Topic: 75' cb750f carburetor troubles, or is it valves? insight needed

[HondaMan]

tried syncing the carbs again myself after setting the timing as best as i could, i noticed that as i apply the throttle, the 4th carb isn't building vacuum like the others do. does this mean there is a vacuum leak that i haven't found yet?

It sorta sounds like it?
The jumpy timing isn't uncommon: it was originally caused when someone turned the engine backward with the big nut by the points. It bends the spark advancer shaft a little bit, making the timing "jitter" back-and-forth. This won't usually cause the trouble you are having, unless it is bent a lot. If you put the timing light on the 2-3 side you'll likely see it jumping, too (and worse than the 1-4 side) with a bent shaft. In an extreme case (like 0.020" bent) or if the spark advancer's cam shims are gone (as many are), then the timing will not settle down until the advancer is re-shimmed and the shaft straightened, and the combination of both together can make the carbs impossible to sync. The vacuum at idle speeds then gets disrupted every time the #3 fires too soon, slowing the crankshaft momentarily and slightly just before the #4 fires, which makes it slightly 'stumble' slow until the #2 fires to speed things back up a bit. This cycle repeats every other revolution of the crank, making a rough, noisy idle and erratic vacuum sync issues.

[proudsnacks]

Thanks hondaman, ill have to double check but I'm pretty sure the 2/3 timing seemed a bit smoother, as for the advancer shims, what exactly are you referring to? I remember inspecting the advancer when I was mounting the new breaker assembly but I was sure what to look for, it seemed fine, but then I've never dealt with one before.
 

[HondaMan]

Thanks hondaman, ill have to double check but I'm pretty sure the 2/3 timing seemed a bit smoother, as for the advancer shims, what exactly are you referring to? I remember inspecting the advancer when I was mounting the new breaker assembly but I was sure what to look for, it seemed fine, but then I've never dealt with one before.

The spark cam has (or had) 2 or 3 thin shims underneath it where it sits on the shaft of the advancer. Many were made from plastic, and most of those have melted and vanished. Only the Hitachi and the TEC post-1972 advancers had metal ones (and, not all of them). The purpose of the shims is to reduce the clearance between the front of the advancer cam and the back of the big nut that holds it onto the crankshaft's mounting shaft (with that little 6mm nut). This clearance can be checked in-situ: slide feeler gage(s) between the back of that big nut and the face of the advancer cam: it should be less than 0.006" clearance. If it is more, the cam can wiggle a little bit, which makes the spark erratic (especially on the 2-3 points). If the shaft that mounts the advancer is bent, it can also make the timing jitter in the same way. To check this, you will need to borrow a dial indicator: if you have my book there is a picture of how to use a mag-base with the indicator dial by setting it on a rear brake stay arm, bolted to the 2 screws on the front of the clutch housing's cover, as a convenient mount: the runout should be less than 0.002" for a "new" feel.

The clutch noise you hear is just due to the jittery timing. It will settle down when you get it sorted.

[proudsnacks]

Ahh that would make sense, I had taken it apart yesterday to inspect, and there is definitely some play in the advancer shaft, I'll check the clearance. I also swapped the breaker plate back to the original and it is a better fit, no play in the plate and I was able to get it timed right on with everything within spec. I'll see what I can do about the shimming. Thanks again, HondaMan.

[proudsnacks]

And the clearance is way beyond .006 so a.shim is needed, what's the best option? Make one from plastic? Or a washer?

[proudsnacks]

Little update, I managed to shim the advancer shaft, helped a little bit but it's still jumpy. I also installed new copper gaskets on the headers and checked for vacuum leaks around the carb hoses but haven't come up with anything, I re synched the carbs after swapping the plate and they were all dialed in except for when the throttle is applied the 4th doesn't build the same vacuum like the other 3. I checked the floats heights again to be sure and they all look good, it only status missing at 2k rpm and above, and it's.only fouling the 4th plug, the others look pretty normal from I can tell. I've about had it with this thing.