Total Motorcycle Community Forums

Seems I've dealt with most of the larger issues surrounding my CB750, but some smallish things are still bugging me. The one I've got in mind right now is that it seems to take quite a while to warm up, and until it does it will stall at idle (I need to keep a bit of throttle on to keep it from happening).
If I adjust the idle high enough to keep it from stalling, it's too high (2,000 plus) once it does warm up. Choke helps a bit (seems to like half choke for the first bit), but doesn't get rid of the problem entirely. To create a happy balance of sorts, I am keeping the post warm-up idle slightly high (around 1,000 or a little more on the bike's tach, rather than 900 per manual)
It may be that I just need to stand there and rev it, or keep some throttle on at the stop lights for a bit, but I'm wondering if this is just a factor of having an old bike (it's a 1976), or indicative of something else.
I went through the carbs last year for a problem that turned out to be unrelated (dipped and compressed air--twice to make sure-- then synched), swapped out the wiring harness recently, adjusted valves, timing, seems to get good spark and the plugs all check out okay.
Anyways, thanks. It's a minor annoyance at best, but I'd just like to know if it's something I can get rid of or something I should just live with.

Your issue is probably due to the EPA and not the bike's fault.

To meet the emissions standards with carbs, the factories had to jet the bike as lean as possible, and this makes them rather cold blooded. Most bikes from the 70's and 80's had this problem. If you find a shop who will work on older bikes, they may be able to re-jet that out for you.

Yeah I was going to say the same thing. Idle problems that improve after the bike gets hot indicate a lean condition.

Are you having any performance problems at higher rpms? If not, you may not need to rejet, just adjust the pilot screws on your carbs (I assume bikes from that era had pilot screws). This will have the effect of richening the mixture at idle.

Thanks, BuzZz and Damian.
The jets I'm running now are pretty big-- rejetted by the previous owner to accommodate some sparkly aftermarket bits. Sounds like I should double check for running lean at regular operating temp (Plug chop, I suppose?) and adjust accordingly, whether it's a big problem (rejet) or an adjustment (mixture screws). No problems at higher RPMs-- once it's up and running, everything's good. Of course, my basis for comparison is how it was running before I did all the other stuff, so there's always room for improvement.
I haven't found a shop that looked at me with anything but pity when I tell them what bike I'm riding, but that's okay. Part of the fun is fixing it myself and learning as I go. That way when it blows up on the highway I've got no one else to blame, but when it doesn't, I've got no one else to credit.
'Cept the folks here, of course. Thanks again for the advice.

Tighten all the clamps on the intake hoses from you air cleaner and the output hoses from your carbs to the motor. If any of these are loose you will be sucking air past the carbs until the bike warms up. This causes the problems as you have described them. Had the same problem with my cb 650. A way to tell is to spray some wd40 around the hoses where they enter or leave the carbs. If this solves the problem your clamps are loose.

Bumping this thread because I also have an idling problem. I am a beginner and don't know very much (if any) about motorcycle maintenance. For all I know, it could be me and not the bike, so any information would help.

I just bought the 2006 Yamaha V-Star 650 Custom on Saturday. It's used and it only has 450 miles, and the previous owner took good care of it. I've ridden it home (40 miles), and to and from work (6 miles total). It seems to take a while to heat up. 5 minutes of idling with the choke half out, and then some more while riding; I think I've gone about half a mile when I put it in.

When I eventually push it all the way in, it will stall when resting unless I'm slightly revving it. Pulling out the choke a little again will cause it to stall, also. I also remember stalling at a light on the way home Saturday after riding with the choke all the way in for about 6 miles, so I would think that it would be hot enough.

The previous owner didn't mention anything about it, and he seemed like he would have told me if something was wrong; he was very helpful, since he knew I was a newish rider, so I don't think he would have kept it from me. So, I kind of feel it might be something I am doing. Any help would be appreciated. Sorry if I left out any needed information.

Turn the idle up slightly.

Do I need to adjust the valve clearance and sync the carburetor before I adjust the idle? (It says to do that in a manual I have.) Or, will it be fine to just adjust the idle for now and tell the mechanics about it at my 600mi checkup? Also, do I need to use a tachometer, or is it fine to do it by ear?

Thanks for the help.

If you know what 1500 rpm sounds like on a v-twin do it by ear.

Personally, I'd just turn it up a little and then tell the service writer what happened when you take it in for the 600mile first service. Do yourself a favour and book it now though, cause I bet they'll be booked a couple of weeks in advance.

Don't worry about playing with the valves or syncing the carbs at this point, I doubt they'll need it for some time. Or they'll get taken care of at the service.