Topic: Choke not Choking

[joegeis]

My '78 CB550 has stock intake, carbs and exhaust. I've noticed this summer that at some point the choke stopped having any affect on the engine. Previously it worked as it should. It would help the bike start cold and would increase the rpms when pulled all the way out. Now it has no affect. I have two videos posted in the album linked below. One video shows that the choke butterfly on carb 4 is in fact moving when I pull the cable. The second video shows the bike just after cold starting it without the choke, and then you can see me engage the choke and notice no impact on idle. (Luckily mine just needs me to feather the throttle to start, at least until the temp drops a bit further!).

I have not yet pulled the carbs out and apart.

Link to Album. Scroll all the way to the bottom.

What in the world is happening here?

[bryanj]

Cant do videos but that is typical of blocked pilot jets, has it stood for some weeks

[joegeis]

Huh, that's strange. I took the carbs apart and cleaned the jets this summer. Bike has been ridden regularly all summer too.

After a min or two of nursing the start with the throttle, it evens out and idles at 1100rpm beautifully.

- coming back to edit my reply -

How exactly does the choke on this bike work? Is there a different pilot for choke vs idle that could be clogged? Or does it simply reduce the amount of air able to flow from the airbox?

[Bodi]

The choke creates a vacuum at the venturi, just by obstructing the intake side of the carb.
That vacuum pulls fuel up the main jet for a rich mixture.
I'm not familiar with your carb stack. But some models have a throttle advance cam coupled to the choke mechanism. This is on the #1 carb, a little mechanism coupled to the choke shaft with a short rod. There is an adjustment for how much throttle is added along with the choke closure. That gives a high RPM idle when the choke is applied - keeps it running while warming, and is a good reminder if you leave the choke on.
If you alter the relationship between the slide lift arms and the throttle shaft, this affects the choke cam function. It's a bit of a dance to get the lift arms, idle screw, and cam to work properly. Unless the lift arm "sync" adjusters are pretty close to what Keihin set originally, the cam may not be adjustable to get much throttle advance or, conversely, can make low idle impossible.
If yours doesn't have that throttle advance cam you have to work the throttle along with the choke to keep it running until warmed up a bit.

[HondaMan]

This might be something to look into, as I've seen it several times: on those 750K7/8 PD4n series carbs the choke cable is just pinched between a screw's nut and a plate. It's not a lot of grip: they can slip a little bit when the choke is set fully if it is not tight enough. Then, the choke cable pulls all the way up, but the choke lever is falling behind a little bit: then it won't high-idle.

To fix it, the choke cable just needs to be adjusted to make sure it is pulling those choke plates fully closed.

[joegeis]

Thanks guys. My choke cable is original and a little chewed up from that screw over the years. I'll check that. My carb does have that idle bump, which is definitely no longer working, so that could be the cause.

I'll also take the carbs off and give them a good clean over the winter (yet again!) to absolutely ensure jets are all perfectly cleaned. I bought a little carb cleaning kit that came with a bunch of tiny little brushes for cleaning jets, so maybe that will also help. It could some combo of a blocked main jet and the cable slipping a bit maybe.