Topic: 1978 CB750 low RPM problem

[Mr. Mike]

Does the "clacking" or clattering sound at your clutch diminish at idle, with the clutch pulled in? Then disappear around 1300 rpm? May very well be the typical clutch clatter these bikes make at idle. The YouTube link was difficult for me to distinguish noises sorry.
Definitely get your carbs serviced. If you do it yourself, be sure to remove all jets and actually clean out the tiny holes as well as the barrel of the jet, don't just " blow them out" with compressed air. Read up on carb cleaning/adjusting if you have doubts about anything. Syncing them is a whole different thing.

[vanhuette]

Mr. Mike, ya sorry that video was not ideal. maybe ill try to take a better one at some point. its not easy to do because it never happens when its sitting in my driveway haha.

Im not totally sure whether this is or isn't the classic "clutch clatter". I thought in the descriptions i read that the clutch clatter tended to go away when the clutch was pulled in (disengaged), and would then happen again when the clutch was re-engaged.

Mine tends to occur when i pull the clutch lever in, and during the "clutch point" where its in between being in and out. And it has never occurred when i start it up and its sitting idling in the driveway (regardless of whether clutch is in or out). it seems to only occur once I've ridden around for a few minutes, and mostly happens as I'm coming to a stop/slowing down and i pull the clutch in. However, it does occur mostly at low RPM, and at low speed.

[flybox1]

I did the cam chain adjustment. Haven't touched the timing and advance, or vacuum synced the carbs (no carb syncing equipment).

looks like I'm buying some new tools!

Im sure the carbs could at the VERY least use a good clean out and rebuild. What are you waiting for?!?!?!     Ill try to get on that this weekend. Just for my sake - you guys suggesting these for the hesitant/shaky throttle problem, none of these could be causing the metallic clacking with the clutch?
You'd be surprised how quiet your bike will run once you do ALL of the necessary maintenance, and follow it up with a nice vacuum sync.  It really can go from sounding like its full of nails to a nice idle.

Im wondering if i should just bite the bullet and rebuild the clutch to see if that helps. Why repair something if you dont know if it needs it?  I'm kind of at a loss for that issue. 

Thanks for the suggestions guys, ill try to keep you updated how it turns out.

While you are waiting for tools....or the cows to come home.....clean and bench sync your carbs!!!
The order of maintenance items also matters as everything works together.
Please follow the steps outlined in the manual.   

[vanhuette]

hahaha, i know flybox, i know. Ill get on the carbs as soon as i get a chance. To be fair, in the original post i thought that the clutch sound was causing the hesitant acceleration, so i wasn't thinking about carbs or ignition i thought there was something in the bottom end that was a problem and i was having trouble diagnosing it.

[vanhuette]

woo, finally got some time this weekend and pulled the carbs apart. Weren't actually in too bad of shape, although two of the main jets were partially plugged. I got new jets for the bike (i went with 120 mains and 38 slow jets. I got that from the fact that my 4into1 exhaust should be up two main jet sizes from 115, and then using the 2:1 jet sizing rule of thumb for main:slow, so I went up one slow jet size from 35). So Im cleaning everything out and putting the new jets in, then rebuilding and bench syncing the carbs.

CARBS!


I got a carb rebuild kit, and there are two small O-rings left over from each carb package. I tried to study the blown up carb diagram in the manual to see where they could go, and I think that one of them is for the float needle seat, but are those seats pressed in? there didn't seem to be any easy way to get them out. And the other O-ring I have no idea what its for. I attached a pic of the carb rebuild package, and then the two leftover O-rings beside the float needle for size reference if anyone has any ideas.

Hopefully Ill have it back on the bike this week

[flybox1]

the small ones are probably for under the fuel screw, and on the drain screw.
there are NO orings in the float needle seat.
the viton tip hits the center waist of the brass float valve seat. 

Keihin jets, right?