Topic: cb500 slow return to idle when warm, poor idle

[kurtkarram]

My 1973 cb500 seems to run very well up at higher rpms, but has a few issues. When warm, engine revs back down to idle pretty slowly. It also runs significantly higher in rpm when warm, at about 1400-1600 rpm on the tach vs the 900-1100 rpm when cold. When cold, rpms move up and down freely. Both warm and cold, the idle fluctuates, bumping up and down in rpm. It is also difficult to start, and will run very rough for about 30 seconds before it warms up at least a little.

My bike is 100% stock. Stock air filter, stock exhaust, stock jetting. I have my fuel levels set, carbs were cleaned, needle at 3rd clip and air screw 1 turn out.
Prior to this I have done a full 3000 mile tune-up. Cam chain, valves, oil, oil filter, timing (with timing light), and carb sync.

Below I have pictures of the plugs. They are in order from 4-3-2-1 left to right. 2 looked a bit lean so I took off the bowl and cleaned out the pilot jet and sprayed up the passage. This honestly seemed to make it worse when I put it all back together. I have also replaced the O-rings between the manifolds and the engine. I have not replaced the rubber intakes, but I doused them all with wd40 and heard no rpm change. And another note, the bike does not like choke. It needs choke to cold start but I have to open it after or the bike will die.

Anyway, I am probably going to take the carbs off and clean them again. Maybe it is the idle circuits, or air screw circuits. I have a road trip thursday and I want to make sure it's running well...or well enough. thanks.

[TwoTired]

Cold engines atomize and burn fuel poorly compared to warm engines.  Cold engines, are therefore less efficient at burning fuel than warm or hot engines. This is why engines without a computer to monitor temp conditions fuel mixtures need a more open throttle at idle when cold to obtain the same idle speed as when hot.

American 500s had a throttle friction knob under the bar control to hold the throttle slightly open without wrist effort during a stationary warm up.  Otherwise hold the throttle a bit more open during warm up using the computer located about two feet above the seat.

After warm up, a slow return to idle can be caused by control binding, or too lean a mixture for idle speed. You can try turning the pilot screws inward a quarter turn from stock setting.  Or, try raising the slide needles a notch.

The stock carb settings assumed pure gas. If you are using alcohol poisoned or oxygenated gas in your bike then try to get your legislator to tune your bike for the gas they forced you to buy that ruins your machine drivability.  Or, buy a new bike that kowtows to your government regime mandates.

[cfairweather]

You may have more than one problem, but I can almost guarantee you that part of the problem is the advance unit.  Take a look at the springs and see if they are a bit loose.  If so, take 1/2 loop off the end of each spring to solve the issue.  The springs should be snug, not tight and able to fully retract.  While you have it out, clean it up and lube it so it works like new. 

[DaveBarbier]

You may have more than one problem, but I can almost guarantee you that part of the problem is the advance unit.  Take a look at the springs and see if they are a bit loose.  If so, take 1/2 loop off the end of each spring to solve the issue.  The springs should be snug, not tight and able to fully retract.  While you have it out, clean it up and lube it so it works like new.

You can check this with a timing light to see when advance kicks in. Should be around 2700-3000rpm. If it comes in too early then the springs have become weak. But no need to modify them if the advance comes in at the right time. Though good to clean and lube if it’s crusty.

And OP, you can also check for air leaks around the carb to head rubber insulators.

[Shtonecb500]

I only now figured out my throttle cable adjustment, I had so much slop in there I always fighting hanging idle and such. My needled drops right where it should go now.

[maxheadflow]

The 2 most common issues for poor return to idle when these were new was sticky advance / loose springs or a intake manifold leak.

[DT Tyson]

I've been fighting this exact same issue for months now. Whats worse is that just when it seems like i've got it all sorted out, it comes back. Damn frustrating!

[david 750f]

Had a hanging idle on a CB500 that I was helping with. Trimmed the advancer springs, went through the carbs, new manifold boots and O-rings. Turns out that the aftermarket push cable was too short, it's worth disconnecting it to see if it makes a difference.

[DT Tyson]

Howdy all!

Been struggling with a similar issue on and off for the past few months with my '77 550k. Not sure even when it became an issue, as my ole girl is only a year into my possession, and I've been doing work on her off and on in between "getting to know you" rides. Almost 8 thousand miles so far for the year.

Reading all the discussions and suggestions related to this issue I could find, dreading the fact that I may need to bench the ole girl for the summer and tear into the carbs (or worse). I've checked my valve gaps, timing chain tension, gone through the points setting, dwell, advance mechanism, the coils, plugs, ignition wires, clear tube measured the floats, cleaned and set the idle circuit to stock (but adjusted for my altitude), cleaned the fuel petcock and filter, checked for intake leaks, new (lubed) throttle cables to boot.

Due to the fact I have thousands of mostly uneventful hwy and mountain miles under her I didn't feel the need to get deeper into the carb settings...yet. Not being a noob to these bikes, I had all but cleaned off the work bench and ordered a carb repair kit, when I did something while out on a grocery getter ride, sitting at a stop light in traffic, out of pure frustration:


I reached down between my legs and..... fully backed off the idle set screw. *GASP*


Problem. Solved. WTF?

While I'm not entirely sure why this was MY fix, because it seems elementary that if this idle setting is off the bike would just idle high, but that wasn't the remedy in MY case. She ran fine in all situations EXCEPT at a stop, after warming up for a bit, and sometimes wouldn't come down fully when shifting through the gears. 

Shes perfect now. 

I have been reluctant to post up my finding until I was sure of the result. But now, a thousand plus road miles later, I have zero issue with the "hanging idle" in any riding situation I've encountered. Maybe someone will find this helpful, maybe someone has some insight as to the why, but either way, the simplest solution was the one I overlooked.


Rubber down, elbows in, throttle on!

[Scott S]

 If the idle screw is fully backed off the stop, the problem isn't "solved". That's just a band aid.

[Scott S]

http://forums.sohc4.net/index.php/topic,184299.msg2133716.html#msg2133716