CB750 K0/K1 Hybrid

[Lucien Harpress]

No on the title, but thankfully Michigan has a decently easy workaround to get one. It's worked for my CA77 and CT90, so I don't see why it won't work on this one.

Carbs are going to be a long-term thing, I think. You're right that they're available, but go for fairly insane prices. The next big purchases for this bike will probably be a correct (single cut) front fender, and a set of clean exhaust pipes- I lucked into correct HM300s, but the header portions are fairly ratty. Any future major purchases will probably be for my Dream and/or KZ1300, so I can circle back around to carbs later. And from a drivability standpoint, I'm okay with sticking with the two-cable carbs for the time being. But they're definitely on my list.

(Sad thing is that way back when, the bike originally came with the 4-cable carbs, albeit in REALLY rough shape. I traded them for a pristine set of carbs I have now. It made sense when I was aiming for a roughly-K2 model, less so now. I don't regret it. Mostly. )

[Lucien Harpress]

Remember what I said about my carb situation? Ignore all of it.



They're going to need a full go-though, and one of the brass floats looks a bit iffy (plus I'll need at least one correct main jet), but hey- at long last this bike has the carbs it originally came with.

[Lucien Harpress]

Been plugging away at this old girl. I got my handlebars swapped over, got all the controls figured out, and managed to get enough done to get the new carbs on the bike!





Now, I can't speak for tuning these monsters, because I hear they can be a pain. That said, from a purely aesthetic standpoint they are SO much nicer. Adjustment points are easier to get to, controls are more straightforward, and the layout just makes a bit more sense. I'm still waiting on a detail or two to actually fire the engine up, but forward progress is still progress.

[wingrider]

Keep at it, looking good so far!

[Lucien Harpress]

Progress continues, bit by bit. I'm managed to get it to run consistently on three out of the four cylinders, but for some reason piston 3 won't stop fouling out. It gets wet enough to not fire after I've let the bike sit overnight, and even if I clean it off it runs like poo-poo. Looks like I'm pulling the carbs again to check things out. On the plus side, I've got it narrowed down to one place, so that's progress I suppose.

[robin1731]

Do you have another set of known good carbs you could put on it?

The year doesn't matter. Just as long as you know they are good.

[cfairweather]

It could be the float valve is leaking. Are you having any gas coming out of the overflow tube? Make sure the overflow tube is clear. The metal float valves on these older CB750s are always a problem and it is normal for them to occasionally leak. I would install clear tubes on the carbs to see where the level is and go from there.

[Lucien Harpress]

Float valves were giving me fits, so I won't rule it out. The brass floats were being a pain, so I've since swapped them out with the plastic floats in my other carbs, along with the float valves. I did do a clear tube bench test right before I installed them last time, and they all matched each other, about 4mm below the lip of the carb body. I may check the level again before I pull the carbs.

If none of that stands out? My other guess is the air jets. At the very least I'll give that whole carb a once-over, see if anything pops up.

Do you have another set of known good carbs you could put on it?

The year doesn't matter. Just as long as you know they are good.

Yes... and no. I've still got the carbs from the later years, but the throttle cable differences would make swapping them an absolute pain. They were also giving me fits ever since I put the earlier airbox on (which has larger openings), so "known good" is pretty relative. This was also the first time I really got my K0 carbs dialed in, so I'm willing to do some experimentation before looking into a carb swap.

[cfairweather]

One nice thing about these carbs is you can do a ton of maintenance and troubleshooting while the carbs are mounted. This includes replacing jets, floats, etc. You are close and I am sure you will figure it out. One thing that I do with these float valves that might help if they are leaking, is to break a cue tip in half and chuck it up into a drill. I use a bit of Mothers Metal Polish on the cotton tip and polish the seat to a high finish. Good luck.

[Lucien Harpress]

Carbs are pulled again, and I separated #3 from the rack just to make it easier to work on. I pulled the plugs to check on how those looked, and results were... mixed. 1, 2 and 4 weren't bad. A bit dark on the edges, but the electrode had a decent tan color starting. It tracks, seeing as how for the last three days this bike was on idle 90% of the time, and my run "at speed" only lasted about 5 minutes.

Plug 3 was impossible to read. I'd hit it with a wire wheel before my run yesterday, and I don't think I ran it long enough to see any change to it. Not to mention it probably wasn't firing for most of the run anyway. I AM getting spark on that plug, so that works at least.

Next up is going over carb 3 with a fine-tooth comb. I'll update if I find anything.

[robin1731]

Not realistic to think a plug that has been fouled that bad can be cleaned and expected to perform a new.

[Rednaxs60]

Following your thread, very interesting. Almost as many issues with carb models as with older FI models, I'm replacing the ECU on my '85 GW Limited Edition FI model. Air-fuel mixture has been an issue since I stated to operate the engine and start the engine tuning process.

Having mentioned this, learned about plug reading from my father and others starting in the '60s, thought that a look at a plug was all that was needed. Have found this to be incorrect and will not give too much information on what is happening. This video is a good view on what the old fellows know about this subject: https://www.youtube.com/watch?v=yvTeyNM_BwY&t=991s Additional searches corroborated what is being presented in this video. Another good web site read is: https://honda-tech.com/forums/forced-in ... A-3063102/. Don't have to tune as per the 1/4 mile, but the reading is the same.

I have destroyed a few plugs to corroborate the information I have found and what is being presented.

The other issue is cleaning plugs, recommended to not use a wire brush, damages the porcelain. Use a propane torch and burn the plug clean, can be used several times before a new set is required. I was under this misconception as well for many years.

Hope the above provides some good information. Good luck.

[Lucien Harpress]

Not realistic to think a plug that has been fouled that bad can be cleaned and expected to perform a new.

Fair enough. Thankfully, my GL and CB both take D8EAs, so I've got more spark plugs than I know what to do with.

I stripped the carb down, figuring I'd see if the air jets were plugged like I thought. Those turned out to be clear to a certain extent, but while looking things over I noticed a bit of schmutz built up under the float valve. Not much, but it might explain my leaky valves. Looking at the main emulsion tube passage, however, there was a BUNCH of crud up and down that whole thing.

I have no idea where it came from- either my IV tank was dirty, parts of the carb I didn't clean well enough got knocked loose by the fuel and collected here, I don't know. Either way, it's getting another clean, the other carbs are getting double-checked, and I suppose we'll see if that's my issue.

Minor Update: I can't fully confirm until I get the carbs back on, but I may have found the culprit. While my carbs got clean, I 100% failed to look hard enough at the fuel Ts. The left one wasn't too bad, but the right (between carbs 3 and 4) was positively fuzzy with crud and gunk. Carb 3 looks like it took the brunt of the garbage as the gasoline knocked it loose, but I saw enough wayward particles in 4 to want to strip that down as well. Thankfully it won't take long to do, and now I have a lesson for the future (namely, when cleaning carbs check EVERYTHING).

[Lucien Harpress]

I was hoping to be able to ride the 750 to Gilmore this weekend, but that isn't going to happen. After about an hour and a half of frankly stupid problems (loose connection on one of the coils, another float not sealing until I whacked the bowl with a stick despite working fine last night) and going around and around in circles, I threw in the towel for the weekend.

It seems like I can get a steady idle, no popping out of the exhaust, or crisp off-idle response and back down to idle without bogging, but never two or more at the same time. Now, I'm sure a lot of this can be blamed on the carb design (there's a reason the four-cable setup went away), but I can only do the pattern of "Sync carbs, set mixtures, sync, now idle is too high so drop it, sync, set mixtures, idle too LOW so raise it, repeat until you pass out from the exhaust fumes" so many times.

About the only good data I got was a couple air-mix screw settings seem a bit far out of whack to keep the motor happy. Manual calls for 1 turn out, and some guys go with 7/8th of a turn. I've got a couple at 7/8, one at just over half, and one at just under half. It's too early to tell if this means anything.

[Lucien Harpress]

Another day in my ongoing K0 carb saga. After a bit of not-going-anywhere-fast, I took a step back and reassessed. I took a look at the idle air screws, and, for the record, these need to be initially set at one turn out. Somehow, in all my going back and forth, all four had ended up stupidly far in- the worst was less that a quarter turn, and all four were less than a half turn.

That might explain a couple of my problems. Namely the tendency to foul plugs if I even think about touching the choke, and black fluffy plugs (at idle) all around. On the plus side, my float seals finally seemed to behave- mostly. Carb 4 will need some attention to prevent the tiniest of fuel weeping on to the ground, but that's easy enough to get to without pulling the rack.

At least the process is interesting.