Topic: Increasing idle speed as bike warms up. I've tried everything I can think of!

[Acoustoman]

Hi folks, I’m new to the forum, and am going crazy with a 1973 CB500f problem (one of many). I’ve spent way too much time and dinero restoring this thing, but I’m having fun and learning a lot. Anyway, I’m having an idle problem when the bike warms up. It starts nicely, and idles well initially – right around 1200 rpm. As the bike warms up though, the idle wants to jump up to around 3000 rpm. If I turn the idle screw down, the thing stalls. I’ve had the carbs apart about 4 times, and everything is clean as a whistle. It has new stock jets (#100 main and #40 slow), float height has been adjusted, pilot screws are out about 1 ½ turns. Carbs were both bench sync’d and vacuum sync’d (which is what I thought the problem was). I’m using PAMCO ignition (timing is right on) with new coils, stock airbox, 4 into 1 MAC exhaust, valves are adjusted. Any ideas of why the idle wants to jump up so high when warm? I’m willing to try anything at this point. This thing’s killin me – glad I have my 650 to get my riding fix. Thanks!

[LesterPiglet]

Idle should be set when the engine is at operating temp. Before that it's a juggling match with the choke and throttle. Make sure your carbs are synchronised.
Wilcommen btw.

[HondaMan]

Try to find some #96 mainjets. The #100 are pretty rich. One way to test it: Remove the air filter, put in low-octane gas, ride it about 50 miles to clear it out well. Then set the idle at about 900-1000 when hot.

These bikes have no auto-choke: your wrist is the high-idle while they warm up. If you'd rather have an auto-choke, you'll have to give up the 500 and get a 550.


Another thing that will help: test your sparkplug caps and make sure they are within 800 ohms of each other, on each coil. If they are not, or if they are more than 8500 ohms, they are finished (they were 7500 ohms when new). You can get new 5000 ohm types at most dealers, about $5 each. Don't get stuck with the 0 ohm types, which some dealers carry (mostly be accident).

[Don R]

 I've found actual float level and vacuum leaks to be the cause of my idle problems. A seeping float valve or slightly heavier float can be an issue. I like the clear tube method to check the float level. It sounds like you've done most of the right things already.

[LesterPiglet]

Whaddayamean autochoke Mark? I have very supple wrists.

[HondaMan]

Whaddayamean autochoke Mark? I have very supple wrists.

I just talk to mine, and let it run for 30 seconds.

[Acoustoman]

Thanks gentlemen, I'll check some more things out, namely the fuel level in the bowls. I did this and adjusted the float "tangs" accordingly (to 22mm), but haven't checked the levels again. Do the floats ever lose bouyancy?
I'm not even using the choke to start the thing, which seems to indicate it's running rich, so I'll look for some of those #96 mains. By the way, needle clips are in middle position.
I'm mostly concerned about how the idle fluctuates though when warm. Seems like a carb sync issue - maybe some of my vacuum tubes were leaking and didn't give me an accurate reading. As far as the wires, I just put on new DynaTek 5 ohm coils and wires/caps, so seems like the wires should be ok?

I've got some things to check out here. I appreciate it!

[Deltarider]

(timing is right on)

Did you test static or dynamic, in other words: is your advancer working as it should?

[calj737]

I did this and adjusted the float "tangs" accordingly (to 22mm), but haven't checked the levels again. Do the floats ever lose bouyancy?
I'm not even using the choke to start the thing, which seems to indicate it's running rich, so I'll look for some of those #96 mains. By the way, needle clips are in middle position.

Regarding your carb brass, make sure to use quality Keihin parts only. If you bought and replaced the stocks with aftermarket junk, you may discover the "100" aftermarket is much different than the stock "100". Especially true if you've changed needles.

Double check for vacuum leaks around the carb boots too. Clear tube method for fuel level is the most accurate means of determining status.

[Vinhead1957]

Are your floats brass?  I have chased problems with leaky floats.  The solder didn't hold up to ethanol.   Also check intake manifold for vacuum leaks

[Acoustoman]

I did use replica main and slow jets from 4into1, assuming they'd be ok, but sounds like that could be a problem.
Floats are not brass, but some kind of other solid synthetic black material.
Timing was dynamic with a timing light and the advancer seems to be working fine.
I haven't had a chance yet to check fuel levels, but will soon.
I don't think there are any leaks around boots - boots are new, clamps are tight, and no popping/backfiring from engine. THanks dudes

[flybox1]

With your carbs off the bike, and with the idle set screw turned out completely, check that your slides sit completely closed.  Any light under any of them will not allow for idle adjustments lower, and you'll need to make bench sync adjustments.

[Acoustoman]

Ok flybox1, I'll check that out. I did the 1/8" drillbit bench sync, but the adjustment screws may not be allowing the slides to come completely down. Man I'm sick of taking these carbs off (although it's easier than the CB650 and vf500f I"ve worked on!). THanks

[xtravbx]

Carb removal on the vf500f is THE WORST.

Keep us updated.

[harisuluv]

This can also happen when your engine is getting too hot, ie starved for oil.  Ever notice any smoke coming out of your vent on top head/valve cover?

[Acoustoman]

Never seen any smoke coming from there, but it vents into the airbox. Maybe I should remove the hose and watch for that. What would cause top end oil starvation? When I had the head, cylinder and everything off a couple weeks ago, all the oil ports were clear, but that could have changed. Thanks

[HondaMan]

Next time you take the carbs off...go get some O-rings for the castings that hold the carb hoses to the head. They are hard as a rock unless you've changed them, and don't seal well. Whichever one leaks will have a very rich sparkplug to tell it: if they all leak, then no amount of jetting changes will fix it!

The O-rings are the same size as the ones that go in the valve adjuster caps on the engine.

[Acoustoman]

Thanks Mark. I'm pretty sure I changed these out when I took the engine apart last year. I did all the o-rings and seals. But I will check. Thankey
By the way, wouldn't a leaky intake make for a lean condition?

[HondaMan]

Thanks Mark. I'm pretty sure I changed these out when I took the engine apart last year. I did all the o-rings and seals. But I will check. Thankey
By the way, wouldn't a leaky intake make for a lean condition?

Not when the engine is run with a pulse-feed type carb, at least below speeds that supply a more-or-less constant vacuum.

Here's how these things work: the idle circuit actually mixes at approximately 8:1 ratio (air:fuel), but in a real short pulse during the intake stroke. This pulse is about 63% of the total intake stroke at low engine speeds, due to having to get the air moving, then slamming the valve shut in its face shortly afterward. This rich mix then works out to (8 / .63 ) = 12.7 air/fuel mix rate at best. Usually, true-mix charts on dyno pulls show it to be closer to 12:1.

As the engine speeds up, the pulses become closer together, and by about 2500 RPM they start to act more like a constant-vacuum carb. It is at this point that the idle jet loses its ability to send gas up its tiny pipe, because the slide (or butterfly, on some carbs) is open so far there isn't much airflow over that jet tip. Instead, it is coming from the mainjet far more. The mainjet is set closer to 14:1 air/fuel ratio. This leans the mix out for a more "normal" type burn, as is found in most IC engines.

So...if the airflow in, say, carbs 1, 3, and 4 is normal, but #2 has a vacuum leak at the head: the #2 carb "sees" less vacuum from the pulse of that one's intake stroke. This makes that carb mix at the lower airspeed (i.e., closer to idle speed while the others are near 1/4 throttle, for example), which makes it run richer on that one cylinder. This is why we ALWAYS see the cylinder with the darkest plug being the one with the biggest vacuum leak, in normal commuter riding.

For those who work on cars with carbs, this seems backward until they realize that there is only one vacuum chamber on all of those cylinders. This creates a much more constant vacuum, much like these bikes see above 4000 RPM.  It is true that if the bike is hiway-ridden only, the vacuum-leaking cylinder will (finally) show up leaner, but that's only because once it passes the 4000 RPM range, all 4 carbs are running above this rich idle circuit. But, it takes about 50 miles of hiway riding to make this show up - or, several laps at the racetrack, where it never goes below 6000 RPM anyway.

Hence, the arguments continue here, while most don't seem to realize that they are not riding the bikes at the speeds needed for car-like carb-and-vacuum readings. This may also be why so many don't believe that these idle air screws run LEANER when turned inward, and RICHER when turned outward. They just don't seem to understand Bernouli's principles that make these little mixers work?

[Deltarider]

It would be interesting to see a CB500 with an A/F meter connected (a simple CO-meter will do), have it run on a stand at say 5000 rpm and then see what small (¹/₈) changes (is 'increments' the right word?) in the airscrewposition will result in.

[Acoustoman]

Thanks Mark for the thorough explanation - I understand things much better now. I'll make sure I check those o-rings.

I design/work on landscape sprinkler systems; Bernouli lives there too!

Good info - thanks again

[HondaMan]

It would be interesting to see a CB500 with an A/F meter connected (a simple CO-meter will do), have it run on a stand at say 5000 rpm and then see what small (¹/₈) changes (is 'increments' the right word?) in the airscrewposition will result in.

It would have to be under a load at 5000 RPM, so the throttle slides are open more than 1/4 throttle. The idle jets quit working at about 1/4 throttle. Most riders who try to "test" this do it with no load on the engine: these engines will hit 6000 RPM at 1/8 throttle if in Neutral, so that's not even close to being a valid test. The easiest way to test it used to be on the hiway: today we have way too much traffic! We'd go out in 4th gear (750s) and thrash them from 2500 to 8500 RPM during these carb charges, and it was extremely revealing.

[Acoustoman]

Ok, so I checked the fuel level in the bowls and there are some differences (1 and 2 are about 2mm higher than 3 and 4). Can anyone tell me at what level they should be, such as at the gasket surface, or is it more important that they are all the same? Right now they are between 2-4 mm below the top of the bowl.

[Spanner 1]

With respect, but you have restored this bike but don't have a manual for it ?... float level is 22mm for that bike AFAIK.
Needs to be set to the stock float level, no exceptions for intake or exhaust variations.

[Acoustoman]

I understand the float level is to be set at 22mm, which is what I adjusted to, but fuel levels in the bowls are not the same. I'm wondering if some needle valves not sealing properly, or float buoyancy is different or something. All floats swivel freely. . . . I thought my 22mm measurements were spot on, but maybe I need to re-visit that.

[calj737]

Have a read through the last page of this thread:
http://forums.sohc4.net/index.php/topic,127186.msg1683359.html#msg1683359

[Acoustoman]

Will do. Thanks

[flybox1]

Float level is a starting point.  the fuel level inside the bowl will change if aftermarket /new float valve parts are added.
as a best practice, fuel level should be verified with the clear tube test.
Stationary, we shoot for a fuel level of 3mm+/-  below the carb bowl gasket seam.

not our carbs, but a useful illustration.

[Acoustoman]

that's perfect - just what I needed. Thanks. How common is it for the needle spring/plunger to weaken and need to be replaced? The needle tips look smooth, but I'm wondering if I'm having trouble with the fuel levels for another reason. It may just be that I'm not seeing things clearly enough to know exactly when the float tab touches the needle plunger (at 22mm). Hope that makes sense.

[flybox1]

The needle and seat are wear items.  Imagine how many times they open and close during a days ride.
springs weaken over time.  aftermarket remakes introduce slight variables in spring strength and post length.
22mm is the starting point.  As long as you understand fuel level is the goal, clear tube test your work, and make adjustments as necessary.

This all can be done off the bike with something to hold the carbs level.

[calj737]

Caveat: if you do replace your needles, be dead certain to buy stock or Keihin brand ONLY brass. Aftermarket needles do differ in their taper/profile and will throw off your settings.

[Acoustoman]

Got busy with end of the school-year stuff and am just now getting back to things. I finally got the fuel levels evened out (much easier with carbs off the bike). I had to take some of the bowls off 3-4 times to get things right, but all are 3mm below gasket level. (thanks flybox1 for the diagram). I did another bench sync after finding that slides were not all the way down with the idle screw turned all the way out. Got the carbs installed and bike started within 45 minutes (personal record!). Did a vacuum sync and fine-tuned a couple. Bike is certainly running better, but idle still wants to creep up. Occasionally I have to let out the clutch a bit to force the idle back down. It stays down until I throttle up the next time. O-rings under intake manifolds are nice and supple, and only a couple months old, so I don't suspect any leaks there.
Anyway, not sure what else to do, but I'm happy for now.  I appreciate everyone's input.

[HondaMan]

I understand the float level is to be set at 22mm, which is what I adjusted to, but fuel levels in the bowls are not the same. I'm wondering if some needle valves not sealing properly, or float buoyancy is different or something. All floats swivel freely. . . . I thought my 22mm measurements were spot on, but maybe I need to re-visit that.

This can be an issue, but first maybe check: the float angs where the tiny valve tip sits get little dimples in them because the brass is so soft. This makes that valve "stick" a little bit, until the bowl fills far enough to force the tip of the valve to roll and snap shut. This results in a too-deep bowl.

The fix for these is: using an equally tiny file, smooth off the tang until the dimple is gone, then polish the tang shiny again with a bit of 400 or 600 grit emery paper (O'Reilly's Auto). Then reset the float height, as this work tends to bend the tang away from you while you work on it!


On the 750K0-K6and F0/1 with non-PD carbs, this is chronic, making dimples much deeper than on the smaller Fours. On these bikes it can cause bowl overflow, often does. The smaller Fours have much more active bowls because the smaller bikes jounce on road bumps more (lighter weight) and help close the bowls valves.

[Acoustoman]

I will do this when I pull the carbs off again. I did notice dimpling on the float tangs and will sand it off. I should probably replace the float needles/seats at the same time because the springs may be weakening-I don't know how old they are.
The jet needles were set at the 3rd clip down position and things seemed to be running lean (based on symptoms I'm reading from other posts). I moved clips to 4th position (stock) and I seem to have better pull at more open throttle. 
I'm still dealing with some burbling off idle, and a variable idle speed which tends to want to creep up. I need to vacuum sync my carbs one more time, and play with the idle/air screws. I have no evidence of air leaks anywhere.
I still need to find a tool tray to go over the air box. This is probably messing with the air flow a bit, although I've put a rag over top of the filter element to restrict air flow just a bit. I'm using MAC 4-1 exhaust, which I understand is close to stock in terms of back pressure. I don't know how much these 2 things could be affecting performance. Thanks again - I really appreciate the light you guys are shedding on my inexperience

[HondaMan]

I will do this when I pull the carbs off again. I did notice dimpling on the float tangs and will sand it off. I should probably replace the float needles/seats at the same time because the springs may be weakening-I don't know how old they are.
The jet needles were set at the 3rd clip down position and things seemed to be running lean (based on symptoms I'm reading from other posts). I moved clips to 4th position (stock) and I seem to have better pull at more open throttle. 
I'm still dealing with some burbling off idle, and a variable idle speed which tends to want to creep up. I need to vacuum sync my carbs one more time, and play with the idle/air screws. I have no evidence of air leaks anywhere.
I still need to find a tool tray to go over the air box. This is probably messing with the air flow a bit, although I've put a rag over top of the filter element to restrict air flow just a bit. I'm using MAC 4-1 exhaust, which I understand is close to stock in terms of back pressure. I don't know how much these 2 things could be affecting performance. Thanks again - I really appreciate the light you guys are shedding on my inexperience

The genuine Keihin springs ARE weak, and for a purpose: this helps the bowls fill better. Today's Keyster versions are usually way too stiff, and this then requires a deeper bowl setting, lest it run low while running at higher RPM. For example, the 750 carbs with the Keyster float valves need a float setting of +2mm to get the same fuel depth at 4000+ RPM as the stock setting. Since we don't have fuel pumps, and the most PSI we EVER see if from a full fuel tank (less than 0.2 PSI then), it's tricky to get the right float depth if the parts are not the OEM style.

[Acoustoman]

Well folks, I have a satisfying update. My increasing idle is no mas problema. I did 2 things: I put new float needles and seats in and made sure my float levels were spot on (~3mm below bowl gasket). Also, I bought new air screws, which I think was the real culprit. The old ones were grooved, apparently from being seated too tightly in the past (see pic). I think this was causing inconsistent air flow and an irregular idle speed. I also noticed that one of them seemed to bottom out, when it really wasn't bottomed out. I'm not sure it was this way with the old air screw, but if so, this one cylinder would have been getting too much air via the idle circuit and it could have caused the idle to creep up. Not sure, but turned past this to gently seat, then turned all screws out 1.25 turns. Simple fix - hope things stay this way. For now, I'm mighty happy with the way she's running.

[HondaMan]

Good find! I have seen once in a while where a spring on those air screws has been lost and replaced with [something else], resulting in a screw that tightened on the SPRING, but not the SEAT. This will really confuse the rider!

[Acoustoman]

Thanks HondaMan. Do you think the idle problems I described earlier (idle creeping up, or being generally inconsistent) could be caused by the grooving on the screws? Thanks for the expertise! Bill