Author Topic: Increasing idle speed as bike warms up. I've tried everything I can think of!  (Read 3634 times)

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Offline Acoustoman

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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!
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline LesterPiglet

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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.
'Then' and 'than' are completely different words and have completely different meanings. Same with 'of' and 'have'. Set and sit. There, their and they're. Draw and drawer. Could care less/couldn't care less. Bought/brought FFS.


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Offline HondaMan

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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).
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline Don R

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 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.
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Offline LesterPiglet

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Whaddayamean autochoke Mark? I have very supple wrists. :)
'Then' and 'than' are completely different words and have completely different meanings. Same with 'of' and 'have'. Set and sit. There, their and they're. Draw and drawer. Could care less/couldn't care less. Bought/brought FFS.


Les Ross.            Certified by a Professional

Offline HondaMan

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Whaddayamean autochoke Mark? I have very supple wrists. :)


I just talk to mine, and let it run for 30 seconds.
:)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Acoustoman

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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!
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline Deltarider

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Quote
(timing is right on)
Did you test static or dynamic, in other words: is your advancer working as it should?
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Offline calj737

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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.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

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Offline Vinhead1957

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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

Offline Acoustoman

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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
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline flybox1

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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.
« Last Edit: May 16, 2015, 12:08:38 PM by flybox1 »
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
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Offline Acoustoman

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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
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline xtravbx

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Carb removal on the vf500f is THE WORST.

Keep us updated.

Offline harisuluv

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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?

Offline Acoustoman

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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
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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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.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Acoustoman

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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?
« Last Edit: May 16, 2015, 07:34:59 PM by Acoustoman »
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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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?
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Deltarider

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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.
« Last Edit: May 16, 2015, 11:59:15 PM by Deltarider »
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Offline Acoustoman

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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
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline HondaMan

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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. ;)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Acoustoman

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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.
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)

Offline Spanner 1

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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.
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Acoustoman

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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.
« Last Edit: May 17, 2015, 10:10:24 PM by Acoustoman »
1973 CB750K (My rider)
1976 CB550K (Next project)
1973 CB500K (Sold)
1981 CB650C (Sold)