Topic: 1978 CB750k Carb rebuild over the winter and tune up questions!

[fendersrule]

Perfect. Thanks Flybox and Black. Very good information here, and you guys know I HUGELY appreciate this!

I'll make a visit to harbor freight tomorrow to look for a palm ratchet. I'd rather see if I can assure that the jets are clean before finally giving in to remove carbs and go throughout that whole process.

Either way, I've got my work ahead of me on this bike with some simple items to do before pulling carbs.

[flybox1]

haha...thanks for the corrections, teach    corrected my post above
yes, caps are 5K ohms, and i assumed  his meter would be switched to read ohms 

Hey Fly I hope you know I did not mean anything negative toward you or Fender

@ Fender For  bowel removal on the bike I use a palm ratchet a short extension.

Plug caps. Bottom of page
http://www.z1enterprises.com/ShopByCategory.aspx?page=2&itemsPerPage=20&name=Ignition&description=Points%2c+condensers%2c+electronic+ignitions%2c+coils%2c+e&category=01010205070507

No way man.   We gotta keep eachother in check.  Mistake on my part that needed correction.  Sometimes I get my thumbs going on my smartphone too fast for my own good.

[flybox1]

You can get yourself the new caps at a local auto store....NAPA, O'Reilly, etc....just take the part no.
1&4 take the VD05F 5Kohm.

[fendersrule]

Flybox, You are not too far away from Wenatchee (where I go sometimes). I'm Boise. If you're ever down in the area, please stop by. I have too many of these 70s Hondas in my garage and I'm always wanting to learn more when it comes to carbs and ignition.

I will say one thing. Ya'll know that I'm a twin fan more than a four fan. One thing we all have to admit: Working on twins (Cb450s are a good example since it has lever released bowls) is 1000000% easier than fours. I can have a compression test done on my CB450 done in 2-2.5 minutes. On a four it takes 20 minutes (if you're good) and a few burn marks.

I can drop the bowls off my 450's (most of them) in less than 5 seconds each without any tools. That means super fast cleaning and checking the jets.

Tuning is easier. More sound that you can hear for each cylinder. More change in the RPM for air/mix adjustments. Every adjustment on carbs are positioned from the outside, so no getting on knees, wearing flashlights, or bending over.

Many times, I stand in the four in awe at its prowess, but also discouraged that it takes so much longer for maintenance, diagnosis, cleaning, and figuring out WTF is happening with the tuning...That's my knowledge anyhow.

Some guy here wants to sell a '78 Super Sport for $1200 "OBO" and it smokes, but looks 80% complete. Someday, I will pick up another SOHC, but I either want either an early 70's K or a 77-78F model. I takes awhile to warm up to the looks of the 77-78F, but once you do, it literally knocks you down and they are arguably the fastest SOHCs produced (I've read in multiple places). Yet, prone to horrible valvetrain wear due to oversized valves. That's what's keeping me from paying $1200.

Ok...rambling. I'll post updates as they come. Thanks guys!

[fendersrule]

You can get yourself the new caps at a local auto store....NAPA, O'Reilly, etc....just take the part no.
1&4 take the VD05F 5Kohm.

Flybox, I was recommended to buy NGK XD05F and SD05F Caps.

But it looks like the XD05Fs need more slant in order to fit. Good catch. I could cancel the XD05s and grab the VD05Fs?

CB750Supply.com recommended XD05Fs.

[flybox1]

I've got family in Eagle, and Star...my cousin owns a real estate company in Boise.
Many steelhead trips up to Riggins over the years.
I know Wenatchee well.  Best fishing buddy lives there.
I'm sure we will cross paths someday.

[andrewk]

I'm not sure what everyone's aversion (not just Fenders) is to removing the carbs on a 750.  Easiest four to do by far.

Remove tank
Disconnect cables/overflow hoses
Remove air filter
Loosen air box clamps
Remove air box bolts
Remove air box
loosen carb boot clamps
Remove carbs.

10 minutes, tops.  It takes 15 minutes of dicking around and cursing to get the bowls off PD carbs while on the bike.  Then once you do you can't really see anything to clean it.  Don't know why anyone would even try.  Remove carbs, clean/check/adjust, re install.  Easy peasy people!

[fendersrule]

While I'm waiting for the new boots, I decided to lube my throttle cables to get them to feel a bit smoother.

I think I may have found the problem.



When I roll on the throttle, [1] moves in the direction as indicated, and [2] moves in that same direction with delay (looks like it's in a bind. If you look right above [2], you will see insect larva stuff that has grabbed ahold it.

Should 1. and 2. move at exactly the same time?

[fendersrule]

Even after cleaning this area, I think that's the intended behavior because it's about the same.

Another question: Now my starter button doesn't work. When I press it, the lights go dim (like it should), but there is no sound or anything. What things should I verify in this case? I did replace the throttle sleeve so I had was moving the wires around in the handlebars. But something tells me the switch and it's connection should be fine because the lights are dimming.

I can start the bike via kick starting. I did check the fuses.

[Stev-o]

Using a long screwdriver, jump the starter switch (solenoid, #7 on fiche) to confirm your battery and starter are good.
Most likely, the starter button is the issue.

http://www.westernhonda.com/fiche_section_detail.asp?section=2451933&category=Motorcycles&make=Honda&year=1978&fveh=131106

[fendersrule]

I can jump the solenoid to start the bike.

Looks like I gotta disconnect the throttle cables yet again to remove the throttle sleeve in order to get to where I can see the starter switch. I run my wires in my bars now. Now it's going to create more work just to remove the sleeve, but still worth it.

[fendersrule]

Another question:

My throttle cables appear original. I see the "Made in Japan" on them.

Why is it that I can lower the RPMs by rolling the throttle backwards? Does this mean an adjustment is needed to one of the cables (either extending or tightening) at the carb? Or does this mean I absolutely need new throttle cables?

[Stev-o]

I've recently noticed the same on my K8 but haven't checked it out yet. I believe my cables are original too, don't think it's a cable issue on mine.  Seems the return spring is not as strong as the earlier models, check that first. And lube the cables...

[fendersrule]

I'll take a look at this and see if I can understand why it does this. My guess is it is probably an adjustment that needs to be made to one of the cables at the carb (either the pull or the push cable is too tight....OR too loose)...OR cable is worn. Usually, speaking from my ass, throttle cables don't usually go bad that often...not anywhere near as often as clutch cables. So this leads me to believe that it's an adjustment that needs to be made. Or, a rare possibility, but the area inside of the handle bar control where the sleeve sits is worn and therefor allows backward travel from the nominal position.

This isn't hasn't been documented anywhere on the web....so Steve, we're a first...

This MUST be fixed for safe riding. Here's why. If your bike does this (able to lower RPMs/kill engine by moving the throttle sleeve backwards), then that means whenever you perform a hard stop you will kill your bike. I know about this because of my braking course that I just completed. I had to turn start the bike after every hard stop...a little embarrassing.

I'll look into this after I fiddle with my starter button. If anyone has any idea about the throttle issue, let me know, otherwise I will have to butt my head against it out in the shop to see if I can come up with something.

[BobbyR]

Another question:

My throttle cables appear original. I see the "Made in Japan" on them.

Why is it that I can lower the RPMs by rolling the throttle backwards? Does this mean an adjustment is needed to one of the cables (either extending or tightening) at the carb? Or does this mean I absolutely need new throttle cables?

On a push pull setup you can lower the RPMs which is great. I have a fast idle cam and I can drop the RPMs to kick it into first and they spring back up when I let go.

Cables start to develop micro breaks  on some of the outer strands and the form whiskers that dig into sheath. Lubing helps for a while. You may also want to check the plastic sleeve, the lube dries out sometimes.

[fendersrule]

I recently just lubed the throttle cables this weekend.

I replaced the plug caps with NGKs.

Still have the same issues.

Being able to lower the RPM enough to where it kills the motor is not very good. I still haven't removed the tank (again) play with this at all to see if it's in need of adjustment at the carbs. There is no documentation that I can find about where the proper setting should be at the carbs...

I also had the choke on, and whenever I wrap the throttle I get the same result....a choking sound as found in my video.

I guess this means it's time for the carbs to come off...

[BobbyR]

I recently just lubed the throttle cables this weekend.

I replaced the plug caps with NGKs.

Still have the same issues.

Being able to lower the RPM enough to where it kills the motor is not very good. I still haven't removed the tank (again) play with this at all to see if it's in need of adjustment at the carbs. There is no documentation that I can find about where the proper setting should be at the carbs...

I also had the choke on, and whenever I wrap the throttle I get the same result....a choking sound as found in my video.

I guess this means it's time for the carbs to come off...

Probably u will have to check the carbs. If you do, take your time and do it right. I was in a hurry and it took me 4 tries. That sucked.

[fendersrule]

The wire to the starter switch became disconnected with all my gruesome tugging. Soldered it back on. All good now.

I want to try one more thing before ripping the carbs out: testing for a vacuum leak. I picked up some starter fluid as advised. Can I start spraying this fluid while the bike is idling or do I need to set the idle speed faster, to maybe 2000 RPM and then start spraying? It probably doesn't matter, but I thought I would ask.


In regards to the throttle cable issue (being able to lower RPM by rotating it backwards)....

I spent about 30 minutes looking over the push/pull mechanism to understand it. Here's what I've found out.

Being able to move the throttle cable backwards is an interaction of three things that have to do with the mechanical throttle system:

1) Idle speed screw. You'll notice that you can screw this in until you remove the play. However, this obviously will raise your idle, so using this to adjust for too much cable slack is incorrect, though it should be mentioned because it is apart of the same mechanical system.

2) Pull cable. This cable is adjustable on the handlebars and via the cable adjuster or at the carburetor connection. Removing too much slack out of this is just going to tighten the throttle and potentially cause it to stick in place. You want to adjust this independently, but it's mentioned because this also interacts with the push cable. So you can't adjust this to fix the problem either.

3) Push cable. This cable is NON ADJUSTABLE. Which means, once slack develops, there is no way to take it out.

That's a lot of information. When it comes down to it, if you can do this on your bike then AT LEAST the push cable needs to be replaced. Not a bad idea to replace the pull cable too, but I suspect that slack in the push cable is the culprit.

[fendersrule]

Guys,

I'm surprised no one has heard about this specific carb issue!

Found some new info. I have a 78 CB125s with the PD carb.

It does the same exact thing. If you wrap the throttle fast, then it will "gulp" and sounds like it's starving. Has anyone confirmed that you can actually wrap the throttle on a 77-78k?

On the CB125s, this still happened after:

1) Set Points SPOT ON. Cleaned them.

2) Set Dynamic/Static Timing. SPOT ON.

3) Valves are SPOT ON.

4) Air cleaner is clean.

5) Adjusted air/mix screw with exhaust analyzer.

6) Pulled carbs, and sprayed it down with carb cleaner. Jets were already clean. Bowl was dirty. I didn't soak anything.

My biggest fear is for my dad to have to tear down the carbs and it still happens....I thought it was interesting that the CB125 does the same thing and it has a PD carb.

What exactly is causing this issue where you cannot quickly wrap on the throttle without stalling? I want to have my dad pay very close attention to those two things. Should he just soak everything in carb cleaner over night (disassemble first of course)?

[flybox1]

These are not FI bikes. 
Even when perfectly tuned....carbs, bikes and all, they will not react like a FI bike.
However, the accel pump does allow for a more 'brisk' roll-on.

Why don't you post a video so we can see your WRAP of the throttle and hear your bikes reaction.
It'd be a good idea to include all your fixes and carb adjustments in your next post....
Update us as to what you have done so far....

[fendersrule]

Here's a video. Let me know if this sounds familiar flybox.

http://forums.sohc4.net/index.php?topic=131396.msg1471826#msg1471826

We already removed the carbs and they are sitting in my dad's car ready to go back home.

[andrewk]

I don't use PD carbs, and have better throttle response than that.  Something's up, because PD's are better than what I have.

My guess is a plugged idle jet(s)/circuit and/or emulsion tubes.  I know you say you checked, and I believe you, but that stumble is a lean stumble- it's not getting gas.  95 percent of the time the culprit is what would be called "incomplete carb cleaning."

Could be something up with the accelerator pump circuit too.  Check to make sure that both the pump arm clearance is correct (spec in manual) and that the little restrictors are still in the hoses that connect the accelerator pump circuit from carb to carb.  Should be one on each side of the number 2 carb. Makes a difference.

[fendersrule]

Thanks guys! I did what was advised and removed the carbs. Pretty easy to do.

I don't even have to split the air box in order to remove it either...I'm able to slowly get it out via the right side of the bike...

I'll send my dad this thread so he knows those few things to order (NO CARB KITS!). Is there a really good resource on 77-78k carbs that he can follow?

[flybox1]

+1 andrewk
Incorrect float height, Blocked pilot jets and or emulsion tubes.

Check all of these...again.

[andrewk]

If you have a factory manual (I found a pdf online) with the supplements in back, page 253 through 258 give the pertinent service specs, but as supplements, they offer little guidance on technique.  Might be a few informative threads in here somewhere too.