IDLE ISSUES: Hanging Idle :(

[Red Fthr]

Bike:
1973 Honda CB500 Four K2 (everything stock except for exhaust and dyna ignition)

Problem:
The bike will idle to 4k (sometimes higher) after it's warmed up and once the throttle is given more than a lil blip and after an additional blip it'll fall to a dead idle and shut off. You can however hold the throttle at the proper idle and once you hear the bike is stable you can let go and it'll stay there but as soon as you give it more than a blip it jumps back up.

Theory:
The bike hadn't been running for several months when I received it. After a carb overhaul/ rebuild (with larger jets; 38) it kicked over with no choke on the second kick. Nothing is sticky and everything is nice and springy. I then benched synced the carbs and hooked them up to a vacuum sync-o-meter (I got pretty close with the syncing) but I haven't gone completely trough the syncing process because of the idle issue. All my plugs indicate the bike is running rich, which could be because of larger jets because the float heights meet manual specifications. I also tested for air leaks and after spraying I got no high revs. I feel it's totally mechanical and not mixture related but I could be wrong.

Questions:
1. I have my adjustment screws loose during this whole process (for syncing purposes), could that be the cause?

2. Could timing be the issue? How do you adjust the timing for dyna ignition?

3. Would a rich mixture cause such terrible idle issues?

4. Could cable adjustment or throttle stop adjustment be the cause? I've never messed with that stuff before so I'm pretty ignorant to it's capabilities. I did disassemble mostly everything so maybe something isn't functioning right there? All though the slides are dropping/ springing back after releasing the throttle just fine.

I've tested out a few different theories and it's now almost 4am here in FL. and I've exhausted my current knowledge on the subject of carbs (which isn't a whole lot I'll admit). This is when you awesome people come in handy and I would appreciate any wisdom you can throw my way.

 

[AlphaDogChoppers]

Get the service manual and follow the procedure for syncing the carbs and adjusting the idle air mixture screws. Use WD-40 sprayed around carb manifold spigots to check for intake leaks. With the engine idling, when you spray where there is a leak, the idle will change.

 

[Toast]

You have an air leak somewhere. The culprit is most likely either old and cracked carb boots or carb boot o-rings (probably both).

As the engine warms up the extra air from the leak leans out the mixture and raises the idle.

 

[Red Fthr]

Thanks for the reply AlphaDog

I am going to follow the manual through the syncing process and will see if it'll bring about the desired result (proper idle). I did spray around the boots before and didn't notice a change in RPMs. Actually maybe a slight decrease but it wasn't much and it could of been total coincidence. I'll check again while syncing them. I'll also take a lil video to see if that helps explain anything.

Before I do this can you help me understand a couple of things the manual isn't very clear about:

1. will tappet clearance cause idle issues or just carb uniform issues?

2. What does the screw/ spring combo found between the throttle cables on the controls do? I took it out and just put it back in and I can't find it on parts list or in my manual.

Again the manual helps but leaves out lil depictions and details like that. At least mine does.

 

[Red Fthr]

Here is a good depiction of what the bike is doing (this is not my bike just a video I found). I'm going to tackle some stuff today and come back with a list of things that have been done in hopes it'll narrow down what could be the cause of the problem.

http://www.youtube.com/watch?v=Nn3co81a_iw

 

[AlphaDogChoppers]

Thanks for the reply AlphaDog

I am going to follow the manual through the syncing process and will see if it'll bring about the desired result (proper idle). I did spray around the boots before and didn't notice a change in RPMs. Actually maybe a slight decrease but it wasn't much and it could of been total coincidence. I'll check again while syncing them. I'll also take a lil video to see if that helps explain anything.

Before I do this can you help me understand a couple of things the manual isn't very clear about:

1. will tappet clearance cause idle issues or just carb uniform issues?

2. What does the screw/ spring combo found between the throttle cables on the controls do? I took it out and just put it back in and I can't find it on parts list or in my manual.

Again the manual helps but leaves out lil depictions and details like that. At least mine does.

Assume everything affects everything else. You can never properly tune carburetors if the valve clearance is fucked up.

That screw/spring is what you adjust to sync the carbs. It adjusts the relative position of the throttle butterfly in the two carbs.

 

[carnivorous chicken]

2. What does the screw/ spring combo found between the throttle cables on the controls do? I took it out and just put it back in and I can't find it on parts list or in my manual.

You're talking about the screw on the handlebar controls? I always thought it was a failed attempt at cruise control, but it might help if you are checking your advance and don't have anyone to help. Not all CBs have them, but I've had a couple that did. If it's what I think you are talking about it's got nothing to do with carb sync and it's not necessary.

 

[Red Fthr]

Yes that screw. The one on the handle bars. Working on the bike this right now/ this afternoon. Ill be back with more details.

 

[teazer]

That screw is a friction adjuster as carnivorous said.

The most likely cause of that sometimes it hangs and sometimes it doesn't idle is either a problem with the cables or more likely a carb synch issue where one or more carbs are staying open. check the slides and see if all 3 snap shut at the same time when you close the throttle slowly versus snapping it shut.

 

[Red Fthr]

Ill try just that Teazer. I'm in the garage now.

I'm about to check tappet clearance/ alignment and timing as well.

The bike has a dyna ignition timing on it which makes the manual kind of irrelevant when it come to the procedure. Does anyone know of any directions online for dyna ignition timing? I did a quick google and didn't find what I needed. Ill get back to y'all soon. Thanks again guys.

 

[Red Fthr]

Teazer. The slides are dropping at even rates and show no sighns of hang up.

 

[Red Fthr]

UPDATE:

Things that have been addressed thus far:
1. Plugs have been gapped
2. Tappet clearness is good
3. Float level heights are spot on.
4. Float action/ needles / seats are fine
5. Slides are a-ok.
6. Carb assembly as a whole is good and set to manual specifications
7. Tested for air leaks

I'm really ruling out a carb issue on this especially since after beating my head against a wall I put my carbs (same exact carbs 627b keihin) which are synced and have been running fine (every day) for the last 6months on the problem bike. And I got the same exact results; no difference at all. So to reiterate, I don't think the carbs are the issue at all.

Here, again, is the problem:

After warming up the bike and once you give it throttle, the idle hangs like crazy and then will drop dead. After the bike has been running for about 10min a carb or two will start pissing gas (shown in video) as if no air or pressure or whatever is coming through and gas isn't being used thus causing a build up. Oh and the plugs indicate richness and you can hear a ill pop/cough around the air box. Lots of smoke out of exhaust when you really lay on the throttle.

I don't know what my next step would be. Two different sets of carbs (one reassembled correctly and the other I know for certain is assembled and functions correctly) show the same results. So my next step is...

Thanks guys.

http://m.youtube.com/#/watch?v=5P5R5UrOOtE&feature=plcp

 

[AlphaDogChoppers]

Well, I have a couple ideas.

To begin with, if your carb(s) are pissing fuel, you have a carburetor problem, period. There is no condition external to the carburetors that can cause that. There is no such things as "gas isn't being used thus causing a build up." That's what the floats do. They regulate the fuel level in the float bowls and shut OFF the flow of fuel into the carbs when fuel is not being used. Carb floats work exactly like the float valve in a toilet tank. When the float bowl gets full, the fuel flow is shut off.

Check to be sure any vent hosed attached to the carbs are clear. Sometimes, mud dauber wasps or other insects will build nests inside vent tubes, and it can cause carburetors to become non-functional. That would explain why you still have a problem with another set of carbs.

 

[Red Fthr]

I do hear what you're saying AlphaDog but is it not strange that a different set of carbs that work fine and don't piss gas had the same issues when put on the problem bike? So I agree only the carbs internal workings will make it piss but something external of the carbs is making the carbs malfunction like that. Both sets of carbs are fine. So maybe I worded it wrong by saying "build up." What I am saying is something external is making the internal workings of the carbs malaunction. What could that be?

I'm thinking peacock/ tank issues? The tank is clean (from what you can see) but maybe particles are fouling up the float needles?

Ill check the screen in the petcock and see if there's anything that would indicate anything Alien to gas.

Any other thoughts though?

 

[Red Fthr]

Oh and ill check for possible clogs/ insect invasions as well. Thanks again.

 

[Rich Ard]

I do hear what you're saying AlphaDog but is it not strange that a different set of carbs that work fine and don't piss gas had the same issues when put on the problem bike? So I agree only the carbs internal workings will make it piss but something external of the carbs is making the carbs malfunction like that. Both sets of carbs are fine. So maybe I worded it wrong by saying "build up." What I am saying is something external is making the internal workings of the carbs malaunction. What could that be?

I'm thinking peacock/ tank issues? The tank is clean (from what you can see) but maybe particles are fouling up the float needles?

Ill check the screen in the petcock and see if there's anything that would indicate anything Alien to gas.

Any other thoughts though?

You could put an inline fuel filter between the petcock and carb - best three dollars' insurance you can buy.

 

[Red Fthr]

Here's some news that might help strengthen my theory about particles in the fuel fouling up the float needles.

The good carbs I took off my bike and put on the bad bike last night to test out of it was a carb issue or another issue are now pissing gas, having bad idle issues, and performing well terrible. My carbs never, ever, had any sort of issues like that up until I put them on the bad bike. So with this added to the equation, any more thoughts?

 

[AlphaDogChoppers]

What I am saying is something external is making the internal workings of the carbs malaunction. What could that be?

Read my post again. Check your external vent hoses.

I'm thinking petcock/ tank issues? The tank is clean (from what you can see) but maybe particles are fouling up the float needles?

Ill check the screen in the petcock and see if there's anything that would indicate anything Alien to gas.

Yes, crap from the gas tank can cause the float needle valves to leak. That most likely isn't what's causing your idle issue, but it could be causing the overflow issue.

Any other thoughts though?

Stop asking questions until you have followed up on what you have already been given, fer fuck sake! It's not like we don't have other things to do. The ball is in your court. Check out the leads you have been given, THEN report back your findings. THEN we may try to come up with other ideas.

In other words, stop wasting our time by arguing and asking the same question again. We've given an avenue to follow, so follow it and report back AFTER you have checked out what we gave you.

Don't make me come over there and bitch-slap you.

 

[teazer]

You suspect dirty fuel, so did you open the carbs after they leaked and drain the bowls to look for dirt?

I'd start there. Excess fuel sounds like the problem. How did you set float height - upside down or on an angle? Regardless of how many pictures show height with carbs inverted, that's not how they are set.

So far it looks like it's dirt or floats set too high.

You could swap the clean tank from your bike onto the "bad bike" and see if that fixes it and drain the bad tank and see what's going on and clean out the tank and petcock/fuel valve.

 

[lingo]

While you are at it, pull your dipstick and see if it smells like fuel. If it does, change the oil and filter