Topic: 1973 cb500 hanging idle

[kurtkarram]

Hello everyone, I have a 1973 cb500 with a hanging idle issue. I have searched around the forums for other answers but this brings questions of my own.

Situation:

Bike idles fine at 1000-1100 rpm. When I give it throttle, the rpm will jump and hang there. It seems to jump right up to 3k rpm even if I just give a little bit of throttle and close immediately. It will hang there for about 2 seconds then drop back to idle. It seems to drop back to idle a bit slow, but relatively close to normal.

I've tried spraying around the boots and intake around the engine with WD-40 to check for air leaks. Not sure if WD40 is able to be used for that, but I had run out of carb cleaner. There was no change in idling from what I could tell

I have set the cam chain tensioner and adjusted the valves. I have not looked at anything on the timing yet, but I don't see how slightly off timing would cause this running issue. Unless it's the advancer, but I'm not really sure of the symptoms of a stuck advancer.

Everything on the bike is stock. 4-4 exhaust baffles in-tact, stock airbox stock paper filter. Stock jets, stock needle position (2nd from bottom) and air screw 2 full 360 degree turns from seated. The carbs are 627b I believe, and the american stock settings were different that the european setting being 3rd needle clip position and 1 1/8 turn on the screw.

Other forums have also said to check the float level with a clear tube but I am not sure how to do that with these style of drain screws as the fuel doesnt come out the nipple on the bowl.

Any help is appreciated, thanks

[kerryb]

I had similar symptoms on my 550f, my cure was timing related.  Getting the points set just right was the first step.  This required a shim on the edge of the points plate.  I think it was twotired that did a nice write up on that.  The second step was clipping one coil off the advancer springs and bending the next coil out to be the new hook.  Apparently these springs get tired and lose a little tension.  The advancer spring fix took care of the hanging idle for me.  Maybe check the FAQ section for more info

Note: when working on the advancer be careful to observe the orientation during disassembly.  I put mine back together backwards (out 180 degrees) and it wouldn't start till I reversed the points wires to the coils!

[kerryb]

Here is the info, under faq the thoughts of Hondaman,  http://forums.sohc4.net/index.php?topic=142860.0.  For the advancer spring stuff, abot 3 pages in.

Ignition coils & plugs FAQ for the sloppy points plate issue,  http://forums.sohc4.net/index.php/topic,5752.0.html
Near the end by twotired.
It helps to have a timing light to spot the advancer issue.

Good luck.

[Ksgfx]

I had the same issue with my 500. Turns out it was also the advancer (which I didn't realize until after the 5th carb rebuild). However mine wasn't an issue with the springs but it was sticking just a bit. I took it apart, cleaned it and added just a little bit of grease to it and bingo, worked perfectly.

[BomberMann650]

+1 sounds like advancer delay

Fwiw - when the high idle is related to an intake leak - the revs can reach well into the midrange rpms.
Motor doesn't sound happy about it when it does. 
Easy way to experience that is back out or remove the plug from the intake sync service port.  Just one.
Its an extreme air leak - but recreates a high hanging idle in a controlled chaos sort of way.

Other audible sign of an intake leak are popping sounds from the exhaust.  Like a little bubble wrap snap.
Easier to observe on a single cylinder bike.  Fjr's do it as a matter of programming.  Like little firecrackers - efi whiz kids think its cool to do with their Acura.  Old honda CB's shouldn't make that noise.

The sound of bad valve or ignition timing is very different.  Would not recommend trying to recreate that.  But trust me, it sounds really scary when its that far outta whack.  Keep your eyeballs away from the carb throat if you ever suspect that is the case.  Fire danger.

[TwoTired]

I didn’t see any mention of a carb sync.  Did I miss that?

Imagine if one or two carbs are set for 1200 rpm idle opening and two carbs are set for a 1000 rpm idle opening.  Might the cylinders fight for idle speed dominance?

In fact, I had very similar symptoms on a 550, that had just one carb not in sync with the others.  Carb sync fixed that one.

Unless otherwise stated, I always assume the problem bike is in stock configuration.  Mixture issues can arise when induction and or exhaust is changed.

Finally, it always amazes me when an ill running bike is totally cured with a complete 3000 mile routine tune up.  …Like what is recommended in the owner’s manual.

Cheers,

[kurtkarram]

Thanks everyone for the help.
I took out the advancer, and it was difficult to move and would barely return to center on its own.
I cleaned the parts, re-greased everything, and shortened the springs by 1/2 as HondaMan has said to do in another post.

This fixed the hanging. I can rev it up and it will drop right back down. It still seems to fall just a tiny bit slow, and it sputters a bit getting up to 2500 rpm, but I think that should be solved with a carb sync. TwoTired you had asked, no I hadn't carb synced, as that's the last step to tuning and I hadn't done anything with the timing. I will get to that tomorrow.

Another question: I might ask this as a new post but I saw on a few forums here that people are setting their timing slightly advance, and that It runs better like this. Is this true? Should I set the timing as perfectly aligned with the mark or slightly advanced?

[MauiK3]

Start with stock, see how it runs.

[BomberMann650]

How much spring did you remove?  1/2 its length?
I may have misinterpreted hondamans mod instructions.  Thought it was meant as 1 or 2 spring windings.
Not big deal so long as the cam is advanced by the appropriate rpm.  Did you do the timing strobe check on that?
The sputter can be jetting and mix related.  But if it didn't do that before the advancer service.  Welp 😵‍💫. Hows the point setting bytheway?

Not sure how out of sync carbs have to be.  In order to create what twotired described.  Most out of sync idles i've heard just kinda "wander around" - like the motor is searching up and down for a sweet spot..  It's a subtle shift in pitch.  Often accompanied by the dreaded primary chain smackle.  With good meters, patience, and a light touch - the 500/550 hondas can be tuned into a solid drone at idle.  Like a modern sport bike.

To address your fuel float level checks.  If you use a tube close to the size of the drain screw threads.  A little kiss from a bic lighter can warm the material enough to thread itself in.  Just be careful, ventilated area, adult supervision, yada yada.

[Deltarider]

I took out the advancer, and it was difficult to move and would barely return to center on its own.
I cleaned the parts, re-greased everything,

That must have helped.

[...] and shortened the springs by 1/2 as HondaMan has said to do in another post.

Why did you do that? Do you have knowledge of good results, experienced by people after they have cut the springs?

Another question: I might ask this as a new post but I saw on a few forums here that people are setting their timing slightly advance, and that It runs better like this. Is this true? Should I set the timing as perfectly aligned with the mark or slightly advanced?

I have a better responding engine with some 3^o extra advance. Back then Honda had to be very conservative, as their bikes were also exported to countries with low octane fuels. I run premium and see no risk in having some extra advance. Was suggested to me by a Honda mec, btw.

[Deltarider]

To address your fuel float level checks.  If you use a tube close to the size of the drain screw threads.  A little kiss from a bic lighter can warm the material enough to thread itself in.  Just be careful, ventilated area, adult supervision, yada yada.

You may want to read this for a much easier method which requires no extra tools or tubes: http://forums.sohc4.net/index.php/topic,187633.msg2176352.html#msg2176352

[dave500]

use a strobe to set your timing,shine it on the marks as you increase rpm watch it move towards the full advance marks,set it to the further of the two,where it ends up at idle forget it,dont worry about the f mark,once your riding itll never be down there,full stop.

[kurtkarram]

How much spring did you remove?  1/2 its length?
I may have misinterpreted hondamans mod instructions.  Thought it was meant as 1 or 2 spring windings.
Not big deal so long as the cam is advanced by the appropriate rpm.  Did you do the timing strobe check on that?
The sputter can be jetting and mix related.  But if it didn't do that before the advancer service.  Welp 😵‍💫. Hows the point setting bytheway?

I didn't explain it correctly. I shortened it by 1/2 of a coil. I just cut the end and bent over the next ring in the spring.

It did sputter before the advancer and timing change. When revving right off idle.

Points were a bit confusing. Spec is .012"-.016" but it was difficult to get a gap size that fired the point on the range of the adjustor plate. I started with .012" and max adjustment on the plate got the light firing a few degrees after the T mark. Raising the gap to .015 .016 helped a tiny bit but wasn't enough to get it in time.

To get timing setup properly I had to set a verrrry tight .012. Possibly too tight I'm not sure. The top part of the point springs up a hair when I put the feeler gauge in which in my head would mean it's actually a .011 or .010. But I'm not sure how to gauge it as the two contact points don't come down perfectly flat to each other. It's rotating down so in the inside will laways be closer than the outside of the contact plates.

[...] and shortened the springs by 1/2 as HondaMan has said to do in another post.

Why did you do that? Do you have knowledge of good results, experienced by people after they have cut the springs?

Followed the thread posted above by someone else. Hondaman goes off about the springs, and everyone else commenting says the same thing that it helps.

[Deltarider]

... and everyone else commenting says the same thing that it helps.

Where?

[RAFster122s]

Thoughts of HondaMan in FAQ

[Deltarider]

Thoughts of HondaMan in FAQ

... and everyone else commenting says the same thing that it helps.

Where?

Kurtkarram: ... and everyone else commenting says the same thing that it helps.
Hence my question: Where?

[kurtkarram]

... and everyone else commenting says the same thing that it helps.

Where?

http://forums.sohc4.net/index.php/topic,142860.0.html

[BomberMann650]

Would someone care to comment or link on points setting?

If op can get those dialed in, he can move onto the fuel system.

[dave500]

use a dwell meter on the points,or just use an electronic saves a lotta hassle.

[kurtkarram]

Thanks everyone for the help.
I have done everything in the standard tune-up except for syncing the carbs. I just didn't have time before I left for college. Here is a video of it running as is.

I will update this forum after I go back home in a few months and sync the carbs in case anyone else has similar problems and wants to see solutions.

[BomberMann650]

Doesn't sound like its hanging anymore!

Idle rpm is a tad low.  They purr nicely when the carbs are synchronized just right.

[Deltarider]

Idle rpm is a tad low.

Idle certainly is too low. Have it @ a true 1100-1200.