Topic: CB500 won't idle

[keelhaul]

Hello folks, long time lurker, first time poster. I picked up a '73 Honda CB500 the other day and I'm having a bit of trouble with it. The PO got it nice and clean- new fuel lines/petcock, new plugs and plug boots, rebuilt and cleaned carbs, the usual stuff. It starts easily with the choke on but when I turn the choke off it dies. Even with the choke on it idles at 1200rpm. I rode it around a bit while choked (not the best idea, but I wanted to make sure nothing else was wrong) and seems fine other than that. I checked the air filter and airbox, everything seems okay there.

I've got a couple of things to try: adjusting the idle up (what should the bike idle at when fully choked?), and making sure the idle mixture screws are set to spec. Am I missing anything? I'll only have a couple of hours to tinker before I go out of town for a few days, I'd like to get the bike set up so that I can ride it around all day on Labor Day.

[dave500]

it wont idle cold,let it get fully warm/hot before you adjust the idle screw,if the po did the carbs theres a chance they wont be vacuum synced.

[keelhaul]

I warmed it up by riding it around the block a few times, it still died when I turned off the choke. Weirdly, it would run at almost zero choke but the instant I turned it fully off it shut down.

If the carbs aren't synced would it still ride correctly with the choke on?

Thanks for the help! I apologize for any dumb questions, this is my first bike and while I know a bit about engines and have a Clymer for my bike I'm not well-versed in the lingo yet and have zero experience.

[jessezm]

Can you get it to tic over reliably at 1200-1400 rpm by holding the throttle open a little?  If so, you probably just need to turn in your idle speed screw a little (not to be mistaken with your idle mixture screws). 

[keelhaul]

I'll try that tonight- I gave it a shot last night but it was already pretty well flooded from riding around with the choke on.

What should the bike be idling at while choked? I used to have an old Porsche with a manual choke that would idle at 2500rpm or so when fully choked, 1100 when the choke was off- I assume bikes work the same way? My bike idles at 1100 while choked, if it's supposed to be 2k+ that would point towards the idle screw as the culprit.

[Zodo]

Yeah, your bike should fast-idle when the choke is out. I think the ideal is in the 2500 range, but you'd have to check the manual to be sure. You may just have the idle screw set way too low.

[wanackg]

My 650 did that when i first got it. The PO had the carbs rebuilt but didnt reinstall them.

After I got them back on, the bike did as you are describing. All you gotta do is turn the idle screw clockwise until it runs completely of of choke.

I dont know about the 500, but my 650 is supposed to be set at 1050-1150rpm, though I like my idle around 1500 or so, but thats just me.

[keelhaul]

I spent some time adjusting the idle and I don't think that's my problem (at least not the only problem). After the bike warms up I crank the throttle to 5k rpms with the choke barely on, turn the choke off, and it dies immediately.

Hmm. Not really sure where to go from here.

[dave500]

itll need everything looked at starting with the ignition,,forget anything the po told you or implied and read your clymer starting with the points and timing adjustment section,if a member is close by they might be able to help you?

[keelhaul]

That sounds like a fun Labor Day project to me. 

Oh, I pulled a spark plug and it was pretty filthy- looked like I had done a good job fouling it in my impatience the other night. Could a fouled plug cause these symptoms? I won't have time to give them a proper cleaning until after the weekend but it'd be nice if the source of my trouble was something so simple.

[dave500]

buy some new ones,fouled plugs are always a bit hit and miss as far as a good clean goes,dont ride the bike on the choke anymore either,it washes oil from the bore and rings and causes early wear,,,umm,id start with the ignition off course,have you got a multi meter?if not go and get a cheap one will do,like 10 bucks is ok for the tests you might need to do,the plug caps should be checked for resistance,,this might be part of your problem?it might not,youll need that multimeter to rule them out.

[keelhaul]

Sorry for the delayed reply, was out of town late last week and finally got back last night.

I am deeply embarrassed to admit this, but I had the choke operation backwards- I thought down was closed and up was for normal operations. I only discovered the correct operation by looking at the cb550 manual on this site. So it turned out the bike wouldn't run or start when fully choked. I checked the idle mixture and the screws were set 2 turns out, so I backed those in to 1 turn per the Clymer manual and it runs good now. Good enough to putter around the neighborhood for a half hour, at least. I still need to replace the plugs, got a set delivered while I was gone and I'll do that when the bike cools down.

Thanks again to everybody, having this site as a resource makes owning this excellent bike even more enjoyable!

[PurduePete]

..... I have been doing the same thing. Been trying to figure out why it won't without full choke, apparently it won't run with choke on. This thread helped me figure that out, consulted the manual to be sure.

I do realize that this hasn't been posted on in a long time. Just thought I would share this bit of insight anyway.