CB750 K0/K1 Hybrid

[Lucien Harpress]

I took a week and a bit off for a vacation to Iowa (Red Power Roundup, a big IH tractor show that travels around the country) looking at trucks, tractors, and dodging flooding, I'm finally back home, picking at the 750.

On a whim I went back to something I thought I'd had sorted. When I first got my carbs, the needles were raised up one notch from stock. I went through everything, but put them back to where they were, in their non-stock position, because after doing a bit of research, some K0 carbs ran them this way from the factory, for... whatever reason.

Well, after getting fed up of constantly fowling plugs, and seeing what happened when I swapped in new plugs and tested them at half throttle (ran great for a minute or two, but started missing and popping after that), I pulled the needles, set them BACK to stock, and tried again. I didn't get a whole lot of run time, but half-throttle was much better than previous. She died immediately on idle, but between the mix screws and idle screws, I figure that won't be too hard to lock down.

I've noticed there's a difference between carbs that are "wrong, but you can get it to run poorly", and "right, but you just need to dial it in". I'm hoping I've finally moved from the former to the latter. I guess we'll see.

[Lucien Harpress]

Okay, so good progress. Dropping the needles solved a whole mess a problems, and while it didn't solve all of them, I'm better than I was. Big win, choke no longer fouls my plugs so bad it kills them. Off-idle response is pretty good, and I've mostly got the syncs (both idle and at throttle) locked in. That said, it's not all sunshine and rainbows.

-Cylinders 1 and 3 are giving me fits. I get the occasional pop out the exhaust from both, but 3 is worse. If I enrichen it too much it starts to not hit on every revolution, but if I lean it out it starts to pop. HOWEVER, there is an overlap in the middle where it does... both. Now, it could be fighting the other cylinders too, so who knows.

-There is a tendency (which gets better and worse, depending on how the adjustment screws are set) for the revs to drop below idle for a bit after blipping the throttle before catching back up to normal. Best explanation I got for why this happens is "something isn't set right".

In any case, things are definitely better than they were.

Side note- I noticed that at one turn out the mix screw on #3 was oddly deeper compared to the other three at the same setting. On the off chance the seat has been goobered up, I found me an affordable spare carb body just in case. I hope I don't need to swap them, but it helps to have insurance.

[Whiskerfish]

Do you have a IR gun? Header temps at idle may shed some light. I think mine was like 25 bucks at Harbor Freight

[Lucien Harpress]

Funny enough, I do actually have one. Now, do I know where it's AT at the moment? Well....

Thankfully, the 4-into-4 pipes are a godsend as far as tracking down individual cylinder problems go. I actually got cylinder 1 to behave decently well a little bit ago (or at the very least shifted the poor performance to cylinder 2), so that one I'm chalking up to an incorrect setting somewhere. 3 is still being stubborn, though. Pulling plugs doesn't seem to tell me too much, as there isn't much difference between the "good" and "bad" cylinders. Of course, a lot of this may be that these particular carbs are a PITA to set up, too (and there's a reason Honda ditched them after two years).

Ah well. I'll see if I can find that IR gun and go from there.

[Lucien Harpress]

Big day the other day: the 750 saw sunlight!





The good news- from off-idle on up, she is 100% perfect. I always worried I'd get some kind of miss or hesitation right on the very beginning on giving throttle, but the bike seemed to do just fine. Idle took a minute to get a good place, but once the bike was warmed up it settled down okay. I ended up doing about 10-12 miles, and at speed she was great.

The ride experience with the straight-pull throttles is... strange. It's a bit less refined than the bell-crank throttle I had earlier, so it really feels like my Trail 90, just scaled up. I got used to it pretty quick, but it's still odd for a bike to feel so familiar, but so different at the same time.

Now, on to the bad- at least one cylinder wasn't QUITE there. In a shocking turn of events, it was cylinder 4, one that's been rather well-behaved in my garage tests. I pulled the plug and compared it to 1, and it definitely looks lean. I figure run the air screw in a touch and see what that gets me. For the record, plug 1 looked pretty good. I want to check plugs 2 and 3, but they are a royal pain to get to. So they can wait.

Two other possibilities with carb 4- I'm still getting a slow fuel weep out the overflow hose (enough to stain my garage floor), and the throttle cable for that carb is kinked over a bit more compared to the other ones by the gas tank. Stuff to check, at least.

Finally, both gauges are now a suggestion. The speedometer is... okay, but the needle wavers quite a bit. The tach is worse- that needle wobbles as well, but if you give it the beans the needle gets to about 7k before falling back to 3-4k and freaking out. It usually settled down, but it took a while. I'm going to start by cleaning out and re-greasing the little gearbox on the back of each of them. If that doesn't work I've got a replacement tach to pull the guts out of, and I can use later (easier to find) model speedometers if I need to replace the guts on that.

[Whiskerfish]

Nice!!

[Track T 2411]

[cfairweather]

Very nice job. I am working on a CB750 K0 right now too. Mine is all in pieces .

[Lucien Harpress]

750 had to go back under the knife to try to clear up some lingering issues. Carb 3 annoyed me enough that I finally just replaced it, and life got in the way long enough that it took me a while to get back to the CB. Well, I am happy to report that- the float valves are still acting up.

For whatever reason this bike REFUSES to keep fuel inside of itself, and I'm fed up enough that I went and bought all new float valves. Carbs will seal up when they want to, and leaks out the overflow tubes will hop from carb to carb, with no real consistency. I can try to tune it, but it wouldn't accomplish much with what I'm assuming are fuel levels hopping all over the place. I'll be sure to update once those arrive.

[cfairweather]

Lucien- I would take a close look at the floats and probably replace them. The angle that the floats are bent and the tab angle at the point of fully closing the valve should be carefully inspected too. If your needles have a wire, make sure it isn't touching the seat when fully closed. I put the carb set in the same position as they are when in use and then use a headlight/magnifying head device, so I can very carefully inspect the operation as I move each float up and down. I also use the clear tube method on these carbs. If you put the bike on a lift, you can do all of this without removing the carbs. If all else fails, you can buy needles that have a rubber tip with wires, but this will require a simple mod to the top of the float valve seat. I can give you details on how to do this if needed. I would be happy to do this mod for you if you send me the float valve seats along with the needles.

[Lucien Harpress]

Finally got fed up with both leaks and carbs that seemed to work when they wanted to, and did a full replace of all 4 float valves AND replaced the later plastic floats with brass ones. While they are technically period-correct, I bought them because they are narrower, and I'm hoping with less chance of hanging up on the carb bowls.



After installing everything and getting the levels set correctly, I decided to rig up a test stand to see how how well the floats worked. Rack nearly made it 20 hours, and even when I started getting gas out of the overflow, it wasn't much.



Re-installed the carbs, and everything I see tells me I've got the slides synced up. I'm not 100% positive on my mixture screw settings, but I have NEVER been good at that. Still, it was enough to move the bike back into the garage, and I look forward to weather good enough to get an extended ride.