I'll need to either enable or permanently disable

I think you may have some wires crossed in your cluster. Those lights shouldn't come on if the switches are disconnected.

_________________
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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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Sat Feb 25, 2017 9:38 pm Reply with quote Back to top

I ended up hetting this right somehow. Its starting like a dream. Choke, start it right up, and as the idle rises I back the choke off. Maybe I burned through the oldest gas but no more idle fiddle.

I had the neutral wire shorted with the sprocket cover. My bad ,it's fixed now too.

The oil light might be oil pressure or maybe the sensor. I'll start a thread for that.

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1981 CB750C - in progress (stock airbox)
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holysmokes
Admin



Joined: Jul 25, 2007
Posts: 35632
Location: ridge road springfield nova scotia

PostPosted: Sun Feb 26, 2017 8:48 am Reply with quote Back to top

If you have to soften up the insulators to install the carbs.your insulators are too hard and are not sealing

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On a steel horse i ride
.mm to inch conversion chart
http://www.carolynrelei.com/inchchart.htm
picture posting
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=127
link to manuals + tech threads>
http://www.cb750c.com/modules.php?name=Forums&file=viewforum&f=5
valve adjust thread>
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=12173&highlight=shim
check charging video
https://www.youtube.com/watch?v=YvRe9g_2gyM
carb cleaning thread>
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=13611&highlight=carb
verifying ground
https://www.youtube.com/watch?v=SyNYlaoInUE
checking stator
https://www.youtube.com/watch?v=5v2LtHlvcqI
checking r/r
https://www.youtube.com/watch?v=kDx3zgOLShY
checking rotor ohms thread>
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=1477&highlight=ohms
torque values thread >
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=81382#81382

http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=52
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=6935&highlight=shadow
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commiehunter
Getting up steam
Getting up steam



Joined: Jul 18, 2016
Posts: 18
Location: Portland, OR

PostPosted: Sun Feb 26, 2017 6:48 pm Reply with quote Back to top

Went out for a spin today. Thanks to this forum for all the assistance in getting this back on the road.

Image

Image

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