Topic: Fast Idle-Choke adjustment '78 CB750k

[Rainmaker]

Greetings....I'm sitting here in my garage with the tank off my 78 CB750k totally frustrated trying to adjust the choke on this bike. My laptop has been my best tool for troubleshooting this beast. I've looked at all the previous posts on choke adjustment and can't find what I'm looking for.  I'm trying to fix a slightly fast idle. The idle adjustment knob has been turned out until it's not even touching the tang but still idles too high. I can put negative pressure on my hand throttle and the RPMs recede.  I tried the cable adjustment trick but couldn't remedy the sitch. One lead in the SOHC forum suggested that perhaps the choke is partially closed. Now I'm trying to get my fat hands in that little space with the tiniest screwdriver in my drawer. What a @#$%*&!!! The Service Manual says turn the adjusting screw until it just touches the cam.............well from what I can tell it will always touch the cam no matter how you adjust the screw due to the return spring pressure. The manual is really helpful (not!) it doesn't tell you which way to turn the screw to achieve final setting.  Anybody know how to do this adjustment correctly?  -Thanks to all who reply....Rainmaker  (Keith)

PS. I did figure out how to get a better grip on the choke adjusting screw.......stick a long screwdriver down there and using a little leverage against the frame, pull the fast idle tang up. It rotates almost perfectly vertical so you can stick a little screwdriver in there.  However, you have to do this hit and miss until you get final adjustment right. 

[Jonesy]

It's much easier if you pull the choke knob all the way up first. This swings the whole works into a much easier position to adjust.

Yeah, it is hit or miss... refer to this other thread:

http://www.sohc4.us/forums/index.php?topic=10938.msg102836#msg102836

I'd suggest backing the screw off until it doesn't touch. Push the choke knob all the way in and run the engine to verify the fast idle issue has been corrected. Then, go back and turn the screw in until you get the desired fast idle for warm-up (when the choke knob is pulled out).

Good luck, and let us know how it goes.

[Rainmaker]

Thanks for the response. I saw your previous post to the 550 owner but I am still confused. From what I can see....the choke adjustment tang will never NOT touch the cam on the fast idle because the return spring always pulls it back to touch it.  Also.......your instructions to turn 2 more times are like what the manual says but it doesn't say which direction; left or right..........to touch or not to touch, that is the question!! Keith

[Jonesy]

We're only dealing with the fast idle cam in this case. 

Here's how things work: when you pull/push the choke knob, you'll see the shaft for the choke butterflies rotate. On this shaft is the fast idle cam. Now, on the throttle shaft (seen rotating when you twist the right-hand grip) is the famous adjustment screw. When the choke knob is pulled, the choke shaft rotates, and the cam rotates into position so the high spot on the cam makes contact with the tip of the adjustment screw. As the cam pushes on the adjustment screw, this twists the throttle shaft a tiny bit, cracking the throttle. The farther in you turn the screw (clockwise), the farther the throttle shaft is twisted and the higher the RPM's at warm-up. The less it's turned in, the lower the RPM's.

When the system is adjusted properly, the adjustment screw on the throttle shaft should not touch the fast idle cam when the choke is in the fully open position, in fact there should be a couple millimeters gap between the two. Back the screw out so the cam doesn't touch it when the choke is open. Then do the "just touch" adjustment with the choke fully closed. Then turn the screw in an additional 2 turns to give you the extra throttle on start-up. If the bike revs into the stratosphere, back the screw out a bit. If it needs more throttle, screw it in a tiny bit more. A little turn makes a big difference. Personally, my '78 550K only needed about 1.5 turns in...

I hope this helps. Sorry for the long-winded explanation but I find if I know how the system works, the easier it is to diagnose and fix problems.

[Rainmaker]

I was able to get a tiny slot screwdriver on the 'famous' adjustment screw.  I couldn't turn it with my fat hand while trying to keep the slot head on the screw.  So I used one finger on the back of the screwdriver pushing to keep enough pressure on the screw head. Then I used some channel locks on the driver handle to rotate the screw in.  Seemed to work though very small turns at a time. 

As I played with pulling and pushing the choke lever while adjusting the scew, I never got it to back off the cam without touching it.  However, whatever I did.......the idle is now normal + I have good adjustment on the main idle screw. Thanks for the detailed explanation and helping me figure this out.  Keith